Question

Given the skeletal drawing of several molecules ( organic chemistry style), how do you determine which molecule is the most acidic (e.g. how does resonance, inductive effect, separation of charge, negative/ positive charge, and other factors that I am not currently aware play a role?-- please state what those other unknown factors are). Conversely, how do you determine which molecule is the most basic given several molecules to compare. Consequently, what are the factors that determine which molecule is the most basic? Please please provide me an answer that goes beyond just stating the factors determining basicity or acidity. Please go to the extent of explaning how these factors help determine which molecule is the most basic or acidic, not just basic or acidic alone.

Answer 1

Here we have to look for certain functional groups first.

If carboxylic group is present, the molecule is acidic. If phenolic group is present, the molecule is acidic. But carboxylic acids are more acidic than phenols.

In carboxylic acids, if electron withdrawing group is present, then acidity increases. Thus, chloro acetic acid is more acidic than acetic acid due to presence of electron withdrawing chlorine atom. Similarly, electron releasing groups decreases the acidity of carboxylic acids. Thus, p-methoxy benzoic acid is less acidic than benzoic acid due to presence of electron releasing methoxy group.

If amino group is present, the molecule is basic.In gas phase, with increase in number of alkyl groups, the basicity of aliphatic amines increases. Thus, decreasing order of basicity is trimethyl amine > dimethyl amine> methyl amine > ammonia. In case of substituted anilines, electron releasing groups (such as methoxy, methyl, ammino) increase the basic strength whereas electron withdrawing groups (such as nitro, cyano, carboxylic, halo) decrease the base strength. Thus, 4-methoxy aniline is more basic than aniline whereas 4-nitroaniline is less basic than aniline.