Chemical Processes And Reactions - Organic Chemistry - Nucleophilic Substitution
Substitution Reactions At sp3 Centres
Halogens are electronegative and therefore pull electrons towards their nucleus. This gives a partial charge difference.
Nucloephile – species which donates electrons density
Electrophile – species which accepts electron density
The halide bonds act as nucleophile
Reactivity follows the series
F << Cl < Br < I
Kinetics - Rate of change of the concentration of reactants.
In this reaction the rate is proportional to the concentration of both OH and MeBr.
Rate = k.[HO-].[MeBr]
Consistent with the two molecules colliding in the rate determining step.
Energy Profile of Transition States
The rate determining step is bimolecular - involves two molecules.
The mechanism of this reaction is therefore called Substitution Nucleophilic Bimolecular-2 and this is denoted as Sn^2.
Sn^2 - Features
It is a one step reaction - leaving group leaves as the nucleophile attacks.
A transition state is depicted.
Rate = k[Nu].[Substrate]
Inversion of the stereogenic centre occurs.
Only primary and secondary alkyl halides can react in this way because of steric crowding.
Reaction Rates and Steric Hindrance At C-X
In the reaction:
R-Br Rel. Rate
CH Br 221 000
CH CH Br 1 350
(CH ) CHBr 1
(CH ) CBr no reaction
Tertiary are the least reactive and primary the most.
Steric Hindrance to Backside Attack of C-X Bond.
Rate Dependent on Steric Hindrance Next To C-X
In the reaction:
R Relative Rate
1. What is electronegativity?
2. How do halogens generally act on electrons within a covalent bond?
3. Describe a nucleophile.
4. Describe an electrophile.
5. Describe the reactivity series between Fluorine and Iodine.
6. Define the word 'kinetics'.
7. What is the rate determining step?
8. What phrase denotes Sn^2
9. How many steps are in an Sn^2 reaction?
10. Why can't tertiary alkyl halides react by Sn^2
11.What happens to the stereogenic centre in an Sn^2 reaction.