AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
The difference in base strength between ammonia, primary aliphatic and…
Full specification wording
Base properties (A-level only): The difference in base strength between ammonia, primary aliphatic and primary aromatic amines.
Base strength here comes down to one question: how available is the lone pair on the nitrogen atom?
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 2 of 2
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Base strength here comes down to one question: how available is the lone pair on the nitrogen atom? Alkyl groups release electron density towards the nitrogen by a positive inductive effect, so in ethylamine the lone pair is pushed out and is more available to a proton than in ammonia, and the cation formed is better stabilised. Ethylamine is therefore the stronger base. In phenylamine the nitrogen's lone pair lies next to the delocalised ring and overlaps with it, so part of the electron density is drawn into the ring and spread over the whole system. The pair is much less available, so phenylamine is a far weaker base than ammonia. The order is primary aliphatic amine, then ammonia, then primary aromatic amine.
What earns the marks
- Explaining base strength in terms of the availability of the lone pair on the nitrogen atom.
- Alkyl groups releasing electrons by a positive inductive effect, making the lone pair more available.
- The lone pair in phenylamine being delocalised into the benzene ring.
- The order ethylamine, then ammonia, then phenylamine.
Common mistakes and how to put them right
Explaining the trend by the size of the groups rather than the electron density at the nitrogen.
Saying the ring withdraws the lone pair by an inductive effect rather than by delocalisation.
Ranking phenylamine as the strongest because the ring is large.
Discussing the stability of the amine molecule rather than the availability of its lone pair.
Exam tips
- Name the effect you are using: 'positive inductive effect' or 'delocalised into the ring'. Saying electrons are 'pushed' rarely scores on its own.
- Answer in comparisons. Every mark here is for one species measured against another.
Try the lesson
The start of the Base properties (A-level only): The difference in base strength between ammonia, primary aliphatic and primary aromatic amines. lesson, made from checked teaching notes for this specification point.
Have a go first
Which order correctly shows the base strength of ammonia, a primary aliphatic amine and a primary aromatic amine?
Listen
Base strength here comes down to one question: how available is the lone pair on the nitrogen atom? Alkyl groups release electron density towards the nitrogen by a positive inductive effect, so in ethylamine the lone pair is pushed out and is more available to a proton than in ammonia, and the cation formed is better stabilised. Ethylamine is therefore the stronger base. In phenylamine the nitrogen's lone pair lies next to the delocalised ring and overlaps with it, so part of the electron density is drawn into the ring and spread over the whole system. The pair is much less available, so phenylamine is a far weaker base than ammonia. The order is primary aliphatic amine, then ammonia, then primary aromatic amine. A strong answer earns marks for these points. explaining base strength in terms of the availability of the lone pair on the nitrogen atom. alkyl groups releasing electrons by a positive inductive effect, making the lone pair more available. the lone pair in phenylamine being delocalised into the benzene ring. the order ethylamine, then ammonia, then phenylamine. Exam tip: Name the effect you are using: 'positive inductive effect' or 'delocalised into the ring'. Saying electrons are 'pushed' rarely scores on its own.
Key words
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availability of the lone pair
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positive inductive effect
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delocalisation into the ring
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order of base strength
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See every definition
- availability of the lone pair
- How available the lone pair on the nitrogen atom is to a proton, which is what base strength comes down to.
- positive inductive effect
- Alkyl groups release electron density towards the nitrogen, so the lone pair is pushed out and is more available to a proton.
- delocalisation into the ring
- In phenylamine the nitrogen's lone pair lies next to the delocalised ring and overlaps with it, so part of the electron density is drawn into the ring and spread over the whole system.
- order of base strength
- Primary aliphatic amine, then ammonia, then primary aromatic amine.
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