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Alcohols, Phenols and Carboxylic Acids

Reactions of the -OH Group in Alcohols, Phenols and Carboxylic Acids

The hydroxyl group -OH can occur in three different environments in organic compounds:

Alcohols: There are three types
- Primary alcohols. In these alcohols the -OH group is on the end of a chain. There are at least two hydrogen atoms attached to the carbon atom that also has the -OH group attached, e.g. propan-1-ol.
- Secondary alcohols. In these alcohols the -OH group is in the middle of the chain. The carbon atom to which the -OH group is attached has one hydrogen atom also attached, e.g. propan-2-ol.
- Tertiary alcohols. In these alcohols the -OH group is attached to a carbon atom which carries no hydrogen atoms, e.g. 2-methylpropan-2-ol.
Phenols: In these compounds the -OH group is directly attached to the benzene ring. Due to this, the -OH group often behaves differently when compared to alcohols. The simplest example is phenol itself.
Carboxylic Acids: Here the -OH group is part of the carboxyl group, e.g. ethanoic acid.

Acidic Properties

An alcohol, like water, is capable of donating or accepting a proton (H+), i.e. acting as an acid or a base. Alcohols are weaker acids than water. The equilibrium lies well over to the left.

R-OH + H₂O <=> R-O^- + H₃O⁺

The only reactions which shews evidence of acidity are those with alkali metals.

C₂H₅OH + Na C₂H₅ONa + ½H₂

With phenols in water the equilibrium lies further to the right. Phenol is more acid than alcohols and water.

Phenol structoral symbol + H₂O <=> Phenol ion structural symbol + H₃O⁺

Carboxylic acids are stronger acids than phenols, equilibrium is even further to the right.

CH₃CO₂H + H₂O <=> CH₃CO₂^- + H₃O⁺

The order of acid strength is:

ethanol < water < phenol < carboxylic acids

It is the stability of the anion formed from the acid which determines how strong the acid is. If the negative charge on the oxygen can be shared with other atoms in the molecule, the anion will be more stable and more of it will be made. In alcohols no such sharing is possible. In phenols the charge can be spread onto the benzene ring. In carboxylic acids the charge can be spread across the carboxyl group.

Phenols and carboxylic acids are strong enough acids to react with strong bases, such as sodium hydroxide, to form salts.

CH₃CO₂H + NaOH		CH₃CO₂Na + H₂O
ethanoic acid		sodium ethanoate

+ NaOH		+ H₂O
phenol		sodium phenoxide

Carboxylic acids are strong enough acids to give off carbon dioxide with carbonates.

Na₂CO₃ + 2CH₃CO₂H 2CH₃CO₂Na + CO₂ + H₂O

Oxidation Reactions

The -OH group can be oxidised with acidified sodium or potassium dichromate (VI). The orange dichromate (VI) ion is reduced to green Cr³⁺.

-CHOH -C=O + 2H

In this reaction two hydrogen atoms are removed. The product is a carbonyl compound.

With primary alcohols such as ethanol, the aldehyde, ethanal, is produced. Under more severe conditions the carboxylic acid, ethanoic acid, is produced.

CH₃CH₂OH
ethanol		ethanol		ethanoic acid

Secondary alcohols are oxidised to ketones.


propan-2-ol		propanone

Tertiary alcohols are not readily oxidised.

Phenols and carboxylic acids are not readily oxidised because they do not have a hydrogen atom on the carbon atom to which the -OH group is attached.

Iron (III) Chloride Test

Compounds containing the C=C-OH (enol) group form a purple complex with neutral Fe3+ ions. Phenols have this arrangement, hence this can be used as a test for them.

Esterification Reaction

Esters are formed when an alcohol reacts with a carboxylic acid.

R-OH +

+ H₂O

The reaction is reversible and is catalysed by concentrated acid. The reverse reaction is called ester hydrolysis.

Esters are named from the alcohol and acid from which they are made.


ethanoate ethyl	propanoate methyl
i.e. ethyl ethanoate	methyl propanoate

The -OH group in phenols is less reactive to esterification than the -OH group in alcohols so it needs a more vigorous reagent to bring about the reaction. A more vigorous reagent is ethanoic anhydride.

This reacts with phenol to produce phenyl ethanoate.

+ (CH₃CO)₂O

O₂C-CH₃ + CH₃CO₂H

An even more reactive ethanoylating agent is ethanoyl chloride.

+ HCl

Ethanoic anhydride and ethanoyl chloride are reagents with the general name acylating agents.

Salicylic acid (2-hydroxybenzoic acid) can be esterified using ethanoic anhydride or ethanoyl chloride. The product is the medicine, aspirin or 2-ethanoylhydroxybenzoic acid.