Alcohols

 
 

Properties of alcohols

Alcohols are a homologous series that all contain the hydroxyl (-OH) functional group and have the general formula, CnH2n+1OH.

Alcohols have a polar –OH bond. The positive charge on the hydrogen can attract lone pairs on an oxygen on another molecule, forming hydrogen bonds. These strong intermolecular forces give water the following properties:

  • Solubility in water – Short-chain alcohols dissolve by forming hydrogen bonds with water molecules. But as the chain length increases, the non-polar portion of the molecule gets larger. This makes longer alcohols less soluble than shorter ones.

  • High boiling points – Compared with alkanes and alkenes, alcohols have much higher boiling points as they form London dispersion forces and hydrogen bonds, which require a large amount of energy to break. Since alcohols are harder to convert from a liquid to a gas, we say they have a lower volatility compared to alkanes or alkenes.

Alcohols can be classed as primary, secondary or tertiary depending on which carbon the hydroxyl group is attached to.

  1. Primary alcohols – the carbon to which the –OH is bound is only bonded to one other carbon.

  2. Secondary alcohols – the carbon to which the –OH is bound is bonded to two other carbon atoms.

  3. Tertiary alcohols – the carbon to which the –OH is bound is bonded to three other carbon atoms.

 
 

Combustion of alcohols

Alcohols can be burned in combustion reactions to release carbon dioxide and water.

This reaction is very exothermic, so alcohols make good fuels. Ethanol is a more sustainable fuel than hydrocarbons obtained from crude oil, as it can be produced through glucose fermentation (which is obtained from plants.)

 
combustion of ethanol.jpg
 

Oxidation of alcohols

Distillation apparatus

Alcohols can be oxidised using acidified potassium dichromate (K2Cr2O7/H+) as the oxidising agent. You get different products depending on the type of alcohol that you are oxidising and the reaction conditions.

Primary alcohols are partially oxidised (through distillation) into aldehydes. Water is formed as a by-product. Distillation involves boiling off the aldehyde immediately before it can be further oxidised.

 
 

Primary alcohols can be completely oxidised into carboxylic acids. Water is formed as a by-product.

Complete oxidation is carried out using reflux apparatus, which repeatedly evaporates and condenses the alcohol, so that it returns to the flask after partial oxidation to be fully oxidised into the carboxylic acid.

 
 

Secondary alcohols are oxidised into ketones by heating with acidified potassium dichromate under reflux. Again, water is also formed.

 
 

Tertiary alcohols cannot be oxidised. In the equations above, notice that oxidation occurs by pulling off hydrogens from the carbon atom that is attached to the hydroxyl group. For tertiary alcohols, this carbon is attached to three other carbon atoms, so there are no hydrogens to be removed for oxidation.

Oxidation of alcohols is accompanied by a colour change due to the reduction of potassium dichromate. Dichromate ions, Cr2O72- are orange in colour but turn to green when they are reduced to chromium (III), Cr3+ ions. You could use this as a test to see whether an alcohol is primary/secondary or tertiary. If an orange to green colour change is seen, you know that you either have a primary or secondary alcohol.

Dehydration of alcohols into alkenes

Water can be removed from alcohols in a dehydration reaction to form alkenes. This reaction requires warm temperatures and the presence of an acid catalyst (either concentrated sulfuric acid or concentrated phosphoric acid).

elimination alcohol.jpg

This is an elimination reaction and the water that is formed comes from the hydroxyl group and a hydrogen atom on either the right or left side of the hydroxyl group. The removal of water therefore creates a double bond that lies to the right or left hand side of where the –OH group used to be, creating two possible products.

Note that if the alkene consists of a C=C bond where each carbon is attached to two different groups, these can form E/Z isomers, producing a total of three products in some cases.

Substitution of alcohols into haloalkanes

Alcohols react with compounds containing halide ions, such as sodium chloride, to form halogenoalkanes (aka haloalkanes). The hydroxyl group is substituted with the halide ion in a nucleophilic substitution reaction. The reaction requires the presence of an acid catalyst, such as sulfuric acid.