halogenoalkane

Question 1

Reagents and conditions to form halogenoalkane from alkane

Answer 1

Mechanism: Free radical substitution

Reagent and conditions: Br2(l), Cl2(g), UV light or heat

Question 2

Reagent and conditions to form halogenoalkane from alkene

Answer 2

Mechanisms: Electrophilic addition

Reagent and conditions: Br2(l)/Cl2(g) or Br2/Cl2 dissolved in CCl4, room temperature

Question 3

Reagent and conditions to form chloroalkanes from alcohols

Answer 3

1. PCl5, room temperature
2. PCl3, room temperature
3. SOCl2, warm

Question 4

Reagent and conditions to form bromoalkanes from alcohols

Answer 4

1. P, Br2(l), heat
2. Concentrated HBr, heat
3. NaBr, concentrated H2SO4, heat

Question 5

Reagent and conditions to form halogenoarenes from arenes

Answer 5

1. Br2(l), FeBr3 as Lewis acid catalyst, heat to form bromoarene
2. Cl2(g), FeCl3 as Lewis acid catalyst, heat to form chloroalkane

Question 6

Stereochemistry of product formed from SN2 nucleophilic substitution

Answer 6

Only one of the enantiomers will be formed as the product. The product will be optically active, if there is a chiral carbon present.

Question 7

Stereochemistry of product formed from SN1 nucleophilic substitution

Answer 7

The carbocation is trigonal planar with respect to the electron-deficient carbon. Hence, the nucleophile is able to attack the electron-deficient carbon from the top and bottom plane with equal probability, forming a racemic mixture.

Question 8

Why SN1 is preferred in terms of stability of carbocation intermediate (usually for tertiary halogenoalkanes)

Answer 8

1. Alkyl groups donate electrons to the carbocation and help to stabilised it.
2. Stability of carbocation increases in the order primary

Question 9

Why SN2 is preferred in terms of steric hindrance of the halogenoalkane (usually for primary halogenoalkanes)

Answer 9

1. The more alkyl groups there are around the central atom, the more crowded the transition state and the higher the activation energy.
2. The rate of SN2 will increase in the order tertiary < secondary < primary

Question 10

Reagent and conditions to form alcohols

Answer 10

Reagent and Condition: Dilute NaOH, heat

Mechanism: Nucleophilic substitution

Question 11

Reagent and condition to form nitrile

Answer 11

Reagent and condition: Ethanolic KCN, heat

Mechanism: Nucleophilic substitution

Question 12

Reagent and condition to form amines

Answer 12

Primary amine: Ethanolic concentrated NH3, heat in sealed tube

Mechanism: Nucleophilic substitution

Question 13

Reagent and conditions to form alkenes

Answer 13

Reagent and conditions: Ethanolic KOH, heat

Mechanism: Elimination

Question 14

Reactivity of halogenarenes vs halogenoalkanes

Answer 14

Halogenoarenes are less susceptible to nucleophilic substitution than halogenoalkanes

1. Lone pair of electrons on halogen atom delocalizes into benzene ring -> Partial double bond character in C-X bond which is shorter and stronger -> Very difficult to break
2. π electron cloud of benzene ring will repel the lone pair of electrons of an incoming nucleophile

Question 15

Distinguishing test for halogenoalkanes: Colour of AgX precipitate

Answer 15

Steps 1: Add NaOH (aq) and heat
Step 2: Add excess dilute HNO3
Step 3: Add AgNO3(aq) and observe the colour of the precipitate

Question 16

Explain the difference in the rate of formation of AgX precipitate.

Answer 16

1. Breaking of the C-X bond is the rate-determining step
2. The stronger the C-X bond, the higher the activation energy of the rate-determining step and hence it will take place at a slower rate.

Question 17

Use of halogenoalkanes

Answer 17

1. Refrigerants
2. Aerosol propellants
3. Fire extinguishers

Question 18

Effects of chlorofluorocarbons (CFCs) on ozone layer

Answer 18

1. Strong UV radiation will cleave the weaker C-Cl bond in CFCs homolytically to generate a reactive Cl radical
2. Catalyses the decomposition of ozone into oxygen, leading to depletion of ozone layer