Carbon–Carbon Bond Formation

Most of the reactions you have met so far interconvert functional groups within a fixed carbon skeleton — oxidation of alcohols, reduction of ketones, esterification, hydrolysis. But synthesis often requires lengthening the carbon chain itself. To do that, you need reactions that form new C–C bonds. OCR expects you to know two of these: the use of nitriles (cyanide addition and alkylation) to add one carbon, and Friedel-Crafts acylation to install an acyl group on an aromatic ring.

This lesson covers the OCR A-Level Chemistry A (H432) specification point 6.2.6: strategic use of carbon–carbon bond-forming reactions in synthesis.

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1. Why C–C Bond Formation Matters

Almost every pharmaceutical, dye, fragrance or material starts from simple feedstocks — ethene, benzene, methane — and is built up by adding carbons. The synthetic chemist's toolkit for extending a chain is deliberately small at A-Level, but the two methods below unlock enormous flexibility.

Summary of the tools:

Reaction	Bond formed	Starting point	Extends chain by
Haloalkane + KCN	C–C	Haloalkane	1 carbon (as –CN)
Carbonyl + HCN	C–C	Aldehyde/ketone	1 carbon (as –CN on new chiral C)
Friedel-Crafts acylation	C–C	Benzene	2+ carbons (as acyl group)

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2. Nitriles: One-Carbon Chain Extension

Both reactions in this section install a cyano group (–CN). The CN group contains one carbon, so attaching it to a molecule extends the carbon chain by one. Subsequent hydrolysis or reduction of the nitrile then converts –CN to –COOH or –CH₂NH₂, giving you access to the carboxylic acid or amine one carbon longer than the starting material.

2.1 Route 1: Haloalkane + KCN (SN2 substitution)

$CH_3CH_2Br + KCN \longrightarrow CH_3CH_2CN + KBr$CH3​CH2​Br+KCN⟶CH3​CH2​CN+KBr

• Reagents: Ethanolic potassium cyanide (KCN).
• Conditions: Heat under reflux.
• Mechanism: Nucleophilic substitution (SN2 for primary haloalkanes).
• Product: A nitrile with one more carbon than the starting haloalkane.

So CH₃CH₂Br (2 carbons) → CH₃CH₂CN (3 carbons, called propanenitrile).

Warning on nomenclature: The nitrile CH₃CH₂CN has three carbons total — the CN carbon counts as C1. This is why it is called propanenitrile, not ethanenitrile. Students get this wrong a lot.

2.2 Route 2: Carbonyl + HCN (nucleophilic addition)

You met this in Lesson 2. A carbonyl reacts with HCN (via KCN + H₂SO₄) to give a hydroxynitrile:

$CH_3CHO + HCN \longrightarrow CH_3CH(OH)CN$CH3​CHO+HCN⟶CH3​CH(OH)CN

• Extends the chain by 1 carbon (the new C comes from CN⁻).
• Creates a new chiral centre if the carbonyl was unsymmetrical — typically gives a racemic mixture.
• Useful because the product has both a hydroxyl group and a nitrile, giving two functional-group handles for further synthesis.

2.3 Hydrolysis of nitriles to carboxylic acids

Once you have a nitrile, you can hydrolyse it to the corresponding carboxylic acid (still one carbon longer than the haloalkane/carbonyl you started with):

$CH_3CH_2CN + 2H_2O + HCl \longrightarrow CH_3CH_2COOH + NH_4Cl$CH3​CH2​CN+2H2​O+HCl⟶CH3​CH2​COOH+NH4​Cl

• Conditions: Dilute HCl, prolonged reflux.
• Alternative: Aqueous NaOH, reflux, then acidify (gives the same carboxylic acid).

So ethanenitrile → ethanoic acid (2C → 2C, the CN was one of the two). Propanenitrile → propanoic acid (3C → 3C). The chain length is preserved in hydrolysis, but you have converted the –CN to a –COOH. The one-carbon extension happened back when you added the CN in the first place.

2.4 Reduction of nitriles to primary amines

Alternatively, the nitrile can be reduced to a primary amine one carbon longer:

$CH_3CH_2CN + 4[H] \longrightarrow CH_3CH_2CH_2NH_2$CH3​CH2​CN+4[H]⟶CH3​CH2​CH2​NH2​

• Conditions: LiAlH₄ in dry ether (or H₂ with Ni catalyst at high pressure/temperature).
• Product: A primary amine with one more carbon than the nitrile would suggest from a first glance.

Wait — careful. The nitrile already has one more C than the starting haloalkane. Reduction adds H atoms but not C atoms. So going haloalkane → nitrile → amine gives an amine with one more carbon than the starting haloalkane. This is the cleanest route to primary amines and is preferred over the haloalkane + NH₃ route from Lesson 6.

2.5 Full synthetic chains using CN

Combining these reactions gives powerful synthetic routes:

graph TD
  A[Haloalkane R-Br] --> B[+ KCN / ethanol, reflux]
  B --> C[Nitrile R-CN, +1 carbon]
  C --> D[+ H2O / H+, reflux: carboxylic acid R-COOH]
  C --> E[+ LiAlH4 / ether: primary amine R-CH2-NH2]

Worked example: Starting from 1-bromopropane (CH₃CH₂CH₂Br, 3C), make butanoic acid (4C).

1. CH₃CH₂CH₂Br + KCN → CH₃CH₂CH₂CN (butanenitrile, 4C).
2. CH₃CH₂CH₂CN + dilute HCl, reflux → CH₃CH₂CH₂COOH (butanoic acid, 4C).

The carbon chain has grown by 1. Exactly what we wanted.

Worked example 2: Starting from ethanal (CH₃CHO, 2C), make 2-hydroxypropanoic acid (lactic acid, 3C).

1. CH₃CHO + KCN/H₂SO₄ → CH₃CH(OH)CN (2-hydroxypropanenitrile, 3C).
2. Hydrolyse with HCl, reflux → CH₃CH(OH)COOH (2-hydroxypropanoic acid, 3C).

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3. Friedel-Crafts Acylation

Benzene rings are wonderfully unreactive — which is a good thing for the stability of aromatic drugs and dyes, but a problem when you need to install a substituent. One of the few reactions that works is Friedel-Crafts acylation, in which an acyl group (R–CO–) is installed on the ring via an electrophilic substitution.

3.1 Overall reaction

$C_6H_6 + CH_3COCl \longrightarrow C_6H_5COCH_3 + HCl$C6​H6​+CH3​COCl⟶C6​H5​COCH3​+HCl

Benzene + ethanoyl chloride → phenylethanone (acetophenone) + HCl.