Transition state diagram ochem

[orangetea]

So I am little bit confused as to how a transition state diagrams should look. For example take a look at this problem

I mainly guessed on this problem but my question is why does H3N have a partial positive charge?
As a nucleophile that is attacking shouldn't it have a negative charge.. I would think that it should be A.


Then in the TPRH book there is a problems : Which of the following best represents the highest energy species on an Sn2 reaction coordinate diagram

HO(partial negative charge)-----------C(two hydrogens attached)-------------------I(partial negative charge)

How are these charges assigned?

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[sillyjoe]

So I am little bit confused as to how a transition state diagrams should look. For example take a look at this problem
View attachment 182941

I mainly guessed on this problem but my question is why does H3N have a partial positive charge?
As a nucleophile that is attacking shouldn't it have a negative charge.. I would think that it should be A.


Then in the TPRH book there is a problems : Which of the following best represents the highest energy species on an Sn2 reaction coordinate diagram

HO(partial negative charge)-----------C(two hydrogens attached)-------------------I(partial negative charge)

How are these charges assigned?

It has a partial positive charge because it partially feels like it is bound to 4 atoms (formal charge of +1) as it is pushing itself towards the nucleus of the alkyl halide. The bromine is being pushed away in this transition state and partially removing electrons so it has a partial negative charge. Out of the answer choices B seems most likely.

I am just as stumped with you in terms of the later question from TPRH. Sorry!

 

[orangetea]

It has a partial positive charge because it partially feels like it is bound to 4 atoms (formal charge of +1) as it is pushing itself towards the nucleus of the alkyl halide. The bromine is being pushed away in this transition state and partially removing electrons so it has a partial negative charge. Out of the answer choices B seems most likely.

I am just as stumped with you in terms of the later question from TPRH. Sorry!

yea that makes sense but I can't apply the same logic to the second problem which is frustrating me.. It just seems like they're interpreting the diagram different ways (one when the NU is attached and one when it it attacking.. idk)

 

[sillyjoe]

yea that makes sense but I can't apply the same logic to the second problem which is frustrating me.. It just seems like they're interpreting the diagram different ways (one when the NU is attached and one when it it attacking.. idk)

Yeah I don't know because the OH is shifting between -1 and 0 while I is shifting between 0 and -1. It seems like both transition states from both examples work and therefore the answers from Kaplan and TPRH are correct, but they are just taking a "snapshot" of the transition state at a different time with the ammonia example being relatively later on in the transition state than the hydroxide example. I doubt you would ever get a question asking you to differentiate between the two... Again, this is the BS of picking "the best" answer.