Methylamine, acid dissociation constant

Van der Linde and Robertson measured the secondary deuterium IEs for the acid dissociation constants of CD3NHj and (CD NHj 52 They found ApAa = 0.056 for the former and 0.12 for the latter, with the deuterated more basic. Surprisingly, these are temperature-independent between 5 and 45°C. Temperature independence requires that the IE lie in the entropy, not the enthalpy, so the zero enthalpy change was attributed to a fortuitous compensation of force constants. A large IE, easily measured, is seen with (CD3)3NH+, which is a stronger base than (CH3)3NH+, with a substantial ApAa, 0.206 at 0°C.53 The ApAa per D increases from 0.017 in methylamine to 0.019 in dimethylamine to 0.021 in trimethylamine. This increase was attributed to a lower IE in a strongly electron-demanding environment. 

Acid dissociation constants appear in Appendix B. Each compound is shown in its fully protonated form. Methylamine, for example, is shown as CH3NH3, which is really the methylammonium ion. The value of (2.33 X 10 given for methyl-amine is actually for the methylammonium ion. To find for methylamine, we write = KJK = (1.0 X 10 V(2.33 X 10 ) = 4.29 X 10 ... 

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). 

As we did with the acid dissociation expression, the concentration of water is omitted from the equilibrium expression, which gives the base dissociation constant, Ky, (or base ionization constant). Thus, for a weak base such as methylamine, the Ky, is written... 

Methylamine on the left hand side of the equation is called the free base, whereas the methyl ammonium ion formed on the right hand side is called the conjugate acid. The extent of ionisation or dissociation in the equilibrium reaction is defined by the equilibrium constant (Keq) ... 

For simple aliphatic amines, like the methylamines, there is a linear inverse correlation between proton affinities and vertical IPs ". A low IP value should therefore indicate high proton affinity and vice versa. The proton affinity PA(B) of a molecule B is related to the homolytic bond energy D(B+—H) in the conjugate acid, as indicated by equation 8. If the homolytic bond dissociation energy is assumed to be constant for a particular functional group, e.g. N—H, the proton affinity wiU exhibit a linear correlation with the quantity IP(H) — IP(B), and such correlations have been reported for the proton affinities and the nitrogen lone-pair ionizations . ... 

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