Organic Chemistry

Compare benzene and cyclohexa-1,3,5-triene

Benzene is more stable; its hydrogenation is 152 kJ mol⁻¹ less exothermic due to electron delocalisation. Benzene has equal C–C bond lengths, whereas cyclohexa-1,3,5-triene has alternating C–C and C=C bond lengths. Both are planar hexagonal structures with 120° bond angles

What are the reagents and conditions for nitration of benzene?

Concentrated HNO₃ and concentrated H₂SO₄ heated under reflux at 50–55°C.

Write the equation for generating the nitronium ion.

HNO₃ + H₂SO₄ → NO₂⁺ + HSO₄⁻ + H₂O

Write the equation for regeneration of the H₂SO₄ catalyst during nitration

H⁺ + HSO₄⁻ → H₂SO₄

What are the reagents and conditions for Friedel–Crafts acylation

An acyl chloride and anhydrous AlCl₃ catalyst, heated under reflux

Write the equation for regeneration of the AlCl₃ catalyst during Friedel–Crafts acylation

H⁺ + AlCl₄⁻ → HCl + AlCl₃

What is the role of AlCl₃ in Friedel–Crafts acylation

AlCl₃ acts as a halogen carrier, helping generate the electrophile

How is the electrophile generated in Friedel–Crafts acylation

RCOCl + AlCl₃ → RCO⁺ + AlCl₄⁻

What are the reagents and conditions for reducing nitrobenzene to phenylamine

Sn and concentrated HCl, heated under reflux. Then add excess NaOH to release phenylamine

Why is benzene less reactive than alkenes towards electrophiles

Benzene's pi electrons are delocalised over six carbon atoms. Benzene resists addition because it would disrupt the stable delocalised system, so it favours electrophilic substitution

What does the enthalpy of hydrogenation of benzene show

Benzene's hydrogenation is 152 kJ mol⁻¹ less exothermic than predicted for three isolated C=C bonds. This shows that benzene has extra stability due to electron delocalisation

Why does benzene undergo electrophilic substitution rather than electrophilic addition

Substitution preserves the stable delocalised pi-electron system, whereas addition would disrupt it and reduce stability

What is the electrophile in benzene nitration

nitronium ion, NO₂⁺

What are the reagents and conditions for converting nitrobenzene into phenylamine

Heat nitrobenzene under reflux with tin and concentrated HCl, then add excess NaOH

What is the electrophile in Friedel–Crafts acylation

The acylium ion, RCO⁺

State the conditions for catalytic cracking.

High temperature and a zeolite (aluminosilicate) catalyst.

How do you test for an alkene?

Add bromine water. It changes from orange to colourless.

Why can alkenes show E/Z isomerism?

The C=C bond prevents free rotation, and each carbon must have two different groups attached.

What is structural isomerism?

Compounds have the same molecular formula but different structural formulae.

What is positional isomerism?

Compounds have the same molecular formula and functional group, but the functional group is in a different position.

What is functional group isomerism?

Compounds have the same molecular formula but different functional groups.

What is stereoisomerism?

Compounds have the same structural formula but different arrangements of atoms in space.

What is optical isomerism?

A type of stereoisomerism where molecules have the same structural formula but differ in the way they rotate plane-polarised light.

How does optical isomerism occur?

When a carbon atom is bonded to four different groups.

What is a chiral molecule?

A molecule that exists as a pair of non-superimposable mirror images.

What is a chiral carbon?

A carbon atom bonded to four different groups.

What are enantiomers?

Non-superimposable mirror images of each other.

What are superimposable structures?

Structures that can be rotated so that every atom matches exactly.

What does non-superimposable mean?

Structures that cannot be rotated to make every atom match exactly.

How do you distinguish between optical isomers?

They rotate plane-polarised light in opposite directions. One rotates it clockwise and the other anticlockwise.

What is a racemic mixture?

A 50:50 mixture of two enantiomers.

Why is a racemic mixture optically inactive?

The equal and opposite rotations of plane-polarised light cancel each other out.

What is an enantiomerically pure substance?

A sample containing only one enantiomer.

What physical properties do enantiomers share?

They have the same melting point, boiling point, solubility, Mr, NMR spectra, IR spectra and mass spectra in an achiral environment.

How can enantiomers behave differently biologically?

They interact differently with chiral biological molecules, so they can have different biological effects.

Why are chiral centres important in pharmaceuticals?

Different enantiomers can have different biological effects. One may be therapeutically active, while the other may be inactive or harmful.

Why may pharmaceutical companies aim to produce a single enantiomer?

To improve therapeutic efficacy and reduce unwanted side effects.

What is a carbonyl group?

A carbon atom double-bonded to an oxygen atom (C=O).

Why can nucleophilic addition to aldehydes and asymmetrical ketones produce a racemic mixture?

The carbonyl group is planar, so the nucleophile can attack from either side with equal probability. If a chiral centre forms, equal amounts of both enantiomers are produced.

What happens when primary, secondary and tertiary alcohols are oxidised?

Primary alcohol → aldehyde → carboxylic acid

Secondary alcohol → ketone

Tertiary alcohol → no oxidation under normal A-level conditions.

How is an aldehyde oxidised to a carboxylic acid?

Reagent: Acidified potassium dichromate(VI), K₂Cr₂O₇ and dilute H₂SO₄.

Conditions: Heat under reflux.

Observation: Orange solution turns green as Cr₂O₇²⁻ ions are reduced to Cr³⁺ ions.

What is the silver mirror test for aldehydes and ketones?

Reagent: Tollens' reagent.

Conditions: Warm gently.

Aldehyde: Silver mirror forms.

Ketone: No visible change.

What is Fehling's test for aldehydes and ketones?

Reagent: Fehling's solution.

Conditions: Warm.

Aldehyde: Blue solution forms a brick-red precipitate.

Ketone: No visible change; solution remains blue.

What happens when aldehydes and ketones are reduced?

Aldehyde → primary alcohol.

Ketone → secondary alcohol.

What are the reagent, mechanism, nucleophile and conditions for reducing aldehydes and ketones?

Reagent: Sodium borohydride (NaBH₄).

Mechanism: Nucleophilic addition.

Nucleophile: Hydride (H⁻), transferred from the borohydride reagent.

Conditions: A suitable aqueous or alcoholic solvent. Acidic conditions are not required.

What are the reagent, mechanism, nucleophile and conditions for the reaction of aldehydes and ketones with cyanide?

Reagents: Potassium cyanide (KCN) and dilute hydrochloric acid (HCl).

Mechanism: Nucleophilic addition.

Nucleophile: Cyanide ion (CN⁻).

Conditions: A suitable aqueous mixture containing KCN and dilute HCl.

What is the product of the reaction of aldehydes and ketones with cyanide?

A hydroxynitrile, containing an –OH group and a –C≡N group.

Why can the reaction of aldehydes and ketones with cyanide produce optical isomers?

The carbonyl group is planar, so CN⁻ can attack from either side. If a chiral centre forms, equal amounts of both enantiomers can be produced, forming a racemic mixture.