Originally posted by Dr. Dodger Dog
I was referring only to carboxylic acids being unreactive in nucleophilic type reactions due to the weak leaving group (hydroxide). The reactions you mention are all very valid, but more specific than just ur everyday nucleophilic addition/substitution processes.
I think I' missed a transition there. Your original statement is that carboxylic acids don't do very many reactions. Now you have restated that they don't nucelophilic type of reactions. The reality is that seven of the eight reactions listed are what you are referring to as "nucleophilic addition/substitution processes", just each has a varying Lewis acid if you will.
The SOCl
2 reaction is a perfect example. The sulfonyl group is attacked by the nucleophilic carbonyl oxygen, which causes the carbonyl carbon to be electron poor. This is the same first step of the mechanism when a carboxylic acid is treated with a strong acid (Lewis or Bro-Lowry). The chloride then attacks the carbonyl carbon to form a tetrahedral intermediate, much like 90% of carbonyl substitution chemistry. The rest just follows standard reactivity.
The fundamental mechanism is simplistic and has appeared on the MCAT on a few occassions. I sincerely hope people reviewing carbonyl chemistry see that the mechanism is similar in almost all carbonyl reactions and thus only study one basic pathway.
As for the mechanism of esterification, it is NOT a simple two step process like other addition elimination reactions. It involves protonation/deprotonation steps, esp in acidic medium.
Actually, it is very simple, and there are only two key steps. As I assume is true where you went to grad school (I have found it to be norm with most chemists), common mechanisms are discussed by their key steps, with catalysis and work-up steps assumed (an thus unstated).
For instance, an SN
1 reaction is said to be a two-step mechanism, despite cases where the leaving group is protonated and/or the nucleophile must be neutralized with a work-up step. You look in most textbooks, and SN
1 reactions are referred to as two-step. Likewise, SN
2 reactions are said to be a concerted mechanism, despite cases where the nucleophile must be neutralized with a work-up step (amines substituting for a halide are a great example).
Hence, whether a carbonyl substitution reaction is carried out in acid (making the carbonyl more electrophilic) or base (making the nucleophile more nucleophilic), the number of key steps is still two: (1) formation of the tetrahedral intermediate following attack by the nucleophile and (2) the loss of the leaving group. Thus, substitution reactions involving carbonyls are commonly referred to as two-step mechanisms.