Alcohols

AS Level Chemistry · Hydroxy compounds. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

Alcohols contain the –OH group. They are classed as primary, secondary or tertiary by the number of alkyl groups (one, two or three) on the carbon atom that carries the –OH. A molecule with two –OH groups is classified one group at a time.

The class decides what happens on oxidation with acidified K2Cr2O7. A primary alcohol gives an aldehyde and then a carboxylic acid. A secondary alcohol gives a ketone. A tertiary alcohol is not oxidised. When oxidation happens, the orange solution turns green.

Alcohols are weaker acids than water. The alkyl group pushes electron density onto the oxygen, so the alkoxide ion, RO−, is less stable than OH− and forms less easily.

Rules to remember

Common mistake

Students give the aldehyde as the product when a primary alcohol is heated under reflux with excess oxidising agent. Reflux keeps the aldehyde in the flask, so it is oxidised further to the carboxylic acid; the aldehyde is obtained only by distilling it off as it forms.

Worked example

An alcohol C4H10O turns warm acidified K2Cr2O7 from orange to green and gives a yellow precipitate with alkaline aqueous iodine. Identify it.

  1. It is oxidised, so it is not the tertiary alcohol 2-methylpropan-2-ol.
  2. The yellow precipitate shows a CH3CH(OH)– group.
  3. Butan-1-ol and 2-methylpropan-1-ol have –CH2OH groups, so they give no precipitate.
  4. Only CH3CH(OH)CH2CH3 has the group needed.

Answer: Butan-2-ol, a secondary alcohol.

Practice questions

Try each one, then open the answer.

1. What are the products when sodium reacts with ethanol?

  1. A
    sodium hydroxide and ethene
  2. B
    sodium ethoxide and hydrogen
  3. C
    sodium ethanoate and hydrogen
  4. D
    sodium ethoxide and water
Show answer

Answer: B. 2CH3CH2OH + 2Na → 2CH3CH2O−Na+ + H2. The hydrogen of the O–H group is replaced, as with water, but the reaction is gentler because ethanol is a weaker acid.

2. Why is ethanol a weaker acid than water?

  1. A
    ethanol has fewer O–H bonds per molecule
  2. B
    ethanol cannot form hydrogen bonds
  3. C
    the O–H bond in ethanol is much stronger
  4. D
    the ethyl group pushes electron density onto the oxygen, making the ethoxide ion less stable than the hydroxide ion
Show answer

Answer: D. The electron-donating alkyl group increases the negative charge density on the oxygen of C2H5O−, so the ion is less stable and ethanol loses its proton less readily than water. Ethanol still reacts with sodium, but more slowly than water.

3. Which reaction produces a secondary alcohol?

  1. A
    reduction of propanal with NaBH4
  2. B
    reduction of propanone with NaBH4
  3. C
    hydrolysis of 1-bromopropane with NaOH(aq)
  4. D
    reduction of propanoic acid with LiAlH4
Show answer

Answer: B. Reducing a ketone puts the OH on a carbon already bonded to two other carbons: propanone gives propan-2-ol. The other three all give the primary alcohol propan-1-ol.

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