Alcohols

Types of Chemical Formulae

To correctly identify and draw alcohols in an exam, you must understand three ways of representing them:

• A molecular formula counts the total number of each atom. While ethanol could be written as $C_2H_6O$, Edexcel strongly prefers writing it as $C_2H_5OH$ to clearly show it belongs to the alcohol family.
• A structural formula shows how the atoms are arranged group by group (e.g., $CH_3CH_2OH$), but it does not explicitly draw out the individual covalent bonds.
• A displayed formula is a complete graphical drawing showing every single atom and every single covalent bond (including the $O-H$ bond) as a distinct line.

The First Four Alcohols

Examiners expect you to recall the specific formulae and draw the displayed structures for the first four members. For propanol and butanol, you only need to know the straight-chain versions (where the $-OH$ group is on the end carbon).

1. Methanol (1 Carbon)

• Molecular Formula: $CH_3OH$
• Structural Formula: $CH_3OH$
• Displayed Formula:

$$\begin{array}{ccccccc}& & \text{H}& & & & \\ & & \mathrm{\mid }& & & & \\ \text{H}& -& \text{C}& -& \text{O}& -& \text{H}\\ & & \mathrm{\mid }& & & & \\ \end{array}$$

2. Ethanol (2 Carbons)

• Molecular Formula: $C_2H_5OH$
• Structural Formula: $CH_3CH_2OH$

$$\begin{array}{ccccccccc}& & \text{H}& & \text{H}& & & & \\ & & \mathrm{\mid }& & \mathrm{\mid }& & & & \\ \text{H}& -& \text{C}& -& \text{C}& -& \text{O}& -& \text{H}\\ & \end{array}$$

3. Propanol (3 Carbons)

• Molecular Formula: $C_3H_7OH$
• Structural Formula: $CH_3CH_2CH_2OH$

$$\begin{array}{ccccccccccc}& & \text{H}& & \text{H}& & \text{H}& & & & \\ & & \mathrm{\mid }& & \mathrm{\mid }& & \mathrm{\mid }& & & & \\ \text{H}& -& \text{C}& -& \text{C}& -& \end{array}$$

4. Butanol (4 Carbons)

• Molecular Formula: $C_4H_9OH$
• Structural Formula: $CH_3CH_2CH_2CH_2OH$

$$\begin{array}{ccccccccccccc}& & \text{H}& & \text{H}& & \text{H}& & \text{H}& & & & \\ & & \mathrm{\mid }& & \mathrm{\mid }& & \mathrm{\mid }& & \mathrm{\mid }& & & & \\ \text{H}& -& \end{array}$$

Worked Example: Drawing Displayed Formulae

Draw the displayed formula for butanol, ensuring you meet all Edexcel mark scheme requirements.

Step 1: Draw the carbon backbone. Butanol has 4 carbon atoms. Draw them in a straight chain connected by single covalent bonds: $C-C-C-C$.

Step 2: Attach the functional group. Add the oxygen atom to the end carbon. Crucial Edexcel Rule: You must draw the single bond between the oxygen and the hydrogen ($O-H$).

Step 3: Fill remaining bonds with hydrogen. Every carbon atom must have exactly four bonds. Add hydrogen atoms above, below, and to the sides of the carbon chain until all slots are filled. (See structure for Butanol above).