How to judge whether a TS is early or late by verifying the bonds length ?

zako

Hi,
According to Hammond's postulate, the transition state of a reaction resembles either the reactants (early transition state) or the products (late transition state), to whichever it is closer in energy, but I always encounter articles judging early or late transition state according to the bonds distances. How to decide wether a transition state is early or late by using bonds length ?

wzkchem5

For atom abstraction reactions and SN2 substitution reactions of the form X + Y-Z -> X-Y + Z, you can measure the X-Y and Y-Z bond lengths of the transition state. If X and Z are the same type of element, then the transition state is early if Y-Z is shorter than X-Y, and late vice versa. If X and Z are different elements, then you can do the comparison after subtracting the covalent radii of X and Z from the bond lengths. For other types of reactions, the situation is less clear-cut but the basic idea is the same.
Note that bond orders are probably more reliable than bond lengths in this aspect, particularly when X and Z are not the same kind of element.

zako

wzkchem5 发表于 2022-3-29 19:22
For atom abstraction reactions and SN2 substitution reactions of the form X + Y-Z -> X-Y + Z, you ca ...

Thank you very much wzkchem5 for your reply, and how about cycloaddition reactions which i'm interested in ?

wzkchem5

zako 发表于 2022-3-29 15:56
Thank you very much wzkchem5 for your reply, and how about cycloaddition reactions which i'm inter ...

As far as I know, there is no clear-cut criterion for early vs. late transition states for cycloaddition reactions. If the bond lengths in the transition state are very close to those of the reactants and very different from those of the products, then it is an early transition state, vice versa; but when the bond lengths of the transition state bear no particular resemblance of either those of the reactants or those of the products, then one probably cannot easily decide whether the transition state is early or late. This is in contrast to substitution reactions where the distinction between early and late is considerably more clear-cut.

sobereva

wzkchem5 发表于 2022-3-29 19:22
For atom abstraction reactions and SN2 substitution reactions of the form X + Y-Z -> X-Y + Z, you ca ...

对于X + Y-Z -> X-Y + Z，“the transition state is early if Y-Z is longer than X-Y” 似乎反了

wzkchem5

sobereva 发表于 2022-3-31 09:22
对于X + Y-Z -> X-Y + Z，“the transition state is early if Y-Z is longer than X-Y” 似乎反了

确实。。。已改