The document summarizes the E2 elimination reaction. 1) The E2 reaction involves the concerted removal of a β-proton by a base and loss of a halide ion in a single step with no intermediate. 2) It is a second-order reaction that depends on both the base and substrate concentrations. The reaction proceeds through a transition state in which the β-proton and halide leave simultaneously to form an alkene. 3) Factors that increase the rate of the E2 reaction include stronger bases, better leaving groups, more substituted substrates, polar aprotic solvents, and bulky or conjugated substrates. The reaction favors antiperiplanar elimination to form the

1. E2 REACTION
RAMKESH CHAUHAN ASU2020010100135 B.PHARM 1st YEAR ORGANIC CHEMISTRY

2. E2 REACTION
• It involve concerted removal of a β-proton by a base and loss of a halide ion.
• In E2 reaction both elimination as well as abstraction of proton take place at same time.
• It is biomolecular elimination reaction(because rate of reaction depend upon base and
substrate)
• This reaction involves a 1 step mechanism with a single transition state.[break down of
carbon-Hydrogen and carbon-halogen and formation of double bond occur (c=c pi bond)]

3. KINETICS
It is 2nd order kinetic reaction.
The rate of reaction depends on base and substrate.
In this case two group leave simultaneously .
There is no intermediate only transition state exist(one step)
Transition state –it is state where you can’t isolate the intermediate.
It is Concerted mechanism.
Reaction goes faster if stronger base and better leaving group
RATE = K[B][R-LG]
B=BASE
R= ALKYL
LG=LEAVING GROUP

4. In this state base attack on proton i.e, ß hydrogen (also called ß
elimination reaction)
Simultaneously leaving group leave and it leads the formation of
alkene(double bond)
Single step mechanism it is complete in SN2 reaction and E2
reaction.(nucleophile act as base in E2 reaction
whereas in nucleophilic reaction it act as nucleophile in SN2)
H C
C LG
Nu-
E2
SN2
C
C
H C
Nu C
H

5. ORDER OF REACTIVITY OF ALKYL HALIDE
THREE ALKYL
SUBSTITUENTS
TWO ALKYL
SUBSTITUENTS
ONE ALKYL
SUBSTITUENTS
• If you have strong electrophile it can easily abstract a proton.it has high capacity of abstract proton though the
reaction is E2 reaction.
• If your nucleophile have high capacity to displace a leaving group it should be slightly weaker than your
nucleophile which is use in E2 reaction than it will go through SN2 reaction.

6. E2 reaction
It is antiperiplanar displacement take place and it is favored by polar aprotic solvent
Reason
• 2nd order reaction
• Single step reaction
• Biomolecular reaction
• Stereospecific reaction
• Favored by polar aprotic solvent(increase in nucleophilicity of nucleophile because polar
protic solvent form a hydrogen bond like H2O and it decrease the nucleophilicity of
nucleophile it leads SN2 reaction)
• Highly/ strong nucleophile/strong base
IF YOU TAKEN NON APROTIC SOLVENT OR POLAR APROTIC SOLVENT WHICH UNABLE TO FORM HYDROGEN BOND
THAT IS SOLVATION EFFECT

7. FACTOR EFFECTING E2 REACTION
1.SUBSTRATE
• Always 3 degree alkyl favored E2 reaction.
• Because it stabilize the intermediate.
• Bulky alkyl halides have an increased rate of reaction.
• 3°(+3 I) > 2°(+2 I) > 1°(+I) > methyl .
• Because of +I effect (indication effect)
2. NATURE OF LEAVING GROUP
• Better the leaving group higher the rate of reaction
• R-I > R-Br >R-Cl > F-R
• Weaker bond present in C and Halide better leaving group.
• In some case Ms > Ts

8. 3. BASE
• Strong base usually favor E2 reaction.
• Strong negatively charged bases like −OH and−OR.
• CH3O- > CH3OH
4.SOLVENT
• Polar aprotic solvent favor E2 reaction
• Polar protic solvent favor E1 reaction
5. EFFECTS OF R-
• Reactivity order : (CH3)3C- > (CH3)2CH- > CH3CH2- > CH3-
• E2 reaction, the reaction transforms two SP3 C atoms into SP2 C atoms.
This moves the substituents away to decreasing any steric interactions. So
more highly substituted undergo E2 eliminations more rapidly.

9. ENERGY DIAGRAM
INTERMEDIATE
Product have lower
energy than starting
material if it have higher
energy it go reverse
reaction.

10. STEREOCHEMISTRY
• In bond H-C and C-LG E2 reaction occurs most rapidly.
• Most often 180* or antiperiplanar.
• More stable alkene will be major product.
LG
H
B:
+ B-H + LG-
ETHENE

11. IMPORTANT POINTS IN E2 REACTION
HOFFMANN ORIENTATION/SAYTZEFF ORIENTATION
If there is bulky leaving group or bulky base – least substituted alkene would be
prefer produced [terminal alkene].
Most substituted product will be major product.
KOH
CH3OH
KOtBu
tBuOH
Saytzeff orientation
Hoffmann orientation

12. THANK YOU