Alcohols contain the hydroxyl group, -OH, bonded to a saturated carbon atom. Alcohols with one -OH group have the general formula CnH2n+1OH. They are named with the ending -ol and a number to show the position of the -OH group, e.g. propan-2-ol.Like halogenoalkanes, alcohols are classified by the number of carbon atoms bonded to the carbon that carries the -OH group:• Primary (1°): the C-OH carbon is bonded to one other carbon. Examples: ethanol, CH3CH2OH, and butan-1-ol. Methanol, CH3OH, is also counted as primary.
• Secondary (2°): the C-OH carbon is bonded to two other carbons. Example: propan-2-ol, CH3CH(OH)CH3.
• Tertiary (3°): the C-OH carbon is bonded to three other carbons. Example: 2-methylpropan-2-ol, (CH3)3COH.Quick check: primary alcohols contain -CH2OH, secondary contain -CH(OH)-, and in a tertiary alcohol the C-OH carbon has no hydrogen atoms.Watch out: branching elsewhere in the molecule does not matter. 2-methylpropan-1-ol, (CH3)2CHCH2OH, is branched, but its C-OH carbon is bonded to only one carbon, so it is primary.Why classify? The class decides how an alcohol reacts with oxidising agents. Primary and secondary alcohols are oxidised by acidified potassium dichromate(VI), but tertiary alcohols are not, because the C-OH carbon has no hydrogen atom to lose. This is covered in a later lesson.All alcohols can form hydrogen bonds, so they have higher boiling points than alkanes of similar Mr, and the smaller alcohols mix with water.
• Secondary (2°): the C-OH carbon is bonded to two other carbons. Example: propan-2-ol, CH3CH(OH)CH3.
• Tertiary (3°): the C-OH carbon is bonded to three other carbons. Example: 2-methylpropan-2-ol, (CH3)3COH.Quick check: primary alcohols contain -CH2OH, secondary contain -CH(OH)-, and in a tertiary alcohol the C-OH carbon has no hydrogen atoms.Watch out: branching elsewhere in the molecule does not matter. 2-methylpropan-1-ol, (CH3)2CHCH2OH, is branched, but its C-OH carbon is bonded to only one carbon, so it is primary.Why classify? The class decides how an alcohol reacts with oxidising agents. Primary and secondary alcohols are oxidised by acidified potassium dichromate(VI), but tertiary alcohols are not, because the C-OH carbon has no hydrogen atom to lose. This is covered in a later lesson.All alcohols can form hydrogen bonds, so they have higher boiling points than alkanes of similar Mr, and the smaller alcohols mix with water.