Methylamine, base strength

Rank methylamine, dimethylamine, and diethylamine in order of increasing base strength. Explain your rankings in relation to molecular structure. 

A straightforward treatment of the base strength of aniline can be given. A saturated aliphatic amine such as methylamine has a base constant Kb of about 5 X 10 4, corresponding to the reaction H H... 

Suggest an explanation for the difference in base strengths between (a) ammonia and methylamine ... 

We recall from Chapter 18 that the relationship K = KJCi, describes the acid and base strengths of any conjugate acid-base pair in aqueous solution. For instance, we can use this relationship for the CH3NH3 /CH3NH2 pair. Appendix G gives Ab for methylamine, CH3NH2. 

The introduction of a trimethylsilyl group into an alkyl group increases the base strength, the pKg, values of both trimethylsilyl-methylamine and di(trimethylsilylmethyl) amine being appreciably higher than the values for methylamine and dimethylamine, respectively. The effect diminishes, however, the further the trimethylsilyl group is removed from the amino group (Table 5). 

The validity of comparisons made in this way was first demonstrated by Brown and Bartholomay who investigated the base strengths of ammonia and the methylamines in the vapour phase by using tri-methylboron as a reference acid. The total pressures set up by equi-molecular mixtures in the vapour state at various temperatures were measured, and the dissociation constants of the addition products calculated from the results. These were found to place the bases in the same relative order as had been found for aqueous solution (Table 17). 

The acidity of alcohols in the gas phase increases with increasing carbon substitution [39, 40] this order is the opposite of the acidity order in solution [40]. The free-energy data are shown in Table 12 they confirm the qualitative gas-phase results first reported by Brauman and Blair [27]. The effects of successive substitution of methyl or t-butyl for hydrogen on the a-C atom are not additive but show a regular saturation effect which is even greater than the one discussed for base strengths in Section 3.2 for the corresponding substitution in methylamine the methyl and t-butyl substituent effects are in the ratio 1.00 1.60 2.00, compared with 1.00 1.85 2.60. 

Arrange the following bases in order of increasing strength on the basis of the pKa values of their conjugate acids, which are given in parentheses (a) ammonia (9.26) (b) methylamine (10.56) ... 

The basic strength of an amine can be measured by its pK value (t5rpically 3-4). The lower the value of pK,, the stronger the base. The pK for ammonia is 4.74, which compares with pK values for methylamine, ethylamine, and propylamine of... 

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