4.2.1 Alcohols

Definitions

Term	Definition
Dehydration	A water molecule is removed from the starting material

Functional group = \(-OH\) (hydroxyl group)
Has polar and non-polar parts
- The \(O-H\) bond is polar (oxygen is more electronegative than hydrogen)
- The side chain is non-polar
- so alcohol can mix with both polar and non-polar liquids

Higher melting & boiling point than alkanes
- Induced dipole-dipole interactions between the non-polar side chains
- Strong hydrogen bonds + permanent dipole-dipole interactions between alcohol molecules hold them together (stronger than London forces)
- Extra heat energy is required to break the strong hydrogen bonds
Greater water solubility than alkenes
- Alkanes are non-polar so they cannot form hydrogen bonds / PDD with water
- \(O-H\) bond in alcohol is polar and forms hydrogen bond with water
- Longer carbon chain = less miscible in water
- More \(-OH\) groups = more miscible in water

Primary: \(-OH\) group attached to a carbon atom attached to 2 hydrogen atoms + 1 alkyl group
Secondary: \(-OH\) group attached to a carbon atom attached to 1 hydrogen atom + 2 alkyl groups
Tertiary: \(-OH\) group attached to a carbon atom attached to no hydrogen atoms + 3 alkyl groups

Alcohol + oxygen \(\rightarrow\) carbon dioxide + water
Exothermic reaction
- A large quantity of energy released in the form of heat
- Burn with a clear blue flame
More carbon atoms in the alcohol chain = more heat energy released per mole
Alcohols undergo complete combustion more often than alkanes due to the oxygen atom in the molecule

Heat with an oxidising agent ([O])
- Normally acidified dichromate (VI) (\(Cr_2O_7^{2-} / H^+\))
- e.g. acidified potassium dichromate (VI) (\(K_2Cr_2O_7 / H_2SO_4\))
Observations
- Cr will be reduced
- Reaction mixture turn from orange (\(Cr^{6+}\) in \(Cr_2O_7^{2-}\)) to green (\(Cr^{3+}\))

Gentle heating + distillation = aldehyde formed
- Aldehyde distilled out of the reaction mixture as it forms to prevent any further reaction (distil)
Heated strongly + reflux + excess of acidified potassium dichromate (VI) = carboxylic acid
- Reflux = ensure that all aldehyde formed initially also oxidised to carboxylic acid
- Excess of oxidising agent = ensure that all alcohol is oxidised

Oxidised to ketones with an oxidising agent (acidified dichromate (VI) ions)
Ketones cannot be further oxidised
Heated under reflux with the oxidising agent to ensure that the reaction goes to completion

Heated under reflux with concentrated acid catalyst (e.g. concentrated \(H_2SO_4 / H_3PO_4\))
Product = an alkene
Type = elimination reaction

Alcohols react with halide ions to form haloalkanes
Heated under reflux with concentrated acid catalyst & halide ion e.g. \(H_2SO_4 + NaBr\)
- Acid need to be concentrated to minimise back reactions