SN2 Mechanism Flashcards
What is an SN2 reaction?
These reactions are concerted
This is a chemical reaction in which all bond breaking and bond making occurs in a single step
Reactive intermediates are not involved
What is the rate equation for this reaction?
Rate= k[RX][Nu]
What does the kinetics suggest about the transition state?
It suggests that both the halogenoalkane and the nucleophile are involved in the transition state or the rate determining step
What type of reaction is the rate determining step?
It is a biomolecular reaction as 2 reactant species are involved in the transition state
Describe an SN2 reaction
The nucleophile attacks the CX bond
The nucleophile forms new bond to the carbon atom at the same time as the CX bond is broken
Describe the structure of the transition state
The carbon is bonded to 3 R groups (in the same plane) and a partial bond to both the nucleophile and the halogen atom
Describe the charge of the transition state
The nucleophile brings charge to the transition state which is spread over the system (1/2 partial negative charge on the partial RX bond and the partial RNu bond)
Draw a reaction profile
See drawing
What are the three possible stereochemical outcomes for SN2 reactions?
Reaction from nucleophile attack on chiral halogenoalkane
- retention
- inversion
- racemisation
Why is the stereochemical product that is produced?
Primary and secondary compounds undergo complete inversion
How does inversion take place?
1) The nucleophile approaches the carbon from the rear
2) This maximises orbital overlap with the nucleophile and the empty antibonding orbital of the CX bond (LUMO) and HUMO of Nu and they interact
3) This weakens and lengthens the CX bond
4) At the same time the NC bond is formed and the R groups bend back in an umbrella like fashion to give the inverted product
How does size of halogen affect the CX bond?
As the halogen increases in size, the bond length increases and bond strength decreases
This is due to poorer overlap
How do C-I bonds undergo nucleophile attack?
They are non polar bonds but when the nucleophile approaches, this polarised the bond and so then nucleophile can attack
How do we known reactions go with inversion?
If every replacement occurs with inversion then the rate of loss of optical activity must he twice the rate of incorporation of I*
The rotation of two molecules goes to zero for every inversion
What do experiments tell us about the inversion mechansim?
The nucleophile must be approaching the substrate from the rear of the departing atom
Inversion takes place through an sp2 hybridised carbon atom
(Attack from the front size would lead to retention)
