CHAPTER 14: ALCOHOLS

Question 1

What are the properties of alcohols?

Answer 1

Polarity:
• Alkane molecules are considered non-polar.
• Electronegativity of H & C is similar.
• IMFs in non-polar molecules are weak London forces.
• Alcohols have polar O⎯H groups.
• Electronegativity of O & H is different.
• Alcohol molecules are polar.
• IMFs are weak London forces & H-bonds.

Water Solubility:
• Due to polarity of O⎯H groups, they’re water soluble.
• H-bonds form between O⎯H & water molecules.
• As chain length increases, water solubility decreases.
• More of the alcohol contains non-polar C⎯H bonds.
• The effect of the polar O⎯H group decreases.

Volatility:
• H-bonds hold alcohol molecules together.
• Must be overcome to change state of alcohol to gas.
• Needs more energy than overcoming London forces.
• Alcohols are less volatile than alkanes with the same number of carbon atoms.

Question 2

How are alcohols classified?

Answer 2

Primary:
• Hydroxyl group attached to C atom with 2 H atoms.
• Carbon atom can have a maximum of 1 alkyl group.

Secondary:
• Hydroxyl group attached to C atom with 1 H atom.
• Carbon atom has 2 alkyl groups.

Tertiary:
• Hydroxyl group attached to C atom with 0 H atoms.
• Carbon atom has 3 alkyl groups.

Question 3

How do alcohols combust?

Answer 3

• Burn completely in excess O₂ to give CO₂ & H₂O.
• Reaction is exothermic = releases lots of heat energy.
• Energy per mole increases with no. of C atoms.

Question 4

How are primary alcohols oxidised?

Answer 4

Primary:
• Oxidised to aldehydes or carboxylic acids.
• Oxidised by an oxidising agent [O].
• [O] = K₂Cr₂O₇/H₂SO₄.
• Gentle heating with [O] forms aldehydes.
• Distillation ensures aldehyde formation.
• It prevents further reaction with [O].
• Orange Cr₂O₇²⁻ are reduced to Green Cr³⁺.
• Water is produced alongside aldehydes.
• Heating under reflux gives carboxylic acids.
• Excess [O] is used to ensure complete oxidation.
• Reflux ensures all of the aldehyde is oxidised.
• Orange Cr₂O₇²⁻ are reduced to Green Cr³⁺.
• Water is produced alongside carboxylic acids.

Question 5

How are secondary alcohols oxidised?

Answer 5

Secondary:
• Heated under reflux oxidised to Ketones.
• [O] = K₂Cr₂O₇/H₂SO₄.
• Reflux ensures reaction goes to completion.
• Orange Cr₂O₇²⁻ are reduced to Green Cr³⁺.
• Water is produced alongside ketones.
• Ketones can’t be oxidised more with K₂Cr₂O₇/H₂SO₄.

Question 6

How are tertiary alcohols oxidised?

Answer 6

Tertiary:
• Don’t undergo oxidation reactions.
• K₂Cr₂O₇/H₂SO₄ remains orange.

Question 7

How are alkenes formed from alcohols?

Answer 7

Alkene Formation (Condensation):
• H₂O molecule is removed from the alcohol.
• It’s heated under reflux with an acid catalyst.
• Acid Catalyst = Concentrated H₂SO₄ or H₃PO₄.
• This is an elimination reaction (dehydration).
• The product is an Alkene.

Question 8

How are haloalkenes formed from alcohols?

Answer 8

Haloalkane Formation (Substitution):
• Heated under reflux with H₂SO₄ & Sodium Halide.
• NaX(s) + H₂SO₄(aq) → NaHSO₄ (aq) + HX(aq).
• Hydrogen halide formed reacts with alcohol.
• CₙH₂ₙ₊₁OH(aq) + HX (aq) → CₙH₂ₙ₊₁X(aq) + H₂O(l).
• The product is a Haloalkane.