CPR Organic Which Mechanism


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Chemical Processes and Reactions - Organic Chemistry - Which Mechanism?

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Which Mechanism Is Operating?

Rule 1.

Mechanism is determined by Substrate

S 2 – mechanism is not viable for very hindered substrates (3°substrates)

S 1 – mechanism is not viable via unstabilised cations (1°substrates)

Rule 2.

If not clear, mechanism is determined by conditions (ie for 2°substrates)

in order of importance:

1. Nucleophile

2. Leaving Group

3. Solvent

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What is solvent? – dissolves things

Reactants are held between solvent molecules

A solvent lets reactant molecules move about


Polar – has dipole – Acetonitrile, (Methanol, THF) 




Bit polar – ether, acetone

Non–polar – pentane (C and H similar electronegativity) 

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Solvents Dissolve Things They Are Similar To

Non-polar dissolves non-polar – interactions similar to sticking to itself

(Van-der-Waals forces – tiny induced dipoles)

Polar dissolves polar– since dipoles can line up – mitigate against 

charge separation (lower the overall energy)



Protic solvents can also H bond to substrates and solvate 

(dissolve) negatively charges substrates.

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Solvent Effects On S 2 Mechanism

Transition state – involves spreading out of charge compared to Nu- – not 

much stabilised by polar or non-polar solvent 

However -

Nucleophile needs to attack substrate in rate step 

– solvent must dissolve Nucleophile and sufficiently polar solvent helps


Nucleophile must also escape the solvent cage to get at substrate

In a protic solvent this is difficult because its too good a solvent

Protic solvent slows down S 2!

Typical solvents favouring S 2 - acetone, acetonitrile, DMF

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Diagram of Solvent Effects on S 2

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Solvent Effects on S 1 Mechanism

Transition state – involves build up of positive charge (t.s. similar to intermediate cation)– stabilised by polar solvent 

Polar solvent accelerates S 1!

Typical S 1 solvents – water!, methanol, formic acid 

Corollary –

For S 1 - Nucleophile not involved in rate step – protic solvents do not slow S 1 


So for substrates capable of reaction via either S 1 or S 2 (2°or benzylic)

Polar protic solvents favour reaction via S 1 mechanism (slows S 2 and speeds up S 1)

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Diagram of Solvent Effects on S 1

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Stereochemical Consequences of S 2








Backside attack at a stereogenic (chiral) centre:

- proceeds with inversion of stereochemistry

- is stereospecific ie substrate controls the stereochemistry of the product

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Stereochemical Consequences of S 1







S 1 reactions proceed with racemisation

produces equal amounts of each non-superimposable mirror image

Since attack can occur on either face of the planar cation

Partial racemisation may occur if the initially formed ion pair reacts 

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Brendt's Rule

Double bonds at bridgeheads are not stable.

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Regioselectivity - S 1

For allylic substrates we can get attack at either end – Under S 1 conditions

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Regioselectivity S 2

Under S 2 conditions - NaI in Acetone

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Anchimeric Assistance

This is the effect of neighbouring groups.

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Retention Of Stereochemistry

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S 2 or S 1, Deciding Which Mechanism is Operating

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