Page 93 - Pharm.Org.Chem-I 02-06-05-101

3- Dehydration of alcohols (-H2O)

• Means elimination of water molecule using sulphuric acid
or phosphoric acid.

• Elimination of OH group from α C and elimination of H from
β C (β-elimination mechanism)

• Alcohols are classified into 3 types:

Primary Secondary Tertiary
alcohol alcohol alcohol

• Primary alcohols undergo dehydration at 160-180 °C to give alkenes
βα

• Secondary alcohols undergo elimination at lower temperatures than
primary alcohols,

• Tertiary alcohols undergo elimination at much lower temperatures than

secondary alcohols.

H3C H2SO4 / 80 oC H3C
H3C C C CH2 + H2O
OH

H3C H3C
tert. Butanol Butene
tertiary alcohol

Ease of dehydration: 3° alc. > 2° alc. > 1° alc.

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