Chemical Processes and Reactions - Organic Chemistry - Which Mechanism?
Which Mechanism Is Operating?
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)
If not clear, mechanism is determined by conditions (ie for 2°substrates)
in order of importance:
2. Leaving Group
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)
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.
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
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
Diagram of Solvent Effects on S 2
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
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)
Diagram of Solvent Effects on S 1
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
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
Double bonds at bridgeheads are not stable.
Regioselectivity - S 1
For allylic substrates we can get attack at either end – Under S 1 conditions
Regioselectivity S 2
Under S 2 conditions - NaI in Acetone
This is the effect of neighbouring groups.