Reasons for Increasing the Inductive Effect

[Crystal037]

Homework Statement

I have been given order of - I effect of different groups. But I don't know why these groups follow these order.

Relevant Equations

I effect is directly proportional to the electron affinity of the group

Here it has been given that NR3+ has more - I effect than NH3+ but according to my knowledge R group has more +I effect that H. So NR3+ will be stabilised by R groups' +I effect and hence NH3+ should have more - I effect.

 

[Crystal037]

+I effect is positive inductive effect and - I effect is negative inductive effect

 

[Crystal037]

Can someone please answer me

 

[TeethWhitener]

Here it has been given

Where? The inductive effect manifests itself in a few different places (acidity, regioselectivity, etc.). Different applications sometimes give different results. At any rate, I doubt that the inductive effects from NH₃ and NR₃ really differ substantially.

 

[Crystal037]

What do you mean by they don't differ substantially

 

[TeethWhitener]

Here’s an example. Compare the pKa’s of NH₄⁺ and Et₃NH⁺. Now compare the pKa’s of glycine (at the carboxyl) and trimethylglycine (aka betaine). Notice anything?

 

[Crystal037]

Here’s an example. Compare the pKa’s of NH₄⁺ and Et₃NH⁺. Now compare the pKa’s of glycine (at the carboxyl) and trimethylglycine (aka betaine). Notice anything?

pKa of nh3+ is 9.2 while while pka of et3nh+ is 10.75 and pka of glycine 2.34(carboxyl) and 9.6(amino) and pka of trimethylglycine is 1.84. What am I supposed to notice?

 

[TeethWhitener]

I was hoping you’d notice there’s not a pattern, thus corroborating my earlier statement:

I doubt that the inductive effects from NH3 and NR3 really differ substantially.

 

[Crystal037]

I was hoping you’d notice there’s not a pattern, thus corroborating my earlier statement:

But pKa of glycine was given in terms of pKa of carboxyl and amino while pKa of trimethylglycine was given in a single term. I don't understand why?

 

[Crystal037]

Where? The inductive effect manifests itself in a few different places (acidity, regioselectivity, etc.). Different applications sometimes give different results. At any rate, I doubt that the inductive effects from NH₃ and NR₃ really differ substantially.

This order has been given in my textbook

of organic chemistry

 

[TeethWhitener]

But pKa of glycine was given in terms of pKa of carboxyl and amino while pKa of trimethylglycine was given in a single term. I don't understand why?

For betaine (trimethylglycine) the only acidic proton is the carbonyl. In glycine, you have the carbonyl as well as the ammonium proton. That’s why I asked you to compare the carboxyls (apples to apples).

I don’t know what organic chemistry book you’re using, but I’ve noticed in some organic chemistry books a tendency to overapply a loose rule of thumb. In this case, my general point was that the inductive effects from NR₃ and NH₃ likely won’t be very different in practice; moreover, they’re likely to be swamped by other, more relevant, considerations in most situations.

 

[Crystal037]

Then methyl group shows +I effect more than hydrogen even if hydrogen is more electropositive than carbon

 

[chemisttree]

No, the trialkylammonium group (-NR₃⁺) is more electron withdrawing than the ammonium group (-NH₃⁺) due to steric effects. Those bulky alkyl groups try harder to achieve a planar structure which makes the bond attached to, whatever, have more ‘p’ character... a shorter bond closer to the positively-charged nitrogen. And closer to the fire usually means warmer!

 

[Crystal037]

No, the trialkylammonium group (-NR₃⁺) is more electron withdrawing than the ammonium group (-NH₃⁺) due to steric effects. Those bulky alkyl groups try harder to achieve a planar structure which makes the bond attached to, whatever, have more ‘p’ character... a shorter bond closer to the positively-charged nitrogen. And closer to the fire usually means warmer!

The structure of nr3+ would be tetrahedral why would it want to achieve a planar structure

 

[chemisttree]

The alkyl groups are big enough to bump into each other. They try to get away from each other. The only way to space out is to move more towards a planar structure (they don’t actually achieve planarity). The bond angles (R-N-R) get a little larger. One way to think of it is that the hybridization of the attached N bond changes slightly from sp₃ to sp_2.?.
Kind of like a VSEPR approach coupled with a bit of quantum mechanics.

 

[Crystal037]

The alkyl groups are big enough to bump into each other. They try to get away from each other. The only way to space out is to move more towards a planar structure (they don’t actually achieve planarity). The bond angles (R-N-R) get a little larger. One way to think of it is that the hybridization of the attached N bond changes slightly from sp₃ to sp_2.?.
Kind of like a VSEPR approach coupled with a bit of quantum mechanics.

Wouldn't they get more space in a 3 d structure than a 2d planar structure. How come hybridisation of the molecule change it depends on the no if atoms attached to the central atom which doesn't vary in this case.

 

[chemisttree]

Review Bent’s rule.