Determination of rate

[hinson]

As we all know, to determine whether a reaction of substitution is Sn1 or Sn2 is by following the reaction rates of different reaction mixtures containing controlled amount of reactants to find out the type of rate law such that if the rate law is first order, the reaction is Sn1 and if that is second order, the reaction is Sn2.

 

My question is, how could we follow the reaction rate that involve intramolecular substitution? I think we can't do this by simply changing the reactant concentration as both the substrate and the nucleophile now are in the same molecule. How can we know whether that substitution is Sn1 or Sn2? e.g. epioxidation of CH3(Br)CHCH(OH)CH3

Edited November 16, 2011 by hinson

[mississippichem]

As we all know, to determine whether a reaction of substitution is Sn1 or Sn2 is by following the reaction rates of different reaction mixtures containing controlled amount of reactants to find out the type of rate law such that if the rate law is first order, the reaction is Sn1 and if that is second order, the reaction is Sn2.

 

My question is, how could we follow the reaction rate that involve intramolecular substitution? I think we can't do this by simply changing the reactant concentration as both the substrate and the nucleophile now are in the same molecule. How can we know whether that substitution is Sn1 or Sn2? e.g. epioxidation of CH3(Br)CHCH(OH)CH3

 

Intramolecular reactions are usually zero order. Think about it, increasing the concentration of the sole reactant [your beta-holahydrin there] doesn't increase the collision frequency. So the rate law is just,

 

[math] -\frac{d[A]}{dt}=k[/math]

 

and after integrating we get,

 

[math] [A]_{f}=-kt+[A]_{i} [/math]

 

If you plot this as a concentration versus time graph, you should get a straight line with a negative slope with respect to reactant concentration.

 

You could probably track this reaction with NMR or FTIR.

 

Good question though.

Edited November 16, 2011 by mississippichem