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.