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cha75632_ch11_363-398.indd Page 363 9/16/09 9:15:04 PM user-s180 /Volumes/MHDQ-New/MHDQ144/MHDQ144-11 CHAPTER 11 HO H N O CH O The burning sensation of chili peppers such as habaneros is mostly due to the organic compound capsaicin (illustrated by its skeletal structure) Introduction to Organic Chemistry CHAPTER OUTLINE ESSENTIAL CONCEPTS 11.1 Classes of Organic Compounds 364 11.2 Aliphatic Hydrocarbons 364 Organic Compounds Organic compounds contain primarily carbon and hydrogen atoms, plus nitrogen, oxygen, sulfur, and atoms of other elements The parent compounds of all organic compounds are the hydrocarbons—the alkanes (containing only single bonds), the alkenes (containing carbon-carbon double bonds), the alkynes (containing carbon-carbon triple bonds), and the aromatic hydrocarbons (containing the benzene ring) Functional Groups The reactivity of organic compounds can be reliably predicted by the presence of functional groups, which are groups of atoms that are largely responsible for the chemical behavior of the compounds Chirality Certain organic compounds can exist as nonsuperimposable mirror-image twins These compounds are said to be chiral The pure enantiomer of a compound can rotate planepolarized light Enantiomers have identical physical properties but exhibit different chemical properties toward another chiral substance Alkanes • Cycloalkanes • Alkenes • Alkynes 11.3 Aromatic Hydrocarbons 379 Nomenclature of Aromatic Compounds • Properties and Reactions of Aromatic Compounds 11.4 Chemistry of the Functional Groups 382 Alcohols • Ethers • Aldehydes and Ketones • Carboxylic Acids • Esters • Amines • Summary of Functional Groups 11.5 Chirality—The Handedness of Molecules 389 STUDENT INTERACTIVE ACTIVITIES Animations Chirality (11.5) Electronic Homework Example Practice Problems End of Chapter Problems 363 cha75632_ch11_363-398.indd Page 364 9/16/09 9:15:23 PM user-s180 364 1A H 2A CHAPTER 11 /Volumes/MHDQ-New/MHDQ144/MHDQ144-11 Introduction to Organic Chemistry 8A 3A 4A 5A 6A 7A B C N O F Si P S Cl Br I Common elements in organic compounds 11.1 Classes of Organic Compounds Carbon can form more compounds than most other elements because carbon atoms are able not only to form single, double, and triple carbon-carbon bonds, but also to link up with each other in chains and ring structures The branch of chemistry that deals with carbon compounds is organic chemistry Classes of organic compounds can be distinguished according to functional groups they contain A functional group is a group of atoms that is largely responsible for the chemical behavior of the parent molecule Different molecules containing the same kind of functional group or groups undergo similar reactions Thus, by learning the characteristic properties of a few functional groups, we can study and understand the properties of many organic compounds In the second half of this chapter we will discuss the functional groups known as alcohols, ethers, aldehydes and ketones, carboxylic acids, and amines All organic compounds are derived from a group of compounds known as hydrocarbons because they are made up of only hydrogen and carbon On the basis of structure, hydrocarbons are divided into two main classes—aliphatic and aromatic Aliphatic hydrocarbons not contain the benzene group, or the benzene ring, whereas aromatic hydrocarbons contain one or more benzene rings 11.2 Aliphatic Hydrocarbons Aliphatic hydrocarbons are divided into alkanes, alkenes, and alkynes, discussed in this section (Figure 11.1) Alkanes For a given number of carbon atoms, the saturated hydrocarbon contains the largest number of hydrogen atoms Alkanes are hydrocarbons that have the general formula CnH2n12, where n 1, 2, The essential characteristic of alkanes is that only single covalent bonds are present The alkanes are known as saturated hydrocarbons because they contain the maximum number of hydrogen atoms that can bond with the number of carbon atoms present The simplest alkane (that is, with n 1) is methane CH4, which is a natural product of the anaerobic bacterial decomposition of vegetable matter under water Because it was first collected in marshes, methane became known as “marsh gas.” A rather improbable Figure 11.1 Classification of hydrocarbons Hydrocarbons Aromatic Aliphatic Alkanes Cycloalkanes Alkenes Alkynes cha75632_ch11_363-398.indd Page 365 9/16/09 9:15:27 PM user-s180 /Volumes/MHDQ-New/MHDQ144/MHDQ144-11 11.2 Aliphatic Hydrocarbons 365 Figure 11.2 H A H O CO H A H H H A A HOC OC OH A A H H H H H A A A HOC OC OC OH A A A H H H Methane Ethane Propane H H H H A A A A HOC OCO COC OH A A A A H H H H H A HOC OH A H A H A A A HO C OC OCOH A A A H H H n-Butane Isobutane Structures of the first four alkanes Note that butane can exist in two structurally different forms, called structural isomers but proven source of methane is termites When these voracious insects consume wood, the microorganisms that inhabit their digestive system break down cellulose (the major component of wood) into methane, carbon dioxide, and other compounds An estimated 170 million tons of methane are produced annually by termites! It is also produced in some sewage treatment processes Commercially, methane is obtained from natural gas Figure 11.2 shows the structures of the first four alkanes (n to n 4) Natural gas is a mixture of methane, ethane, and a small amount of propane We discussed the bonding scheme of methane in Chapter 10 The carbon atoms in all the alkanes can be assumed to be sp3-hybridized The structures of ethane and propane are straightforward, for there is only one way to join the carbon atoms in these molecules Butane, however, has two possible bonding schemes resulting in different compounds called n-butane (n stands for normal) and isobutane n-Butane is a straight-chain alkane because the carbon atoms are joined in a continuous chain In a branched-chain alkane like isobutane, one or more carbon atoms are bonded to a nonterminal carbon atom Isomers that differ in the order in which atoms are connected are called structural isomers In the alkane series, as the number of carbon atoms increases, the number of structural isomers increases rapidly For example, C4H10 has two isomers; C10H22 has 75 isomers; and C30H62 has over 400 million possible isomers! Obviously, most of these isomers not exist in nature nor have they been synthesized Nevertheless, the numbers help to explain why carbon is found in so many more compounds than any other element Termites are a natural source of methane EXAMPLE 11.1 How many structural isomers can be identified for pentane, C5H12? Strategy For small hydrocarbon molecules (eight or fewer C atoms), it is relatively easy to determine the number of structural isomers by trial and error Solution The first step is to write the straight-chain structure: H H H H H A A A A A HOCOCOCOCOCOH A A A A A H H H H H n-pentane (b.p 36.1°C) (Continued) n-pentane cha75632_ch11_363-398.indd Page 366 9/16/09 9:15:32 PM user-s180 366 CHAPTER 11 /Volumes/MHDQ-New/MHDQ144/MHDQ144-11 Introduction to Organic Chemistry The second structure, by necessity, must be a branched chain: H CH3 H H A A A A HOCOCO C OOCOCOH A A A A H H H H 2-methylbutane (b.p 27.9°C) 2-methylbutane Yet another branched-chain structure is possible: H CH3 H A A A HOCOCOOOCOH A A A H CH3 H 2,2-dimethylpropane (b.p 9.5°C) 2,2-dimethylpropane Similar problems: 11.11, 11.12 We can draw no other structure for an alkane having the molecular formula C5H12 Thus, pentane has three structural isomers, in which the numbers of carbon and hydrogen atoms remain unchanged despite the differences in structure Practice Exercise How many structural isomers are there in the alkane C6H14? Table 11.1 shows the melting and boiling points of the straight-chain isomers of the first 10 alkanes The first four are gases at room temperature; and pentane through decane are liquids As molecular size increases, so does the boiling point Drawing Chemical Structures Shortly we will discuss the nomenclature of alkanes Before proceeding further, it is useful to learn different ways of drawing the structure of organic compounds Consider the alkane 2-methylbutane (C5H12) To see how atoms are connected in this molecule, we need to first write a more detailed molecular formula, Table 11.1 Crude oil is the source of many hydrocarbons The First 10 Straight-Chain Alkanes Name of Hydrocarbon Molecular Formula Methane CH4 Ethane Number of Carbon Atoms Melting Point (°C) Boiling Point (°C) 2182.5 2161.6 CH3OCH3 2183.3 288.6 Propane CH3OCH2OCH3 2189.7 242.1 Butane CH3O(CH2)2OCH3 2138.3 20.5 Pentane CH3O(CH2)3OCH3 2129.8 36.1 Hexane CH3O(CH2)4OCH3 295.3 68.7 Heptane CH3O(CH2)5OCH3 290.6 98.4 Octane CH3O(CH2)6OCH3 256.8 125.7 Nonane CH3O(CH2)7OCH3 253.5 150.8 Decane CH3O(CH2)8OCH3 10 229.7 174.0 cha75632_ch11_363-398.indd Page 367 9/16/09 9:15:35 PM user-s180 /Volumes/MHDQ-New/MHDQ144/MHDQ144-11 11.2 Aliphatic Hydrocarbons H A HOCOH C A H A H H A A A A HOCOCOCOCOH C C C C A A A A H H H H CH3 A CH CH D G D H3C CH2 (a) (b) (c) (d) 367 Figure 11.3 CH3CH(CH3)CH2CH3, and then draw its structural formula, shown in Figure 11.3(a) While informative, this structure is time-consuming to draw Therefore, chemists have devised ways to simplify the representation Figure 11.3(b) is an abbreviated version and the structure shown in Figure 11.3(c) is called the skeletal structure in which all the C and H letters are omitted A carbon atom is assumed to be at each intersection of two lines (bonds) and at the end of each line Because every C atom forms four bonds, we can always deduce the number of H atoms bonded to any C atom One of the two end CH3 groups is represented by a vertical line What is lacking in these structures, however, is the three-dimensionality of the molecule, which is shown by the molecular model in Figure 11.3(d) Depending on the purpose of discussion, any of these representations can be used to describe the properties of the molecule Conformation of Ethane Molecular geometry gives the spatial arrangement of atoms in a molecule However, atoms are not held rigidly in position because of internal molecular motions For this reason, even a simple molecule like ethane may be structurally more complicated than we think The two C atoms in ethane are sp3-hybridized and they are joined by a sigma bond (see Section 10.5) Sigma bonds have cylindrical symmetry, that is, the overlap of the sp3 orbitals is the same regardless of the rotation of the COC bond Yet this bond rotation is not totally free because of the interactions between the H atoms on different C atoms Figure 11.4 shows the two extreme conformations of ethane Conformations are different spatial arrangements of a molecule that are generated by rotation about single bonds In the staggered conformation, the three H atoms on one C atom are pointing away from the three H atoms on the other C atom, whereas in the eclipsed conformation the two groups of H atoms are aligned parallel to one another A simpler and effective way of viewing these two conformations is by using the Newman projection, also shown in Figure 11.4 Look at the COC bond end-on The two C atoms are represented by a circle The COH bonds attached to the front C atom are the lines going to the center of the circle, and the COH bonds attached to the rear C atom appear as lines going to the edge of the circle The eclipsed form of ethane is less stable than the staggered form Figure 11.5 shows the variation in the potential energy of ethane as a function of rotation The rotation of one CH3 group relative to the other is described in terms of the angle between the COH bonds on Different representations of 2-methylbutane (a) Structural formula (b) Abbreviated formula (c) Skeletal formula (d) Molecular model Skeletal structure is the simplest structure Atoms other than C and H must be shown explicitly in a skeletal structure cha75632_ch11_363-398.indd Page 368 9/16/09 9:15:38 PM user-s180 368 CHAPTER 11 /Volumes/MHDQ-New/MHDQ144/MHDQ144-11 Introduction to Organic Chemistry Figure 11.4 Molecular models and Newman projections of the staggered and eclipsed conformations of ethane The dihedral angle in the staggered form is 60° and that in the eclipsed form is 0° The COC bond is rotated slightly in the Newman projection of the eclipsed form in order to show the H atoms attached to the back C atom The proximity of the H atoms on the two C atoms in the eclipsed form results in a greater repulsion, and hence its instability relative to the staggered form Staggered conformation Eclipsed conformation H HH Molecular models H H H H Newman projections H H H H H front and back carbons, called the dihedral angle The dihedral angle for the first eclipsed conformation is zero A clockwise rotation of 60° about the COC bond generates a staggered conformation, which is converted to another eclipsed conformation by a similar rotation and so on Conformational analysis of molecules is of great importance in understanding the details of reactions ranging from simple hydrocarbons to proteins and DNAs Alkane Nomenclature The nomenclature of alkanes and all other organic compounds is based on the recommendations of the International Union of Pure and Applied Chemistry (IUPAC) The first four alkanes (methane, ethane, propane, and butane) have nonsystematic names As Table 11.1 shows, the number of carbon atoms is reflected in the Greek Potential energy diagram for the internal rotation in ethane Here the dihedral angle is defined by the angle between the two COH bonds (with the red spheres representing the H atoms) Dihedral angles of 0°, 120°, 240°, and 360° represent the eclipsed conformation, while those of 60°, 180°, and 300° represent the staggered conformation Thus, a rotation of 60° changes the eclipsed conformation to the staggered one and vice versa The staggered conformation is more stable than the eclipsed conformation by 12 kJ/mol However, these two forms interconvert rapidly and cannot be separated from each other Potential energy Figure 11.5 12 kJ/mol 0° 60° 120° 180° Dihedral angle 240° 300° 360° cha75632_ch11_363-398.indd Page 369 10/28/09 4:56:02 PM user-s176 /Volumes/MHDQ-New/MHDQ144/MHDQ144-11 369 11.2 Aliphatic Hydrocarbons prefixes for the alkanes containing to 10 carbons We now apply the IUPAC rules to the following examples: The parent name of the hydrocarbon is that given to the longest continuous chain of carbon atoms in the molecule Thus, the parent name of the following compound is heptane because there are seven carbon atoms in the longest chain CH 4A CH 3OCH 2OCH 2OCHOCH 2OCH 2OCH An alkane less one hydrogen atom is an alkyl group For example, when a hydrogen atom is removed from methane, we are left with the CH3 fragment, which is called a methyl group Similarly, removing a hydrogen atom from the ethane molecule gives an ethyl group, or C2H5 Table 11.2 lists the names of several common alkyl groups Any chain branching off the longest chain is named as an alkyl group When one or more hydrogen atoms are replaced by other groups, the name of the compound must indicate the locations of carbon atoms where replacements are made The procedure is to number each carbon atom on the longest chain in the direction that gives the smaller numbers for the locations of all branches Consider the two different systems for the same compound shown below: CH Table 11.2 Common Alkyl Groups Name Formula Methyl OCH3 Ethyl OCH2OCH3 n-Propyl O(CH2)2OCH3 n-Butyl O(CH2)3OCH3 CH3 A Isopropyl OCOH A CH3 t-Butyl* CH3 A OCOCH3 A CH3 *The letter t stands for tertiary CH 2A 4A CH 3OCHOCH 2OCH 2OCH CH 3OCH 2OCH 2OCHOCH 2-methylpentane 4-methylpentane The compound on the left is numbered correctly because the methyl group is located at carbon of the pentane chain; in the compound on the right, the methyl group is located at carbon Thus, the name of the compound is 2-methylpentane, and not 4-methylpentane Note that the branch name and the parent name are written as a single word, and a hyphen follows the number When there is more than one alkyl branch of the same kind present, we use a prefix such as di-, tri-, or tetra- with the name of the alkyl group Consider the following examples: CH CH 3A CH 3OCHOCHOCH 2OCH 2OCH 2A CH 3A CH 3OCH 2OCOCH 2OCH 2OCH A CH 2,3-dimethylhexane 3,3-dimethylhexane When there are two or more different alkyl groups, the names of the groups are listed alphabetically For example, CH C H 4A CH 3OCH 2OCHOCHOCH 2OCH 2OCH 3A 4-ethyl-3-methylheptane Of course, alkanes can have many different types of substituents Table 11.3 lists the names of some substituents, including bromo and nitro Thus, the compound NO2 Br 2A 3A CH 3OCHOCHOCH 2OCH 2OCH Table 11.3 Names of Common Substituent Groups Functional Group Name ONH2 Amino OF Fluoro OCl Chloro OBr Bromo OI Iodo ONO2 Nitro OCHPCH2 Vinyl cha75632_ch11_363-398.indd Page 370 9/16/09 9:15:56 PM user-s180 370 CHAPTER 11 /Volumes/MHDQ-New/MHDQ144/MHDQ144-11 Introduction to Organic Chemistry is called 3-bromo-2-nitrohexane Note that the substituent groups are listed alphabetically in the name, and the chain is numbered in the direction that gives the lowest number to the first substituted carbon atom EXAMPLE 11.2 Give the IUPAC name of the following compound: CH3 CH3 A A CH 3OCOCH 2OCHOCH 2OCH A CH3 Strategy We follow the IUPAC rules and use the information in Table 11.2 to name the compound How many C atoms are there in the longest chain? Solution The longest chain has six C atoms so the parent compound is called hexane Note that there are two methyl groups attached to carbon number and one methyl group attached to carbon number CH3 2A CH3 4A CH 3OCOCH 2OCHOCH 2OCH A CH3 Similar problems: 11.28(a), (b), (c) Therefore, we call the compound 2,2,4-trimethylhexane Practice Exercise Give the IUPAC name of the following compound: CH3 C2H5 C2H5 A A A CH 3OCHOCH 2OCHOCH 2OCHOCH 2OCH Example 11.3 shows that prefixes such as di-, tri-, and tetra- are used as needed, but are ignored when alphabetizing EXAMPLE 11.3 Write the structural formula of 3-ethyl-2,2-dimethylpentane Strategy We follow the preceding procedure and the information in Table 11.2 to write the structural formula of the compound How many C atoms are there in the longest chain? Solution The parent compound is pentane, so the longest chain has five C atoms There are two methyl groups attached to carbon number and one ethyl group attached to carbon number Therefore, the structure of the compound is CH3 C2H5 3A CH 3OCOOCHOCH 2OCH A CH3 Similar problems: 11.27(a), (c), (e) 2A Practice Exercise Write the structural formula of 5-ethyl-2,6-dimethyloctane cha75632_ch11_363-398.indd Page 371 10/27/09 5:00:08 PM user-s180 /Volumes/MHDQ-New/MHDQ144/MHDQ144-11 11.2 Aliphatic Hydrocarbons Reactions of Alkanes Alkanes are generally not considered to be very reactive substances However, under suitable conditions they react For example, natural gas, gasoline, and fuel oil are alkanes that undergo highly exothermic combustion reactions: CH4 (g) 2O2 (g) ¡ CO2 (g) 2H2O(l) DH° 2890.4 kJ/mol 2C2H6 (g) 7O2 (g) ¡ 4CO2 (g) 6H2O(l) DH° 23119 kJ/mol These, and similar combustion reactions, have long been utilized in industrial processes and in domestic heating and cooking Halogenation of alkanes—that is, the replacement of one or more hydrogen atoms by halogen atoms—is another type of reaction that alkanes undergo When a mixture of methane and chlorine is heated above 100°C or irradiated with light of a suitable wavelength, methyl chloride is produced: CH4 (g) Cl2 (g) ¡ CH3Cl(g) HCl(g) methyl chloride If an excess of chlorine gas is present, the reaction can proceed further: CH3Cl(g) Cl2 (g) ¡ CH2Cl2 (l) HCl(g) methylene chloride CH2Cl2 (l) Cl2 (g) ¡ CHCl3 (l) HCl(g) chloroform CHCl3 (l) Cl2 (g) ¡ CCl4 (l) HCl(g) carbon tetrachloride A great deal of experimental evidence suggests that the initial step of the first halogenation reaction occurs as follows: Cl2 energy ¡ Cl ? Cl ? Thus, the covalent bond in Cl2 breaks and two chlorine atoms form We know it is the ClOCl bond that breaks when the mixture is heated or irradiated because the bond enthalpy of Cl2 is 242.7 kJ/mol, whereas about 414 kJ/mol are needed to break COH bonds in CH4 A chlorine atom is a radical, which contains an unpaired electron (shown by a single dot) Chlorine atoms are highly reactive and attack methane molecules according to the equation CH4 Cl ? ¡ ? CH3 HCl This reaction produces hydrogen chloride and the methyl radical ? CH3 The methyl radical is another reactive species; it combines with molecular chlorine to give methyl chloride and a chlorine atom: ? CH3 Cl2 ¡ CH3Cl Cl ? The production of methylene chloride from methyl chloride and any further reactions can be explained in the same way The actual mechanism is more complex than the scheme we have shown because “side reactions” that not lead to the desired products often take place, such as Cl ? Cl ? ¡ Cl2 ? CH3 ? CH3 ¡ C2H6 371 cha75632_ch11_363-398.indd Page 372 9/16/09 9:16:06 PM user-s180 372 CHAPTER 11 /Volumes/MHDQ-New/MHDQ144/MHDQ144-11 Introduction to Organic Chemistry Figure 11.6 Structures of the first four cycloalkanes and their simplified forms H H HH H C C H C H C C C C H H H H H H C C C C C H H H H HH H H Cyclobutane Cyclopentane H Cyclopropane The systematic names of methyl chloride, methylene chloride, and chloroform are chloromethane, dichloromethane, and trichloromethane, respectively H H H H H H C C H H C C H H C C H H H H Cyclohexane Alkanes in which one or more hydrogen atoms have been replaced by a halogen atom are called alkyl halides Among the large number of alkyl halides, the best known are chloroform (CHCl3), carbon tetrachloride (CCl4), methylene chloride (CH2Cl2), and the chlorofluorohydrocarbons Chloroform is a volatile, sweet-tasting liquid that was used for many years as an anesthetic However, because of its toxicity (it can severely damage the liver, kidneys, and heart) it has been replaced by other compounds Carbon tetrachloride, also a toxic substance, serves as a cleaning liquid, for it removes grease stains from clothing Methylene chloride is used as a solvent to decaffeinate coffee and as a paint remover Cycloalkanes In addition to C, atoms such as N, O, and S may also occupy the ring positions in these compounds Alkanes whose carbon atoms are joined in rings are known as cycloalkanes They have the general formula CnH2n, where n 3, 4, The simplest cycloalkane is cyclopropane, C3H6 (Figure 11.6) Many biologically significant substances such as antibiotics, sugars, cholesterol, and hormones contain one or more such ring systems Cyclohexane can assume two different conformations called the chair and boat that are relatively free of angle strain (Figure 11.7) By “angle strain” we mean that the bond angles at each carbon atom deviate from the tetrahedral value of 109.5° required for sp3 hybridization Alkenes The alkenes (also called olefins) contain at least one carbon-carbon double bond Alkenes have the general formula CnH2n, where n 2, 3, The simplest alkene Figure 11.7 Axial The cyclohexane molecule can exist in various shapes The most stable shape is the chair conformation and a less stable one is the boat conformation Two types of H atoms are labeled axial and equatorial, respectively Equatorial Chair conformation Boat conformation cha75632_ch11_363-398.indd Page 373 9/16/09 9:16:10 PM user-s180 /Volumes/MHDQ-New/MHDQ144/MHDQ144-11 11.2 Aliphatic Hydrocarbons is C2H4, ethylene, in which both carbon atoms are sp2-hybridized and the double bond is made up of a sigma bond and a pi bond (see Section 10.5) Geometric Isomers of Alkenes In a compound such as ethane, C2H6, the rotation of the two methyl groups about the carbon-carbon single bond (which is a sigma bond) is quite free The situation is different for molecules that contain carbon-carbon double bonds, such as ethylene, C2H4 In addition to the sigma bond, there is a pi bond between the two carbon atoms Rotation about the carbon-carbon linkage does not affect the sigma bond, but it does move the two 2pz orbitals out of alignment for overlap and, hence, partially or totally destroys the pi bond (see Figure 10.15) This process requires an input of energy on the order of 270 kJ/mol For this reason, the rotation of a carbon-carbon double bond is considerably restricted, but not impossible Consequently, molecules containing carbon-carbon double bonds (that is, the alkenes) may have geometric isomers, which have the same type and number of atoms and the same chemical bonds but different spatial arrangements Such isomers cannot be interconverted without breaking a chemical bond The molecule dichloroethylene, ClHCPCHCl, can exist as one of the two geometric isomers called cis-1,2-dichloroethylene and trans-1,2-dichloroethylene: resultant m 88 n888 m 88 dipole moment m Cl Cl G D CPC D G H H cis-1,2-dichloroethylene ϭ 1.89 D b.p 60.3ЊC m 88 88 m 88 m 88 88 m 88 m 88 Cl G D CPC D G Cl H m H trans-1,2-dichloroethylene ϭ0 b.p 47.5ЊC where the term cis means that two particular atoms (or groups of atoms) are adjacent to each other, and trans means that the two atoms (or groups of atoms) are across from each other Generally, cis and trans isomers have distinctly different physical and chemical properties Heat or irradiation with light is commonly used to bring about the conversion of one geometric isomer to another, a process called cis-trans isomerization, or geometric isomerization (Figure 11.8) A H B H 90° rotation A H H 90° rotation B A H H B Figure 11.8 Breaking and remaking the pi bond When a compound containing a CPC bond is heated or excited by light, the weaker pi bond is broken This allows the free rotation of the single carbonto-carbon sigma bond A rotation of 180° converts a cis isomer to a trans isomer or the other way around Note that a dashed line represents a bond axis behind the plane of the paper, the wedged line represents a bond axis in front of the paper, and the solid line represents bonds in the plane of the paper The letters A and B represent atoms (other than H) or groups of atoms Here we have a cis-to-trans isomerization 373 cha75632_ch11_363-398.indd Page 374 9/16/09 9:16:14 PM user-s180 374 CHAPTER 11 /Volumes/MHDQ-New/MHDQ144/MHDQ144-11 Introduction to Organic Chemistry Figure 11.9 The primary event in the vision process is the conversion of 11-cis retinal to the all-trans isomer on rhodopsin The double bond at which the isomerization occurs is between carbon-11 and carbon-12 For simplicity, most of the H atoms are omitted In the absence of light, this transformation takes place about once in a thousand years! Cis-trans Isomerization in the Vision Process The molecules in the retina that respond to light are rhodopsin, which has two components called 11-cis retinal and opsin (Figure 11.9) Retinal is the light-sensitive component and opsin is a protein molecule Upon receiving a photon in the visible region the 11-cis retinal isomerizes to the all-trans retinal by breaking a carbon-carbon pi bond With the pi bond broken, the remaining carbon-carbon sigma bond is free to rotate and transforms into the all-trans retinal At this point an electrical impulse is generated and transmitted to the brain, which forms a visual image The all-trans retinal does not fit into the binding site on opsin and eventually separates from the protein In time, the trans isomer is converted back to 11-cis retinal by an enzyme (in the absence of light) and rhodopsin is regenerated by the binding of the cis isomer to opsin and the visual cycle can begin again Alkene Nomenclature An electron micrograph of rod-shaped cells (containing rhodopsins) in the retina In naming alkenes we indicate the positions of the carbon-carbon double bonds The names of compounds containing CPC bonds end with -ene As with the alkanes, the name of the parent compound is determined by the number of carbon atoms in the longest chain (see Table 11.1), as shown here: CH2“CHOCH2OCH3 H3COCH“CHOCH3 1-butene 2-butene The numbers in the names of alkenes refer to the lowest numbered carbon atom in the chain that is part of the CPC bond of the alkene The name “butene” means that there are four carbon atoms in the longest chain Alkene nomenclature must specify whether a given molecule is cis or trans if it is a geometric isomer, such as In the cis isomer, the two H atoms are on the same side of the CP C bond; in the trans isomer, the two H atoms are across from each other CH H 3C 4A CH C OCH OCH G 3D CPC D G H H cis-4-methyl-2-hexene H 3C H G2 3D CPC D G4 C OCH 2OCH H CH A CH trans-4-methyl-2-hexene Properties and Reactions of Alkenes Ethylene is an extremely important substance because it is used in large quantities in manufacturing organic polymers (very large molecules) and in preparing many other cha75632_ch11_363-398.indd Page 375 9/16/09 9:16:18 PM user-s180 /Volumes/MHDQ-New/MHDQ144/MHDQ144-11 11.2 Aliphatic Hydrocarbons organic chemicals Ethylene and other alkenes are prepared industrially by the cracking process, that is, the thermal decomposition of a large hydrocarbon into smaller molecules When ethane is heated to about 800°C in the presence of platinum, it undergoes the following reaction: Pt C2H6 (g) ¡ CH2 “CH2 (g) H2 (g) catalyst The platinum acts as a catalyst, a substance that speeds up a reaction without being used up in the process and therefore does not appear on either side of the equation Other alkenes can be prepared by cracking the higher members of the alkane family Alkenes are classified as unsaturated hydrocarbons, compounds with double or triple carbon-carbon bonds Unsaturated hydrocarbons commonly undergo addition reactions in which one molecule adds to another to form a single product An example of an addition reaction is hydrogenation, which is the addition of molecular hydrogen to compounds containing CPC and CqC bonds H H H A A D G H2 ϩ CPC 888n HOCOCOH D G A A H H H H H Hydrogenation is an important process in the food industry Vegetable oils have considerable nutritional value, but many oils must be hydrogenated to eliminate some of the CPC bonds before they can be used to prepare food Upon exposure to air, polyunsaturated molecules—molecules with many CPC bonds—undergo oxidation to yield unpleasant-tasting products (vegetable oil that has oxidized is said to be rancid) In the hydrogenation process, a small amount of nickel catalyst is added to the oil and the mixture is exposed to hydrogen gas at high temperature and pressure Afterward, the nickel is removed by filtration Hydrogenation reduces the number of double bonds in the molecule but does not completely eliminate them If all the double bonds are eliminated, the oil becomes hard and brittle (Figure 11.10) Under controlled (a) (b) Figure 11.10 Oils and fats have side chains that resemble hydrocarbons (a) The side chains of oils contain one or more CPC bonds The cis form of the hydrocarbon chains prevents close packing of the molecules Therefore, oils are liquids (b) Upon hydrogenation, the saturated hydrocarbon chains stack well together As a result, fats have higher density than oils and are solids at room temperature 375 cha75632_ch11_363-398.indd Page 376 9/16/09 9:16:22 PM user-s180 376 CHAPTER 11 /Volumes/MHDQ-New/MHDQ144/MHDQ144-11 Introduction to Organic Chemistry Figure 11.11 When ethylene gas is bubbled through an aqueous bromine solution, the reddish brown color gradually disappears due to the formation of 1,2-dibromoethane, which is colorless conditions, suitable cooking oils and margarine may be prepared by hydrogenation from vegetable oils extracted from cottonseed, corn, and soybeans Other addition reactions to the CPC bond involve the hydrogen halides and halogens (Figure 11.11): H2C“CH2 X2 ¡ CH2XOCH2X H2C“CH2 HX ¡ CH3OCH2X in which X represents a halogen atom Figure 11.12 shows the electron density maps of HCl and ethylene When the two molecules react, the interaction is between the electron-rich region (pi electrons of the double bond) and the electron-poor region of HCl, which is the H atom The steps are H A ϩ H2 CPCH2 ϩ HCl 888n H2C COCH2 ϩ Cl Ϫ 888n CH3 —CH2 Cl The addition of a hydrogen halide to an unsymmetrical alkene such as propene is more complicated because two products are possible: H H H A A D G ϩ HBr 888n HOCOCOCH C C CPC D G A A H CH3 Br H H propene Figure 11.12 The addition reaction between HCl and ethylene The initial interaction is between the positive end of HCl (blue) and the electron-rich region of ethylene (red), which is associated with the pi electrons of the CPC bond 1-bromopropane and/or H H A A HOCOCOCH C C A A H Br 2-bromopropane In reality, however, only 2-bromopropane is formed This phenomenon was observed in all reactions between unsymmetrical reagents and alkenes In 1871 the Russian chemist Vladimir Markovnikov postulated a generalization that enables us to predict the outcome of such an addition reaction This generalization, now known as Markovnikov’s rule, states that in the addition of unsymmetrical (that is, polar) reagents to alkenes, the positive portion of the reagent (usually hydrogen) adds to the carbon atom in the double bond that already has the most hydrogen atoms As the marginal figure on p 377 shows, the C atom to which the two H atoms are cha75632_ch11_363-398.indd Page 377 9/16/09 9:16:26 PM user-s180 /Volumes/MHDQ-New/MHDQ144/MHDQ144-11 11.2 Aliphatic Hydrocarbons 377 Figure 11.13 Structure of polyethylene Each carbon atom is sp3-hybridized attached has a higher electron density Therefore, this is the site for the H1 ion (from HBr) to form a COH bond, followed by the formation of the COBr bond on the other C atom Finally we note that ethylene undergoes a different type of addition reaction that leads to the formation of a polymer In this process, first an initiator molecule (R2) is heated to produce two radicals: R2 ¡ 2R ? The reactive radical attacks an ethylene molecule to generate a new radical (it is the pi bond that is broken in the polymerization of ethylene): R ? CH2“CH2 ¡ ROCH2OCH2 ? which further reacts with another ethylene molecule, and so on: The electron density is higher on the carbon atom in the CH2 group in propene This is the site of hydrogen addition by hydrogen halides ROCH2OCH2 ? CH2“CH2 ¡ ROCH2OCH2OCH2OCH2 ? Very quickly a long chain of CH2 groups is built Eventually, this process is terminated by the combination of two long-chain radicals to give the polymer called polyethylene (Figure 11.13): ROCH ( ) nCH2CH2? ROCH ( ) nCH2CH2? ¡ 2OCH2O 2OCH2 O ROCH ( OCH O ) CH CH ( ) nR 2 n 2OCH2CH2OCH 2OCH2O where O ( CH2OCH2O ) n is a convenient shorthand convention for representing the repeating unit in the polymer The value of n is understood to be very large, on the order of thousands Under different conditions, it is possible to prepare polyethylene with branched chains Today, many different forms of polyethylene with widely different physical properties are known Polyethylene is mainly used in films in frozen food packaging and other product wrappings A specially treated type of polyethylene, called Tyvek, is used in home construction and mailer envelopes Alkynes Alkynes contain at least one carbon-carbon triple bond They have the general formula CnH2n22, where n 2, 3, Alkyne Nomenclature Names of compounds containing CqC bonds end with -yne Again the name of the parent compound is determined by the number of carbon atoms in the longest chain (see Table 11.1 for names of alkane counterparts) As in the case of alkenes, the names of alkynes indicate the position of the carbon-carbon triple bond, as, for example, in HC‚COCH2OCH3 H3COC‚COCH3 1-butyne 2-butyne Common mailing envelopes made of Tyvek cha75632_ch11_363-398.indd Page 378 378 CHAPTER 11 10/27/09 5:00:18 PM user-s180 /Volumes/MHDQ-New/MHDQ144/MHDQ144-11 Introduction to Organic Chemistry Properties and Reactions of Alkynes The simplest alkyne is ethyne, better known as acetylene (C2H2) The structure and bonding of C2H2 were discussed in Section 10.5 Acetylene is a colorless gas (b.p 284°C) prepared in the laboratory by the reaction between calcium carbide and water: CaC2 (s) 2H2O(l) ¡ C2H2 (g) Ca(OH) (aq) Industrially, it is prepared by the thermal decomposition of ethylene at about 1100°C: C2H4 (g) ¡ C2H2 (g) H2 (g) Acetylene has many important uses in industry Because of its high heat of combustion 2C2H2 (g) 5O2 (g) ¡ 4CO2 (g) 2H2O(l) The reaction of calcium carbide with water produces acetylene, a flammable gas DH° 22599.2 kJ/mol acetylene burned in an “oxyacetylene torch” gives an extremely hot flame (about 3000°C) Thus, oxyacetylene torches are used to weld metals (see p 200) Acetylene is unstable and has a tendency to decompose: C2H2 (g) ¡ 2C(s) H2 (g) In the presence of a suitable catalyst or when the gas is kept under pressure, this reaction can occur with explosive violence To be transported safely, it must be dissolved in an inert organic solvent such as acetone at moderate pressure In the liquid state, acetylene is very sensitive to shock and is highly explosive Being an unsaturated hydrocarbon, acetylene can be hydrogenated to yield ethylene: C2H2 (g) H2 (g) ¡ C2H4 (g) It undergoes these addition reactions with hydrogen halides and halogens: CHqCH(g) HX(g) ¡ CH2“CHX(g) CHqCH(g) X2 (g) ¡ CHX“CHX(g) CHqCH(g) 2X2 (g) ¡ CHX2OCHX2 (l) Methylacetylene (propyne), CH3OCqCOH, is the next member in the alkyne family It undergoes reactions similar to those of acetylene The addition reactions of propyne also obey Markovnikov’s rule: H 3C CH OCqCOH ϩ HBr 888n Propyne Can you account for Markovnikov’s rule in this molecule? propyne H G D CPC D G Br H 2-bromopropene R EVIEW OF CONCEPTS How could an alkene and an alkyne be distinguished by using only a hydrogenation reaction? cha75632_ch11_363-398.indd Page 379 9/16/09 9:16:34 PM user-s180 /Volumes/MHDQ-New/MHDQ144/MHDQ144-11 379 11.3 Aromatic Hydrocarbons 11.3 Aromatic Hydrocarbons Benzene (C6H6) is the parent compound of this large family of organic substances As we saw in Section 9.8, the properties of benzene are best represented by both of the following resonance structures (p 304): mn Benzene is a planar hexagonal molecule with carbon atoms situated at the six corners All carbon-carbon bonds are equal in length and strength, as are all carbon-hydrogen bonds, and the CCC and HCC angles are all 120° Therefore, each carbon atom is sp2-hybridized; it forms three sigma bonds with two adjacent carbon atoms and a hydrogen atom (Figure 11.14) This arrangement leaves an unhybridized 2pz orbital on each carbon atom, perpendicular to the plane of the benzene molecule, or benzene ring, as it is often called So far the description resembles the configuration of ethylene (C2H4), discussed in Section 10.5, except that in this case there are six unhybridized 2pz orbitals in a cyclic arrangement Because of their similar shape and orientation, each 2pz orbital overlaps two others, one on each adjacent carbon atom According to the rules listed on p 351, the interaction of six 2pz orbitals leads to the formation of six pi molecular orbitals, of which three are bonding and three antibonding A benzene molecule in the ground state therefore has six electrons in the three pi bonding molecular orbitals, two electrons with paired spins in each orbital (Figure 11.15) In the ethylene molecule, the overlap of the two 2pz orbitals gives rise to a bonding and an antibonding molecular orbital, which are localized over the two C atoms The interaction of the 2pz orbitals in benzene, however, leads to the formation of delocalized molecular orbitals, which are not confined between two adjacent bonding atoms, but actually extend over three or more atoms Therefore, electrons residing in any of these orbitals are free to move around the benzene ring For this reason, the structure of benzene is sometimes represented as An electron micrograph of benzene molecules, which shows clearly the ring structure Electrostatic potential map of benzene shows the electron density (red color) above and below the plane of the molecule For simplicity, only the framework of the molecule is shown H C H C C C H Top view H C Side view C H H Figure 11.14 The sigma bond framework of the benzene molecule Each C atom is sp2-hybridized and forms sigma bonds with two adjacent C atoms and another sigma bond with an H atom (a) (b) Figure 11.15 (a) The six 2pz orbitals on the carbon atoms in benzene (b) The delocalized molecular orbital formed by the overlap of the 2pz orbitals The delocalized molecular orbital possesses pi symmetry and lies above and below the plane of the benzene ring Actually, these 2pz orbitals can combine in six different ways to yield three bonding molecular orbitals and three antibonding molecular orbitals The one shown here is the most stable cha75632_ch11_363-398.indd Page 380 9/16/09 9:16:41 PM user-s180 380 CHAPTER 11 /Volumes/MHDQ-New/MHDQ144/MHDQ144-11 Introduction to Organic Chemistry in which the circle indicates that the pi bonds between carbon atoms are not confined to individual pairs of atoms; rather, the pi electron densities are evenly distributed throughout the benzene molecule As we will see shortly, electron delocalization imparts extra stability to aromatic hydrocarbons We can now state that each carbon-to-carbon linkage in benzene contains a sigma bond and a “partial” pi bond The bond order between any two adjacent carbon atoms is therefore between and Thus, molecular orbital theory offers an alternative to the resonance approach, which is based on valence bond theory Nomenclature of Aromatic Compounds The naming of monosubstituted benzenes, that is, benzenes in which one H atom has been replaced by another atom or a group of atoms, is quite straightforward, as shown next: CH2CH3 A ethylbenzene Cl A NH2 A chlorobenzene NO2 A aminobenzene (aniline) nitrobenzene If more than one substituent is present, we must indicate the location of the second group relative to the first The systematic way to accomplish this is to number the carbon atoms as follows: Three different dibromobenzenes are possible: Br A Br Br A Br A E H Br 1,2-dibromobenzene (o-dibromobenzene) 1,3-dibromobenzene (m-dibromobenzene) A Br 1,4-dibromobenzene (p-dibromobenzene) The prefixes o- (ortho-), m- (meta-), and p- (para-) are also used to denote the relative positions of the two substituted groups, as just shown for the dibromobenzenes Compounds in which the two substituted groups are different are named accordingly Thus, NO2 A H Br is named 3-bromonitrobenzene, or m-bromonitrobenzene cha75632_ch11_363-398.indd Page 381 9/16/09 9:16:49 PM user-s180 /Volumes/MHDQ-New/MHDQ144/MHDQ144-11 11.3 Aromatic Hydrocarbons 381 Finally we note that the group containing benzene minus a hydrogen atom (C6H5) is called the phenyl group Thus, the following molecule is called 2-phenylpropane: This compound is also called isopropylbenzene (see Table 11.2) A CH3OCHOCH3 Properties and Reactions of Aromatic Compounds Benzene is a colorless, flammable liquid obtained chiefly from petroleum and coal tar Perhaps the most remarkable chemical property of benzene is its relative inertness Although it has the same empirical formula as acetylene (CH) and a high degree of unsaturation, it is much less reactive than either ethylene or acetylene The stability of benzene is the result of electron delocalization In fact, benzene can be hydrogenated, but only with difficulty The following reaction is carried out at significantly higher temperatures and pressures than are similar reactions for the alkenes: H H H A EH H H H GD H G DH HO O Pt ϩ 3H2 8888n catalyst OH HO G E HH D H H DG H A H H H cyclohexane We saw earlier that alkenes react readily with halogens and hydrogen halides to form addition products, because the pi bond in CPC can be broken more easily The most common reaction of halogens with benzene is substitution For example, HH H A EH E HH H A H FeBr3 ϩ Br2 8888n catalyst HH Br A EH E HH H A H ϩ HBr bromobenzene Note that if the reaction were addition, electron delocalization would be destroyed in the product H H H A Br D H O OBr E G H A H H and the molecule would not have the aromatic characteristic of chemical unreactivity A catalyst is a substance that can speed up the rate of a reaction without itself being used up More on this topic in Chapter 14 cha75632_ch11_363-398.indd Page 382 9/16/09 9:16:57 PM user-s180 382 CHAPTER 11 /Volumes/MHDQ-New/MHDQ144/MHDQ144-11 Introduction to Organic Chemistry Figure 11.16 Some polycyclic aromatic hydrocarbons Compounds denoted by * are potent carcinogens An enormous number of such compounds exist in nature Naphthalene Anthracene Benz(a)anthracene* Phenanthrene Naphthacene Dibenz(a,h)anthracene* Benzo(a)pyrene Alkyl groups can be introduced into the ring system by allowing benzene to react with an alkyl halide using AlCl3 as the catalyst: CH2CH3 A AlCl ϩ CH3CH2Cl 8888n catalyst ethyl chloride ϩ HCl ethylbenzene An enormously large number of compounds can be generated from substances in which benzene rings are fused together Some of these polycyclic aromatic hydrocarbons are shown in Figure 11.16 The best known of these compounds is naphthalene, which is used in mothballs These and many other similar compounds are present in coal tar Some of the compounds with several rings are powerful carcinogens—they can cause cancer in humans and other animals R EVIEW OF CONCEPTS Benzene has sp2-hybridized carbon atoms and multiple bonds However, unlike ethylene, geometric isomerism is not possible in benzene Explain 11.4 Chemistry of the Functional Groups We now examine some organic functional groups, groups that are responsible for most of the reactions of the parent compounds In particular, we focus on oxygen-containing and nitrogen-containing compounds Alcohols All alcohols contain the hydroxyl functional group, OOH Some common alcohols are shown in Figure 11.17 Ethyl alcohol, or ethanol, is by far the best known It is produced biologically by the fermentation of sugar or starch In the absence of oxygen the enzymes present in bacterial cultures or yeast catalyze the reaction enzymes C6H12O6 (aq) O¡ 2CH3CH2OH(aq) 2CO2 (g) C2H5OH ethanol ... 21 82. 5 21 61.6 CH3OCH3 21 83.3 28 8.6 Propane CH3OCH2OCH3 21 89.7 24 2.1 Butane CH3O(CH2)2OCH3 21 38.3 20 .5 Pentane CH3O(CH2)3OCH3 21 29.8 36.1 Hexane CH3O(CH2)4OCH3 29 5.3 68.7 Heptane CH3O(CH2)5OCH3 29 0.6... alkanes that undergo highly exothermic combustion reactions: CH4 (g) 2O2 (g) ¡ CO2 (g) 2H2O(l) DH° 28 90.4 kJ/mol 2C2H6 (g) 7O2 (g) ¡ 4CO2 (g) 6H2O(l) DH° 23 119 kJ/mol These, and similar combustion... by the combination of two long-chain radicals to give the polymer called polyethylene (Figure 11.13): ROCH ( ) nCH2CH2? ROCH ( ) nCH2CH2? ¡ 2OCH2O 2OCH2 O ROCH ( OCH O ) CH CH ( ) nR 2 n 2OCH2CH2OCH