Organic Bromination

[sdm33]

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12)*If the bromination of 1,3 butadiene is carried out at - 80*o C the major product is Compound A. However, if the reaction is carried out at 40 o C Compound B becomes the major product. Which of the following statements most accurately explains this transformation?*

A) A is the thermodynamic product and B is the kinetic product.
*B) A is the kinetic product and B is the thermodynamic product.
C) A has a disubstituted double bond and is somewhat less stable than B which has a monosubstituted double bond.
D) A has a monosubstituted double bond and is more stable than B which has a disubstituted double bond.

the more stable intermediate requires the*smallest*activation energy and thus forms a product*faster*(=*the kinetic product, A),
while the less stable intermediate requires more*energy (higher temperature,*i.e. 40*o*C in the problem) to form a product (=thermodynamic product, B).
How do you know is low Ea?

 

[mehc012]

What is your question, specifically?

Basically what you have here is a system with 2 possible reactions...one of which has a lower Ea and one of which has a lower energy (more stable) final product. Even without knowing anything about the reaction at hand, you should be able to answer this question by figuring out what they mean by "kinetic product" and "thermodynamic product", and why it would be true.

 

[sdm33]

the more stable intermediate requires the*smallest*activation energy and thus forms a product*faster*(=*the kinetic product, A),
while the less stable intermediate requires more*energy (higher temperature,*i.e. 40*o*C in the problem) to form a product (=thermodynamic product, B).
How do you know is low Ea?
Which chapters should I read to review this concept??
Thanks

 

[mehc012]

the more stable intermediate requires the*smallest*activation energy and thus forms a product*faster*(=*the kinetic product, A),
while the less stable intermediate requires more*energy (higher temperature,*i.e. 40*o*C in the problem) to form a product (=thermodynamic product, B).
How do you know is low Ea?
Which chapters should I read to review this concept??
Thanks

That's it right there, though! The one which forms under low temperatures is going to be the kinetic product (with a low activation energy), whereas the one forming under high temperatures will be the more stable thermodynamic product. That is always true; the thermodynamic and kinetic products are identified by the temperatures at which they form, so you know for a fact that A is kinetic and B is thermodynamic here.

I have no idea which chapters you should read, as I a) have no clue which textbook you have, or which review book you're using, and b) last went over this myself in an upper-level Physical Organic chem class, and couldn't tell you what it was learned under before that.

 

[polyatomic]

http://www.youtube.com/watch?v=DrToRsXmjP4


Basically, as said above -

Kinetic reactions = have lower activation energies, have less stable products. Typically favored at lower temperatures, because we need less energy to achieve the activation energy.

Thermodynamic reactions = have higher activation energies, have more stable products. Typically favored at higher temperatures, because we need more energy to achieve the high activation energy.

In questions they'll have you compare to products or reactions. Like in this question, they'll say one reaction occurs at higher temperatures and another at lower temperatures. If it occurs at higher temperatures, it will be the thermodynamic product and so forth. It's that simple.

 

[sdm33]

Thanks a lot guys, But
1. why larger products have higher activation energies are more stable ?
2. can we say endothermic Rxn is kinetic and Exothermic Rxn is thermodynamic ?

That You tube was awesome, is his other videos as good

 

[mehc012]

Thanks a lot guys, But
1. why larger products have higher activation energies are more stable ?
2. can we say endothermic Rxn is kinetic and Exothermic Rxn is thermodynamic ?

That You tube was awesome, is his other videos as good

1. This is not always true; it's just that when there is a situation where 2 products are possible, one via a lower energy reaction and one with a high Ea but a more stable product, then you will form different products at different temperatures. When that is true, the kinetic (low Ea) product will always form at a lower temperature than the thermodynamic product, by definition.

2. Again, not every reaction can be classified as forming either a kinetic or thermodynamic product. You have to have a very specific set of characteristics for those terms to be applicable. So no, you can't generalize that all exo/endothermic rxns form one or the other product, because most of the time, it's not applicable.


Besides, endothermic rxns would likely have a higher Ea than exothermic ones anyway, so even if you found an incredibly rare situation where there were two possible mechanisms, one exothermic and one endothermic, resulting in different products, you'd be labelling them backwards.

I highly suggest looking in the index of your book for "kinetic product" and finding the section that way.
Alternatively, try Wikipedia for an overview: http://en.wikipedia.org/wiki/Thermodynamic_versus_kinetic_reaction_control

 

[sdm33]

Thanks mehc012