Halogenoalkanes

Question 1

Why does boiling point increases down the halogen group

Answer 1

• number of electron increases, which results in a greater ease of distortion of the electron cloud
• more energy is required to overcome the stronger instantaneous dipole-induced dipole attraction between the molecules

Question 2

Preparations of halogenoalkane

Answer 2

• Free radical substitution from alkanes
• Electrophilic addition from alkanes (HX or X₂)
• Nucleophilic substitution from alcohols *( HX, PX₃, PCl₅, SOCl₂

Question 3

Chemical reaction of halogenoalkanes

Answer 3

• Elimination to form alkenes
• Nucleophilic substitution to form alcohols(hydrolysis)
• Nucleophilic substitution to form nitriles
  —Acid hydrolysis to form Carboxylic acid
  —Base hydrolysis to form Carboxylic acid
  —Reduction
• Nucleophilic substitution to form primary amines(Ammonolysis)

Question 4

Nucleophilic substitution mechanisms

Answer 4

• S_N2: Bimolecular Nucleophilic substitution mechanism
• S_N1: Unimolecular Nucleophilic substitution mechanism

Question 5

S_N2 mechanism
- factors affecting

Answer 5

• one step mechanism
• bimolecular
• reactivity: tertiary<secondary<primary<CH₃X
• steric hindrance: the more alkyl groups attached to the C atom bearing the X atom, the more crowded the electrophilic centre, more difficult for nucleophile to approach it
• weak electrophilic centre: the more electron-donating groups attached to the C atom bearing X atom, the less electron deficient is the electrophilic centre, hence less attractive nucleophiles

Question 6

S_N1

Answer 6

• two step mechanism forming carbocation intermediate
• unimolecular
• reactivity: CH₃X<primary<secondary<tertiary
  — a tertiary carbocation intermediate has more electron donating alkyl groups bonded to C+
  — positive charge is dispersed to a greater extent
  — a more stable carbocation intermediate is formed

Question 7

Stereochemistry of S_N1 reaction(chiral carbon)

Answer 7

• a racemic mixture is formed
• since carbocation intermediate has a trigonal planar structure, the nucleophile attacks from either side of the plane with equal probabilities, yielding equal quantities of both enantiomers, thus racemic mixture is formed

Question 8

Reactivity among different alkyl halides in Nucleophilic substitution

Answer 8

• the weaker the C-X bond and the greater the stability of X^- formed, the more reactive is the halogenoalkane

Question 9

Factors affecting reactivity among halogenoalkanes

Answer 9

• strength of C-X bond
• bond energy of C-X bond decreases for F to I
• ease of breaking C-X bond increases
• formation of X^- will be faster
• reactivity decreases in the order RI>RBr>RCl>RF

Question 10

Preparation of halogenoarene

Answer 10

• Electrophilic substitution from benzene

Question 11

Why is halogenoarene less susceptible to Nucleophilic substitution

Answer 11

• delocalisation of lone pair of electrons on halogen
  — lone pair of electrons on X delocalise into benzene ring
  — partial double bond to the C-X bond, C-X bond is stronger and more difficult to cleave
  — reduced partial positive charge on C of C-X bond
  — C is less susceptible to attack by nucleophiles
• steric hindrance
  — rear side of C-X bond in halogenoarene sis blocked by benzene ring
  — pie electron cloud of benzene will repel the lone pair of electrons of the incoming nucleophile, making it difficult for the nucleophile to approach
• hybridisation state of C

Question 12

Reactivity of halogenoarene towards elimination

Answer 12

• do not undergo
• disrupt the stable delocalised pie electron system of the benzene ring which require a lot of energy

Question 13

Electrophilic substitution of halogenoarene

Answer 13

• halogen is deactivating and 2-,4- directing