s&r topic 10

Question 1

what is an SN2 reaction

Answer 1

a reaction mechanism that happens in 1 step

Question 2

SN2 reaction

describe the first step around a tetrahedral carbon

Answer 2

nucleophile approaches empty σ* orbital at the C-X

nucleophile donates pair of electrons into this orbital

Question 3

SN2 reaction

what is happening in the transition state in terms of

• bonds forming/breaking
• steric effects

Answer 3

as a new sigma bond is forming between carbon and the nucleophile, the bond between the carbon and the leaving group is breaking

this is the highest energy stage of the reaction

lots of steric demand because carbon is very crowded
- there are 5 substituents around the carbon at this time, angles are 90° and 120° like a trigonal bipyramidal shape

Question 4

SN2 reaction

on the starting molecule, what R substituents are preferred on the carbon and why

Answer 4

reaction is favoured when R = H

smaller atoms around the carbon mean less steric strain, makes reactions easier, faster ROR

when carbon has smaller substituents to start with its more likely to undergo SN2 reactions

Question 5

SN2 reaction

what is the stereochemical outcome of the reaction when it occurs on a stereogenic centre?

Answer 5

results in inverse of configuration at the stereogenic centre

eg if centre is (R) configuration at the start, it will be an (S) configuration at the end

Question 6

what is an SN1 reaction

Answer 6

a reaction mechanism that occurs in 2 stages

Question 7

SN1 reaction

what is the 1st step

Answer 7

breaking of C-X bond to form carbocation intermediate

this is the slower step in the reaction = rate determining step

Question 8

SN1 reaction

what is the 2nd step

Answer 8

nucleophile attacks carbocation to form substituted product

faster step in the reaction

Question 9

SN1

what two things determine ROR

Answer 9

carbocation stability and overall carbocation skeleton

leaving group ability, the easier the C-X bond breaks, the quicker the reaction

Question 10

what is the order of carbocation stability and why

Answer 10

tertiary > secondary > primary

stabilised by hyperconjugation:
- tertiary carbocation has filled sigma C-H orbitals adjacent to it

• this donates electrons into the empty p-orbital and stabilises the C+

species that form a tertiary C+ are more likely to undergo SN1 reactions

Question 11

SN1 reaction

what is the stereogenic outcome of this reaction and why does this occur

Answer 11

when the reactions occurs at a chiral centre, the outcome is racemisation = when the products formed are a 50:50 mixture of the stereoisomers

this occurs because at the sp2 planar carbocation, attack on the empty p-orbital above and below the plane is equally likely so both stereoisomers are equally as likely