Hydrogenation of Alkenes

  1. Hi

    In the Sebastier Senderen hydrogenation of alkenes, the general rule is that the more stable the alkene, the less reactive it is (the stability being assessed by the degree of hyperconjugation in the alkene). The decreasing order of reactivity as told to us in class is:

    ethylene > 1-propene > 2-butene > 3-methyl but-2-ene > 2,3-dimethyl but-2-ene

    The problem arises when the stability and reactivity of butenes is considered. One source says that vanderwalls strain is a destabilizing factor when alkyl groups are cis to each other. Hence, according to this source, the order of stability is

    isobutene > trans-2-butene > cis-2-butene > 1-butene

    However, in class the order of stability given to us is,

    trans-2-butene > isobutene > cis-2-butene

    Can someone tell me which is correct?

    Thanks and cheers,
    Vivek
     
    Last edited: Sep 15, 2004
  2. jcsd
  3. GCT

    GCT 1,769
    Science Advisor
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    The stability of each can be determined experimentally through measuring heats of hydrogenation, for one. Your book should have a listing of such values in the introduction chapter of alkenes and the topic regarding their stability.

    Of course, common sense tells you that both factors influence relative thermodynamic stability...in this sense one has to be measuring a common analog of reactions, such as hydrogenation or combustion. In this case I believe that the main emphasis is on the degree of conjugation to determine stability, the van-derwall factor, while it may be of influence in the sense of a higher energy transition state (that is influences the rate of reaction), may not have so much of a factor as the degree of conjugation. It's kind of similiar to the attraction repulsion aspect of things; electron deficiency is alleviated through proximate electron density, yet it still has a bit of a repulsion effect....in this particular reaction (observe the mechanism), the former is important since it reactivity and stability is centered on the electron deficient carbon.

    I don't have the time to browse through my text at the moment (which I am sure has the answer to this question).
     
  4. Hi GCT

    Thanks for your reply.......

    Well I have a textbook which states order 1, in which the determining factor is vanderwalls strain. My chemistry teacher however, gave me order 2, which was not explained but given to us to remember--according to him, the order was experimentally determined. Now some books suggest order 1 while others suggest order 2.

    Which is the text you were referring to in the last line of your post?

    Cheers
    mav
     
  5. chem_tr

    chem_tr 613
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    Gold Member

    Hello,

    Firstly, please remember that books are written by expert guidance and through peer reviewing; I don't think there is a false statement there, but your teacher might have wanted to express something else.

    If you view the attachment, you'll find that stability is directly proportional to the carbocation class. In my opinion, isobutene would be the most stable one in the series as it has a tertiary carbocation. The others all have secondary carbocations, so another determination factor needs to be considered. Trans- alkenes have the advantage of being free from steric hindrance and as such more stable. Cis-alkenes are however more reactive (i.e., less stable). But-1-ene is a terminal alkene and is therefore more vulnarable to attacks, that is, more reactive (less stable). In this viewpoint, we may conclude that the overall stability is isobutene>trans-2-butene>cis-2-butene>1-butene.

    To sum up, I think your teacher has said something else, explainable by another mechanism, or misjudged about the stability order.

    Regards
    chem_tr
     
  6. movies

    movies 284
    Science Advisor

    As an addition to what chem_tr wrote, my textbook suggests that the reactivity follows the stability of the corresponding radical, not the carbocation. The trend for radical stability is the same as for carbocation stability, so the answer is the same. I think it is more consistent with the supposed mechanism of hydrogenation, which is thought to be radical.

    Tri- and tetra-substituted olefins are more stable based on hyperconjugation; it seems that this effect outweighs the steric effect of cis substitutions.

    Info is from this book: http://www.amazon.com/exec/obidos/A...67333/sr=ka-1/ref=pd_ka_1/002-8608785-4560853
     
  7. GCT

    GCT 1,769
    Science Advisor
    Homework Helper

    It may be the case that your text was emphasizing the degree of repulsion for the sake of discussing the topic in and of itself, perhaps you had misunderstood the text's intentions.

    You should trust the experimentally determined results, it's a real problem if your text has different values for such experiments, however I don't believe this is the case. They cannot ignore the experimentally determined results which should be mainstream knowledge.

    Hope this helps
     
  8. First off, thanks GCT, chem_tr and movies for your help. My observations:

    I am still looking for the experimental results. I do not think I misunderstood my text's intentions. However, it is likely that there is a misprint. Though experimentally determined results are mainstream knowledge, they are too many to be mentioned. As I said, the problem comes not in judging stabilities of alkenes in general (which, for small sized alkenes, can be determined by hyperconjugation according to texts), but the reactivity of alkenes in hydrogenation.

    It is surprising that you mention the supposed mechanism of hydrogenation as radical-based. My text states that the mechanism essentially involves absorption of the olefin over a nickel/platinum/paladium catayst surface and that of hydrogen much in the same way. Pi bonds are broken and a hydrogen atom attaches on either side of the pi bond. The surface then allows the hydrogenated molecule to leave. I do not know if this is correct, but it was indeed one of the supposed mechanisms sometime ago (again, my knowledge of organic chemistry is limited to what is given in my texts).

    I hope you were right chem_tr. But the textbook I referred to is not of the same class. It has been written not by an organic chemist but by a person who is self-taught (and in that way, it is not even a textbook but a cookbook written by someone with a smattering knowledge of the subject). The extent of contradictions in organic chemistry increases with lack of knowledge, and so does the probability of equally appealing answers. However, I agree with you as far as the better known, established and technically correct textbooks are concerned...Morrison & Boyd, Finar, Sykes etc.

    Let me mention the problem again, to make it more specific:

    What is the order of reactivity of alkenes (and particularly of 2-butenes) in hydrogenation?


    Thanks and cheers
    Vivek
     
  9. chem_tr

    chem_tr 613
    Science Advisor
    Gold Member

    Okay, we have settled one thing I think; 1-butene, and afterwards, cis-butene are the most reactive ones. Then comes the problem, which is more reactive, trans-butene or isobutene?

    In my opinion, trans-butene should be a bit more reactive as it contains a secondary carbocation. But the steric relief has diminished the reactivity a lot. Then comes isobutene, that's what I speculate.

    The final order of reactivity in my opinion is this:

    1-butene > cis-butene >> trans-butene ~> isobutene

    Regards, chem_tr
     
  10. movies

    movies 284
    Science Advisor

    From my class notes I have these relative rates for hydrogenation:
    (the examples I have are for generic olefins, but I will relate them to this discussion for simplicity)

    1-butene > isobutene > cis-2-butene > trans-2-butene > 2-methyl-2-butene > 2,3-dimethyl-2-butene

    relative rates:

    90-300 > 25 > 10-20 > 2 > 1 >> (something small)

    So it looks like isobutene is slightly faster, probably because of the radical/carbocation stability.

    Mav, the way that you have described the hydrogenation mechanism is pretty much as I think of it as well, but it is unclear how the breaking of the pi bonds and making of the C-H bonds actually occurs. Palladium does a lot of reactions where the mechanism favors some sort of radical process (in this case, cleave the Pd-H bond homolytically). It is relatively easy to break a pi bond into two radicals. Furthermore, olefins are much less reactive towards attack by charged nucleophiles and electrophiles, while the attack of a radical on an olefin is one of the fastest reactions known. The short is that no one seems to know the hydrogenation mechanism precisely, but I think that olefin and Pd reactivity suggest a radical mechanism rather than a polar one.
     
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