• (n.) Any one of a series of compounds of mercaptans with aldehydes.
Mer·cap'tal noun [ Mercapt an + al dehyde.] (Chemistry) Any one of a series of compounds of mercaptans with aldehydes. 
Type: Term Pronunciation: mer-kap′tăl Definitions: 1. A substance derived from an aldehyde by the replacement of the bivalent oxygen by two thioalkyl (-SR) groups.