Functional Group Reactions
Chemistry (Year 12) - Organic Chemistry
Oxidation of Alcohols
When primary or secondary alcohols are reacted with an oxidising agent such as KMnO4 or K2Cr2O7, they will produce an aldehyde, carboxylic acid or a ketone.
The product we end up with depends on the type of alcohol we started with, as well as the reaction conditions.
Oxidation of Primary Alcohols
Primary alcohols can oxidise in two stages; partial or complete oxidation. These reactions require an acidic condition and an oxidising agent.
Partial oxidation forms an aldehyde from the primary alcohol. The product is favoured when we use weaker oxidising agents such as Cr2O7-2/H+. The amount of Cr2O7-2/H+ is usually limited, or the aldehyde is distilled as soon as it is formed.
Complete oxidation forms a carboxylic acid from the aldehyde. This product is favoured when MnO4-/H+ is used, or Cr2O7-2/H+ is used in excess.
Oxidation of Secondary Alcohols
Secondary alcohols do not have a partial oxidation stage. Their oxidation goes directly to completion, producing a ketone.
As all four bonding sites from the C in the C = O group are occupied, ketones are resistant to further oxidation.
Oxidation of Tertiary Alcohols
Tertiary alcohols do not undergo oxidation. This is because the tertiary C atom's bonding sites are fully occupied by neighbouring C atoms, and the fourth site is occupied by the O atom from the alcohol group.
Carboxylic Acids as Acids
Carboxylic acids as indicated by the name are acids. During ionisation, the functional group -COOH of a carboxylic acid will donate a proton in an aqueous solution.
The second type of reaction is between carboxylic acids and other substances such as metals, carbonates and hydroxides to produce a salt and other by-products such as CO2 or H2O.
As shown above, when naming the salts of these reactions, we use the '-oate' suffix.
Esterification is the reaction between an alcohol and a carboxylic acid to produce an ester. In esterification, the H atom from the alcohol and the OH group in the carboxylic acid reacts. The remaining ions of the ester and carboxylic acid join together, releasing a water molecule.
Esterification requires a concentrated acid such as concentrated H2SO4 to allow the reaction to occur.
Ester hydrolysis is the reaction between an ester and water or a base to produce the original alcohol and carboxylic acid. To catalyse the reaction a dilute acid such as H2SO4 is used to provide both the acid and water.
As an alternative to water, we can use a strong base to speed up the rate of hydrolysis. In this case, the ester will be split to form the original alcohol and salt (rather than a carboxylic acid).