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Module 4: Core organic chemistry›Alcohols (4.2.1)

Alcohols (4.2.1)

Bonding, structure, properties, and reactions of alcohols.

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Alcohols are organic compounds which contain the hydroxyl () functional group.

Chemical structure of methanol, showing the arrangement of atoms and partial charges on the molecule.

The hydroxyl group contains a oxygen making the adjoining hydrogens and carbon; the overall alcohol molecule is polar.

Alcohols exhibit solubility in polar and non-polar solvents. This varies with the length of their carbon chain and the number of groups present.

The group participates in hydrogen bonding and creates molecular polarity. This leads to interaction with polar solvents like water.

The hydrocarbon chain is non-polar and can interact with non-polar solvents.

Chemical structures of three alcohols: Methanol (top left), very soluble in water; Butanol (top right), slightly soluble in water; and Octanol (bottom), very insoluble in water. Each structure shows carbon (C) and oxygen (O) atoms with hydrogen (H) atoms attached.

Short-chain alcohols have good solubility in water and other polar solvents. As the length of the non-polar hydrocarbon chain increases, the solubility of the alcohol in water decreases.

Alcohols exhibit much stronger intermolecular forces than alkanes and alkenes of an equivalent molecular weight.

Alongside the London forces exhibited in both alcohols and similarly sized hydrocarbons, alcohols also exhibit permanent dipole–dipole interactions and hydrogen bonds that are significantly stronger and require more energy to break.

As a result alcohols have higher boiling points and are less volatile than alkanes and alkenes of an equivalent molecular weight.

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 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.

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.

When oxidising a primary alcohol, aldehydes are isolated through distillation. A carboxylic acid product can be obtained by heating under reflux.

A diagram illustrating a laboratory setup for the oxidation of a primary alcohol to an aldehyde. The setup includes a round-bottom flask containing a blue liquid labeled 'Primary alcohol and [O]', connected to a condenser with arrows indicating water flow in and out. Below the flask, a heat source is shown, and a receiving flask is positioned to collect the resulting aldehyde.

Hydrogen bonding is present in the alcohol but not the aldehyde. As a result, the aldehyde has the lowest boiling point and can be collected by distillation as it forms, preventing further oxidation.

Under reflux, the aldehyde returns to the reaction mixture as it is produced, allowing the secondary oxidation to occur. The carboxylic acid is formed.

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.

During the elimination reaction the group is removed along with a proton from an adjacent (beta) carbon.

Where more than one adjacent proton exists, the double bond can form in either location.

Both structural isomers and stereoisomers can be formed through elimination reactions of alcohols. Structural isomers form due to double bond location within the molecule, and stereoisomers form due to potential configurations around the double bonds formed.

Question walkthrough

Reactions of alcohols

Prediction of products formed

The group of an alcohol can be substituted with a halide ion.

The alcohol must be acidified and exposed to a halide nucleophile.

Direct addition of a hydrogen halide (e.g. ) provides both acidity and the halide nucleophile.

Chemical reaction diagram showing the conversion of an alcohol (with a hydroxyl group) and hydrochloric acid (HCl) into an alkyl chloride and water (H2O).

The resulting product is a haloalkane.

Alcohols can undergo nucleophilic substitution with the salt of a halide and a strong acid, to form haloalkanes.

Alcohols are acidified with a strong acid, such as or , and then mixed with a salt of the desired halide.

The strong acid provides protons needed to improve quality of the leaving group whilst the salt provides the halide nuclephile. This route is preferred in the formation of bromoalkanes, as is highly corrosive and provides risk of explosion.

Chemical reaction diagram showing the conversion of an alcohol and sodium chloride into an alkyl chloride and sodium hydroxide, with the addition of a proton (H+).

is not a suitable acid due to the ability of to act as a competing nucleophile.