Halogen derivatives

Question 1

Halogenoalkanes are polar compounds

Answer 1

Molecule contains a polar c-x bond, which causes PD-PD and ID-ID forces of attraction

Question 2

Boiling point of halogenoalkanes RX increases as R/X increases

Answer 2

As size of R/X increases, number of electron per molecule increases, size of electron cloud increases, ease of polarisation of electron cloud increases, strength of ID-ID increases.

Question 3

Halogenoalkanes are polar, yet insoluble in water but soluble in organic solvent

Answer 3

RX is unable to attract H2O molecules to undergo hydrogen bonding. RX is able to interact with solvent molecules via ID-ID attractions

Question 4

Formation of R-Cl from R-OH

Answer 4

PCl3/PCl5/SOCl2, room temperature
Special case: conc. HCl with ZnCl2 catalyst and heat

Question 5

Formation of R-Br from R-OH

Answer 5

PBr3 anhydrous
HBr, KBr (S), conc H2SO4, heat

Question 6

Formation of R-I from R-OH

Answer 6

KI(S), conc H3PO4, heat
Red P, I2, in situ reaction

Question 7

R-X undergoes nucleophilic substitution

Answer 7

Due to the electronegativity of X, carbon bonded to X is electron deficient. Therefore, being susceptible to attack by nucleophiles.

Nucleophilic substitution then undergoes Sn1 or Sn2 mechanism

Question 8

Definition of Sn2 Mechanism

Answer 8

SN ~ nucleophilic substitution
2 ~ two reactants involved in the rate determining step

Rate=K[RX][OH-]

Question 9

Description of Sn2 Mechanism

Answer 9

Step 1. Nucleophilic attack
- Nucleophile attacks the partial positive carbon attached to the X atom from the side directly opposite

Step 2. Transition state
- Bond breaking and bond formation occurs at the same time. Formation of the transition state where, simultaneous partial formation of C-O bond and partial cleavage of C-X bond

Step 3: Product
- Eventually C-X bond breaks and forms X- ions and subsequently alcohol product. Reaction results in stereochemical inversion of tetrahedral arrangement of bonds to the C atom.

Question 10

Factors affecting reactivity of RX in Sn2 Mechanism

Answer 10

1. Strength of RX bond
   - Weaker RX bond, the faster the rate of reaction is. Thus reactivity increases as RX bond is weaker.
2. Steric hindrance from R groups in Sn2 reactions
   - Greater steric hindrance, rate of substitution decreases. Thus reactivity decreases.

Question 11

Definition of Sn1 mechanism

Answer 11

SN~ Nucleophilic substitution
1~ 1 reactant involved in rate determining step

Rate=K[RX]

Question 12

Description of Sn1 mechanism

Answer 12

Step 1: heterolytic fission
- C-X bond undergoes fission to form carbocation. This is rate determining step since it involves endothermic bond breaking. Carbocation formed is trigonal planar.

Step 2: Nucleophilic attack
- Nucleophile attacks carbocation from both sides of the planar structure with equal probability. Forming a racemic mixture.

Question 13

Factors affecting reactivity of RX in Sn1 mechanism

Answer 13

1. Strength of RX bond
   - Strength of RX bond decreases down the group, Rate of Sn1 reaction increases.
2. Stability of carbocation in Sn1 reaction
   - Stability of carbocation increases due to the R groups attached to it, lower activation energy required to form the carbocation, rate of Sn1 reaction increases.

Question 14

Reactions of RX: Formation of alcohol

Answer 14

NaOH/KOH (aq), Heat
Note: Aq medium is used to help ensure nucleophilic substitution occurs.

Question 15

Reactions of RX: Formation of Nitrile

Answer 15

Alcoholic/Ethanolic KCN, Heat

Question 16

Reactions of RX: Formation of Amines

Answer 16

Excess conc NH3 in alcohol, heated in a sealed tube

Note: Excess of NH3 minimises polyalkylation while excess of RX increases polyalkylation

Question 17

Reactions of RX: Formation of alkoxide ions (R’O-)

Answer 17

R’-O-Na+ or R’-OH with Na, Heat
Product: Ester

Question 18

Reactions of RX: Elimination

Answer 18

Alcoholic/Ethanolic KOH (NaOH) and heat.
Products: H2O and X-

Question 19

Preparation of halogenoarenes: Electrophilic substitution

Answer 19

AlX3, FeX3, Fe

Question 20

Why do halogenoarenes not undergo NS

Answer 20

1. Lone pair of electrons from P orbital of the X atom to delocalise into the benzene ring. Resulting in a partial double bond in the C-X and is thus strengthened.
2. Benzene ring of the halogenoarene causes steric hindrance. While the Pi electron cloud of the benzene ring will repel the lone pair of electron of the nucleophile.
3. Halogenoarenes are unable to form stable carbocations.