OCR A Level Chemistry A: Carbon–carbon bond formation (H432) – Revision Notes

These notes condense section 6.2.4, Carbon–carbon bond formation, of the OCR A Level Chemistry A (H432) specification, Version 3.1 (May 2026), for first assessment in 2017. Outcomes 6.2.4(a) to (d) are all here in condensed form; they belong to the Organic chemistry and analysis module (Module 6). The whole of H432 is one A level, and Module 6 is examined on Synthesis and analytical techniques (02), while Unified chemistry (03) can draw on it too because that paper spans every module. For full explanations and five worked examples, use the study guide.

Reach the rest of the course through the OCR A Level Chemistry hub page. When you can do a 6.2.4 outcome unaided, tick it on the printable list of H432 outcomes, then attempt the 6.2.4 question set. For a course-wide check, use the free diagnostics.

6.2.4(a): The point of C–C bond formation

Idea	What to remember
Why it matters	Most organic reactions keep the carbon skeleton; a C–C step makes the chain longer
Nitrile steps	Add exactly one carbon, as –C≡N
Friedel–Crafts steps	Join a whole alkyl or acyl group to a benzene ring
After a nitrile step	Reduce to RCH₂NH₂ or hydrolyse to RCOOH; the extra carbon stays

Specification cross-references: 6.1.1 d (arenes) and 6.1.2 b (carbonyl compounds).

Key definitions

Term	Meaning
Nucleophile	An electron pair donor, here :CN⁻
Electrophile	An electron pair acceptor, here R⁺ or RCO⁺
Nitrile	Compound with a –C≡N group; the CN carbon is C1 in the name
Hydroxynitrile	–OH and –C≡N on the same carbon
Halogen carrier	AlCl₃, FeCl₃, FeBr₃, iron or another aluminium or iron halide; makes the electrophile
Acyl chloride	R–COCl

Reagents and conditions at a glance

Outcome	Starting compound	Reagent and conditions	Product	Mechanism to know
(b)(i)	Haloalkane, R–X	NaCN or KCN in ethanol, reflux	Nitrile, R–CN	Nucleophilic substitution
(b)(ii)	Aldehyde or ketone	HCN, as NaCN(aq)/H⁺(aq)	Hydroxynitrile	Nucleophilic addition
(c)(i)	Nitrile	H₂ with Ni catalyst	Primary amine, R–CH₂NH₂	Not required
(c)(ii)	Nitrile	Dilute acid, e.g. HCl(aq), reflux	Carboxylic acid, R–COOH	Not required
(d)	Benzene	Haloalkane + halogen carrier, dry	Alkylbenzene	Electrophilic substitution
(d)	Benzene	Acyl chloride + halogen carrier, dry	Aromatic ketone	Electrophilic substitution

Equations to learn

• Haloalkane: R–Br + CN⁻ → R–CN + Br⁻
• Carbonyl: RCHO + HCN → RCH(OH)CN, or R₂CO + HCN → R₂C(OH)CN
• Reduction: R–CN + 2H₂ → R–CH₂NH₂
• Hydrolysis: R–CN + 2H₂O + H⁺ → R–COOH + NH₄⁺ (with HCl: R–CN + 2H₂O + HCl → R–COOH + NH₄Cl)
• Alkylation: C₆H₆ + R–Cl → C₆H₅R + HCl
• Acylation: C₆H₆ + RCOCl → C₆H₅COR + HCl
• Electrophile: R–Cl + AlCl₃ → R⁺ + AlCl₄⁻, or RCOCl + AlCl₃ → RCO⁺ + AlCl₄⁻
• Carrier regenerated: H⁺ + AlCl₄⁻ → AlCl₃ + HCl

6.2.4(b)(i): Substitution mechanism in steps

1. Label the C–X bond Cδ+ and Xδ−.
2. Arrow: cyanide carbon lone pair → δ+ carbon.
3. Arrow: C–X bonding pair → X.
4. Products: R–CN and X⁻.

Reminder: 1-chlorobutane gives pentanenitrile, CH₃CH₂CH₂CH₂CN. The stem goes up by one carbon.

6.2.4(b)(ii): Addition mechanism in steps

1. Label the C=O bond Cδ+ and Oδ−.
2. Arrow: cyanide carbon lone pair → carbonyl carbon.
3. Arrow: C=O π pair → O, leaving an O⁻ intermediate.
4. Protonate O⁻ with H₂O (giving OH⁻ as well) or with H⁺.

Reminder: 2,2-dimethylpropanal, (CH₃)₃CCHO, gives 2-hydroxy-3,3-dimethylbutanenitrile, (CH₃)₃CCH(OH)CN.

Naming in steps

1. Find the longest chain that includes the CN carbon.
2. Number from the CN carbon as C1.
3. Name substituents (hydroxy, methyl, phenyl) with locants, in alphabetical order.
4. End with “-nitrile” after the full alkane name: butanenitrile, not butanitrile.

6.2.4(c): Nitrile to amine or acid

	Reduction	Acid hydrolysis
Reagent	H₂, Ni catalyst	Dilute HCl or H₂SO₄, reflux
Nitrile carbon becomes	–CH₂– next to NH₂	–COOH carbon
By-product	None	NH₄⁺ (as NH₄Cl with HCl)
Example	Propanenitrile → propan-1-amine	Propanenitrile → propanoic acid

A hydroxynitrile hydrolyses to a 2-hydroxy carboxylic acid: the –OH is untouched and only –CN changes. Reduce the same hydroxynitrile with H₂/Ni instead and you get an amino alcohol, with –OH on one carbon and –CH₂NH₂ next to it. In both cases, check your answer by counting: the product has the same number of carbon atoms as the nitrile.

6.2.4(d): Friedel–Crafts in steps

1. Identify the reagent: haloalkane (alkylation) or acyl chloride (acylation).
2. Write the electrophile equation with AlCl₃ (or FeCl₃, FeBr₃).
3. Write the overall equation, ending in HCl.
4. Show the carrier regenerated from H⁺ and AlCl₄⁻.
5. For a mechanism (6.1.1(g) lets you be asked to predict one), follow the halogenation pattern: π electrons to the electrophile, then loss of H⁺.

Reminder: benzene and 2-chloropropane with AlCl₃ give (propan-2-yl)benzene, C₆H₅CH(CH₃)₂, and HCl.

Route planning: spot the C–C step

Start every synthesis question by counting carbons in the starting compound and the target. Then match the change to a pattern.

Change from start to target	Likely route
One more carbon, ending –COOH	Haloalkane → nitrile (CN⁻, ethanol) → acid (dilute acid, reflux)
One more carbon, ending –CH₂NH₂	Haloalkane → nitrile → amine (H₂/Ni)
One more carbon, OH and COOH on the same carbon	Aldehyde or ketone → hydroxynitrile (HCN) → 2-hydroxy acid
One more carbon, OH and CH₂NH₂ on neighbouring carbons	Aldehyde or ketone → hydroxynitrile → reduce with H₂/Ni
Carbon chain joined to a benzene ring	Friedel–Crafts alkylation
C=O joined to a benzene ring	Friedel–Crafts acylation

Small reminders:

• 1-Bromo-3-phenylpropane, C₆H₅CH₂CH₂CH₂Br → 4-phenylbutanenitrile → 4-phenylbutanoic acid.
• Pentanal → 2-hydroxyhexanenitrile → 2-hydroxyhexanoic acid.
• Iodomethane → ethanenitrile → ethylamine.

For several steps, multiply the step yields as decimals to get the overall yield. To find the starting mass for a target mass, divide the moles of target by the overall yield, then convert to grams.

Must-know distinctions

Pair	Difference
CN⁻ with haloalkane vs with carbonyl	Substitution (halide leaves) vs addition (nothing leaves)
Ethanol vs NaCN(aq)/H⁺(aq)	Ethanol for haloalkanes; aqueous cyanide with acid for carbonyls
Reduction vs hydrolysis of nitriles	Amine (–CH₂NH₂) vs acid (–COOH); both keep the nitrile carbon
Alkylation vs acylation	Haloalkane gives alkylbenzene; acyl chloride gives aromatic ketone
Nucleophile vs electrophile	CN⁻ donates a pair; R⁺ and RCO⁺ accept one
Arrow start for CN⁻	Carbon lone pair, never nitrogen

Quick self-test

1. Define a nucleophile.
2. Name the nitrile formed when 2-iodopropane reacts with KCN in ethanol.
3. State the reagent and conditions for converting a haloalkane into a nitrile.
4. Cyclobutanone, C₄H₆O, reacts with HCN. Give the molecular formula and Mᵣ of the product (Aᵣ: N 14.0; C 12.0; O 16.0; H 1.0).
5. Write the equation for the reduction of ethanenitrile with hydrogen.
6. Write the equation for the hydrolysis of butanenitrile with dilute hydrochloric acid.
7. Give the electrophile formed when an acyl chloride, RCOCl, reacts with AlCl₃.
8. Name the halogen carriers the specification lists.
9. Which of these reactions lengthens a carbon chain: (A) chloroethane with NaOH(aq); (B) propanal with HCN; (C) propanal with NaBH₄?
10. A two-step route has step yields of 75% and 80%. What is the overall yield?
11. Benzene reacts with an acyl chloride and AlCl₃. What type of compound forms?
12. Explain why a dry apparatus is used for Friedel–Crafts reactions.

Answers

1. An electron pair donor.
2. 2-Methylpropanenitrile, (CH₃)₂CHCN.
3. NaCN or KCN in ethanol, heated under reflux.
4. HCN adds across C=O, so the product is C₄H₆O + HCN = C₅H₇NO, with OH and CN on the same ring carbon; Mᵣ = 5(12.0) + 7(1.0) + 14.0 + 16.0 = 97.0.
5. CH₃CN + 2H₂ → CH₃CH₂NH₂ (Ni catalyst); the product is ethylamine (ethanamine).
6. CH₃CH₂CH₂CN + 2H₂O + HCl → CH₃CH₂CH₂COOH + NH₄Cl.
7. RCO⁺, an acylium ion, with AlCl₄⁻.
8. Iron, iron halides and aluminium halides.
9. B only.
10. 0.75 × 0.80 = 0.60, so 60.0%.
11. An aromatic ketone, C₆H₅COR.
12. Aluminium chloride reacts with water, so it would no longer work as a halogen carrier.

Where marks are usually lost

• Drawing the CN⁻ arrow from nitrogen, or from the charge rather than a lone pair.
• Missing the δ+/δ− dipole on C–X or C=O when the question asks for a mechanism.
• Forgetting the protonation step in nucleophilic addition, or drawing the O⁻ intermediate without a charge.
• Giving “KCN(aq)” for the haloalkane step instead of cyanide in ethanol.
• Naming nitriles without counting the CN carbon, such as calling CH₃CH₂CN “ethanenitrile”.
• Balancing nitrile reduction with one H₂, or nitrile hydrolysis without NH₄⁺.
• Leaving HCl off a Friedel–Crafts equation, or not showing AlCl₃ regenerated.
• Writing the acylation product as an aldehyde, or putting the acyl group on the ring through oxygen.

Official syllabus

OCR A Level Chemistry A (H432) specification, Version 3.1 (May 2026), for first assessment in 2017, section 6.2.4 Carbon–carbon bond formation, learning outcomes (a) to (d). Published by OCR.