Alcohols, phenols and ethers: study guide

Key ideas

Alcohols have an –OH on a saturated carbon, phenols have it on a benzene ring, and ethers have an oxygen between two carbons. How the carbon bearing the –OH is substituted decides much of the chemistry.

• A primary alcohol has the –OH carbon attached to one other carbon, a secondary to two, and a tertiary to three.
• Lucas test (ZnCl₂ in HCl): tertiary alcohols turn cloudy at once, secondary after a few minutes, and primary stay clear at room temperature.
• Oxidation: primary alcohols give aldehydes and then carboxylic acids, secondary give ketones, and tertiary do not oxidize. PCC stops at the aldehyde.
• Phenols are more acidic than alcohols and dissolve in NaOH. The Williamson synthesis makes ethers from an alkoxide and a primary haloalkane.

Key equations

• alcohol CₙH₂ₙ₊₁OH
• 1° RCH₂OH → RCHO → RCOOH
• 2° R₂CHOH → R₂C=O
• RO⁻ + R′X → ROR′ + X⁻

Worked example

Butan-2-ol is a secondary alcohol. With K₂Cr₂O₇/H⁺ it is oxidized to butanone, and its Lucas test is slow. The C₄H₁₀O family has four alcohols (butan-1-ol, butan-2-ol, 2-methylpropan-1-ol and 2-methylpropan-2-ol) and three ethers, seven isomers in all.

Common mistakes

• Trying to oxidize a tertiary alcohol with dichromate. It has no hydrogen on the carbinol carbon.
• Mixing up the order of reactivity in the Lucas test. Tertiary is fastest.
• Thinking ethers can hydrogen-bond like alcohols. They accept hydrogen bonds but cannot donate them.

Practice

Test yourself with practice questions on this topic: Alcohols, phenols and ethers