Alcohols

Alcohols

Alcohols are organic compound that have at least one hydroxyl group (-OH) attached to the carbon chain. Names of alcohols can be identified by the suffix "-ol", which follows the normal prefix used for identifying the number of carbons. Hence, hexanol refers to an alcohol with six carbons and butanol is an alcohol with four carbons. Outside of methanol and ethanol (one and two carbon alcohols respectively), the hydroxyl group could be located in several different places on the carbon chain. As a result, a number is used to indicate the location of the hydroxyl group. Consider butanol, which was noted earlier as an alcohol with four carbons. Note the two structures below, both of which depict a four carbon chain with a hydroxyl group.

The first alcohol has the hydroxyl group on the first carbon in the chain. As a result it goes by the name 1-butanol or butan-1-ol. The alcohol below it has the hydroxyl group located on the second carbon, therefore its name is 2-butanol or butan-2-ol. These are the only two straight-chain isomers possible for butanol. Moving the hydroxyl group to the third carbon is merely the mirror image of 1-butanol. The same is true if the hydroxyl group from 1-butanol is placed at the other end of the chain. The structure would merely be a mirror image of the own already shown, and would still be 1-butanol.

Primary, Secondary, and Tertiary Alcohols

Alcohols can be classified as either primary (1°), secondary (2°) or tertiary (3°). The nature of the carbon atom that is directly bonded to the hydroxyl group determines if the alcohol is primary, secondary, or tertiary. Recall 1-butanol from earlier on this page. Since the hydroxyl group appeared on the last carbon in the chain, that final carbon is only attached to one other carbon. This makes 1-butanol a primary alcohol. In fact, all n-alcohols, or 1-alcohols, are primary.

2-butanol is an example of a secondary alcohol. The hydroxyl group is bonded to the second carbon. On either side of this carbon are carbons, the first and third carbons in the chain. Due to these two carbons, 2-butanol is secondary.

Below is a picture of a tertiary alcohol, 2-methyl-2-propanol. It is a tertiary alcohol.

The reason why it is tertiary is because the carbon bonded to the hydroxyl group is bonded to three other carbons in its own right.

Oxidation of Alcohols

Oxidation of alcohols describes the process in which primary and secondary alcohols can be made into aldehydes and ketones respectively. The aldehyde produced from the primary alcohol can be further oxidized to a carboxylic acid. Only primary and secondary alcohols are oxidized. Tertiary alcohols cannot be oxidized. An acidified dichromate solution, usually potassium dichromate or sodium dicrhomate is utilized, works as an oxidizer for the alcohol. If a primary alcohol is to be oxidized, the aldehyde that is produced is merely a step on the path to full oxidation to the carboxylic acid. Environmental conditions and concentration of the acidified dichromate must be strictly maintained if the aldehyde is the desired product.

Consider the general reaction for the oxidation of primary alcohols shown below. First the hydroxyl is converted to a carbonyl, and then to a carboxyl group.

Below is shown the general reaction for the oxidation of secondary alcohols. Note that after the hydroxyl group is converted to a carbonyl, the reaction stops.