896 Chapter 21 Amines H H N R R R N R H R N R H O H R 3 N CH 2 NH 2 NHCH 3

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1 896 Chapter 21 Amines Summary Section 21.1 Section 21.2 Section 21.3 Section 21.4 Alkylamines are compounds of the type shown, where R, R, and R are alkyl groups. ne or more of these groups is an aryl group in s. R amine R R Secondary amine R R R Tertiary amine Alkylamines are named in two ways. ne method adds the ending -amine to the name of the alkyl group. The other applies the principles of substitutive nomenclature by replacing the -e ending of an alkane name by -amine and uses appropriate locants to identify the position of the amino group. Arylamines are named as derivatives of aniline. itrogen s unshared electron pair is of major importance in understanding the structure and properties of amines. Alkylamines have a pyramidal arrangement of bonds to nitrogen, with an unshared electron pair in an sp 3 -hybridized orbital. The geometry at nitrogen in s is somewhat flatter, and the unshared electron pair is delocalized into the π system of the ring. Delocalization binds the electron pair more strongly in s than in alkylamines. Arylamines are less basic and less nucleophilic than alkylamines. Amines are less polar than alcohols. ydrogen bonding in amines is weaker than in alcohols because nitrogen is less electronegative than oxygen. Amines have lower boiling points than alcohols, but higher boiling points than alkanes. amines have higher boiling points than isomeric secondary amines; tertiary amines, which cannot form intermolecular hydrogen bonds, have the lowest boiling points. Amines resemble alcohols in their solubility in water. The basicity of amines is conveniently expressed in terms of the pk a of their conjugate acids. R 3 R 3 Conjugate acid of amine Amine The stronger base is associated with the weaker conjugate acid. The greater the pk a of the conjugate acid, the stronger the base. The pk a s of the conjugate acids of alkylamines lie in the 9 11 range. Arylamines are much weaker bases than alkylamines. The pk a s of the conjugate acids of s are usually 3 5. Strong electron-withdrawing groups can weaken the basicity of s even more. C 2 2 C 3 Benzylamine (alkylamine: pk a of conjugate acid 9.3) -Methylaniline (: pk a of conjugate acid 4.8) Section 21.5 Quaternary ammonium salts, compounds of the type R 4 X, find application as phase-transfer catalysts. A small amount of a quaternary ammonium salt promotes the transfer of an anion from aqueous solution, where it is highly solvated, to an organic solvent, where it is much less solvated and much more reactive. Sections Methods for the preparation of amines are summarized in Table Sections The reactions of amines are summarized in Tables 21.6 and

2 21.20 Summary 897 TABLE 21.5 Preparation of Amines Alkylation methods Alkylation of ammonia (Section 21.7) Ammonia can act as a nucleophile toward primary and some secondary alkyl halides to give primary alkylamines. Yields tend to be modest because the primary amine is itself a nucleophile and undergoes alkylation. Alkylation of ammonia can lead to a mixture containing a primary amine, a secondary amine, a tertiary amine, and a quaternary ammonium salt. RX 2 3 R 2 4 X Alkyl halide C 6 5 C 2 Cl Benzyl chloride (1 mol) Ammonia 3 (8 mol) Alkylamine C 6 5 C 2 2 Benzylamine (53%) Ammonium halide (C 6 5 C 2 ) 2 Dibenzylamine (39%) Alkylation of phthalimide. The Gabriel synthesis (Section 21.8) The potassium salt of phthalimide reacts with alkyl halides to give -alkylphthalimide derivatives. ydrolysis or hydrazinolysis of this derivative yields a primary alkylamine. RX K R Alkyl halide -Potassiophthalimide -Alkylphthalimide R 2 2 R 2 -Alkylphthalimide ydrazine amine Phthalhydrazide C 3 C CC 2 Cl 1-Chloro-2-butene 1. -potassiophthalimide, DMF , ethanol C 3 C CC Buten-1-amine (95%) Reduction methods Reduction of alkyl azides (Section 21.9) Alkyl azides, prepared by nucleophilic substitution by azide ion in primary or secondary alkyl halides, are d to primary alkylamines by lithium aluminum hydride or by catalytic hydrogenation. R Alkyl azide F 3 C 3 Ethyl 2-azido-4,4,4- trifluorobutanoate R 2 amine 2, Pd F 3 C 2 Ethyl 2-amino-4,4,4- trifluorobutanoate (96%) Reduction of nitriles (Section 21.9) itriles are d to primary amines by lithium aluminum hydride or by catalytic hydrogenation. R C itrile RC 2 2 amine C 1. LiAl C 2 2 Cyclopropyl cyanide Cyclopropylmethanamine (75%) Continued

3 898 Chapter 21 Amines TABLE 21.5 Preparation of Amines (Continued) Reduction of aryl nitro compounds (Section 21.9) The standard method for the preparation of an is by nitration of an aromatic ring, followed by reduction of the nitro group. Typical reducing agents include iron or tin in hydrochloric acid or catalytic hydrogenation. Ar 2 Ar 2 itroarene Arylamine C Fe, Cl 2. C itrobenzene Aniline (97%) Reduction of amides (Section 21.9) Lithium aluminum hydride s the carbonyl group of an amide to a methylene group., secondary, or tertiary amines may be prepared by proper choice of the starting amide. R and R may be either alkyl or aryl. RCR' 2 Amide RC 2 R' 2 Amine 1. LiAl tert-butylacetamide -tert-butyl--ethylamine (60%) Reductive amination (Section 21.10) Reaction of ammonia or an amine with an aldehyde or a ketone in the presence of a reducing agent is an effective method for the preparation of primary, secondary, or tertiary amines. The reducing agent may be either hydrogen in the presence of a metal catalyst or sodium cyanoborohydride. R, R, and R may be either alkyl or aryl. RCR' Aldehyde or ketone Acetone R" 2 Ammonia or an amine 2 Cyclohexylamine reducing agent 2, Pt R'' 2 RCR' Amine -Isopropylcyclohexylamine (79%) TABLE 21.6 Reactions of Amines Discussed in This Chapter Alkylation (Section 21.12) Amines act as nucleophiles toward alkyl halides. amines yield secondary amines, secondary amines yield tertiary amines, and tertiary amines yield quaternary ammonium salts. R 2 R'C 2 X RC2 R' amine Secondary amine R'C 2 X R(C2 R') 2 Tertiary amine R'C 2 X R(C2 R') 3 X Quaternary ammonium salt Cl 2-Chloromethylpyridine Pyrrolidine 2-(Pyrrolidinylmethyl)pyridine (93%) Continued

4 21.20 Summary 899 TABLE 21.6 Reactions of Amines Discussed in This Chapter (Continued) ofmann elimination (Section 21.13) Quaternary ammonium hydroxides undergo elimination on being ed. It is an anti elimination of the E2 type. The regioselectivity of the ofmann elimination is opposite to that of the Zaitsev rule and leads to the less highly substituted alkene. RC 2 CR' RC CR' (C 3 ) 3 (C 3 ) 3 Alkyltrimethylammonium hydroxide 2 Alkene Trimethylamine Water (C 3 ) 3 Cycloheptyltrimethylammonium hydroxide Cycloheptene (87%) Electrophilic aromatic substitution (Section 21.14) Arylamines are very reactive toward electrophilic aromatic substitution. It is customary to protect s as their -acyl derivatives before carrying out ring nitration, chlorination, bromination, sulfonation, or Friedel Crafts reactions. Ar E ArE Arylamine Electrophile Product of electrophilic Proton aromatic substitution acetic acid 2 2 p-itroaniline 2 2,6-Dibromo-4-nitroaniline (95%) itrosation (Sections ) itrosation of amines occurs when sodium nitrite is added to a solution containing an amine and an acid. amines yield alkyl diazonium salts. Alkyl diazonium salts are very unstable and yield carbocation-derived products. Aryl diazonium salts are exceedingly useful synthetic intermediates. Their reactions are described in Table R 2 a 2 R amine 2 3 Diazonium ion a 2, 2 S 4 2, 0 _ 5 o C S m-itroaniline m-itrobenzenediazonium hydrogen sulfate Secondary alkylamines and secondary s yield -nitroso amines. R 2 Secondary amine a 2 3 R 2 -itroso amine 2,6-Dimethylpiperidine a 2, Cl 2 2,6-Dimethyl-nitrosopiperidine (72%) Tertiary alkylamines illustrate no useful chemistry on nitrosation. Tertiary s undergo nitrosation of the ring by electrophilic aromatic substitution. (C 3 ) 2 a 2, Cl 2 (C 3 ) 2,-Dimethylaniline,-Dimethyl-4-nitrosoaniline (80 89%)

5 900 Chapter 21 Amines TABLE 21.7 Synthetically Useful Transformations Involving Aryl Diazonium Ions (Section 21.17) Reaction and comments Preparation of phenols eating its aqueous acidic solution converts a diazonium salt to a phenol. This is the most general method for the synthesis of phenols. Ar 2 1. a 2, 2 S 4, , Ar Phenol 2 1. a 2, 2 S 4, , 2 m-itroaniline 2 m-itrophenol (81 86%) Preparation of aryl fluorides Addition of fluoroboric acid to a solution of a diazonium salt causes the precipitation of an aryl diazonium fluoroborate. When the dry aryl diazonium fluoroborate is ed, an aryl fluoride results. This is the Schiemann reaction; it is the most general method for the preparation of aryl fluorides. Ar a 2, 3 Ar BF 4 2. BF 4 Aryldiazonium fluoroborate 1. a 2, Cl, 2 BF 4 ArF Aryl fluoride F C 3 2. BF 4 C 3 C 3 m-toluidine m-methylbenzenediazonium fluoroborate (76 84%) m-fluorotoluene (89%) Preparation of aryl iodides Aryl diazonium salts react with sodium or potassium iodide to form aryl iodides. This is the most general method for the synthesis of aryl iodides. Ar 2 1. a 2, 3 2. ai or KI ArI Aryl iodide 2 1. a 2, 2 S 4 2 I 2. ai 2 2 2,6-Dibromo-4-nitroaniline 1,3-Dibromo-2-iodo-5-nitrobenzene (84 88%) Preparation of aryl chlorides In the Sandmeyer reaction, a solution containing an aryl diazonium salt is treated with copper(i) chloride to give an aryl chloride. Ar 2 1. a 2, Cl, 2 2. CuCl ArCl Aryl chloride C a2, Cl, 2 2. CuCl C 3 Cl o-toluidine o-chlorotoluene (74 79%) Continued

6 21.20 Summary 901 TABLE 21.7 Synthetically Useful Transformations Involving Aryl Diazonium Ions (Section 21.17) (Continued ) Reaction and comments Preparation of aryl bromides The Sandmeyer reaction using copper(i) bromide converts primary s to aryl bromides. Ar 2 1. a 2,, 2 2. Cu Ar Aryl bromide 2 1. a 2,, 2 m-omoaniline 2. Cu m-dibromobenzene (80 87%) Preparation of aryl nitriles Copper(I) cyanide 1. a 2, 2 converts aryl diazonium salts to aryl nitriles. Ar 2 2. CuC ArC Aryl nitrile 2 2 m-itroaniline 1. a 2, Cl, 2 2. CuC C 2 o-itrobenzonitrile (87%) Reductive deamination of primary s The amino group of an can be replaced by hydrogen by treatment of its diazonium salt with ethanol or with hypophosphorous acid. Ar 2 1. a 2, 3 2. C 3 C 2 or 3 P 2 Ar Arene C 3 C Methyl-2-nitroaniline 1. a 2, Cl, P 2 2 m-itrotoluene (80%) Section The stretching frequency of primary and secondary amines appears in the infrared in the cm 1 region. In the MR spectra of amines, protons and carbons of the type C are more shielded than C. 3 C C 2 3 C C Amines have odd-numbered molecular weights, which helps identify them by mass spectrometry. Fragmentation tends to be controlled by the formation of a nitrogen-stabilized cation. C C C C

Alkylamines are compounds of the type shown, where R, R, and R are alkyl groups. One or more of these groups is an aryl group in arylamines.

22.20 Summary 955 22.20 SUMMARY Section 22.1 Alkylamines are compounds of the type shown, where R, R, and R are alkyl groups. ne or more of these groups is an aryl group in s. R R R R amine R Secondary