Nucleophillic substitutions Flashcards
Two types of nucleophillic substitution
Sn^1 and Sn^2
First order nucleophillic substitution and second order nucleophillic substitution.
Why are haloalkanes susceptible to nucleophillic attack
Due to the polar nature of the carbon-halogen bond.
Process of nucleophillic substitution
The nucleophile donates a pair of electrons forming a bond with the carbon atom of the C-X bond. The halogen atom is ‘thrown out’ and substituted by the nucleophile.
Sn1 Mechanism
2 Step process, step 1 slow step 2 fast.
Step 1: carbon - halogen bond breaks forming carbocation and negative halogen ion.
Step 2:
The nucleophile will attack the C+ ion forming a covalent bond between the pair. Leaving nucleophile in the halogens place.
What to remember when drawing ions
Show free electron dots
Sn1 info
Only one species in rate determining step, the monohaloalkane. So rate = k[monohaloalkane].
Usually tertiary or bulky secondary monohaloalkanes.
How many steps for Sn1?
Two
How many steps for Sn2?
One (and a transition state)
Sn2 mechanism
Nucleophile attacks the slightly positive carbon from side opposite halogen and begins to form a covalent bond with it, at the time the carbon-halogen bond begins to break.
Transition state is then reached with dotted lines from the carbon to both the nucleophile and the halogen, in square brackets with - charge outside. Nucleophile now substituted for the halogen with halogen ion released.
What to remember for Sn2 transition state
Square bracket, dotted lines and charge
Sn2 info
Reaction between a primary or not bulky secondary monohaloalkane and nucleophile.
Two species involved in rate determining step so:
rate = k[monohaloalkane][nucleophile]
In Sn2 nucleophile attacks carbon from side….. the halogen
Opposite
Primary monohaloalkanes undergo
Sn2
Tertiary monohaloalkanes undergo
Sn1
What determines what a secondary monohaloalkane will undergo
How bulky the secondary monohaloalkane is.
