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Chemical Reaction: Elimination Reactions & Reaction with Metals | Chemistry Class 12 - NEET

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Elimination Reactions

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

Chemical Reaction: Elimination Reactions & Reaction with Metals | Chemistry Class 12 - NEET

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

Dehydration of Alcohol (E1)

Chemical Reaction: Elimination Reactions & Reaction with Metals | Chemistry Class 12 - NEET

Characteristics of E1 reaction

  1. It is a unimolecular, two-step process
  2. It is a first-order reaction
  3. Reaction intermediate is carbocation, so rearrangement is possible
  4. In the second step, a base abstracts a proton from the carbon atom adjacent to the carbocation and forms alkene.
  5. Kinetics → Rate ∝ [Substrate]
    Rate = k[Substrate]

Chemical Reaction: Elimination Reactions & Reaction with Metals | Chemistry Class 12 - NEET

E2- elimination:Chemical Reaction: Elimination Reactions & Reaction with Metals | Chemistry Class 12 - NEET

Chemical Reaction: Elimination Reactions & Reaction with Metals | Chemistry Class 12 - NEET

Question for Chemical Reaction: Elimination Reactions & Reaction with Metals
Try yourself:What is the correct order of reactivity of haloalkanes towards β-elimination reactions?
View Solution

Bimolecular reaction, second-order kinetic:

  1. Leaving group leads when the base is taking proton from adjacent carbon.
  2. It is a single step reaction
  3. It shows elemental as well as kinetic isotopic effect with lg as well as at b-position.
  4. Normally Saytzaff product is major.
  5. Transition state mechanism therefore rearrangement is not possible.
  6. The orientation of the proton & leaving group should be antiperiplanar for E2.
  7. 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)
  8. E2-elimination is favour by:
    a. Strong base (RO-, Alc. KOH)
    b. Polar aprotic solvent.
    c. High conc. of base.
    d. High temperature

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

Question 1: Predict the elimination products of the following reactions.

(a) Sec. butyl bromide + Chemical Reaction: Elimination Reactions & Reaction with Metals | Chemistry Class 12 - NEET Chemical Reaction: Elimination Reactions & Reaction with Metals | Chemistry Class 12 - NEET
(b) 3-Bromo-3-ethylpentane + CH3OH Chemical Reaction: Elimination Reactions & Reaction with Metals | Chemistry Class 12 - NEET
(c) 2-Bromo-3-ethylpentane + MeONa Chemical Reaction: Elimination Reactions & Reaction with Metals | Chemistry Class 12 - NEET 
(d) 1-Bromo-2-methylcyclohexane + EtONa Chemical Reaction: Elimination Reactions & Reaction with Metals | Chemistry Class 12 - NEET

Sol. (a) CH3 - CH = CH -CH3 
(b) Chemical Reaction: Elimination Reactions & Reaction with Metals | Chemistry Class 12 - NEET
(c) Chemical Reaction: Elimination Reactions & Reaction with Metals | Chemistry Class 12 - NEET 
(d) Chemical Reaction: Elimination Reactions & Reaction with Metals | Chemistry Class 12 - NEET

Question 2:  Chemical Reaction: Elimination Reactions & Reaction with Metals | Chemistry Class 12 - NEET major + minor

Write the structure of major and minor product.

Sol. Chemical Reaction: Elimination Reactions & Reaction with Metals | Chemistry Class 12 - NEET (minor)

 Chemical Reaction: Elimination Reactions & Reaction with Metals | Chemistry Class 12 - NEET(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



Question 3: Chemical Reaction: Elimination Reactions & Reaction with Metals | Chemistry Class 12 - NEET P + Q + R

Sol. P is Chemical Reaction: Elimination Reactions & Reaction with Metals | Chemistry Class 12 - NEET, Q is Chemical Reaction: Elimination Reactions & Reaction with Metals | Chemistry Class 12 - NEET, R Chemical Reaction: Elimination Reactions & Reaction with Metals | Chemistry Class 12 - NEET

Question 4: Arrange the compounds of each set in order of reactivity towards dehydrohalogenation by a 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

Question for Chemical Reaction: Elimination Reactions & Reaction with Metals
Try yourself:The SN1 reaction cannot be carried out in which of the following media?
View Solution

Reactions with MetalsHaloarenes undergo few reactions with metals. Two primary reactions are:

  • Wurtz-Fittig Reaction
  • Fittig Reaction

Wurtz-Fittig Reaction:

In this reaction, a mixture of alkyl halide reacts with an aryl halide in the presence of dry ether and sodium. The resultant product is alkyl arene.
Chemical Reaction: Elimination Reactions & Reaction with Metals | Chemistry Class 12 - NEET

Question for Chemical Reaction: Elimination Reactions & Reaction with Metals
Try yourself:Which is the metal involved in Wurtz-Fittig reaction?
View Solution

Fittig Reaction:

In this reaction, a mixture of haloarenes reacts with sodium in the presence of dry ether. The resultant product is diaryl.

Chemical Reaction: Elimination Reactions & Reaction with Metals | Chemistry Class 12 - NEET


Question for Chemical Reaction: Elimination Reactions & Reaction with Metals
Try yourself: Fittig reaction results in the formation of a diphenyl.
View Solution

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