Which of these arrangements of atoms is found in all alcohols?

• A. C-OH
• B. C=C
• C. C=O
• D. C-O-C

Answer: (A)

Alcohols are defined by a hydroxyl group attached to a carbon skeleton, so the arrangement of a carbon bonded to an oxygen that is also bonded to hydrogen (C–O–H) is the universal feature. This -OH group is what makes an alcohol, regardless of what the rest of the molecule looks like. The other patterns don’t appear in every alcohol: a C=C indicates a double bond between carbons (unsaturation, not required for all alcohols); a C=O is a carbonyl group found in aldehydes and ketones (not present in all alcohols); and a C–O–C bond is an ether linkage (not necessary for alcohols and not universal). Thus, the C–O–H arrangement is the consistent hallmark across all alcohols.