Nucleophilic properties (A-level only) — AQA A-Level Chemistry
Test yourself on Nucleophilic properties (A-level only) with AQA A-Level practice questions.
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Nucleophilic properties (A-level only) explained
A nucleophile is an electron pair donor that is attracted to an atom carrying a partial positive charge.
Read the full explanation
An amine has exactly what that requires: a lone pair sitting on the nitrogen atom, ready to form a new bond. It is the same lone pair that makes the amine a base, and the only difference is what it attacks. Offered a proton, the amine behaves as a base; offered a δ⁺ carbon, such as the carbon bonded to a halogen in a halogenoalkane or the carbonyl carbon of an acyl chloride, it behaves as a nucleophile. Anything that makes the lone pair more available makes the amine both a stronger base and a better nucleophile, which is why aliphatic amines outperform phenylamine in both roles.
The nucleophilic substitution reactions of ammonia and amines with halogenoalkanes to form primary, secondary, tertiary amines and quaternary ammonium salts.
The halogen is more electronegative than carbon, so the carbon it is bonded to carries a δ⁺ charge and attracts a lone pair. Ammonia attacks that carbon, the carbon-halogen bond breaks heterolytically and the halide ion leaves, giving an alkylammonium salt. A second ammonia molecule then removes a proton, freeing the primary amine. But that amine still has a lone pair, so it attacks another halogenoalkane, and the sequence runs on through the secondary amine and the tertiary amine to the quaternary ammonium salt, which has four alkyl groups, no N-H bond and so no proton left to lose. The product ratio is set by the conditions: excess ammonia favours the primary amine, excess halogenoalkane the quaternary salt.
The use of quaternary ammonium salts as cationic surfactants.
A quaternary ammonium ion has four groups bonded to the nitrogen and a permanent positive charge, because there is no lone pair left and no N-H bond to lose. Make one of those four groups a long hydrocarbon chain and you have a cationic surfactant: an ionic head that is attracted to water and a non-polar tail that is not. Unlike a soap, whose charge depends on the pH of the water, this charge is fixed. Wet hair and wet fabric fibres carry a slight negative charge on their surface, so the cations are attracted head-first onto the surface and their hydrocarbon tails stick outwards. That coating lubricates the fibres, so they slide over one another, and it carries away the charge that would otherwise build up as static.
The nucleophilic addition–elimination reactions of ammonia and primary amines with acyl chlorides and acid anhydrides.
Ammonia and primary amines attack the carbonyl carbon of an acyl chloride or an anhydride, and the chloride or carboxylate is then eliminated, so the product keeps the C=O and gains a nitrogen. Ammonia with ethanoyl chloride gives ethanamide, CH₃CONH₂; a primary amine such as methylamine gives the N-substituted amide CH₃CONHCH₃. The inorganic product is hydrogen chloride, but it never survives free: a second mole of ammonia or amine takes it up as the ammonium or alkylammonium salt, so two moles of the nitrogen compound are used per mole of acyl chloride, and a white smoke of the solid salt is seen. Ethanoic anhydride gives the same amide but releases ethanoic acid instead, and reacts less violently.
Students should be able to outline the mechanisms of: these nucleophilic substitution reactions the nucleophilic addition–elimination reactions of ammonia and primary amines with acyl chlorides.
Two mechanisms, told apart by whether the carbon under attack is saturated or a carbonyl carbon. With a halogenoalkane it is a one-step nucleophilic substitution: an arrow from the nitrogen lone pair to the δ⁺ carbon, and at the same time an arrow from the carbon-halogen bond to the halogen, which leaves as a halide ion; a second molecule of ammonia or amine then removes H⁺ from the salt. With an acyl chloride it is addition-elimination: the nitrogen lone pair attacks the carbonyl carbon while the C=O electrons move onto the oxygen, giving a tetrahedral intermediate with a negative oxygen and a positive nitrogen; then the oxygen's pair comes back down to reform C=O and pushes out Cl⁻, and H⁺ is lost at the end.
Your focus
- Define a nucleophile and identify the feature of an amine that lets it act as one.
- Explain why the same lone pair accounts for both the basic and the nucleophilic behaviour of an amine.
- Predict which of two amines is the better nucleophile and justify the choice.
Show all 15 objectives
- Write equations for the successive reactions of ammonia with a named halogenoalkane, through to the quaternary salt.
- Explain why a mixture of amines is obtained and how the conditions shift which product dominates.
- Identify the quaternary ammonium salt in a set of structures and say why it cannot be deprotonated.
- Draw a quaternary ammonium ion that would work as a cationic surfactant and label its head and tail.
- Explain how such an ion binds to hair or fabric and what effect the coating has.
- Contrast a cationic surfactant with soap in terms of the charge on the head group.
- Draw and name the amide formed when a named acyl chloride reacts with ammonia or with a primary amine.
- Write a balanced equation that accounts for both moles of the nitrogen compound consumed.
- Predict how the products change when an acid anhydride replaces the acyl chloride.
- Draw the nucleophilic substitution mechanism of ammonia or an amine with a named halogenoalkane.
- Draw the addition-elimination mechanism of ammonia or a primary amine with a named acyl chloride.
- Explain why the second mole of ammonia or amine is needed and show the step in which it acts.
Nucleophilic properties (A-level only) exam tips
Marking Points
- one mark for defining a nucleophile as an electron pair donor
- one mark for identifying the lone pair on nitrogen as the pair that is donated
- one mark for the attack being at a carbon atom carrying a δ⁺ charge
- one mark for linking nucleophile strength to the same lone pair availability that controls base strength
- one mark for the carbon being δ⁺ because the halogen is more electronegative
- one mark for the halide ion identified as the leaving group
- one mark for a second molecule of ammonia or amine removing H⁺ to free the amine
- one mark for the quaternary ammonium salt having four alkyl groups and no N-H bond
- one mark for choosing excess ammonia to maximise the yield of the primary amine
- one mark for the cation drawn with four groups on nitrogen, one of them a long hydrocarbon chain
- one mark for the negative charge on wet hair or fabric attracting the positive head
- one mark for the hydrocarbon tails pointing away from the surface
- one mark for a stated effect, such as reduced friction between fibres or less static
- one mark for the amide drawn with the nitrogen bonded directly to the carbonyl carbon
- one mark for the N-substituted amide when a primary amine is used in place of ammonia
- one mark for two moles of ammonia or amine per mole of acyl chloride
- one mark for ethanoic acid rather than hydrogen chloride as the second product with an anhydride
- one mark for the curly arrow from the lone pair on nitrogen to the δ⁺ carbon atom
- one mark for the arrow from the carbon-halogen bond to the halogen in the substitution
- one mark for the arrow from the C=O bond to the oxygen in the addition step
- one mark for the intermediate drawn with the negative charge on oxygen and the arrows that reform C=O and release Cl⁻
- one mark for the second molecule of ammonia or amine removing H⁺ from the intermediate salt
Examiner Tips
- 💡In any amine mechanism the first arrow starts at the lone pair on the nitrogen; draw the lone pair in before you draw the arrow.
- 💡If a question asks why phenylamine is a poorer nucleophile, reuse the base strength argument word for word.
- 💡Draw all four organic products in order when the question says 'a mixture of products is formed'; the marks are usually one per species.
- 💡State the conditions with the product: excess ammonia, in ethanol, in a sealed tube.
- 💡Sketch the ion as a charged circle with one long zig-zag tail; that diagram earns the structure mark on its own.
- 💡Name the surface you are coating. 'Hair' or 'fabric' with its negative charge is what turns a description into an explanation.
- 💡Check the mole ratio before you balance: if the nitrogen compound is also a base, you need a second mole of it.
- 💡An amide has the nitrogen on the carbonyl carbon; an amine has it on a saturated carbon. Look at what the nitrogen is bonded to before you name the product.
- 💡Name the mechanism exactly as AQA does: nucleophilic substitution, or nucleophilic addition-elimination. 'Nucleophilic elimination' is not accepted.
- 💡The intermediate plus its lone pair and charges is worth a mark in itself, so always draw it as a separate structure.
Common Mistakes
- saying the amine is attracted to the halogen atom rather than to the δ⁺ carbon
- drawing the curly arrow from an N-H bond rather than from the lone pair
- calling an amine an electrophile because it contains hydrogen atoms
- claiming a nucleophile must carry a full negative charge
- stopping at the alkylammonium salt and calling it the amine
- breaking a C-H bond instead of the carbon-halogen bond
- claiming the quaternary ammonium salt can lose a proton to give a 'quaternary amine'
- predicting a single clean product from a one-to-one mixture of ammonia and halogenoalkane
- drawing nitrogen with three groups and a lone pair, which is an amine and not a quaternary salt
- saying the hydrocarbon tail binds to the fibre and the charged head points outwards
- describing soap's micelle mechanism instead of surface coating
- claiming the charge on the head depends on pH, when a quaternary ion is permanently charged
- giving an amine salt rather than the amide as the organic product
- writing the equation with one mole of ammonia and free hydrogen chloride alongside the amide
- writing HCl as a product when the reagent given was ethanoic anhydride
- naming CH₃CONHCH₃ as an amine because it contains an N-H bond
- starting the arrow at the amine as a whole, or at a charge sign, rather than at the lone pair
- omitting the formal charges from the tetrahedral intermediate
- expelling the chloride ion in the same step as the nucleophile attacks the carbonyl carbon
- leaving the halide ion and the H⁺ unaccounted for instead of showing the ammonium salt
- drawing a skeletal mechanism in which a C-H bond breaks without drawing in the bond and the H