Steric hindrance
(by methyl groups ) inhibits
formation of (trigonal bipyramid)
transition state/attack by
nucleophile with tertiary compound

^That's from the mark scheme but I still cannot full understand even after reading the wiki page. :frown:

Reply 4

AtomicMan

13

Original post by upthegunners

Steric hindrance
(by methyl groups ) inhibits
formation of (trigonal bipyramid)
transition state/attack by
nucleophile with tertiary compound

^That's from the mark scheme but I still cannot full understand even after reading the wiki page. :frown:

Steric hinderance in a Sn1 reaction is basically that the CH3 groups around the central carbon atom are large and bulky, so the nucleophile cant directly attack the delta-positive carbon atom. An analogy would be like trying to get to a celebrity around his body guards, you cant get to him directly because the big bulky guards are surronding him.

(edited 11 years ago)

Reply 5

upthegunners

OP

10

Original post by AtomicMan

Steric hinderance in a Sn1 reaction is basically that the CH3 groups around the central carbon atom are large and bulky, so the nucleophile cant directly attack the delta-positive carbon atom. An analogy would be like trying to get to a celebrity around his body guards, you cant get to him directly because the big bulky guards are surronding him.

wow brilliant response

thanks

what about the bit about the trigonal bipyramid

I dont get it fully

Reply 6

AtomicMan

13

Original post by upthegunners

wow brilliant response

thanks

what about the bit about the trigonal bipyramid

I dont get it fully

Ok, as an example lets take bromoethane (CH3CH2BR). In Sn2, the intermediate will have 2 hydrogen atoms, a carbon atom (of the CH3 group), bromine atom and the nucleophile all bonded to the delta-positive carbon atom(the one with the Br on it). Because you have 5 things bonded to that one carbon atom, if you have done shapes of molecules, it will form a trigonal bipryramidal shape.

Its saying that because in Sn1 you have 3 bulkly CH3 groups attached to the delta-positive carbon atom, they are preventing the nucleophile from joining on to the delta-positive carbon atom while the halogen is bonded to it at the same time. So basically, the 3 CH3 groups take up so much space that the nucleophile cant join onto the d+ carbon so that you get 5 things bonded to the carbon, preventing a trigonal bipryamidal intermediate from forming.

So an analogy would be like say you had 5 seats in a row on an aeroplane. Normally they will seat 5 people (leading to trigonal byprimadal), but if you get 3 large people sitting there (CH3 groups), they take up so much space that only 1 normal person can also sit there in the same row (preventing trigonal byprimadal).

(edited 11 years ago)

what is steric hendrance?

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