Alcohols (HSC SSCE Chemistry): Revision Notes

Alcohols

Introduction to alcohols

Alcohols are a family of organic compounds characterised by the presence of a hydroxyl group ($-\text{OH}$). This hydroxyl group is the functional group that defines alcohols and gives them their distinctive properties.

The simplest alcohol is methanol ($\text{CH}_3\text{OH}$), also called methyl alcohol. The next member of the series is ethanol ($\text{CH}_3\text{CH}_2\text{OH}$), also known as ethyl alcohol.

Uses of ethanol

Ethanol is one of the most widely used carbon compounds. Its main applications include:

• Alcoholic beverages (wine, beer, spirits)
• Fuel additive for petrol or as an alternative vehicle fuel
• Solvent in medicines, antiseptics, and household cleaners
• Industrial reactant and solvent for manufacturing plastics, adhesives, pharmaceuticals, and perfumes

Naming alcohols

The naming of alcohols follows similar conventions to alkanes and other hydrocarbons. The process involves these steps:

Step 1: Identify the longest continuous carbon chain. This becomes your parent chain. Use the standard stems (meth-, eth-, prop-, but-, etc.).

Step 2: Remove the final 'e' from the alkane name and add 'ol'. For example:

• Butane becomes butanol
• Ethane becomes ethanol
• Hexane becomes hexanol

Step 3: Number the carbon chain to give the hydroxyl group the lowest possible position number. Place this number before the alcohol name. For example, if the $-\text{OH}$ group is on the second carbon of butanol, the name becomes $2$-butanol.

Step 4: Name and number any substituents (branches) using the same rules as for hydrocarbons. List substituents alphabetically in the final name.

Worked example: naming an alcohol

Isomerism in alcohols

Alcohols exhibit isomerism just like other hydrocarbon families. Isomers are molecules with the same molecular formula but different structural arrangements.

Positional isomers

Positional isomers occur when the carbon skeleton remains the same but the position of the hydroxyl group changes.

Chain isomers

Chain isomers result from rearranging the carbon skeleton itself whilst maintaining the same molecular formula.

$2$-methyl-$2$-butanol is a chain isomer of $1$-pentanol (the carbon chain has been shortened and branched), whilst $1$-pentanol is a positional isomer of $2$-pentanol (the $-\text{OH}$ group has moved position).

Classification of alcohols

Alcohols can be classified into three types based on how many carbon atoms are attached to the carbon bearing the hydroxyl group.

Primary alcohols

A primary alcohol has only one carbon atom bonded to the carbon that carries the $-\text{OH}$ group. The carbon with the hydroxyl group is attached to one other carbon and two hydrogen atoms.

Example: $1$-butanol

Secondary alcohols

A secondary alcohol has two carbon atoms bonded to the carbon that carries the $-\text{OH}$ group. The carbon with the hydroxyl group is attached to two other carbons and one hydrogen atom.

Example: $2$-butanol

Tertiary alcohols

A tertiary alcohol has three carbon atoms bonded to the carbon that carries the $-\text{OH}$ group. The carbon with the hydroxyl group is attached to three other carbons and no hydrogen atoms.

Example: $2$-methyl-$2$-propanol

Physical properties of alcohols

The hydroxyl group in alcohols contains a highly electronegative oxygen atom, making the $\text{C-O}$ and $\text{O-H}$ bonds very polar. This polarity, combined with the non-polar hydrocarbon chain, determines the physical properties of alcohols.

Two key factors influence alcohol properties:

1. The presence of the $-\text{OH}$ group (capable of forming hydrogen bonds)
2. The size of the hydrocarbon chain

Boiling points

The boiling point of an organic compound depends on the energy needed to overcome intermolecular forces between molecules. Unlike alkanes, alkenes, and alkynes (which only form weak dispersion forces), alcohols can form hydrogen bonds.

Comparison with alkanes

Consider butane and $1$-butanol. Both have similar length carbon chains, so dispersion forces are approximately equal. However, $1$-butanol also forms hydrogen bonds, requiring much more energy to separate the molecules. Therefore, $1$-butanol has a much higher boiling point than butane.

Trend within the alcohol series

As the carbon chain length increases within the alcohol homologous series, the boiling point also increases. This occurs because:

• Larger molecules have more electrons
• More electrons create stronger dispersion forces between hydrocarbon chains
• More energy is needed to overcome these additional forces

Solubility in water

Alcohol solubility depends on the balance between:

• The polar hydroxyl group (water-soluble)
• The non-polar hydrocarbon chain (water-insoluble)

The polar $-\text{OH}$ group can form hydrogen bonds with water molecules, promoting solubility. The non-polar hydrocarbon chain cannot form hydrogen bonds with water, reducing solubility.

Small alcohols

For smaller alcohols like methanol, ethanol, and propanol, the effect of the polar hydroxyl group dominates. These alcohols are infinitely soluble in water.

Larger alcohols

As the hydrocarbon chain increases in size, its non-polar effect begins to outweigh the polar effect of the single hydroxyl group. This makes larger alcohols progressively less soluble:

• $1$-butanol: $7.7\,\text{g}/100\,\text{mL}$
• $1$-pentanol: $2.2\,\text{g}/100\,\text{mL}$
• $1$-hexanol: $0.59\,\text{g}/100\,\text{mL}$

Very large alcohols like $1$-octanol and $1$-decanol become insoluble in water.

Multiple hydroxyl groups

Some large molecules remain water-soluble despite having a large hydrocarbon chain. This occurs when the molecule contains multiple hydroxyl groups, multiplying the effect of the polar functional group.

Investigation: Solubility of alcohols

Aim

To investigate how alcohol solubility changes with increasing carbon chain length.

Safety considerations

Method summary

1. Label six test tubes ($1$-$6$) corresponding to the number of carbons in each alcohol
2. Add $3\,\text{mL}$ deionised water and two drops of food colouring to each tube
3. Add $2\,\text{mL}$ of the appropriate alcohol to each tube
4. Observe and photograph any layers that form
5. Add another $2\,\text{mL}$ of alcohol and note any changes

Key observations

The investigation reveals that:

• Short-chain alcohols (methanol, ethanol, propanol) mix completely with water
• Medium-chain alcohols show partial solubility
• Long-chain alcohols form separate layers (immiscible)

Remember!