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The halogen is more electronegative than carbon, so the C–X bond has a partially positive carbon. A nucleophile can attack that carbon and displace a halide ion. Aqueous hydroxide produces an alcohol; cyanide produces a nitrile with one extra carbon; ammonia can produce an amine. In a substitution mechanism show the incoming electron pair and the bond pair moving onto the departing halogen. The leaving group carries both bonding electrons away.
Video by Eliot Rintoul. The video’s own board label is retained; follow the specification reference and scope notes on this lesson for assessed content.
Viewing focus: Explain nucleophilic substitution
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: Which conditions favour an alkene from a haloalkane?
Hot ethanolic hydroxide. Hydroxide acts as a base in elimination.
After watching 2: Why does an iodoalkane often hydrolyse faster than its chloro analogue?
Its C–I bond is weaker. Bond strength is a key factor.
What you'll be able to do
The halogen is more electronegative than carbon, so the C–X bond has a partially positive carbon. A nucleophile can attack that carbon and displace a halide ion. Aqueous hydroxide produces an alcohol; cyanide produces a nitrile with one extra carbon; ammonia can produce an amine. In a substitution mechanism show the incoming electron pair and the bond pair moving onto the departing halogen. The leaving group carries both bonding electrons away.
Heating a halogenoalkane with hydroxide in ethanol favours elimination to an alkene. Hydroxide acts as a base, removing a hydrogen from a carbon adjacent to the carbon bearing the halogen. The C–H electrons form the double bond while the C–X bond breaks. More than one alkene can form if different adjacent carbons are available. Conditions matter: aqueous and ethanolic hydroxide should not be treated as interchangeable instructions, and temperature also affects competition between pathways.
Hydrolysis can be followed by release of halide ions and formation of a silver-halide precipitate in a suitable comparison method. Control concentration, temperature and solvent. The C–I bond is weaker than C–Br, which is weaker than C–Cl, so iodoalkanes often hydrolyse faster for otherwise comparable structures. Electronegativity alone would give the wrong reactivity prediction. Structure also matters: primary, secondary and tertiary substrates can follow different mechanistic routes, so do not compare unrelated compounds as if only halogen identity changed.
Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.
Common mistakes to avoid
Key takeaways
Question 1: How many carbon atoms are in the nitrile made from bromoethane and CN⁻?
Three; the product is propanenitrile.
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.