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

Elimination Reactions

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

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

Dehydration of Alcohol (E₁)

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]

E₂- elimination:

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 l_g 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 E_2: R - I > R - Br > R - Cl > R - F

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

(a) Sec. butyl bromide +  
(b) 3-Bromo-3-ethylpentane + CH₃OH
(c) 2-Bromo-3-ethylpentane + MeONa  
(d) 1-Bromo-2-methylcyclohexane + EtONa

Sol. (a) CH₃ - CH = CH -CH₃ 
(b)
(c)  
(d)

Question 2:   major + minor

Write the structure of major and minor product.

Sol. (minor)

 (major)

Comparison of E₁ and E₂ elimination:

Question 3: P + Q + R

Sol. P is , Q is , R

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

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.

Fittig Reaction:

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