Sam_Chem
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Literally have no idea where to start with this question :mad:

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Maker
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Its due to the stability of the cation. Think about the electron structure of the 3 different cations and how they influence the stability of the cation.

Its interesting to note phenylamine has a benzene ring.
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Pigster
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It is surely due to the electron donating nature of CH3 groups vs the electron withdrawing nature of C6H5 groups which make the lone pair on the N more or less available to accept a proton and hence more or less basic.

And don't call me Shirley.
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motivatedshroom
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Yeah, the phenylamine cation kafter accepting a proton) has delocalisation across its structure which means it resonates between several structures - this is very srable. For ethylamime , the methyl is electron-donating and hence stablilises the positive charge but this isn't as stabilising as delocalisation/resonance. (Also there's hyper conjugation for both I think)

This is a guess by the way !
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Sam_Chem
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(Original post by Maker)
Its due to the stability of the cation. Think about the electron structure of the 3 different cations and how they influence the stability of the cation.

Its interesting to note phenylamine has a benzene ring.
Could the stability be attributed to conjugation, as you say, phenylamine has a bezene ring so it is more conjugated than the others.

Strength of the base relates to the stability of the cation when the base accepts a H+ ion. (LB definition of bases)

Phenylamine forms the most stable cation as it is conjugated, allowing the charge to be spread across the molecule, making it the most stable base. OR Phenylamine includes a bezene ring, which has delocalisation of electrons making it a stronger base by being able to accept the H+ easier?

And ammonia, with the lone pair of electrons, makes a strong base/strong acceptor of H+.

Bah, im getting different concepts mixed up!
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Maker
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(Original post by Sam_Chem)
Could the stability be attributed to conjugation, as you say, phenylamine has a bezene ring so it is more conjugated than the others.

So, in answering this question for 3 marks I could say roughly:

Strength of the base relates to the stability of the cation when the base accepts a H+ ion. (LB definition of bases)

Phenylamine forms the most stable cation as it is conjugated, allowing the charge to be spread across the molecule, making it the most stable base. OR Phenylamine includes a bezene ring, which has delocalisation of electrons making it a stronger base by being able to accept the H+ easier?

And ammonia, with the lone pair of electrons, makes a strong base/strong acceptor of H+.

Bah, im getting different concepts mixed up!
Its been ages since I did this so I can't remember it all.
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Sam_Chem
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(Original post by Maker)
Its been ages since I did this so I can't remember it all.
THanks anyways! Its 3% of the paper and marking everything else I got 92% so I might just throw away those marks how i've only ever done conjugation in the other module exam :P
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langlitz
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(Original post by Sam_Chem)
Could the stability be attributed to conjugation, as you say, phenylamine has a bezene ring so it is more conjugated than the others.

So, in answering this question for 3 marks I could say roughly:

Strength of the base relates to the stability of the cation when the base accepts a H+ ion. (LB definition of bases)

Phenylamine forms the most stable cation as it is conjugated, allowing the charge to be spread across the molecule, making it the most stable base. OR Phenylamine includes a bezene ring, which has delocalisation of electrons making it a stronger base by being able to accept the H+ easier?

And ammonia, with the lone pair of electrons, makes a strong base/strong acceptor of H+.

Bah, im getting different concepts mixed up!
You do realise a larger pKb means it is a weaker base?

The methyl group in methylamine is electron donating so it stabilises the +ive charge better then ammonia thus making it a stronger base. Phenylamine is a very weak base compared to ammonia because the lone pair orbitals interact with the oribitals of the benzene ring, making them less available for a proton to form a bond with the nitrogen
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Sam_Chem
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(Original post by langlitz)
You do realise a larger pKb means it is a weaker base?
Ah... just realised what I typed, thought I was typing about pKa, I stand corrected!
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Sam_Chem
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(Original post by langlitz)
The methyl group in methylamine is electron donating so it stabilises the +ive charge better then ammonia thus making it a stronger base. Phenylamine is a very weak base compared to ammonia because the lone pair orbitals interact with the oribitals of the benzene ring, making them less available for a proton to form a bond with the nitrogen
That makes A LOT more sense - Thanks!
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Maker
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(Original post by langlitz)
You do realise a larger pKb means it is a weaker base?

The methyl group in methylamine is electron donating so it stabilises the +ive charge better then ammonia thus making it a stronger base. Phenylamine is a very weak base compared to ammonia because the lone pair orbitals interact with the oribitals of the benzene ring, making them less available for a proton to form a bond with the nitrogen
Makes sense, its the reverse of why phenol is a stronger acid than cyclohexanol.
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