Methyl iodide addition to alkenes....?

I'm sure I should know the answer to this but it has been a long summer....

Why does methyl iodide react preferentially with an amine than an alkene (chemoselectively)? Because the amine is more nucleophilic (?)

Does MeI ever react with alkenes?

Thanks

Reply 1

Alchem

1

Original post by Kenny135

I'm sure I should know the answer to this but it has been a long summer....

Why does methyl iodide react preferentially with an amine than an alkene (chemoselectively)? Because the amine is more nucleophilic (?)

Does MeI ever react with alkenes?

Thanks

Interesting question! It's probably because the amine nitrogen is more nucleophilic than the pi electrons in an alkene. Since the methyl group is bulky, I was wondering if it could be due to steric hindrance. But then, the pi electrons are probably more accessible than the lone pair on nitrogen. What do you think?

Reply 2

Kenny135

OP

2

Original post by Alchem

Interesting question! It's probably because the amine nitrogen is more nucleophilic than the pi electrons in an alkene. Since the methyl group is bulky, I was wondering if it could be due to steric hindrance. But then, the pi electrons are probably more accessible than the lone pair on nitrogen. What do you think?

I would imagine it was because the nitrogen is more nucleophilic although there must be another factor as Br2 would react with the double bond and not the amine (well I guess the productive reaction would be addition to the alkene because the addition of Br2 to the amine would be reversible?

Reply 3

illusionz

20

Original post by Kenny135

I'm sure I should know the answer to this but it has been a long summer....

Why does methyl iodide react preferentially with an amine than an alkene (chemoselectively)? Because the amine is more nucleophilic (?)

Does MeI ever react with alkenes?

Thanks

Original post by Alchem

Interesting question! It's probably because the amine nitrogen is more nucleophilic than the pi electrons in an alkene. Since the methyl group is bulky, I was wondering if it could be due to steric hindrance. But then, the pi electrons are probably more accessible than the lone pair on nitrogen. What do you think?

It's been a very long summer indeed!

The non bonding orbital on the nitrogen is much higher energy than the pi bonding molecular orbital on an alkene, which makes it more nucleophilic.

At first glance, one could expect methyl iodide (soft electrophile due to very diffuse iodine orbitals) to react with ethene (soft nucleophile) over ammonia (relatively hard), but the reality is that the pi bonding electrons in ethene are too low in energy.

Also, for the record, MeI is the most unhindered alkyl iodide, and the methyl group itself is not bulky (when thinking about sterics, bulky groups are t-Bu and the like, a single methyl group does very little).

The above explanation is relatively basic, but another way I've just thought about it which makes more sense to me - think about the mechanism of bromination of ethene, you form a bromonium ion intermediate with one arrow from the alkene to the bromine, and one back from bromine to the alkene. You could not get an interaction like this with methyl iodide as the carbon does not have a lone pair it can donate. The result of this is that the intermediate formed would be incredibly unstable and hence there is a kinetic barrier to the reaction (even though I'd have thought it would be thermodynamically favourable).

Methyl iodide addition to alkenes....?

Quick Reply

Related discussions

Latest

Trending

Trending

Articles for you