Effect of alkyl substituents

The steric effects of alkyl substituents (R= methyl, ethyl, i-propyl, f-butyl) on the nitrogen have been related to the steric factors of these same groups as measured in kinetic studies (152). 

The effects of alkyl substituents in positions 6 and 7 on the rates of hydration and dehydration of 2-hydroxypteridine are not very great. The observed decrease in the equilibrium ratio of [HY]/[HX] has been... 

Figure 3.82 (a) The effect of or/Ao-substituents on substitution reactions of cis-Pt(PR3)2ArCl complexes (b) the effect of alkyl substituents on substitution reactions of dien complexes. 

Saitoh, T. Yamazaki, Y. Kamidate, T. Watanabe, H. Haraguchi, K. Steric effect of alkyl substituent on the extraction of nickel(II) and cobalt(II) by N-alkylcarbonyl substituted N-phenylhydroxylamines. Anal. Sci. 1992, 8, 767-771. 

It would be interesting to assess the relative effects of alkyl substituents vs. protons on the directional flexibility of amine nucleophiles. Data are available... 

It is well known that alkyl substitution changes the basicity of amines. However, solvation effects lead to an anomalous order of basicities in solution (NH3 tertiary amine < primary amine < secondary amine). From gas-phase proton affinity data the intrinsic effects of alkyl substituents can be evaluated and a quite regular order (NH3 < primary amine < secondary amine < tertiary amine) is obtained91. 

Effects of Alkyl Substituents on the Rotational Barriers and Equilibrium Constants of 2,4,6-Trialkylacetanilides (13)... 

Condensations of ammonia and methylamine with 1-alkyll. 1 -epoxycycloalkanea were likewise examined thoroughly by Mouseeron and Granger i to test the directive effect of alkyl substituents on the sense of ring opening. In every instance ring cleavage followed I hr... 

The lesser magnitude of the -effect in the secondary cation (22.1 kcalmol-1 compared to 38 kcalmol-1 for the primary cation) is attributed to reduced electron demand in the secondary cation, as a consequence of the stabilizing effect of alkyl substituents. 

Thiazoles readily undergo AT-alkylation by alkyl halides or tosylates (Menshutkin reaction). The sensitivity of this SN2 quaternization reaction to the molecular environment of the nitrogen atom has been used to evaluate, in a quantitative way, steric and electronic effects of ring substituents. The electronic effect of alkyl substituents (unperturbed by any steric effect) may be evaluated from the rate constants for the reaction of 5-alkylthiazoles with methyl iodide (in nitrobenzene at 30 °C) Table 19 shows that introducing a methyl group at the 5-position corresponds to an acceleration of the rate by a factor of 2 but that each addition of a methyl ramification to the 5-alkyl group enhances the rate only by a factor of 1.1. The data in Table 19 fit well with a Hammett-Taft equation (3) ... 

Effect of alkyl substituent on carbocation stability. A carbocation is stabilized by overlap of filled orbitals on an adjacent alkyl group with the vacant p orbital of the carbocation. Overlap between a cr bond and a p orbital is called hyperconjugation. 

Let us analyse the effect of alkyl substituents on the basicity of vinylamine and compare it with those of saturated amines bearing equivalent substituents. In principle, the alkyl substituents will exert their action through their polarizability effect, so that introducing them at roughly equivalent positions relative to the nitrogen atom should result in comparable effects if the nitrogen atoms were protonated. On analysing data (in kcal mol" ) for the methylation of vinylamine, at the a position, it is clearly apparent that these compounds are protonated at their carbon atoms. 

The effect of alkyl substituents at C-3 in acyclic substrates was studied and here the stereoselectivity depends on the steric bulk of the alkyl group. The ds-isomer is generally favored under kinetic conditions, although with moderate diastereoselectivity, while a bulky isopropyl sub-... 

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