8● CHAPTER 22 AMINES itrogen-containing compounds are essential to life. Their ultimate source is atmo- spheric nitrogen which, by a process known as nitrogen fixation, is reduced to ammonia, then converted to organic nitrogen compounds. This chapter describes the chemistry of amines, organic derivatives of ammonia. Alkylamines have their nitro- gen attached to sp-hybridized carbon; arylamines have their nitrogen attached to an sp--hybridized carbon of a benzene or benzene-like ring R=alkyl group Ar= aryl group Amines, like ammonia, are weak bases. They are, however, the strongest uncharged bases found in significant quantities under physiological conditions. Amines are usually the bases involved in biological acid-base reactions; they are often the nucleophiles in biological nucleophilic substitutions Our word""was coined in 1912 in the belief that the substances present in the diet that prevented scurvy, pellagra, beriberi, rickets, and other diseases were"vital amines. "In many cases, that belief was confirmed; certain vitamins did prove to amines. In many other cases, however, vitamins were not amines. Nevertheless, the name vitamin entered our language and stands as a reminder that early chemists recognized the crucial place occupied by amines in biological processes. 858 Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
CHAPTER 22 AMINES Nitrogen-containing compounds are essential to life. Their ultimate source is atmospheric nitrogen which, by a process known as nitrogen fixation, is reduced to ammonia, then converted to organic nitrogen compounds. This chapter describes the chemistry of amines, organic derivatives of ammonia. Alkylamines have their nitrogen attached to sp3 -hybridized carbon; arylamines have their nitrogen attached to an sp2 -hybridized carbon of a benzene or benzene-like ring. Amines, like ammonia, are weak bases. They are, however, the strongest uncharged bases found in significant quantities under physiological conditions. Amines are usually the bases involved in biological acid–base reactions; they are often the nucleophiles in biological nucleophilic substitutions. Our word “vitamin” was coined in 1912 in the belief that the substances present in the diet that prevented scurvy, pellagra, beriberi, rickets, and other diseases were “vital amines.” In many cases, that belief was confirmed; certain vitamins did prove to be amines. In many other cases, however, vitamins were not amines. Nevertheless, the name vitamin entered our language and stands as a reminder that early chemists recognized the crucial place occupied by amines in biological processes. R N R alkyl group: alkylamine Ar N Ar aryl group: arylamine 858 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
22.1 Amine Nomenclature 22.1 AMINE NOMENCLATURE Unlike alcohols and alkyl halides, which are classified as primary, secondary, or tertiary according to the degree of substitution at the carbon that bears the functional group, amines are classified according to their degree of substitution at nitrogen. An amine with one carbon attached to nitrogen is a primary amine, an amine with two is a secondary amine, and an amine with three is a tertiary amine Teri The groups attached to nitrogen may be any combination of alkyl or aryl groups Amines are named in two main ways, in the IUPAC system: either as alkylamines or as alkanamines. When primary amines are named as alkylamines, the ending -amine is added to the name of the alkyl group that bears the nitrogen. When named as alkan- amines, the alkyl group is named as an alkane and the -e ending replaced by -amine CH3CHNH CH3CHCH,CH,CH3 Ethylamine Cyclohexylamine I-Methylbutylamine (ethanamine) (cyclohexanamine) (2-pentanamine) PROBLEM 22 1 Give an acceptable alky lamine or alkanamine name for each of the following amines: (a)CH5 CH2NH (b)CsHs CHNH2 (c CH2=CHCH2NH SAMPLE SOLUTION (a)The amino substituent is bonded to an ethyl group that ears a phenyl substituent at C-2. the compound CsHs CH2 CH2 NH2 may be named as either 2-phenylethylamine or 2-phenylethanamine Aniline is the parent IUPAC name for amino-substituted derivatives of benzene Substituted derivatives of aniline are numbered beginning at the carbon that bears the 1826 as a degradation prod amino group. Substituents are listed in alphabetical order, and the direction of number- uct of indigo, a dark blue g is governed by the usual"first point of difference"rule. ye obtained from the West from which the name aniline NH NH, Br CHCH p-Fluoroaniline 5-Bromo-2-ethylaniline Arylamines may also be named as arenamines. Thus, benzenamine is an alterna tive, but rarely used, name for aniline. Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
22.1 AMINE NOMENCLATURE Unlike alcohols and alkyl halides, which are classified as primary, secondary, or tertiary according to the degree of substitution at the carbon that bears the functional group, amines are classified according to their degree of substitution at nitrogen. An amine with one carbon attached to nitrogen is a primary amine, an amine with two is a secondary amine, and an amine with three is a tertiary amine. The groups attached to nitrogen may be any combination of alkyl or aryl groups. Amines are named in two main ways, in the IUPAC system: either as alkylamines or as alkanamines. When primary amines are named as alkylamines, the ending -amine is added to the name of the alkyl group that bears the nitrogen. When named as alkanamines, the alkyl group is named as an alkane and the -e ending replaced by -amine. PROBLEM 22.1 Give an acceptable alkylamine or alkanamine name for each of the following amines: (a) C6H5CH2CH2NH2 (b) (c) CH2œCHCH2NH2 SAMPLE SOLUTION (a) The amino substituent is bonded to an ethyl group that bears a phenyl substituent at C-2. The compound C6H5CH2CH2NH2 may be named as either 2-phenylethylamine or 2-phenylethanamine. Aniline is the parent IUPAC name for amino-substituted derivatives of benzene. Substituted derivatives of aniline are numbered beginning at the carbon that bears the amino group. Substituents are listed in alphabetical order, and the direction of numbering is governed by the usual “first point of difference” rule. Arylamines may also be named as arenamines. Thus, benzenamine is an alternative, but rarely used, name for aniline. F 4 NH2 1 p-Fluoroaniline NH2 Br CH2CH3 5 1 2 5-Bromo-2-ethylaniline C6H5CHNH2 CH3 CH3CH2NH2 Ethylamine (ethanamine) NH2 Cyclohexylamine (cyclohexanamine) CH3CHCH2CH2CH3 NH2 1-Methylbutylamine (2-pentanamine) R N H H Primary amine N R H R Secondary amine N R R R Tertiary amine 22.1 Amine Nomenclature 859 Aniline was first isolated in 1826 as a degradation product of indigo, a dark blue dye obtained from the West Indian plant Indigofera anil, from which the name aniline is derived. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
CHAPTER TWENTY-TWo Amines Compounds with two amino groups are named by adding the suffix -diamine to the name of the corresponding alkane or arene. The final -e of the parent hydrocarbon H2NCH, CHCH3 H, CH, CH2CH, CH,CH,, H,N 1. 6-Hexanediamine 14-Benzenediamine for a. Amino groups rank rather low in seniority when the parent compound is identified naming purposes. Hydroxyl and carbonyl groups outrank amino groups. In these cases, the amino group is as a substituent HOCH, CHNH NH2 (4-Aminobenzenecarbaldehyde) Secondary and tertiary amines are named as N-substituted derivatives of primary mines.The parent primary amine is taken to be the one with the longest carbon chain. The prefix N- is added as a locant to identify substituents on the amino nitrogen as NHCH,CH3 N(CH3h2 CHANHCHCH N-Methylethylamine 4-Chloro-N-ethyl-3- N, N-Dimethylcyclo- nitroaniline (a secondary amine) (a secondary amine) (a tertiary amine) TPROBLEM 22.2 Assign alkanamine names to N-methylethylamine and to N, N- methylcycloheptylamine SAMPLE SOLUTION N-Methylethylamine (given as CH3NHCH2 CH3 in the pre ceding example)is an N-substituted derivative of ethanamine; it is N- PROBLEM 22.3 Classify the following amine as primary, secondary, or tertiary, and give it an acceptable IUPAC nam CH)2CH→ CHCH A nitrogen that bears four substituents is positively charged and is named as an ammonium ion. The anion that is associated with it is also identified in the name Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
Compounds with two amino groups are named by adding the suffix -diamine to the name of the corresponding alkane or arene. The final -e of the parent hydrocarbon is retained. Amino groups rank rather low in seniority when the parent compound is identified for naming purposes. Hydroxyl groups and carbonyl groups outrank amino groups. In these cases, the amino group is named as a substituent. Secondary and tertiary amines are named as N-substituted derivatives of primary amines. The parent primary amine is taken to be the one with the longest carbon chain. The prefix N- is added as a locant to identify substituents on the amino nitrogen as needed. PROBLEM 22.2 Assign alkanamine names to N-methylethylamine and to N,Ndimethylcycloheptylamine. SAMPLE SOLUTION N-Methylethylamine (given as CH3NHCH2CH3 in the preceding example) is an N-substituted derivative of ethanamine; it is Nmethylethanamine. PROBLEM 22.3 Classify the following amine as primary, secondary, or tertiary, and give it an acceptable IUPAC name. A nitrogen that bears four substituents is positively charged and is named as an ammonium ion. The anion that is associated with it is also identified in the name. (CH3)2CH N CH2CH3 CH3 CH3NHCH2CH3 N-Methylethylamine (a secondary amine) NO2 Cl 4 1 3 NHCH2CH3 4-Chloro-N-ethyl-3- nitroaniline (a secondary amine) N(CH3)2 N,N-Dimethylcycloheptylamine (a tertiary amine) HOCH2CH2NH2 2-Aminoethanol HC NH2 O 1 4 p-Aminobenzaldehyde (4-Aminobenzenecarbaldehyde) H2NCH2CHCH3 NH2 1,2-Propanediamine H2NCH2CH2CH2CH2CH2CH2NH2 1,6-Hexanediamine H2N NH2 1,4-Benzenediamine 860 CHAPTER TWENTY-TWO Amines Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
22.2 Structure and Bonding 861 47 ppm GURE 22.1A ball- and-stick model of methyl- pyramidal arrangement of stable conformation has the staggered arrangement of bonds shown. Other alkyl- CH CHINH2 CI chacha cfco CHS CH,N(CH3)3 I Methylammonium N-Ethyl-N-methylcyclopentyl Benzyltrimethyl chloride ammonium trifluoroacetate ammonium iodide Ammonium salts that have four alkyl groups bonded to nitrogen are called quaternary ammonium salts 22.2 STRUCTURE AND BONDING Alkylamines: As shown in Figure 22 1 methylamine, like ammonia, has a pyramidal arrangement of bonds to nitrogen. Its H-N-H angles(106)are slightly smaller than the tetrahedral value of 109.5, whereas the C-N-H angle (112 )is slightly larger The C-N bond distance of 147 pm lies between typical C-C bond distances in alkanes (153 pm)and C-O bond distances in alcohols(143 pm) An orbital hybridization description of bonding in methylamine is shown in Fig- ure 22.2. Nitrogen and carbon are both sp-hybridized and are joined by a o bond. The FIGURE 22.2 Orbital hybridization description of bonding in methylamine. (a)Carbon has four valence electrons; each of four equivalent sp-hybridized orbitals contains one electron Nitrogen has five valence electrons. Three of its sp hybrid orbitals contain one electron each; the fourth sp hybrid orbital contains two electrons. (b)Nitrogen and carbon are connected by a o bond in methylamine. This o bond is formed by overlap of an sp hybrid orbital on each atom. the five hy drogen atoms of methylamine are joined to carbon and nitrogen by o bonds. The two remaining electrons of nitrogen occupy an sp-hybridized orbital Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
Ammonium salts that have four alkyl groups bonded to nitrogen are called quaternary ammonium salts. 22.2 STRUCTURE AND BONDING Alkylamines: As shown in Figure 22.1 methylamine, like ammonia, has a pyramidal arrangement of bonds to nitrogen. Its H±N±H angles (106°) are slightly smaller than the tetrahedral value of 109.5°, whereas the C±N±H angle (112°) is slightly larger. The C±N bond distance of 147 pm lies between typical C±C bond distances in alkanes (153 pm) and C±O bond distances in alcohols (143 pm). An orbital hybridization description of bonding in methylamine is shown in Figure 22.2. Nitrogen and carbon are both sp3 -hybridized and are joined by a bond. The CH3NH3 Cl Methylammonium chloride NCH2CH3 CH3 H CF3CO2 N-Ethyl-N-methylcyclopentylammonium trifluoroacetate C6H5CH2N(CH3)3 I Benzyltrimethylammonium iodide (a quaternary ammonium salt) 22.2 Structure and Bonding 861 147 ppm 112 106 C N H H H H H (a) (b) FIGURE 22.1 A balland-stick model of methylamine showing the trigonal pyramidal arrangement of bonds to nitrogen. The most stable conformation has the staggered arrangement of bonds shown. Other alkylamines have similar geometries. FIGURE 22.2 Orbital hybridization description of bonding in methylamine. (a) Carbon has four valence electrons; each of four equivalent sp3 -hybridized orbitals contains one electron. Nitrogen has five valence electrons. Three of its sp3 hybrid orbitals contain one electron each; the fourth sp3 hybrid orbital contains two electrons. (b) Nitrogen and carbon are connected by a bond in methylamine. This bond is formed by overlap of an sp3 hybrid orbital on each atom. The five hydrogen atoms of methylamine are joined to carbon and nitrogen by bonds. The two remaining electrons of nitrogen occupy an sp3 -hybridized orbital. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
CHAPTER TWENTY-TWo Amines unshared electron pair on nitrogen occupies an sp-hybridized orbital. This lone pair is involved in reactions in which amines act as bases or nucleophiles. The graphic that You can opened this chapter is an electrostatic potential map that clearly shows the concentration cluding its electros of electron density at nitrogen in methylamine tial, in more detail By Modeling Arylamines: Aniline, like alkylamines, has a pyramidal arrangement of bonds around nitrogen, but its pyramid is somewhat shallower. One measure of the extent of this flat tening is given by the angle between the carbon-nitrogen bond and the bisector of the l80° Methylamine O=cHNH) For sp-hybridized nitrogen, this angle(not the same as the c-N--H bond angle)is 125, and the measured angles in simple alkylamines are close to that. The correspond ing angle for sp- hybridization at nitrogen with a planar arrangement of bonds, as in amines is discussed in an arti. amides, for example, is 180. The measured value for this angle in aniline is 142.5, sug cle entitled "What Is the gesting a hybridization somewhat closer to sp than to sp Geometry at Trigonal Nitro. gen?in the January 1998 is- The structure of aniline reflects a compromise between two modes of binding the nitrogen lone pair(Figure 22.3). The electrons are more strongly attracted to nitrogen Chemical Education, pp when they are in an orbital with some s character--an sp-hybridized orbital, for exam- ple--than when they are in a p orbital. On the other hand, delocalization of these elec trons into the aromatic T system is better achieved if they occupy a p orbital. A p orbital of nitrogen is better aligned for overlap with the p orbitals of the benzene ring to form a) FIGURE 22. 3 Electrostatic potential maps of the aniline in which the geometry at nitrogen is(a)nonplanar and(b)planar. In the nonplanar geometry, the unshared pair occupies an brid orbital of nitrogen. The region of highest In the planar geometry, nitrogen is sp 2-hybridize the electron pair is delocalized between p orbital of nitrogen and the t system of the he region of highest electron density in( b) encompasses both the ring and nitrogen the actual structure combines features of both; nitro- n adopts a hybridization state between sp and Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
unshared electron pair on nitrogen occupies an sp3 -hybridized orbital. This lone pair is involved in reactions in which amines act as bases or nucleophiles. The graphic that opened this chapter is an electrostatic potential map that clearly shows the concentration of electron density at nitrogen in methylamine. Arylamines: Aniline, like alkylamines, has a pyramidal arrangement of bonds around nitrogen, but its pyramid is somewhat shallower. One measure of the extent of this flattening is given by the angle between the carbon–nitrogen bond and the bisector of the H±N±H angle. For sp3 -hybridized nitrogen, this angle (not the same as the C±N±H bond angle) is 125°, and the measured angles in simple alkylamines are close to that. The corresponding angle for sp2 hybridization at nitrogen with a planar arrangement of bonds, as in amides, for example, is 180°. The measured value for this angle in aniline is 142.5°, suggesting a hybridization somewhat closer to sp3 than to sp2 . The structure of aniline reflects a compromise between two modes of binding the nitrogen lone pair (Figure 22.3). The electrons are more strongly attracted to nitrogen when they are in an orbital with some s character—an sp3 -hybridized orbital, for example—than when they are in a p orbital. On the other hand, delocalization of these electrons into the aromatic system is better achieved if they occupy a p orbital. A p orbital of nitrogen is better aligned for overlap with the p orbitals of the benzene ring to form ≈125 Methylamine (CH3NH2) Aniline (C6H5NH2) Formamide (OœCHNH2) 142.5 180 862 CHAPTER TWENTY-TWO Amines The geometry at nitrogen in amines is discussed in an article entitled “What Is the Geometry at Trigonal Nitrogen?” in the January 1998 issue of the Journal of Chemical Education, pp. 108–109. (a) (b) FIGURE 22.3 Electrostatic potential maps of the aniline in which the geometry at nitrogen is (a) nonplanar and (b) planar. In the nonplanar geometry, the unshared pair occupies an sp3 hybrid orbital of nitrogen. The region of highest electron density in (a) is associated with nitrogen. In the planar geometry, nitrogen is sp2 -hybridized and the electron pair is delocalized between a p orbital of nitrogen and the system of the ring. The region of highest electron density in (b) encompasses both the ring and nitrogen. The actual structure combines features of both; nitrogen adopts a hybridization state between sp3 and sp2 . You can examine the structure of methylamine, including its electrostatic potential, in more detail on Learning By Modeling. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website