CH3- vs NH2-

[mikexima]

which one is more nucleophilic and why?

i know the answer as to which, i just want to understand the reasoning behind it a little better.

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[PreDental88]

which one is more nucleophilic and why?

i know the answer as to which, i just want to understand the reasoning behind it a little better.

Correct me if I'm wrong, but I believe that when comparing the nucleophilicity of compounds from the same period, nucleophilicity increases with basicity. Since basicity increases going to the left, I think CH3- would be the better nucleophile.

 

[mikexima]

well here is where im confused. why does it increase going to the left?

 

[2PacClone23]

Honestly, I've never seen :CH3 anything so don't expect it to...

 

[loveoforganic]

http://en.m.wikipedia.org/wiki/Methyllithium?wasRedirected=true

 

[mx41]

Grignard reagants or Lithium reagents like CH3-Li are good nucleophiles, but not CH3 by itself. It should be more basic than NH3, but def not more nucleophilic.
The reason, not sure exactly, I'd like to know as well.

 

[gigawatt]

A nucleophile is a Lewis base by definition: that is, an electron pair donor.

In the same period, the biggest factor is electronegativity. The less electronegative, the better able you are to donate electrons as opposed to keeping them for yourself.

In the same group, the biggest factor is size because it increases polarizability of the electron cloud. The more polarizable the better able you are to donate a region of your electron cloud to an electrophilic center. This increases going down a group. (Electronegativity also decreases going down so the two go hand in hand).

 

[mikexima]

well the answer to the question is CH3 but the description merely mentions because C is to the left of N, its more basic.... it just doesnt make sense to me. N has more lone pairs of electrons AND it is more electronegative so how could it be a less basic base !????

 

[gigawatt]

I'll offer this but I'm open to being corrected here:

In Grignard and organolithium reagents, the carbon-metal bond is ionic. So there really is a negative charge on the carbon as indicated by Li+ -CH3, which is how you sometimes see it written.

Going on electronegativity for elements in the same period, C is less electronegative, so CH3- should be the better nucleophile compared to NH2-.

 

[mikexima]

the way i see it, a better nucleophile is the one that has more e- or the one that is happier with an extra bond.... N likes to have combined 3 bonds and a lone pair. C likes to have 4 bonds, correct so far? so now we havea H3C: and a H2:N: two pairs vs 1 pair, more e- vs less e-....... it just doesnt add up for me.

 

[DQLEUCD]

I'll offer this but I'm open to being corrected here:

In Grignard and organolithium reagents, the carbon-metal bond is ionic. So there really is a negative charge on the carbon as indicated by Li+ -CH3, which is how you sometimes see it written.

Going on electronegativity for elements in the same period, C is less electronegative, so CH3- should be the better nucleophile compared to NH2-.

Yup, electronegativity is the key.

 

[RSD2014]

Honestly, I've never seen :CH3 anything so don't expect it to...

What exactly is :CH3? I know :CH2 is carbene.

 

[DQLEUCD]

What exactly is :CH3? I know :CH2 is carbene.

Looks like a carbanion if i'm not mistaken. Also Carbon is known to be electron donating.

 

[Troyvdg]

Yup, electronegativity is the key.

agreed with this.
nitrogen can compensate the negative charge better than carbon, due to its electronegativity.

think of acetilyde, its a great carbon nucleophile