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basicity of amines?

I understand that primary and secondary amines are stronger bases than aromatics.

but is phenylmethylamine stronger than ammonia? and why?

(edited 6 years ago)

Reply 1

HateOCR

The alkyl group is electron released. The lone pair on N is too far away from the delocalised electron system in Benzene.
Ammonia has no alkyl groups in comparison.

Reply 2

username3756768

Phenylmethylamine would behave like a primary amine so it would be stronger than ammonia due to the positive inductive effect. The methyl group would donate electron density to the nitrogen making the lone pair on nitrogen more diffuse thus increasing basicity by facilitating donation of electrons.

Reply 3

ljcrisp

I think phenylmethylamine is weaker than ammonia because it’s a primary aromatic amine.

Yes you’re right in saying primary and secondary amines are stronger than aromatics however the phenyl tells us that a benzene ring is attached to the NH2 and methyl group. The lone pair is less available due to the benzene ring drawing the electrons from the nitrogen atom towards itself therefore electron density on the nitrogen is lowered and the nitrogen’s lone pair is partially delocalised onto the ring.

I hope this helps (Do let me know if I’ve gone wrong anywhere) :smile:

(edited 6 years ago)

Reply 4

Lia22

@ljcrisp
@DrXavier

ahh I'm confused, two contradicting points.
is phenylmethylamine weaker or stronger?? 😕

(edited 6 years ago)

Reply 5

J...H

Oh god I somehow misread the title as 'best of animes' and was seriously confused by the content here. Pretty ashamed tbh 😂

Reply 6

ljcrisp

Original post by Lia22

@ljcrisp
@DrXavier

ahh I'm confused, two contradicting points.
is phenylmethylamine weaker or stronger?? 😕

@DrXavier without being rude at all, even though you’re right in saying primary amines are stronger bases than aromatics, the phenyl bit in phenylmethylamine indicates that there’s a benzene involved therefore phenylmethylamine is an aromatic.

@Lia22 This diagram may help
Reference: CGP AQA A Level Chemistry: Year 2
9708BC48-5378-42F9-978B-6B0CE81CAC41.jpg.jpeg

9708BC48-5378-42F9-978B-6B0CE81CAC41.jpg.jpeg

Reply 7

ljcrisp

Original post by J...H

Oh god I somehow misread the title as 'best of animes' and was seriously confused by the content here. Pretty ashamed tbh 😂

Don’t be ashamed 😂😂

Reply 8

username3434964

Oh crap I saw the word anime...

Reply 9

gjd800

I've got an amine burn scar on my leg.

Reply 10

username3756768

Original post by ljcrisp

I am afraid you are mistaken. If the phenyl was alpha to the nitrogen this would be correct, since it is beta, the aromatic group no longer withdraws electron density from the nitrogen.

Without being rude, I am quite certain about the argument made (I tutor chemistry and one of my students got into Oxford for next year).

In fact I just went and checked their respective pKb values phenylmethylamine has a pKb of 4.66 and ammonia a pKb of 4.75 which again tells you the phenylmethylamine is marginally a stronger base.

(edited 6 years ago)

Reply 11

ljcrisp

Original post by DrXavier

Okay then if you say so :smile:

I only meant if you’re looking from it as an A Level perspective. We’ve not learnt about pKb values for our specification but that’s good to know (I’m guessing then what it says in the book is another lie then?) Thank you!

(edited 6 years ago)

Reply 12

charco

Study Forum Helper

Original post by ljcrisp

Okay then if you say so :smile:

These concepts ARE an A level perspective.

The "availabiity" of the lone pair to attract protons determines the base strength.

This in turn depends induction and withdrawal, as outlined above.

Reply 13

username3756768

Original post by ljcrisp

Okay then if you say so :smile:

it isn't wrong - it applies to phenylamine (1) in the diagram I have drawn) but not phenylmethylamine (2) in the diagram I have drawn

you can see how in the first case the lone pair becomes less available so you can't donate it as well (thus less basic)

in the second (this is phenylmethylamine) the phenyl group cannot do this but the methyl group can act as a donating group (hyperconjugation and all that jazz)

Reply 14

ljcrisp

Original post by charco

These concepts ARE an A level perspective.

The "availabiity" of the lone pair to attract protons determines the base strength.

This in turn depends induction and withdrawal, as outlined above.

Yeah I agree with you on that. I just didn’t understand the alpha and beta that @DrXavier was on about in terms of the nitrogen cause I’ve not been taught that for my A Level spec.

Reply 15

ljcrisp

Original post by DrXavier

it isn't wrong - it applies to phenylamine (1) in the diagram I have drawn) but not phenylmethylamine (2) in the diagram I have drawn

That makes sense! Thank you!!

Reply 16

charco

Study Forum Helper

Original post by ljcrisp

Yeah I agree with you on that. I just didn’t understand the alpha and beta that @DrXavier was on about in terms of the nitrogen cause I’ve not been taught that for my A Level spec.

Alpha and beta are just old terminology used to refer to the relative positions of carbon atoms and their substituents.

A carbon "next door" is referred to as an alpha carbon atom, and the next one along the chain is referred to as a beta carbon and so on through the Greek alphabet.

It allows you to discuss carbon atoms that are near functional groups without having to use absolute numbering.

For example CH3CH2CH2COCH2CH2CH3

The alpha carbons are next to the carbonyl group. The hydrogen atoms on these carbons are also referred to as alpha hydrogens.

If the chain is long. eg fatty acids, they may refer to the last carbon as the omega carbon and count using conventional number back from this one.