The hydroxyl group -OH can occur in three different environments in organic compounds:
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.
+ H2O + 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|
Carboxylic acids are strong enough acids to give off carbon dioxide with carbonates.
Na2CO3 + 2CH3CO2H 2CH3CO2Na + CO2 + H2O
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.
Secondary alcohols are oxidised to ketones.
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.
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.
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.
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.
Esters can be hydrolysed very slowly with water. The reaction can be speeded up by using either acid or alkali.
With acid the alcohol and carboxylic acid are formed. With alkali the alcohol and salt of the carboxylic acid are formed.
|+ NaOH||+ C2H5OH|
|ethyl ethanoate||sodium ethanoate|
Part of this site was last updated on 21st January 2009.
This work is licensed under a Creative Commons Attribution-NonCommercial 2.0 England & Wales Licence.
Copyright © Article Gems 2006-09.