2
$\begingroup$

For the reaction of 1,3-butadiene with DBr (D = heavy hydrogen), I found that several possible products could be formed.

Kinetic products

A pair of enantiomers is formed. One of the pi bonds attacks the heavy proton and the leaving group, bromide anion, comes back around to take care of the allylic carbocation. Because the bromide can attack from both above and below the plane of the carbocation, both enantiomers are formed.

Thermodynamic products

2) Now, because there is resonance, the carbocation possesses partial primary allylic carbocation character. As a result, bromide anion may also attack at a different position. This results in an internal alkene - the more stable alkene (the Zaitsev alkene).

Diastereomers are formed as well because single bonds can rotate, and the partial, terminal double bond can be "flipped" upward in the resonance depiction (I know that resonance depictions do not exist independently; hence "partial"). This allows the Z alkene to be formed in addition to the E alkene, as illustrated by my mechanism.

Questions

1) Are the products all valid and correctly categorized?

2) Is the E and Z isomer each produced, and in the way I think both are produced?

enter image description here

Improve this question
$\endgroup$

1 Answer 1

Reset to default
5
$\begingroup$

You have only considered reaction from the s-trans (here the prefix "s" is used to describe the arrangement of substituents around the central "single" bond) conformation of 1,3-butadiene, where the 2 double bonds are in a trans relationship about the central single bond. The s-trans conformation is in equilibrium with the s-cis conformation. The s-cis conformation can also react to produce the terminal olefin, but also (by the mechanism you described) it can react to produce the z 2-butene as one of its kinetic products. Hence, all 3 butenes (1-butene and e and z 2-butene) can be formed as the kinetic products.

Improve this answer
$\endgroup$
8
  • 1
    $\begingroup$ What does the prefix "s" mean in s-trans? $\endgroup$
    – Dissenter
    Sep 16, 2014 at 16:34
  • 2
    $\begingroup$ It stands for "single" bond. In other words there is a trans arrangement of ethylenes about the central single bond in the s-trans conformation. $\endgroup$
    – ron
    Sep 16, 2014 at 16:35
  • $\begingroup$ No problem! So is considering the resonance structure as rotating to give E/Z isomers incorrect? $\endgroup$
    – Dissenter
    Sep 16, 2014 at 16:36
  • 2
    $\begingroup$ Under typical reaction conditions (something around STP) there is restricted rotation about the bonds in or attached to the allyl framework. $\endgroup$
    – ron
    Sep 16, 2014 at 16:38
  • $\begingroup$ Makes sense given double bond character. $\endgroup$
    – Dissenter
    Sep 16, 2014 at 16:41

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.