Quantcast

Alkyl group vs Phenyl group effect on carbocation

This forum made possible through the generous support of SDN members, donors, and sponsors. Thank you.

plusalpha

Full Member
5+ Year Member
Joined
Oct 19, 2015
Messages
657
Reaction score
388
Which is more stable, a carbocation bonded to two methyl groups, or bonded to one methyl group and one phenyl group?

Put in a different way, in the image I've attached, will there be a hydride shift to produce a more stable carbocation or no?

20151207_163955.jpg
 

fittycent

Full Member
10+ Year Member
7+ Year Member
Joined
Aug 15, 2007
Messages
29
Reaction score
16
I think I get where you're coming from... though in the pic you provided they'd both be tertiary carbocations if a hydride shift happened... so I don't think a hydride shift would occur here. The carbocation in the one you drew is a benzylic carbocation, so it would be more stable due to resonance (it's one carbon away from pi electrons). The other carbocation wouldn't be able to form any resonance structures, if a hydride shift occurred. Since we're just making a comparison between 2 tertiary carbocations, and only one of them has benzylic, the one with resonance stabilization (is benzylic) wins.

This might not be what you meant, but I think to make the comparison of stabilizing effect of hyperconjugation vs stabilizing effect of resonance you'd have to have a benzylic secondary carbocation attach to Ph on one side, and attached to a t-butyl group on the the other side. Then the question would be does a methyl shift occur to form a tertiary carbocation vs a secondary, but resonance stabilized carbocation?

((H3C)3C)-(HC+)-(Ph)

if a methyl shift occurred:

((H3C)2C+)-(HC(H3C))-(Ph)



And with that I'm not sure. If I had to guess, I'd say resonance has more of stabilizing effect than donating via hyperconjugation. So you wouldn't have a methyl shift. I'm definitely just guessing, though. I'm sort of hoping this isn't a comparison we'd have to make :p

Page 5 on this link says this:

When considering the importance of hyperconjugation versus resonance as the more important stabilizing feature, resonance usually wins out. For example, a primary carbocation with resonance is more stable than a secondary carbocation without resonance. A secondary carbocation with resonance is usually more stable than a tertiary carbocation without resonance. The general rules for carbocation stability can be summarized as follows. (a) Increasing substitution increases stability. CH3 + (methyl; least stable) < RCH2 + (1o ) < R2CH+ (2o ) < R3C+ (3o ; most stable) (b) Resonance is more important than substitution. For example, a secondary carbocation without resonance is generally less stable than a primary carbocation with resonance.
 
Last edited:
  • Like
Reactions: 1 user

orgoman22

DAT DESTROYER
Lifetime Donor
Sponsor
Vendor
15+ Year Member
  • Joined
    Mar 12, 2005
    Messages
    4,595
    Reaction score
    3,367
    Which is more stable, a carbocation bonded to two methyl groups, or bonded to one methyl group and one phenyl group?

    Put in a different way, in the image I've attached, will there be a hydride shift to produce a more stable carbocation or no?

    View attachment 198529

    I will be more than happy to help you. I have been studying carbocations for over 30 years, and have read much work done on them by Nobel Scholar George Olah. You have presented a HIGHLY stable tertiary benzylic carbocation.....this would not shift. The energy barrier for its formation is very low !!!!! It doesnt get much better than this. Carbocations are at the cornerstone of organic reaction mechanisms and theory.

    Hope this helps.

    Dr. Jim "Orgoman" Romano
     
    Last edited:
    • Like
    Reactions: 1 users

    plusalpha

    Full Member
    5+ Year Member
    Joined
    Oct 19, 2015
    Messages
    657
    Reaction score
    388
    Thank you both for you replies!
    So to reiterate the concepts provided by your replies, the carbocation in the diagram below (notice that I've changed the benzylic carbocation from tertiary to secondary), STILL wouldn't shift to the adjacent tertiary carbocation due to resonance stability, correct?
    20151207_184848 (1).jpg
     

    rak173

    Full Member
    Joined
    May 14, 2014
    Messages
    222
    Reaction score
    46
    I think the order is like this (in decreasing stability): Resonance>tertiary> secondary >primary>methyl

    Correct me if I am wrong.
     
    • Like
    Reactions: 1 user

    orgoman22

    DAT DESTROYER
    Lifetime Donor
    Sponsor
    Vendor
    15+ Year Member
  • Joined
    Mar 12, 2005
    Messages
    4,595
    Reaction score
    3,367
    Thank you both for you replies!
    So to reiterate the concepts provided by your replies, the carbocation in the diagram below (notice that I've changed the benzylic carbocation from tertiary to secondary), STILL wouldn't shift to the adjacent tertiary carbocation due to resonance stability, correct? View attachment 198533

    You have a great point. A hydride shift would result in a tertiary carbocation. The stability , although close would NOT be the same. I will use The Advanced Organic text by Francis A. Carey from Univ of Virginia as my reference. The tertiary carbocation was assigned a value of 230 kcal/mole,,,,,,when the secondary benzylic pictured above was assigned a value of 226 kcal/mole/ . The lower the value, the greater the carbocation stability. This data is from their newest addition.

    Hope this helps.

    Dr. Romano
     
    • Like
    Reactions: 1 users
    Top