Halogeno Derivatives

Question 1

How to prepare halogenoalkanes from alkanes?

Answer 1

Reaction: Free radical substitution

Reagents and conditions: Limited X2 + UV light

Question 2

How to prepare halogenoalkanes from alkenes?

(There are 2 ways)

Answer 2

1. Reaction: Electrophilic addition
   Reagents and conditions: X2 in CCl4, in the dark
2. Reaction: Electrophilic addition
   Reagents and conditions: Dry HX/ HX in CCl4

Question 3

How to prepare halogenoalkanes from alcohols?

Answer 3

Reaction: Nucleophilic substitution
For Cl,
Reagents and conditions: HCl, heat/ anhydrous PCl5/ anhydrous SOCl2/anhydrous PCl3
For Br
Reagents and conditions: NaBr, conc. H2SO4, heat or HBr, heat / anhydrous PBr3
For I,
Reagents and conditions: Red P + I2

Question 4

What is the reaction when it is first-order/second-order?

What and how are the features of the products formed if a chiral C atom is obtained?

Answer 4

First order: Sn1
Second order: Sn2

Sn1: There would be a racemic mixture formed due to the equal probability of nucleophile attacking the sp2 hybridised trigonal planar carbocation from either top or bottom of the plane
Sn2: The optical isomerism of the product is reversed because the nucleophile attacks the C atom from the side opposite the halogen atom.

Question 5

What type of halides favour which mechanism?

Answer 5

Methyl and primary halides: Sn2

Tertiary halides: Sn1

Question 6

State the reaction, reagents, and conditions for:

R-X +NaOH

Answer 6

Reaction: Nucleophilic substitution

Reagents and conditions: NaOH, KOH, heat

Question 7

State the reaction, reagents, and conditions for:

R-O + R’O- Na+ (ester)

Answer 7

Reaction: Nucleophilic substitution

Reagents and conditions: Na in excess alcohol (R’O-Na+, heat

Question 8

State the reaction, reagents, and conditions for:

R-X +KCN

Answer 8

Reaction: Nucleophilic substitution

Reagents and conditions: KCN in ethanol, heat under reflux

Question 9

State the reaction, reagents, and conditions for:

R-X + NH3

Answer 9

Reaction: Nucleophilic substitution

Reagents and conditions: Excess conc. ammonia in ethanol, heat in sealed tube.

Question 10

State the reagents, and conditions for:

Elimination

Answer 10

Reagents and conditions: KOH or NaOH in ethanol. heat

Eliminate from halogenoalkane to alkene

Question 11

State the reagents, and conditions for:

Acidic hydrolysis of RCN

Answer 11

Reagents and conditions: HCl or H2SO4, heat under reflux

Question 12

State the reagents, and conditions for:

Basic hydrolysis of RCN

Answer 12

Reagents and conditions: NaOH or KOH, heat under reflux

Question 13

State the reagents, and conditions for :

Reduction of RCN

Answer 13

Reagents and conditions:

1. LiAlH4 in dry ether
2. H2 in Pd/Pt catalyst
3. H2 in Ni catalyst

Question 14

Describe a chemical test for halogenoalkanes

Answer 14

1. Heat the compound with aq NaOH, followed by HNO3
2. Add AgNO3(aq)
   RCl: white ppt
   RBr: cream ppt
   RI: yellow ppt

Question 15

State the order of reactivity of halogenoalkanes (RCl, RBr, RI) and why

Answer 15

RCl > RBr > RI
Bond energy: RCl > RBr > RI
Bond strength: RCl > RBr > RI

Question 16

How to prepare halogenoarenes from arenes?

Answer 16

Reaction: Electrophilic substitution

Reagents and conditions: Cl2 (g), anhydrous AlCl3

Question 17

Why are halogenoarenes less reactive towards nucleophilic substitution

Answer 17

• C-X bond is shorter and stronger than halogenoalkanes.
• The lone pair of electrons on the halogen atom is delocalised into the benzene ring. This strengthens the carbon-halogen (C-X) bond in the halogenoarenes due to the presence of double bond character.
• The pi electron cloud of the benzene ring would repel the lone pair of electrons of the incoming nucleophile, hence rendering attack of the nucleophile difficult.