Chemistry Y12 Reaction Pathways

Alkane → haloalkane: reaction and conditions?

Free-radical substitution; Cl₂ or Br₂ and UV light.

Haloalkane → alkane: reverse route?

Reduction (e.g. hydrogenolysis with H₂ and suitable catalyst); extension, not simply reverse UV substitution.

Alkene → alkane?

Hydrogenation: H₂, Ni catalyst, heat.

Alkane → alkene?

Cracking or catalytic dehydrogenation under suitable industrial conditions; not a simple reverse hydrogenation.

Alkene → haloalkane?

Hydrohalogenation: add HX (HCl, HBr); electrophilic addition.

Haloalkane → alkene?

Elimination: ethanolic KOH, heat/reflux.

Alkene → dihaloalkane?

Add Br₂ or Cl₂ across C=C; electrophilic addition.

Dihaloalkane → alkene?

Reductive dehalogenation, e.g. Zn; extension.

Alkene → alcohol?

Hydration: steam, H₃PO₄ catalyst, high temperature and pressure (ethene: ~300°C, 6–7 MPa).

Alcohol → alkene?

Dehydration/elimination: concentrated H₂SO₄ and heat (or heated Al₂O₃).

Haloalkane → alcohol?

Nucleophilic substitution: aqueous NaOH/KOH, heat.

Alcohol → haloalkane?

Substitution using HX or suitable halogenating reagent; e.g. ethanol + HBr → bromoethane + water.

Primary alcohol → aldehyde?

Oxidise using acidified K₂Cr₂O₇; warm and distil aldehyde as it forms.

Aldehyde → primary alcohol?

Reduction: NaBH₄ (commonly aqueous/alcoholic solvent).

Aldehyde → carboxylic acid?

Oxidise using acidified K₂Cr₂O₇; heat under reflux.

Carboxylic acid → aldehyde?

Not a straightforward reverse of oxidation; specialised partial reduction, extension.

Primary alcohol → carboxylic acid?

Oxidise using excess acidified K₂Cr₂O₇ and reflux.

Carboxylic acid → primary alcohol?

Reduction using LiAlH₄ followed by work-up; extension.

Secondary alcohol → ketone?

Oxidise using acidified K₂Cr₂O₇ and heat/reflux.

Ketone → secondary alcohol?

Reduction using NaBH₄.

Can tertiary alcohols be oxidised readily with acidified dichromate?

No: they lack a hydrogen on the carbon bearing OH; no straightforward oxidation under normal school conditions.

Carboxylic acid + alcohol → ester?

Esterification: concentrated H₂SO₄ catalyst, heat/reflux; produces ester + water.

Ester → carboxylic acid + alcohol?

Acid hydrolysis: dilute aqueous acid, heat/reflux; reversible.

Ester → carboxylate + alcohol?

Base hydrolysis/saponification: aqueous NaOH, heat/reflux; acidify carboxylate to obtain carboxylic acid.

Haloalkane → primary amine?

Nucleophilic substitution with excess ethanolic ammonia, heat (often sealed/under pressure).

Primary amine → haloalkane?

No standard simple reverse of ammonia substitution; do not assume same conditions backwards.

Carboxylic acid + amine → amide?

Amide formation requires suitable activation/dehydrating conditions or an acyl chloride; simply mixing usually first forms an ammonium carboxylate salt.

Acyl chloride + amine → amide?

Nucleophilic acyl substitution; typically room temperature, with base/excess amine to neutralise HCl.

Amide → carboxylic acid + amine/ammonium?

Hydrolysis: aqueous acid, heat/reflux; products depend on pH.

Amide → carboxylate + amine?

Base hydrolysis: aqueous NaOH, heat/reflux.

Alkene → addition polymer?

Addition polymerisation: C=C opens to form saturated backbone; conditions depend on monomer and catalyst/initiator.

Addition polymer → original alkene?

Not a standard simple reversible reaction; chemical recycling/depolymerisation needs specialised conditions.

Diol + dicarboxylic acid → polyester?

Condensation polymerisation; ester links form and small molecules (usually water) are eliminated.

Diamine + dicarboxylic acid → polyamide?

Condensation polymerisation under suitable heating/process conditions; amide links form and water is eliminated.

Polyester → monomers?

Hydrolysis of ester links using acid/base and heat; products depend on conditions.

Polyamide → monomers?

Hydrolysis of amide links using acid/base and heat; products depend on conditions.

Haber: equation?

N₂(g) + 3H₂(g) ⇌ 2NH₃(g); forward reaction exothermic.

Haber: typical temperature, pressure and catalyst?

~400–500°C, ~20 MPa, iron-based catalyst.

Why does high pressure favour ammonia?

4 mol gaseous reactants → 2 mol gaseous products; higher pressure shifts equilibrium right.

Why not use very low temperature for Haber?

Lower T favours exothermic forward equilibrium but slows reaction rate; compromise needed.

How is Haber overall conversion improved?

Cool/condense and remove NH₃; recycle unreacted N₂ and H₂.

Contact stage 1: equation?

S(s) + O₂(g) → SO₂(g); burn sulfur in air/oxygen.

Contact stage 2: equation and catalyst?

2SO₂(g) + O₂(g) ⇌ 2SO₃(g); exothermic, V₂O₅ catalyst.

Contact stage 2: temperature and pressure?

~450°C; typically near atmospheric or modest pressure. Check your course text for its inconsistent pressure figures.

Why does pressure favour SO₃?

3 mol gaseous reactants → 2 mol gaseous products; increasing pressure shifts equilibrium right.

Contact stage 3: why not add SO₃ straight to water?

Direct reaction forms difficult-to-control acid mist; absorb SO₃ in concentrated H₂SO₄ to make oleum first.

Contact: oleum equations?

SO₃ + H₂SO₄ → H₂S₂O₇; then H₂S₂O₇ + H₂O → 2H₂SO₄.

Fermentation: overall equation and conditions?

C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂; yeast/zymase, anaerobic, warm conditions.

What is saccharification?

Enzymatic hydrolysis of polysaccharides such as starch into smaller soluble sugars; amylase is an example.

Why does fermentation stop and how is ethanol concentrated?

Accumulated ethanol inhibits yeast (textbook ~13%, variable); distillation increases ethanol concentration.

Ethene hydration: equation and conditions?

C₂H₄(g) + H₂O(g) ⇌ C₂H₅OH(g); H₃PO₄ catalyst, ~300°C, 6–7 MPa.

Hydration vs fermentation: key difference?

Hydration uses ethene/steam, high T/P and acid catalyst; fermentation uses sugar/yeast and anaerobic warm conditions.

Hydrogen fuel cell: anode half-equation (acidic PEM)?

H₂ → 2H⁺ + 2e⁻; oxidation at anode.

Hydrogen fuel cell: cathode half-equation (acidic PEM)?

O₂ + 4H⁺ + 4e⁻ → 2H₂O; reduction at cathode.

Hydrogen fuel cell: overall equation?

2H₂ + O₂ → 2H₂O; electrons travel through external circuit.

Catalyst effect on equilibrium yield?

Catalyst speeds both directions and attainment of equilibrium; does not change equilibrium constant or equilibrium yield.