Reactions of Alcohols (OCR A-Level Chemistry A): Revision Notes

Reactions of Alcohols

Introduction

Alcohols are versatile organic compounds that undergo several important types of reactions. Understanding these reactions and the conditions required is essential for A-Level chemistry. The main reaction types you need to know are:

• Combustion - complete burning in oxygen
• Oxidation - converting alcohols to aldehydes, ketones, or carboxylic acids
• Elimination - removing water to form alkenes (dehydration)
• Substitution - replacing the hydroxyl group with halogens

The products formed depend on the type of alcohol (primary, secondary, or tertiary) and the specific reaction conditions used.

Combustion of alcohols

When alcohols undergo complete combustion in a plentiful supply of oxygen, they produce carbon dioxide and water. This reaction releases significant amounts of energy, making alcohols useful as fuels.

The complete combustion of ethanol can be represented by the balanced equation:

$\ce{C2H5OH(l) + 3O2(g) -> 2CO2(g) + 3H2O(l)}$

This is an exothermic reaction that releases substantial heat energy. As the carbon chain length of the alcohol increases, the quantity of heat energy released per mole also increases. This is because longer chain alcohols contain more carbon-hydrogen bonds that can be broken and reformed as carbon dioxide and water, releasing more energy overall.

Oxidation of alcohols

Overview of oxidation reactions

Both primary and secondary alcohols can be oxidised when treated with suitable oxidising agents. The most commonly used oxidising mixture in organic chemistry is acidified potassium dichromate(VI), written as $\ce{K2Cr2O7/H2SO4}$.

During oxidation reactions, a visible colour change occurs that serves as evidence the reaction has taken place:

• Orange dichromate(VI) ions, $\ce{Cr2O7^{2-}}$, are reduced to
• Green chromium(III) ions, $\ce{Cr^{3+}}$

This distinctive colour change from orange to green is an important observation in practical work.

Oxidation of primary alcohols

Primary alcohols can undergo oxidation to form two different types of products depending on the reaction conditions employed:

1. Aldehydes - formed under gentle heating with distillation
2. Carboxylic acids - formed when heated under reflux with excess oxidising agent

The key factor determining which product forms is whether the aldehyde intermediate is allowed to remain in contact with the oxidising agent. Aldehydes are themselves susceptible to further oxidation to carboxylic acids, so careful control of conditions is essential.

Formation of aldehydes

To prepare an aldehyde from a primary alcohol, the reaction mixture must be heated gently, and the aldehyde product must be removed from the reaction mixture as soon as it forms. This is achieved by distillation, which separates the aldehyde before it can undergo further oxidation.

The oxidation of butan-1-ol to butanal is shown below:

Formation of carboxylic acids

If a primary alcohol is heated strongly under reflux with an excess of acidified potassium dichromate(VI), a carboxylic acid is formed as the final product. The excess oxidising agent ensures complete oxidation, and reflux ensures that any aldehyde formed initially remains in the reaction mixture long enough to be further oxidised to the carboxylic acid.

The complete oxidation of butan-1-ol to butanoic acid is shown below:

Summary of primary alcohol oxidation conditions:

Product desired	Conditions required	Technique
Aldehyde	Acidified $\ce{K2Cr2O7/H2SO4}$	Distillation (gentle heating)
Carboxylic acid	Excess acidified $\ce{K2Cr2O7/H2SO4}$	Reflux (strong heating)

The order of oxidation for primary alcohols is: $\text{Primary alcohol} \rightarrow \text{Aldehyde} \rightarrow \text{Carboxylic acid}$

Oxidation of secondary alcohols

Secondary alcohols are oxidised to form ketones when treated with acidified potassium dichromate(VI). Unlike primary alcohols, secondary alcohols cannot be oxidised further beyond the ketone stage under normal conditions. This is because ketones lack the hydrogen atom bonded to the carbonyl carbon that would be needed for further oxidation.

To ensure complete conversion of the secondary alcohol to the ketone, the reaction mixture is heated under reflux with the oxidising agent. Once again, the characteristic colour change from orange dichromate(VI) to green chromium(III) ions confirms that oxidation has occurred.

The oxidation of propan-2-ol to propanone is illustrated below:

Oxidation of tertiary alcohols

Tertiary alcohols are resistant to oxidation under normal laboratory conditions. When acidified potassium dichromate(VI) is added to a tertiary alcohol, no reaction occurs. The dichromate solution remains orange, showing that no oxidation has taken place and no reduction of the chromium ions has occurred.

This lack of reactivity is due to the structural features of tertiary alcohols - they have no hydrogen atoms bonded to the carbon carrying the hydroxyl group, which means the oxidation pathway available to primary and secondary alcohols is not possible.

Summary table for alcohol oxidation:

Alcohol type	Product of oxidation	Observable colour change
Primary	Aldehyde (with distillation) → Carboxylic acid (with reflux + excess)	Orange to green
Secondary	Ketone	Orange to green
Tertiary	No reaction	Remains orange

Dehydration of alcohols

What is dehydration?

Dehydration is an elimination reaction in which a water molecule ($\ce{H2O}$) is removed from an organic compound. When applied to alcohols, dehydration results in the formation of alkenes - unsaturated hydrocarbons containing a carbon-carbon double bond.

The general process involves:

• Removal of the hydroxyl group (-OH) from one carbon
• Removal of a hydrogen atom (H) from an adjacent carbon
• Formation of a $\ce{C=C}$ double bond between these carbons
• Release of water as a small molecule

This type of reaction is classified as an elimination because a small molecule (water) is eliminated from the starting material, creating a double bond in its place.

Conditions for dehydration

To achieve dehydration of an alcohol, the following conditions are required:

• Acid catalyst: Either concentrated sulfuric acid ($\ce{H2SO4}$) or concentrated phosphoric acid ($\ce{H3PO4}$)
• Heat under reflux: The mixture must be heated to provide activation energy for the reaction

The acid catalyst plays a crucial role in activating the alcohol molecule, making it easier for the hydroxyl group to leave. Reflux ensures that any volatile components don't escape before the reaction is complete.

Example: dehydration of cyclohexanol

The dehydration of cyclohexanol to form cyclohexene demonstrates this reaction clearly:

Substitution reactions of alcohols

Formation of haloalkanes

Alcohols undergo substitution reactions with hydrogen halides to produce haloalkanes. In these reactions, the hydroxyl group (-OH) of the alcohol is replaced by a halogen atom (such as bromine or chlorine), forming a new carbon-halogen bond.

When preparing a haloalkane from an alcohol, the hydrogen halide is typically generated in situ (meaning "in place" - formed in the reaction mixture rather than added directly). This is commonly achieved using a sodium halide salt combined with sulfuric acid.

Formation of hydrogen bromide in situ

For example, to generate hydrogen bromide (HBr) in the reaction mixture, sodium bromide is reacted with sulfuric acid:

$\ce{NaBr(s) + H2SO4(aq) -> NaHSO4(aq) + HBr(aq)}$

The HBr produced immediately reacts with the alcohol present in the mixture, converting it to a bromoalkane.

Example: formation of 2-bromopropane

The substitution reaction of propan-2-ol with hydrogen bromide illustrates this process:

Overall reaction equation

The complete reaction of propan-2-ol with sodium bromide and sulfuric acid can be written as:

$\ce{CH3CHOHCH3 + NaBr + H2SO4 -> CH3CHBrCH3 + NaHSO4 + H2O}$

This equation shows:

• Propan-2-ol as the starting alcohol
• Sodium bromide and sulfuric acid generating HBr in situ
• 2-bromopropane as the organic product
• Sodium hydrogensulfate and water as by-products

Key points about substitution reactions:

• The hydroxyl group is replaced by a halogen
• The carbon skeleton remains unchanged
• The reaction typically requires heating under reflux
• Sulfuric acid acts as both a catalyst and a reactant (forming the hydrogen halide)

Summary of key reactions

This table summarises the main reactions of alcohols you need to know:

Reaction type	Reagent	Conditions	Product type	Colour change
Combustion	Oxygen ($\ce{O2}$)	Burn	$\ce{CO2}$ and $\ce{H2O}$	-
Oxidation (primary alcohol)	Acidified $\ce{K2Cr2O7/H2SO4}$	Distillation	Aldehyde	Orange → green
Oxidation (primary alcohol)	Excess acidified $\ce{K2Cr2O7/H2SO4}$	Reflux	Carboxylic acid	Orange → green
Oxidation (secondary alcohol)	Acidified $\ce{K2Cr2O7/H2SO4}$	Reflux	Ketone	Orange → green
Oxidation (tertiary alcohol)	Acidified $\ce{K2Cr2O7/H2SO4}$	-	No reaction	Remains orange
Dehydration (elimination)	Conc. $\ce{H2SO4}$ or $\ce{H3PO4}$	Reflux	Alkene + $\ce{H2O}$	-
Substitution	NaX + $\ce{H2SO4}$	Reflux	Haloalkane + $\ce{H2O}$	-