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Organic chemistry (Topics 6, 17 and 18)›Alcohols (Topic 6E)

Alcohols (Topic 6E)

Classification, properties, and reactions of alcohols, including oxidation, combustion, elimination and nucleophilic substitution.

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Illustration showing the structural formulas of primary, secondary, and tertiary alcohols. The primary alcohol has one carbon atom bonded to the hydroxyl group, the secondary alcohol has two carbon atoms, and the tertiary alcohol has three carbon atoms bonded to the hydroxyl group.

If in an alcohol is connected to:

one other carbon, it is a primary alcohol, 1^o
two other carbons, it is secondary alcohol, 2^o
three other carbons, it is tertiary alcohol, 3^o

Alcohols can be oxidised to carbonyl compounds during redox reactions with oxidising agents.

Acidified potassium dichromate VI () is commonly used in the oxidation of alcohols.

Dichromate ions, , turn from orange to green as they are reduced to . The oxidation state of chromium reduces from +6 to +3.

Two laboratory flasks side by side. The left flask contains a yellow solution labeled with 'Cr2O7^2-' indicating chromium in the +6 oxidation state. The right flask contains a green solution labeled with 'Cr^3+' indicating chromium in the +3 oxidation state, with both flasks emitting vapor.

Alcohols can be oxidised to carbonyl compounds during redox reactions with oxidising agents.

Acidified potassium permanganate, (), is a commonly used oxidising agent.

Permanganate ions, -, turn from deep purple to colourless as they are reduced to .

Two laboratory flasks side by side. The left flask contains a purple liquid, labeled with +7 MnO4-, indicating the presence of permanganate ions. The right flask is empty, labeled with +2 Mn2+, indicating manganese ions in a lower oxidation state.

When oxidised, primary alcohols form aldehydes first before further oxidation to carboxylic acids.

A diagram illustrating the oxidation of a primary alcohol. The first structure shows a primary alcohol with an -OH group attached to a carbon atom. The second structure, labeled 'Aldehyde,' shows the alcohol oxidized to an aldehyde with a carbonyl group (C=O). The third structure, labeled 'Carboxylic acid,' shows further oxidation to a carboxylic acid, which retains the carbonyl group and adds an -OH group.

Secondary alcohols are oxidised to form ketones.

Diagram illustrating the conversion of a secondary alcohol to a ketone. The structure on the left shows a secondary alcohol with an -OH group, while the structure on the right depicts the resulting ketone after oxidation, indicated by the [O] arrow.

Tertiary alcohols CANNOT be oxidised as they do not have a proton on the carbon bearing the group.

Diagram illustrating a tertiary alcohol with the hydroxyl group (OH) attached to a carbon atom (C) that is connected to three other groups (R, R', R'). The diagram indicates that when oxidized ([O]), there is no reaction.

can be used along the arrow or as a balanced reagent in a chemical equation to represent the oxidising agent.

Alcohols react with phosphorus pentachloride () to produce chloroalkanes.
reacts with the alcohol, replacing the with a chlorine atom.

is a strong chlorinating agent suitable for converting primary, secondary, and tertiary alcohols.

The reaction is typically carried out in an inert solvent, such as dichloromethane

Alcohols react with concentrated sulfuric acid and potassium bromide () to form bromoalkanes.

Sulfuric acid reacts with potassium bromide to generate hydrobromic acid, , in situ.

then acts as the nucleophile, substituting the group with a bromine atom.

Alcohols react with red phosphorus and iodine () to produce iodoalkanes.

Red phosphorus reacts with iodine to generate phosphorus triiodide, (), in situ.

facilitates the substitution of the hydroxyl group with an iodine atom.

This reaction is particularly useful for primary alcohols, yielding primary iodoalkanes, which are common intermediate products in organic synthesis.

The removal of water from an alcohol is an elimination reaction resulting in the formation of an alkene.

Alcohols can also be synthesised from alkenes using an acid catalyst and steam. This process is reversible.

Chemical reaction diagram showing the conversion of an alcohol (OH group) to an alkene using sulfuric acid (H2SO4) and heat.

The acid catalyst is or .

or are not used due to the tendency of and to act as competing nucleophiles.

Alcohols can undergo oxidation in the form of combustion when heated in oxygen, producing water and carbon dioxide.

The complete combustion of butanol is:

Combustion of alcohol is considered cleaner than alkane combustion because it is less likely to produce particulate carbon. This is due to lower oxygen demand for the complete combustion of an alcohol, compared to an alkane of similar molecular weight.

Butanol, 74, burns completely in oxygen with a 1:6 mole ratio, whereas pentane, 72, requires a 1:8 mole ratio.

Question walkthrough

Reactions of alcohols

Prediction of products formed

Heating under reflux

Key features

Reflux involves heating a reaction mixture to its boiling point and continuously condensing the vapour back into the liquid phase using a condenser.

It allows reactions to proceed at elevated temperatures without loss of solvent.

Provides continuous heating, ensuring complete reaction of reactants.

Procedure

Set up a reflux apparatus with a round-bottom flask, heat source or mantle, and open vertical condenser.

A labeled diagram of a laboratory setup for distillation, featuring a round bottom flask containing a blue reaction mixture, a heat source beneath it, a condenser above with water in and out connections, and arrows indicating the flow of water.

Add the reactants, appropriate solvent, and antibumping granules to the flask.

Heat the mixture to boiling; vapours rise into the condenser and return to the flask.

Maintain reflux for the required duration to ensure reaction completion.

Safety precautions

DO NOT stopper the condenser. This could create a pressure build-up.

Use appropriate heat-resistant gloves and eye protection.

Choose a suitable heat source and environment for the chemicals selected, e.g. no open flames with flammable chemicals and fume hoods for toxic gases.

Distillation

Key features

Distillation is a technique used to separate components of a liquid mixture based on differences in their boiling points.

It is effective for purifying liquids and separating liquid mixtures.

Procedure

Set up the distillation apparatus with a distillation flask, thermometer, condenser, and receiving flask.

An illustration of a distillation setup, featuring a distillation vessel containing a blue mixture, a thermometer for measuring temperature, and a collection vessel for the pure product. The setup includes water inlets and outlets for cooling, and a heat source represented by a flame beneath the distillation vessel.

Heat the mixture to the first boiling point; that of the lowest boiling component.

Remove the heat when the temperature starts to rise. At this point, all the lower boiling component has evaporated.

Collect the purified liquid in the receiving flask.

Add the collected mixture to a distillation flask and repeat as necessary to achieve desired purity.

Safety precautions

Use appropriate heat-resistant gloves and eye protection.

Choose a suitable heat source and environment for the chemicals selected, e.g. no open flames with flammable chemicals and fume hoods for toxic gases.

The distillation flask should be stoppered to prevent escape of gases as the set-up is open at the collection vessel.

Liquid–liquid extraction with a separating funnel

Key features

Liquid–liquid extraction separates compounds based on their solubility in two immiscible solvents, typically an aqueous phase and an organic phase.

Liquid–liquid extraction allows the isolation of a desired organic compound from an aqueous solution or the removal of impurities.

Procedure

Transfer the reaction mixture to a separating funnel.

Add a suitable solvent that is immiscible with the primary solvent.

Stopper and invert the funnel several times, allowing the layers to interact. Open the tap frequently to prevent pressure buildup.

Allow the layers to separate and identify the layer containing your product.

Illustration of a separating funnel labeled with components: a stopper at the top, a separating funnel body containing two layers of liquids (orange for low density liquid and green for high density liquid), and a stopcock at the bottom.

Drain the denser lower (usually aqueous) layer and collect.

Drain the less dense upper (organic) layer into a separate vessel.

Safety precautions

Handle solvents in a well-ventilated area or fume hood.

Avoid overfilling the separating funnel to prevent spills during inversion.

Open the tap frequently during inversion to prevent pressure buildup.

Drying with an anhydrous salt

Key features

Drying agents (anhydrous salts) are used to remove residual water from organic solvents and compounds.

Simple and efficient method to remove moisture.

Reversible process; drying agents can often be regenerated.

Common drying agents

Magnesium sulfate (): Suitable for many organic solvents.

Sodium sulfate (): Commonly used for less sensitive applications.

Procedure

Add the anhydrous drying agent to the organic solution.

Swirl or stir the mixture to allow water absorption by the drying agent.

Continue adding until the drying agent no longer clumps together.

Allow the drying agent to settle or remove with a filter to obtain the dried organic layer.

Safety precautions

Observe the specific hazards associated with any chemicals used.

Boiling temperature determination

Key features

Boiling point determination is used to assess the purity of a liquid organic compound and to identify it by comparison with known values.

Pure compounds have sharp, characteristic boiling points.

Impurities typically cause boiling point elevation and broadening of the boiling range.

Procedure

Set up a simple distillation apparatus with a thermometer.

Slowly heat the sample and observe the temperature at which vaporisation begins.

Record the temperature where a steady stream of vapour is observed.

The gap between these values indicates the boiling point range.

Compare the observed boiling point with literature values to determine compound identity and/or purity.

Safety precautions

Use appropriate heat sources. Open flames must not be used with flammable chemicals.

Handle hot apparatus with care, using appropriate gloves to avoid burns.