Chemistry » Organic Molecules » Organic Molecular Structures

The Alcohols

The Alcohols

An alcohol is any organic compound where there is a hydroxyl functional group $$(-\text{OH})$$ bound to a carbon atom. The general formula for a simple alcohol is $$\color{red}{\textbf{C}_{\textbf{n}}\textbf{H}_{\textbf{2n+1}}\textbf{OH}}$$.

The simplest and most commonly used alcohols are methanol and ethanol (see figure below).

The (a) structural, (b) condensed structural and (c) molecular formula representations of methanol.

The (a) structural, (b) condensed structural and (c) molecular formula representations of ethanol. (d) An atomic model of ethanol.

There are three possible types of carbon atoms – primary, secondary and tertiary. A primary carbon is attached to only one other carbon atom. A secondary carbon is attached to two other carbon atoms, while a tertiary carbon atom is attached to three other carbon atoms.

Note:

Make sure you understand the difference between primary, secondary and tertiary carbons:

• Primary

A primary carbon is a carbon atom bonded to one other carbon atom.

• Secondary

A secondary carbon is a carbon atom bonded to two other carbon atoms.

• Tertiary

A tertiary carbon is a carbon atom bonded to three other carbon atoms.

For example:

$$\text{CH}_{3}\text{CH}_{2}\text{CH}_{3}$$, carbon 1 and carbon 3 are primary carbon atoms because they are only bonded to one other carbon atom. Carbon 2 is a secondary carbon atom because it is bonded to two other carbon atoms.

$$\text{C}(\text{CH}_{3})_{3}\text{X}$$, The central carbon atom in this compound is a tertiary carbon atom because it is bonded to three other carbon atoms.

A primary alcohol has the hydroxyl ($$-\text{OH}$$) group bonded to a primary carbon atoms. Similarly, a secondary alcohol has the hydroxyl bonded to a secondary carbon atom and a tertiary alcohol has the hydroxyl bonded to a tertiary carbon atom.

There can be a functional group attached to these different types of carbon atom. When a hydroxyl $$(-\text{OH})$$ functional group is attached to a primary carbon atom it is called a primary alcohol. For a secondary alcohol the hydroxyl is bonded to a secondary carbon atom. When the hydroxyl is bonded to a tertiary carbon atom it is a tertiary alcohol. Examples are given below.

A (a) primary (butan-1-ol), (b) secondary (butan-2-ol) and (c) tertiary (2-methylpropan-2-ol) alcohol.

The alcohols have a number of different uses:

• methylated spirits is ethanol with methanol added

Fact:

Methanol is toxic. If ingested it forms formic acid or formate salts, which damage the central nervous system and can cause blindness, coma or death.

• all alcohols are toxic, but in low concentrations ethanol can be used in alcoholic drinks

• ethanol is the only alcohol used in alcoholic drinks

• ethanol is used as an industrial solvent

• methanol and ethanol can both be used as a fuel and they burn more cleanly than petrol or diesel

• ethanol is used as a solvent in medical drugs, perfumes and plant essences

• ethanol is an antiseptic

• ethanol can be used as a preservative for fixing and preserving specimens as well as for preserving food.

Extra Notes: Alcohols or Alkanols

Alcohols or alkanols are the compounds containing one or more hydroxyl groups (-OH). The alcohols contain the -OH group attached to alkyl group

Alkanols are classified as mono-, di- and trihydric alcohols according to the number of -OH groups’ contained in their molecules. Some examples of mono-, di- and trihydric alcohols are as follows:

Primary (1°), Secondary (2°) and Tertiary (3°) Alkanols

Monohydric alcohols are classified as primary, secondary or tertiary alcohols depending upon whether the hydrogen group is attached to a primary, secondary or tertiary carbon atom, For example: