[davidenarb]

Hi all,

I would like to understand why does E2 reaction react more rapidly with tertiary substrate than primary substrates?

I know that, in the case of E1, the reason is because the carbocation is more stable when it is tertiary as the alkyl groups are electrons donating, but what about E2?

[Dan]

I would look at two factors here:

1. Statistical probability of a successful molecular collision
2. Charge distribution in the transition state

Have a think about it and tell us your ideas...

[davidenarb]

I would look at two factors here:

1. Statistical probability of a successful molecular collision
2. Charge distribution in the transition state

Have a think about it and tell us your ideas...

Thank you for your help, but I didn't understand the meaning of charge distribution in the transition state. For me, I need to look for the stability of intermediates, but there is no intermediates in E2.

Thank you in advance for further clarification.

[spirochete]

Think about factors effecting the thermodynamic stability of the product. These factors also influence the stability of the transition state leading to that product. It's for this reason that (all other things equal) a more exothermic reaction will have a lower kinetic barrier, also.

[orgopete]

Re: charge distribution

Let us consider this like bond length. If you had a C-Br bond, which do you think would be longer, MeBr, EtBr, iPrBr, or t-BuBr? How will that bond length affect the carbon to which it is attached? How might that affect the neighboring C-H bonds (except MeBr)?

[davidenarb]

Re: charge distribution

Let us consider this like bond length. If you had a C-Br bond, which do you think would be longer, MeBr, EtBr, iPrBr, or t-BuBr? How will that bond length affect the carbon to which it is attached? How might that affect the neighboring C-H bonds (except MeBr)?

For me, bond length doesn't depends on the carbon skeleton of the compound unless it is double or triple bonds, so I expect to have the same C-Br bond length for MeBr, EtBr, iPrBr, and t-BuBr.

[orgopete]

For me, bond length doesn't depends on the carbon skeleton of the compound unless it is double or triple bonds, so I expect to have the same C-Br bond length for MeBr, EtBr, iPrBr, and t-BuBr.

Okay, I don't have lit values for the bromides, so let me give you carbon-chlorine bond lengths. The R-groups are any carbons. The values, except MeCl are averages reported in the paper.

MeCl, 177.7 pm
RCH2Cl, 179 pm
R2CHCl, 180.5 pm
R3CCl, 184.3 pm
and for your interest
C=CCl, 171.3 pm

Now, I consider this data revealing about the nature of the atoms and helpful in telling us how they might react. First off, what do the bond lengths tell you about the substituents attached to the carbon with the chlorine attached? Are they donating or withdrawing? Which bond would you predict to be more easily broken? If we were to form a carbocation or only party form one by stretching the bond, which group would you expect to see hyperconjugation-like properties? How might a hyperconjugation-like structure/intermediate/transition state affect an E2 reaction? Which do you think will react faster in an E2 reaction?

[davidenarb]

For me, bond length doesn't depends on the carbon skeleton of the compound unless it is double or triple bonds, so I expect to have the same C-Br bond length for MeBr, EtBr, iPrBr, and t-BuBr.

Okay, I don't have lit values for the bromides, so let me give you carbon-chlorine bond lengths. The R-groups are any carbons. The values, except MeCl are averages reported in the paper.

MeCl, 177.7 pm
RCH2Cl, 179 pm
R2CHCl, 180.5 pm
R3CCl, 184.3 pm
and for your interest
C=CCl, 171.3 pm

Now, I consider this data revealing about the nature of the atoms and helpful in telling us how they might react. First off, what do the bond lengths tell you about the substituents attached to the carbon with the chlorine attached? Are they donating or withdrawing? Which bond would you predict to be more easily broken? If we were to form a carbocation or only party form one by stretching the bond, which group would you expect to see hyperconjugation-like properties? How might a hyperconjugation-like structure/intermediate/transition state affect an E2 reaction? Which do you think will react faster in an E2 reaction?

If we were to form a carbocation, the tertiary carbocation is the most stable by hyperconjugation. However, there is no carbocation formed in E2 (no intermediate formed in E2), and I still don't get the idea of bond length

[orgopete]

If we were to form a carbocation, the tertiary carbocation is the most stable by hyperconjugation. However, there is no carbocation formed in E2 (no intermediate formed in E2), and I still don't get the idea of bond length

Time is a variable in reactions. Carbocations are not an either/or proposition. If a bond breaks, it does so over time. It begins at the measured bond length and ends at some point in which the separation is so large that a bond no longer exists. Therefore, hyperconjugation should also participate in a proportional degree. As a bond breaks, the neighboring electrons can be donated to the growing carbocation. This electron donation also increases the acidity of the hyperconjugated proton.

We should see two effects here, first, neighboring electrons can compete with a nucleophile in an attack to the backside of a C-X bond. This will retard an attack. Second, this attack by neighboring electrons should increase as a bond breaks.

How do we know a bond is breaking? Check the distance. Let us assume the methyl chloride bond length represents no bond breakage and no polarization. Therefore, any increase in bond length represents a weaker bond, a greater degree of polarization, a greater amount of hyperconjugation, a greater amount of backside interference, and a greater acidity of the beta hydrogens. If that were so, then I would argue a tertiary halide should react faster via an E2 reaction than a secondary or primary.

Those are assumptions for completely synchronous reactions. Since we know tertiary halides can break faster than nucleophiles can attack, if bond actually stretched rather than remain fixed and motionless, this stretching would increase the amount of bond breakage, hyperconjugation, etc.. The net effect would actually be increased for a tertiary halide in an E2 reaction.

I don't know if that helps or not, but that is how I envision these elimination reactions taking place. I also use this model to predict Zaitsev versus Hoffmann elimination. More bond breakage is more carbocation-like, more hyperconjugation, and more Zaitzev products. The reverse is Hoffmann products. Ethanol/ethoxide with bromide or iodide favor Zaitzev products as rationalized above. Fluorides and ammonium salts have less bond polarization, hence depend on CH acidity and favor Hoffmann products.