E1 and E1cB Reactions

Hafiz Muhammad Athar RizwanOrganic Chemistry

In an elimination reaction some fragments of a molecule are removed.

Elimination reaction are classified into two general headings

α-eliminations β-eliminations i

Elimination Reactions:

α-eliminations in which two groups are eliminated from the same atom.

In this case unstable species are formed which undergo further reactions.

β-eliminations in which groups on adjacent atoms are eliminated with the formation of multiple bonds.

C C

H

X

BaseC C

β-eliminations proceeds through two mechanisms whichare

E2 bimolecular elimination reactions E1 unimolecular elimination reaction

The E2 and E1 mechanisms differ in the timing of bond cleavage and bond formation, analogous to the SN2

and SN1 mechanisms.

β-eliminations

E1 indicates a elimination, unimolecular reaction. The E1 reaction proceeds via a two-step mechanism: the bond to the leaving group breaks first before the π bond is formed. The slow step is unimolecular, involving only the alkyl halide.

E1, Unimolecular Elimination Reactions

The dehydrohalogenation of (CH3)3CI with H2O to form (CH3)2C=CH2 can be used to illustrate the E1 mechanism.

Energy diagram for E1 reaction

Types of elimination reactions:

1. Dehydrohalogenation (-HX) and 2. Dehydration (-H2O)

1.Dehydrohalogenation (-HX)

CCH3

CH3

CH3

Br

slow

B:-

CCH3

CH3

C H

H

H

+ rapidC C

CH3 H

CH3H

+ B H + Br-

Br--

2.Dehydration (-H2O)

strong acid+ H2OR C C R

R

OH

H

R

C C

R

R R

R

1)

2)

3)

+ H+

+

+

slow

++ H2O

+

+ H+

CH3 C CH3

CH3

OH

CH3 C CH3

CH3

OH2

CH3 C CH3

CH3

OH2

CH3 C CH3

CH3

CH3 C CH3

CH3

C CH2

CH3

CH3

Alcohols also undergo Elimination under Acidic Conditions:

1) Substrate effect2) Base effect3) Isotope effect4) Orientation of elimination

Factors affecting the rate of E1 mechanism

The order of the reactivity of the alkyl groupsis Tertiary> Secondary>Primary

This is because the rate determining step is the formation of carbocation and the stability of these ions increases.

1) Substrate effect:

Tertiary (3o) > secondary (2o) > primary (1o)

It is hard (but not impossible) to get primary compounds to go by E1. The reason for this is that primary carbocations are not stable!

Order of stability of Carbocation

Increased Substitution Favors Elimination

Bulky Bases Favor Elimination

2) Base effect:

E1 reactions do not show an isotope effect: kH/kD = 1

This tells us that the C-D or C-H bonds are not broken in the rate determining step (step 1). They are broken in the fast step (step 2) in the mechanism).

3) Isotop effect:

E1 reactions faithfully follow Zaitsev’s rule! This means that the major product should be the

product that is the most highly substituted.

4. Orientation of elimination: Regiochemistry/ Zaitsev’s Rule

In reactions of removal of hydrogen halides from alkyl halides or the removal of water from alcohols, the hydrogen which is lost will come from the more highly-branched b-carbon.

1. Kinetics2. Stereochemistry3. Rerangement

Evidences of E1 reactins

E1 reactions follow first order (unimolecular) kinetics:

Rate = k [R-X]1

The solvent helps to stabilize the carbocation, but it doesn’t appear in the rate law!!

1. Kinetics:

Rate law: rate = k [R-Br]1

Kinetics:1)

++ Br

_slow

+

2)..

:

+fast

O.. +O

C C

Br

C C

H

C C

HC C

H

H H

H

H

H

rate determining step

E1 reactions do not require an anti coplanar orientation of H and X.

Diastereomers give the same products with E1 reactions, including cis- and trans products.

E1 reactions usually give the thermodynamically most stable product as the major product. This usually means that the largest groups should be on opposite sides of the double bond. Usually this means that the trans product is obtained.

2. Stereochemistry of the reactants:

Alkyl groups and hydrogen can migrate in rearrangement reactions to give more stable intermediate carbocations.

3. Rearrangements:

+

+

major

minor

trace

CH3 C C CH3

CH3

CH3

H

CH3 C CH CH2

CH3

CH3

CH3 C C CH3

CH3

HCH3

CH2 C CH

CH3

CH3

CH3

C C

CH3

CH3 CH3

CH3

tertiary carbocationsecondary carbocation

Factors to Consider:

1. How Basic is the Nucleophile?

2. 2. Steric Hindrance at Reacting Carbon

3. 3. Steric Hindrance at Nucleophile

Substitution or Elimination

Summary of Reactivity

E1cB mechanism (E-elimination, 1cB-first order with respect to conjugate base) is one of the three limiting mechanisms of 1,2-elimination.  It is a two-step mechanism.

E1cB Mechanism:

Step 1

Step 2

Step two is first order and its reactant is the conjugate base of the substrate, hence the notation 1cB.

A 1,2-elimination occurring via E1cB mechanism is called and E1cB reaction.  Stand-alone E1cB reactions are not common, and they have a complex rate law, meaning that the rate-limiting step is the second step.

However, in the most common E1cB reactions, the base is ‾OH and the solvent is water, in which case the rate law simplifies to

rate = k[substrate][base]

When the base is strong enough and the leaving group is very poor(such as fluorine or hydroxyl groups), E1CB is preferred.