Extending carbon chain length
Quick Notes
- Carbon–carbon Bond Formation is essential in organic synthesis to increase chain length.
- Nitrile formation (adding CN a group) enables carbon–carbon bond to be formed.
- Nitriles from haloalkanes: nucleophilic substitution with CN− in ethanol.
- Nitriles from aldehydes/ketones: nucleophilic addition with HCN.
- Nitrile Reactions
- Reduction: nitrile to amine (e.g. H2/Ni catalyst).
- Acid hydrolysis: nitrile to carboxylic acid.
- Aromatic C–C Bond Formation
- Friedel–Crafts alkylation: benzene + haloalkane.
- Friedel–Crafts acylation: benzene + acyl chloride.
- Requires halogen carrier (e.g. AlCl3).
Full Notes
Creating new carbon–carbon bonds is vital for making organic molecules and extending the carbon chain.
Nitrile formation creates a C–C bond and is a common reaction used to increase carbon chain length.
Formation of Nitriles
Nitriles (–C≡N) are useful intermediates because the –CN group can be converted into other functional groups after the C–C bond has been formed.
Nitriles can be formed from haloalkanes and aldehydes or ketones. You need to know both methods of preparation.
From Haloalkanes (via nucleophilic substitution)
Reagent: KCN or NaCN
Conditions: Ethanolic solution, reflux
General reaction: R–X + CN− → R–CN + X−
Mechanism: Nucleophilic substitution (see Topic 4.2.2).
Example Bromoethane to propanenitrile
CH3CH2Br + KCN → CH3CH2CN + KBr
From Aldehydes/Ketones (via nucleophilic addition)
Reagents: HCN or NaCN/H+
Conditions: Aqueous, pH ~4–5
Mechanism: Nucleophilic addition.
Product: Hydroxynitrile
Example Ethanal with HCN
CH3CHO + HCN → CH3CH(OH)CN
Reactions of Nitriles
Once formed, nitriles can undergo the following useful conversions:
Reduction to Amines
Reagents: H2 gas and Ni catalyst
Conditions: High pressure and temperature
Reaction: R–CN + 2H2 → R–CH2NH2
Used to form primary amines from nitriles.
Acid Hydrolysis to Carboxylic Acids
Reagents: Dilute HCl or H2SO4
Conditions: Heat under reflux
Reaction: R–CN + 2H2O + H+ → R–COOH + NH4+
Converts nitrile into carboxylic acid.
Friedel–Crafts Reactions for Aromatic C–C Bond Formation
Friedel–Crafts reactions enable attachment of alkyl or acyl groups onto a benzene ring, forming substituted aromatic compounds and making a new C–C bond.
Alkylation (C–C bond from haloalkane)
Reagents: Haloalkane (e.g. CH3Cl)
Catalyst: AlCl3 (halogen carrier)
Reaction: Benzene + R–Cl → Alkylbenzene + HCl
Forms a new C–C bond.
Acylation (C–C bond from acyl chloride)
Reagents: Acyl chloride (e.g. CH3COCl)
Catalyst: AlCl3
Reaction: Benzene + RCOCl → Aryl ketone + HCl
Important in forming aromatic ketones (e.g. phenyl ethanone).
Both alkylation and acylation are electrophilic substitution reactions. Mechanisms follow the general electrophilic attack of the aromatic ring followed by loss of a proton to regenerate aromaticity.
Summary
- Nitrile formation increases carbon chain length by creating a new C–C bond.
- Nitriles are made from haloalkanes (nucleophilic substitution) and carbonyls (nucleophilic addition).
- Nitriles can be converted into amines (reduction) or carboxylic acids (acid hydrolysis).
- Friedel–Crafts alkylation and acylation form new C–C bonds in aromatic systems.