Nucleophilic acyl substitution and basicity of groups

[lifeiseasy]

I'm studying the reaction mechanisms for carboxylic acid and its derivatives and here it says whether a compound with a C=O bond undergoes nucleophilic addition (as in aldehydes and ketones) or nucleophilic acyl substitution depends on the relative basicities of the substituent group. For instance, it's very difficult for an aldehyde to eject a H- (hydride) ion because it's a powerful base. But what does it mean by that? Is it talking about Lewis bases?

 

[alxm]

Well not necessarily a Lewis base, but just a strong base. (see http://en.wikipedia.org/wiki/Leaving_group" )

Basically, something's a better leaving group if it's more stable as an ion in solution. A strong base, such as H^-, is a strong base precisely because it's not very stable in solution. (In water, it'd snatch a proton instantly and form H₂ + OH^-)

So if you compare the halogens (which are all very weak bases), for instance, F^- is less stable than Cl^-, which is less stable than Br^- which is less stable than I^-, since they gain stability from their larger sizes. Correspondingly, bromide is a better leaving group than chloride, and so forth.

 

[quicknote]

I can explain that the concept of basicity in nucleophilic acyl substitution refers to the ability of a substituent group to donate an electron pair to the carbonyl carbon atom. This donation of electrons can lead to the formation of a new bond with a nucleophile, resulting in the substitution of the leaving group.

In this context, the term "base" refers to a Lewis base, which is a species that can donate an electron pair to form a new bond. The relative basicity of a group is determined by its ability to stabilize the negative charge that is formed upon donation of an electron pair. This can be influenced by various factors such as electronic effects, steric hindrance, and resonance.

For example, a highly electronegative group, such as a halogen, can withdraw electron density from the carbonyl carbon, making it less basic and therefore less likely to undergo nucleophilic acyl substitution. On the other hand, a group with electron-donating properties, such as an alkyl group, can increase the basicity of the carbonyl carbon and promote nucleophilic acyl substitution.

In summary, the basicity of a substituent group plays a crucial role in determining the mechanism of reaction for carboxylic acid and its derivatives. It is an important concept to consider in understanding and predicting the behavior of these compounds in various chemical reactions.