Halogenoalkanes

• Halogenoalkane basics

  • general formula: C(n)H(2n+1)X
  • bond polarity: C-X -> Cδ+-Cδ-
  • Cδ+ is electron deficient->attacked by electron-rich reagents
  • the reagents = nucleophiles
  • nucleophile = electron pair donor
  • can use this to predict reactivity
  • bond enthalpy decreases going down group
  • bond enthalpy overrides bond polarity in terms of reactivity

  A) 484

  B) 4

  C) very

  D) 338

  E) 3

  F) less

  G) 276

  H) 2.8

  I) less

  J) 238

  K) 2.5

  L) not

  
• Physical properties

  Solubility

  • only have dipole-dipole forces -> not polar enough to be soluble in water
  • soluble in organic solvents

  Boiling point

  • increases going down group
  • increases with increased chain length

  Melting point

  • higher than MP of alkanes of similar length
  • increased branching -> lower melting point (not packed closely together)
• Nucleophilic substitution

  nucleophiles: :OH-, :NH3, :CN-

  reagent: NaOH

  conditions: aqueous

  A) shared electron pair leaves with Br ion

  B) -ve lone pair moves to +ve region

  
• Types of halogenoalkane

  primary halogenoalkane = halogen bonded to 1 end of alkane

  • carbon attached to 1 other alkyl group

  secondary halogenoalkane = halogen attached to a carbon attached to 2 other alkyl groups

  tertiary halogenoalkane = halogen attached to a carbon which is attached to 3 other alkyl groups
• Elimination reaction

  • OH- can act as a base as well as a nucleophile
  • differing conditions determine an elimination or substitution reaction
  • conditions: hot, dissolved in ethanol
  • can only happen in 2° & 3° halogenoalkanes
  • can form many isometric products

  A) O's lone pair donated to bond with partially +ve H

  B) shared electron pair in C-H move to C-C to form C=C

  C) Br takes electron pair & leaves as Br: