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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: qwerty on April 19, 2004, 07:49:42 PM

Title: reaction mechanisms using HBr
Post by: qwerty on April 19, 2004, 07:49:42 PM

*delete me*  Can anybody draw this??

Propose a reaction mechanism for the reaction of HBr with 2,4-hexadiene.  Include structures for all intermediates and indicate any mixture products.

Propose a reaction mechanism for the radical mono-bromination of 2-butene.  Include structures for all intermediates and indicate any mixture of products.

Discuss how these two reactions are similar.

Discuss how these two reactions are different.

Thanks!


Edit: edited title for better indexing. Mitch

Title: Re:reaction mechanisms
Post by: Donaldson Tan on April 20, 2004, 04:10:55 AM

Quote from: qwerty on April 19, 2004, 07:49:42 PM

Propose a reaction mechanism for the reaction of HBr with 2,4-hexadiene.  Include structures for all intermediates and indicate any mixture products.

Any organic chemistry textbook would've the mechanism for electrophillic addition of hydrogen halides to alkenes. There will be major & minor products thou...

Quote from: qwerty on April 19, 2004, 07:49:42 PM

Propose a reaction mechanism for the radical mono-bromination of 2-butene.  Include structures for all intermediates and indicate any mixture of products.

duh.. it's free radical substitution

 ;D

Title: Re:reaction mechanisms
Post by: TJAL on April 20, 2004, 02:12:24 PM

I think there are 2 proposed mechanisms for the first reaction, i have heard of this one using a bromonium ion intermediate:

http://courses.chem.psu.edu/chem38/mol-gallery/bromonium/bromonium.html

Fortunately for this reaction i don't think Markovnikov or Zaitsev rules apply when adding here, but they will be a mix of products.

Title: Re:reaction mechanisms
Post by: GCT on April 20, 2004, 02:22:05 PM

The former question is a classic issue of thermodynamic vs rate product.  However in this case the rate product is also the thermodynamic product...I'm pretty sure.

The reaction for the former can either be a 2,3 adduct or 2,5 adduct.  Your Organic book should have a section devoted to this subject.

For the radical reaction, transfer of radical character occurs with the addition of bromine and thus the most stable product will occur when the radical is on the most stable carbon; and thus bromine will add to the less stable carbon in relevance to its degree of substitution.

The similarities:
the initial addiction is a electrophilic addition to the alkene.

Differences:
Bromine is the electrophile in the latter and a nucleophile in the former.  The former has a resonance stabilized intermediate.  The latter is a anti-markovnikov product.  You should keep on looking for other differences.

Title: Re:reaction mechanisms
Post by: qwerty on April 20, 2004, 07:07:54 PM

I think I understand the reaction mechanisms but I am having trouble finding similarities between the two reactions?? :-\

Also, would the second reaction be hard to control due to the radical?

Title: Re:reaction mechanisms
Post by: AWK on April 21, 2004, 01:22:45 AM

In both reactions alpha-bromoalkenes are formed.

Title: Re:reaction mechanisms
Post by: qwerty on April 21, 2004, 08:00:28 AM

doesn't the second reaction form an alkane?

Title: Re:reaction mechanisms
Post by: AWK on April 22, 2004, 09:39:05 AM

Radical bromination of alkenes gives alpha-bromoalkenes - this is radical substitution, not addition.

Title: Re:reaction mechanisms using HBr
Post by: qwerty on April 28, 2004, 08:14:57 AM

ANSWER-

1.  1,2 addition (4-bromo-2-hexene)
     1,4 addition (5-Bromo-3-hexene)

2.   1-bromo-2-butene & 3-bromo1butene

3.   similarities- both make more then one product
                       both are resonance stabilized (allylic)

4.   differences- 2 butene forms a radical intermediate
      2,4 hexadiene forms a carbocation intermediate

       These are just a few- there are others...