Identifying Functional Groups

A functional group is the group of atoms responsible for the characteristic reactions of a compound. Many molecules used in synthesis and medicine contain several functional groups, and you need to pick out each one.Key groups at AS level:
• alkene: C=C
• alcohol: C-OH (primary, secondary or tertiary)
• haloalkane: C-X, where X is F, Cl, Br or I
• aldehyde: -CHO, a C=O at the end of a chain
• ketone: a C=O between two carbon atoms
• carboxylic acid: -COOH
• ester: -COO- between two carbon groupsLook carefully at what is attached to each C=O group. A C=O bonded to an H atom is an aldehyde; bonded to two carbon atoms it is a ketone; bonded to an -OH it is a carboxylic acid; bonded to an -O-C it is an ester. An -OH on the carbon of a C=O is part of a carboxylic acid, not an alcohol.Classify an alcohol by counting the carbon atoms joined to the C-OH carbon: one for primary, two for secondary, three for tertiary.Worked example: identify the functional groups in HOCH₂CH=CHCOCH₃.
• HOCH₂-: the C-OH carbon is joined to one other carbon, so this is a primary alcohol.
• -CH=CH-: an alkene.
• -COCH₃: the C=O is joined to two carbon atoms, so it is a ketone.
The molecule has three functional groups: a primary alcohol, an alkene and a ketone.