CPR Organic Nucleophilic Substitution


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Chemical Processes And Reactions - Organic Chemistry - Nucleophilic Substitution

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Substitution Reactions At sp3 Centres

Halogens are electronegative and therefore pull electrons towards their nucleus. This gives a partial charge difference.

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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

least                             most

reactive                        reactive

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Biomolecular Mechanism

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.

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Energy Profile of Transition States

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Mechanism Nomenclature

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.

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.

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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.

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Steric Hindrance to Backside Attack of C-X Bond.

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Rate Dependent on Steric Hindrance Next To C-X

In the reaction:




R                            Relative Rate



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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.

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