Alcohols


Alcohols have the functional group CnH(2n+1)OH and are a homologous series. Alcohols have the functional group OH and short chain alcohols dissolve in water (the OH forms a hydrogen bond with the water molecules), which gives them a high miscibility with water, meaning they are able to mix well. This can be compared to other organic compounds e.g. alkanes which float on the top of water, think of an oil spill.

Alcohols have strong hydrogen bond intermolecular forces, which give them a higher boiling point, or a lower volatility, when compared to other hydrocarbons of a similar chain length.

Classification of Alcohols
Primary 1° One Carbon atom attached to the C-OH
Secondary 2° Two Carbon atoms attached to the C-OH
Tertiary 3° Three Carbon atoms attached to the C-OH

Naming Alcohols
These are named similar to alkanes or alkenes, except you give preference to the OH and specify the position of the OH bond (you start counting from whichever end gives the lowest number to the OH bond).

Other hydrocarbons


Alcohol Reactions


Alcohols react in several different ways. For the following reactions the alcohol has been assumed to be ethanol.

1. Combustion
Conditions: None

CH3CH2OH + 3O2 2CO2 + 3H2O

Uses: Fuel, mainly as a petrol substitute in countries such as Brazil, which have low natural crude oil reserves but can produce lots of glucose from plants to convert to alcohols via fermentation.


2. Reaction with halogen alkanes (e.g. HBr to from bromoalkane)
Conditions: HBr is made in situ. by heating concentrated H2SO4 with sodium bromide:

NaBr +H2SO4 NaHSO4 + HBr

The HBr then reacts with the alcohol.

CH3CH2OH + HBr CH3CH2Br +H2O

Uses: No specific uses – can undergo further reactions to form other organic compounds


3. Reaction with Sodium to form a salt
Conditions: None

CH3CH2OH + Na CH3CH2O-Na+ + ½H2

Note: How the sodium and oxygen form an ionic bond

Uses: To dispose of small quantities of sodium as the reaction is slow and controlled, than for example the reaction of sodium with water!


4. Dehydration
Conditions: Hot concentrated sulphuric acid or hot aluminium oxide catalyst

CH3CH2OH CH2=CH2 + H2O

Uses: No specific uses


5. Esterification
Conditions: Hot concentrated sulphuric acid catalyst

Uses: Esters are used in solvents and have fruity smells and are therefore used as food additives and in perfumes.

 

Oxidation of Alcohols


Alcohols oxidise in different ways dependent on whether they’re primary, secondary or tertiary. In all cases the alcohols are heated with a mixture of potassium dichromate (VI) and sulphuric acid.

Primary alcohols
Conditions: Potassium dichromate (VI) and sulphuric acid catalysts

These can be oxidised twice
Step One - Removal of hydrogen via distillation.

Colour change – Orange to Green

Step Two – Addition of oxygen via reflux.


Distillation Separate a mixture based on their boiling points.


Reflux Process of continuous heating followed by evaporation and then condensation of the mixture back into the reacting flask.

Secondary alcohols
Conditions: Potassium dichromate (VI) and sulphuric acid

These can only be oxidised once
Removal of hydrogen via distillation.



Colour change – Orange to Green

Tertiary alcohols
These cannot be oxidised, as there is no hydrogen atom on the Carbon atom attached to the OH bond. Therefore no colour change will occur.