Elimination Reactions And Polyhalogen Compounds | Chemistry for JEE Main & Advanced PDF Download

Elimination Reactions:

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

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

Dehydration of Alohol (E1):

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]

E₂- elimination :

Bi-molecular reaction, second order kinetic.

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

2. It is a single step reaction.

3. Rate a single step reaction.

Rate a Leaving group tendency-

4. It shows elemental as well as kinetic isotopic effect with leaving group (lg) as well as at β-position.

5. Normally, Saytzeff product is major.

6. Transition state mechanism therefore rearrangement is not possible.

7. The orientation of proton & leaving group should be anti-planar for E2.

8. Positional orientation of elimination: Mostly, E₁ and E₂ eliminations give 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 leaving group (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 +  
(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)

Ex.11  major + minor

Write the structure of major and minor product.

Sol. (minor)

 (major)

Comparison of E₁ and E₂ elimination:

Ex.12 P + Q + R

Sol. P is , Q is , R

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.

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

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

Polyhalogen derivatives:

Trichloromethane (Chloroform), CHCl₃

Preparation:

CH₄ + Cl₂  CH₃Cl + HCl
                              Chloromethane

CH₃Cl + Cl₂  CH₂Cl₂ + HCl
                                  Dichloromethane

CH₂Cl₂ + Cl₂  CHCl₃ + HCl
                                    Trichloromethane

CHCl₃ + Cl₂  CCl₄ + HCl
                                Tetrachloromethane

The mixture of CH₃Cl, CH₂Cl₂, CHCl₃ and CCl₄ can be separated by fractional distillation.

2. From chloral hydrate, Pure chloroform can prepare.

NaOH + CCl₃CHO → HCOONa + CHCl₃
                                      chloral

NaOH + CCl₃CH(OH)₂ → HCOONa + CHCl₃ + H₂O
Chloral hydrate               sodium formate Chloroform

3. Laboratory Method : From ethanol or acetone, by reaction with a paste of bleaching powder and water.

In case of ethanol, the reaction occurs as follows:

CaOCl₂ + H₂O → Ca(OH)₂ + Cl₂

CH₃CH₂OH + Cl₂  CH₃CHO + 2HCl

CH₃CHO + 3Cl₂  CCl₃CHO + 3HCl

                                                              Chloral

Ca(OH)₂ + 2CCl₃ CHO  2CHCl₃ + (HCOO)₂Ca

                                                               Chloroform Calcium formate

4. From carbon tetrachloride:

CCl₄ + 2[H]  CHCl₃ + HCl (partial reduction)

5. Haloform reaction:

 (Haloform)

Step 1 : Attack of the

Step 2 : Elimination

Step 3 : Proton transfer

nucleophile of the leaving group

Prob. Compare rate of elimination (Dehydro halogenation in presence of alcoholic KOH ) i.e., E2 :

1. (a)  (b)  (c)  (d)

c > b > a > d

2. (a)  (b)  (c)

c > b > a

3. (a)  (b)  (c)

c > b > a

4. (a)  (b)  (c)

b > a > c

Dehalogenation : - (-X₂) E2

Dehalogenation : - (-X₂) E2

E_c or E_i (Intramolecular or cyclic elimination mechanism):

(1) Lg and Base present in same molecule.

(2) It proceeds by cyclic transition state.

(3) Overall it is syn elimination.

(4) Hoffmann is major product as it is obtained by least hindered site of cyclic transition state.

(5) No rearrangement.

Example of E_c/E_i:

Pyrolysis of Ester:

1.

2.

1.8.2 Physical properties of chloroform

Chloroform is a colourless, heavy liquid which has sweetish, sickly odour and taste. It boils at 334º K and is slightly soluble in water. It is heavier than water. As inhaling of the vapours of chloroform induces unconsciousness therefore it can be used as an anaesthetic agent for surgery. 

1.8.3 Chemical properties of chloroform:

1. Action of sun light and air-

2 CHCl₃ + O₂  2COCl₂ + 2HCl

Phosgene

As chloroform is used for anaesthetic purposes, therefore in order to maintain a high purity of chloroform, this reaction can be avoided by storing it in dark bottles, completely filled upto the brim. The use of dark bottles (brown or blue) cuts off active light radiations and filling upto brim keeps out air. Apart from this a small amount of ethanol (1%) is usually added to bottles of chloroform. Addition of a little ethanol fixes the toxic COCl₂ as non-poisonous diethyl carbonate.

COCl₂ + 2C₂H₅OH  O = C(OC₂H₅) + 2HCl

diethyl carbonate

2. Hydrolysis :

H - CCl₃ + (aq.) 3KOH  HCOOK

3. Reduction :

Zn + 2HCl  ZnCl₂ + 2[H]

CHCl₃ + 2[H]  CH₂Cl₂ + HCl
                                Dichloromethane
                              (Methylene chloride)

CHCl₃  CH₄ + 3HCl

4. Reaction with acetone :

(CH₃)₂C = O + CHCl₃  

                                                                     Chlroetone

Use : Chloretone is used as hypnotic (a sleep inducing) drug.

5. Reaction with nitric acid:

2CHCl₃ + HONO₂  CCl₃. NO₂ + H₂O
                                               (Chloropicrin)

Use: Chloropicrin is used as an insecticide and war gas.

6. Reaction with silver powder :

7. Chlorination :

CHCl₃ + Cl₂ CCl₄ + HCl

8. Reimer-Tiemann reaction:

 + CHCl₃ + 3NaOH   + 2NaCl + 2H₂O

1.8.4 Uses of chloroform

1. As solvent in oils and varnishes.

2. As preservative for anatomical specimens.

3. As laboratory reagent.

4. As an anesthetic.