Rate determining step & transition state

[Div_MD2B]

hi everyone--
so i am reviewing some basic biochem stuff for the mcat...and i am a bit confused about this whole idea of rate determining step of the reaction. Is transition state considered to be the rate determining step?? In gen chem, i learned that rate determining step is the slowest step of the reaction and determines the rate (obviously)...i thought transition state was the same thing but in the book, it says that transition state exists for a really short time (so i am thinking that it can't really be the slowest part of the rxn). so, i'm a bit confused now...and i would really appreciate it if sumone could clarify this. thanks!

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[jcohen85]

i'm not exactly the best at chem, but i've been doing a lot of studying so i hope i can answer this. i don't think the two are explicitly related. it's my understanding that the rate determining step [like you said] is the slowest step of the reaction [the first step a lot of times, but doesn't have to be].

a transition state occurs in some reactions, not all. i feel like it occurs a lot in organic reactions [SN2 and E2 for instance]. a transition state, like you said, occurs for a short time, and is not the same as a rate determining step. in the case of SN2 for instance, the transition state occurs right after the nucleophilic attack. keep in mind it is NOT the same as an intermediate as in SN1.

hope that was somewhat helpful!

 

[TMP-SMX]

hi everyone--
so i am reviewing some basic biochem stuff for the mcat...and i am a bit confused about this whole idea of rate determining step of the reaction. Is transition state considered to be the rate determining step?? In gen chem, i learned that rate determining step is the slowest step of the reaction and determines the rate (obviously)...i thought transition state was the same thing but in the book, it says that transition state exists for a really short time (so i am thinking that it can't really be the slowest part of the rxn). so, i'm a bit confused now...and i would really appreciate it if sumone could clarify this. thanks!

Transition states are unstable and high energy molecules. Therefore without a catalyst it would be the rate determining step of a chemical reaction. Hence you have to put energy into a reaction in order to get to a transition state. Once the transition state is reached it's the fast step. With enzymes the activation energy required to start the reaction is reduced (and life becomes possible).

Think of it this way. You have a sheet of paper. The carbon in that paper will easily combust and it is certainly a spontaneous reaction. However, it has a relatively high activation energy required to start the cascade effect reaction. Light a match and set it on fire and it will combust... Quite well. It's getting past that activation energy that is the slow step.

Of course I don't think these two subjects are entirely related, but transition states and rate determining steps are the same.

 

[Condeeleza Rice]

Look over some of the basic orgo in your organic textbook. I had an actual test question on an exam that said this graph contains how many intermediates and transition states. It had more than one transition state and intermediate, I forget but one is always one less than the other(I think intermediate is always one less than transition state). Now relating rate rate determining steps and transition states, the rate determining step is the transition state with the largest energy hill(if looking at a graph), meaning that the reactant is less stable than the intermediate product, and the other transition states with lower energy hills are fast steps meaning that the reactant intermediates are less stable than the products in these fast steps. So there is a possibility of more than 1 transition state.

 

[TMP-SMX]

Look over some of the basic orgo in your organic textbook. I had an actual test question on an exam that said this graph contains how many intermediates and transition states. It had more than one transition state and intermediate, I forget but one is always one less than the other(I think intermediate is always one less than transition state). Now relating rate rate determining steps and transition states, the rate determining step is the transition state with the largest energy hill(if looking at a graph), meaning that the reactant is less stable than the intermediate product, and the other transition states with lower energy hills are fast steps meaning that the reactant intermediates are less stable than the products in these fast steps. So there is a possibility of more than 1 transition state.

I'm pretty sure intermediates are actually stable, but in terms of a long multiple reaction, transition states are the arcs where the curve is high and intermediates are where the curve is low. The end product and original reactant are pretty obvious.

 

[UMP]

look in to your organic book... mine talked about it a lot while explaining the Sn1 ... transition state is the top of the hills, intermediate is the little valley between the transition states (transition state - intermediate - transition)...
the intermediate is the carbocation...
and in the Sn1 reaction the rate limiting step is the first step, so it is the big first hill, whatever has the higher activation energy will be the rate limiting step...
BTW, transition states as far as I know are impossible to isolate and are only theoretical... intermediate states are hard to, but possible to isolate...


these things are way too hard to explain without those nice little energy curves