22.5 Basicity of Amines Recall from Section 4.6 that acid-base reactions are char. H—OCCH3 OCCH cterized by equilibrium con Methylamine Acetate the left side of the equation (stronger acid; pKa= 4.7) eaker acid; pK=10.7) ind the weaker acid on the Conversely, adding sodium hydroxide to an ammonium salt converts it to the free CH →CH3NH2+ H-OH Methylammor Hydroxide ion Methylamine (stronger acid; pKa =10.7) ply the Henderson-Hasselbalch equation (see"Quantitative Relationships Involving Carboxylic Acids, the box accompanying Section 19.4)to calculate the CHaNH3 T/CH3 NH, ratio in water buffered at pH 7. Their basicity provides a means by which amines may be separated from neutral organic compounds. A mixture containing an amine is dissolved in diethyl ether and shaken with dilute hydrochloric acid to convert the amine to an ammonium salt. The ammonium salt, being ionic, dissolves in the aqueous phase, which is separated from the ether layer. Adding sodium hydroxide to the aqueous layer converts the ammonium salt back to the free amine, which is then removed from the aqueous phase by extraction with a fresh portion of ether. 22.5 BASICITY OF AMINES Amines are weak bases, but as a class, amines are the strongest bases of all neutral mol- ecules. Table 22 1 lists basicity data for a number of amines. The most important rela- tionships to be drawn from the data are 1. Alkylamines are slightly stronger bases than ammonia. 2. Alkylamines differ very little among themselves in basicity. Their basicities cover a range of less than 10 in equilibrium constant (1 pK unit). 3. Arylamines are much weaker bases than ammonia and alkylamines. Their basicity constants are on the order of 10 smaller than those of alkylamines(6 pK units) The differences in basicity between ammonia, and primary, secondary, and tertiary alkylamines result from the interplay between steric and electronic effects on the mole- cules themselves and on the solvation of their conjugate acids. In total, the effects are mall, and most alkylamines are very similar in basicity Arylamines are a different story, however; most are about a million times weaker as bases than ammonia and alkylamines As unfavorable as the equilibrium is for cyclohexylamine acting as a base in nh h,o NH3 HO (Kb44×10;pKb3.4) Cyclohexylamine Water Cyclohexylammonium Hydroxide Back Forward Main MenuToc Study Guide ToC Student o MHHE WebsiteConversely, adding sodium hydroxide to an ammonium salt converts it to the free amine: PROBLEM 22.6 Apply the Henderson–Hasselbalch equation (see “Quantitative Relationships Involving Carboxylic Acids,” the box accompanying Section 19.4) to calculate the CH3NH3 /CH3NH2 ratio in water buffered at pH 7. Their basicity provides a means by which amines may be separated from neutral organic compounds. A mixture containing an amine is dissolved in diethyl ether and shaken with dilute hydrochloric acid to convert the amine to an ammonium salt. The ammonium salt, being ionic, dissolves in the aqueous phase, which is separated from the ether layer. Adding sodium hydroxide to the aqueous layer converts the ammonium salt back to the free amine, which is then removed from the aqueous phase by extraction with a fresh portion of ether. 22.5 BASICITY OF AMINES Amines are weak bases, but as a class, amines are the strongest bases of all neutral mol￾ecules. Table 22.1 lists basicity data for a number of amines. The most important rela￾tionships to be drawn from the data are 1. Alkylamines are slightly stronger bases than ammonia. 2. Alkylamines differ very little among themselves in basicity. Their basicities cover a range of less than 10 in equilibrium constant (1 pK unit). 3. Arylamines are much weaker bases than ammonia and alkylamines. Their basicity constants are on the order of 106 smaller than those of alkylamines (6 pK units). The differences in basicity between ammonia, and primary, secondary, and tertiary alkylamines result from the interplay between steric and electronic effects on the mole￾cules themselves and on the solvation of their conjugate acids. In total, the effects are small, and most alkylamines are very similar in basicity. Arylamines are a different story, however; most are about a million times weaker as bases than ammonia and alkylamines. As unfavorable as the equilibrium is for cyclohexylamine acting as a base in water, CH3N  H H H Methylammonium ion (stronger acid; pKa 10.7)  OH  Hydroxide ion CH3NH2 Methylamine  H OH Water (weaker acid; pKa 15.7) CH3NH2 Methylamine  H OCCH3 O Acetic acid (stronger acid; pKa 4.7) CH3NH3  Methylammonium ion (weaker acid; pKa 10.7)  OCCH3 O  Acetate ion 22.5 Basicity of Amines 865 NH2 Cyclohexylamine  H2O Water NH3  Cyclohexylammonium ion  HO Hydroxide ion (Kb 4.4 104 ; pKb 3.4) Recall from Section 4.6 that acid–base reactions are char￾acterized by equilibrium con￾stants greater than unity when the stronger acid is on the left side of the equation and the weaker acid on the right. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website