This is why this reaction type is called a nucleophilic acyl substitution: one acyl X group is substituted for another. Notice that in the product, the nucleophile becomes the new acyl X group. The tetrahedral intermediate rapidly collapses: the carbon-oxygen double bond re-forms, and the acyl X group is expelled. What this means is that the tetrahedral product formed from attack of the nucleophile on the carbonyl carbon is not the product: it is a reactive intermediate. In carboxylic acid derivatives, the acyl X group is a potential leaving group. Just like in aldehydes and ketones, carboxylic acid derivatives are attacked from one side of their trigonal planar carbonyl carbon by a nucleophile, converting this carbon to tetrahedral (sp 3) geometry. The fact that the atom adjacent to the carbonyl carbon in carboxylic acid derivatives is an electronegative heteroatom – rather than a carbon like in ketones or a hydrogen like in aldehydes - is critical to understanding the reactivity of these functional groups. The nucleophilic acyl substitution reaction Notice that the acyl X groups are simply deprotonated forms of other functional groups: in an amide, for example, the acyl X group is an amine, while in an ester the acyl X group is an alcohol. One the other side is the heteroatom-containing group: in this text, we will sometimes refer to this component as the ‘acyl X' group (this, however, is not a standard term in organic chemistry). One side is the carbonyl group and the attached alkyl group: this is called an acyl group (in the specific case where R is a methyl group, the term acetyl group is used). You can think of a carboxylic acid derivative as having two sides. The carboxylic acid derivatives can be distinguished from aldehydes and ketones by the presence of a group containing an electronegative heteroatom - usually oxygen, nitrogen, or sulfur – bonded directly to the carbonyl carbon. Two more examples of carboxylic acid derivatives which are less biologically relevant but important in laboratory synthesis are carboxylic acid anyhydrides and acid chlorides. The functional groups that undergo nucleophilic acyl substitutions are called carboxylic acid derivatives: these include carboxylic acids themselves, carboxylates (deprotonated carboxylic acids), amides, esters, thioesters, and acyl phosphates. \)Ĭarboxylic acid derivatives and acyl groups
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |