Doc: Elimination Reactions and Reaction with Metals Class 12 Notes | EduRev

Chemistry Class 12

Class 12 : Doc: Elimination Reactions and Reaction with Metals Class 12 Notes | EduRev

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1.7.2 Elimination reactions:

In an elimination reaction two atoms or groups (YZ) are removed from the substrate with formation of pi bond.

Doc: Elimination Reactions and Reaction with Metals Class 12 Notes | EduRev

depending on the reagents and conditions involved, an elimination may be a first order (E1) or second order (E2).

Dehydration of Alohol (E1)

Doc: Elimination Reactions and Reaction with Metals Class 12 Notes | EduRev

Characteristics of E1 reaction :

(i) It is unimolecular, two step process

(ii) It is first order reaction

(iii) Reaction intermediate is carbocation, so rearrangement is possible

(iv) In the second step, a base abstracts a proton from the carbon atom adjacent to the carbocation, and forms alkene.

(v) Kinetics ° Rate ∝ [Substrate]

Rate = k[Substrate]

Doc: Elimination Reactions and Reaction with Metals Class 12 Notes | EduRev

E2- elimination :

Doc: Elimination Reactions and Reaction with Metals Class 12 Notes | EduRev

Doc: Elimination Reactions and Reaction with Metals Class 12 Notes | EduRev

Bimolecular reaction, second order kinetic.

1. Leaving group leads when base is taking proton from adjecent carbon.

2. It is a single step reaction

3. Rate a single step reaction

Rate a Leaving group tendency

4. It shows elimental as well as kinetic isotopic effect with lg as well as at b-position.

5. Normally saytzaff product is major.

6. Transition state machenism therefor rearrangement is not possible.

7. The orientation of proton & leaving group should be antiperiplanar for E2.

8. Positional orientation of elimination ° In most E1 and E2 eliminations gives two or more possible elimination products, the product with the most highly substituted double bond will predominate. This rule is called the saytzeff or zaitsev rule (i.e., most stable alkene will be the major product)

9. E2-elimination is favour by :

(1) Moderate lg

(2) Strong base (RO!, Alc. KOH)

(3) Polar aprotic solvent.

(4) High conc. of base.

(5) High temperature

Reactivity towards E2 ° R - I > R - Br > R - Cl > R - F

Ex. Predict the elimination products of the following reactions.

(a) Sec. butyl bromide + Doc: Elimination Reactions and Reaction with Metals Class 12 Notes | EduRev Doc: Elimination Reactions and Reaction with Metals Class 12 Notes | EduRev
(b) 3-Bromo-3-ethylpentane + CH3OH Doc: Elimination Reactions and Reaction with Metals Class 12 Notes | EduRev
(c) 2-Bromo-3-ethylpentane + MeONa Doc: Elimination Reactions and Reaction with Metals Class 12 Notes | EduRev 
(d) 1-Bromo-2-methylcyclohexane + EtONa Doc: Elimination Reactions and Reaction with Metals Class 12 Notes | EduRev

Sol. (a) CH3 - CH = CH -CH3 
(b) Doc: Elimination Reactions and Reaction with Metals Class 12 Notes | EduRev
(c) Doc: Elimination Reactions and Reaction with Metals Class 12 Notes | EduRev 
(d) Doc: Elimination Reactions and Reaction with Metals Class 12 Notes | EduRev

Ex.11 Doc: Elimination Reactions and Reaction with Metals Class 12 Notes | EduRev major + minor

Write the structure of major and minor product.

Sol. Doc: Elimination Reactions and Reaction with Metals Class 12 Notes | EduRev (minor)

 Doc: Elimination Reactions and Reaction with Metals Class 12 Notes | EduRev(major)

Comparison of E1 and E2 elimination:

Promoting factors

E1

E2

(i) Base

Weak base

Strong base required

(ii) Solvent

Good ionizing solvent

Wide variety of solvent

(iii) Substrate

3° > 2° > 1°

3° > 2° > 1°

(iv) Leaving group

Better one required

Better one required

Characteristics

 

 

(i) Kinetics

K[R- X], I order

K[R - X] [Base], IIst order

(ii) Orientation

Saytzeff alkene

Saytzeff alkene

(iii) Stereochemistry

No special geometry is required

transition state must be co-planar



Ex.12 Doc: Elimination Reactions and Reaction with Metals Class 12 Notes | EduRev P + Q + R

Sol. P is Doc: Elimination Reactions and Reaction with Metals Class 12 Notes | EduRev, Q is Doc: Elimination Reactions and Reaction with Metals Class 12 Notes | EduRev, R Doc: Elimination Reactions and Reaction with Metals Class 12 Notes | EduRev

Q.6 Arrange the compounds of each set in order of reactivity towards dehydrohalogenation by strong base

(a) 2-Bromo-2-methylbutane, 1-Bromopentane, 2-Bromopentane

(b) 1-Bromo-3-methylbutane, 2-bromo-2-methylbutane-2-Bromo-3-methylbutane

(c) 1-Bromobutane,1-Bromo-2,2-dimethylpropane, 1-bromo-2-methylbutane, 1-Bromo-3-methylbutane

(C) mechanism of E1 CB reaction (Unimolecular conjugate base reaction) :

The E1 CB or carbanion mechanism : In the E1 CB, H leaves first and then the X. This is a two step process, the intermediate is a carbanion.

Mechanism:

Step-1 : Consists of the removal of a proton, H+  by a base generating a carbanion

Doc: Elimination Reactions and Reaction with Metals Class 12 Notes | EduRev

Step-2 : Carbanion loses a leaving group to form alkene

Doc: Elimination Reactions and Reaction with Metals Class 12 Notes | EduRev

Conditiion: For the E1 CB, substrate must be containing acidic hydrogens and poor leaving groups (i.e., bad lg)

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