To Differentiate Different Types of Alcohol

Objective:

1. To differentiate different types of alcohol.

Introduction:

In organic chemistry, alcohols from the functional group of hydroxyl groups are organic compounds which are characterized by one or more hydroxyl (−OH) groups attached to a carbon atom of an alkyl group (hydrocarbon chain). Alcohols have the general formula CnH2n+1OH (where n represent the number of carbon atom in the compound) or R-OH (where R represent an alkyl group).

Alcohols is usually classified into three classes, the primary alcohols (1°), secondary alcohols (2°) and tertiary alcohols (3°).Alcohols fall into different classes depending on how the -OH group is positioned on the chain of carbon atoms. In a primary (1°) alcohol, the carbon which carries the -OH group is only attached to one alkyl group. In a secondary (2°) alcohol, the carbon with the -OH group attached is joined directly to two alkyl groups. In a tertiary (3°) alcohol, the carbon atom holding the -OH group is attached directly to three alkyl groups. The attached alkyl groups may be from the combination of same or different groups.

R = alkyl group

The boiling points of the normal alcohols increase regularly with the increase of the molecular weights. The primary alcohols boil at a higher temperature than the secondary alcohols that are isomeric with them. Similarly, the latter have higher boiling points than their tertiary isomerides. Among isomeric alcohols, the boiling point decreases with increase in branching in the alkyl group. Boiling points of alcohols are much higher than those of alkenes, halo alkenes or ethers of comparable molecular masses. This is because in alcohols intermolecular hydrogen bonding exists due to which a large amount of energy is required to break these bonds. Smaller molecules of alcohols are soluble in water. However, when the number of carbon reaches to four and beyond, its solubility in water reduced and will form two obvious layers.

The chemical properties of alcohols generally involve the reactions of -OH group. They can undergo substitution as well as elimination reaction. In this experiment, few tests including Lucas test (substitution by halogens) and oxidation test is carried out to distinguish between the different classes of alcohols. For Lucas test, the reagents are a mixture of concentrated hydrochloric acid and zinc chloride while the reagents used for oxidation test is sodium dichromate. The end product formed can determine the types of alcohol each reactant belong to. Other condition taken into consideration includes the colour of product formed and also the time taken for reaction.