Haloalkanes and haloarenes

• Haloalkanes


  Alkyl halides (R-X) where halogen atom is bonded to an alkyl group
• Haloarenes


  Aryl halides where halogen atom is directly bonded to an aromatic ring
• Classification of haloalkanes based on hybridisation


  • Primary alkyl halides (halogen attached to primary carbon)
  • Secondary alkyl halides (halogen attached to secondary carbon)
  • Tertiary alkyl halides (halogen attached to tertiary carbon)
  • Allylic halides (halogen attached to carbon adjacent to C=C)
  • Benzylic halides (halogen attached to carbon attached to aromatic ring)
• Classification of haloarenes


  • Vinylic halides (halogen attached to sp2 hybridised carbon of C=C)
  • Aryl halides (halogen directly bonded to sp2 hybridised carbon of aromatic ring)
• Geminal dihalides


  Dihaloalkanes with both halogen atoms on the same carbon
• Vicinal dihalides


  Dihaloalkanes with halogen atoms on adjacent carbons
• Haloalkanes and haloarenes are named using common names or IUPAC system
• Carbon-halogen bond is polarised due to higher electronegativity of halogen
• Bond length, bond enthalpy and dipole moment of C-X bond increases from C-F to C-I
• Preparation of haloalkanes from alcohols


  1. Reaction with conc. halogen acids

  2. Reaction with phosphorus halides

  3. Reaction with thionyl chloride
• Preparation of haloalkanes from hydrocarbons


  Free radical halogenation
• Preparation of haloalkanes from alkenes


  1. Addition of hydrogen halides

  2. Addition of halogens
• Preparation of haloalkanes by halogen exchange


  1. Finkelstein reaction (alkyl iodides)

  2. Swarts reaction (alkyl fluorides)
• Preparation of haloarenes


  1. Electrophilic aromatic substitution

  2. Sandmeyer's reaction (from aromatic amines)
• Preparation of Haloarenes


  1. Electrophilic substitution of arenes with chlorine and bromine in the presence of Lewis acid catalysts

  2. Separation of ortho and para isomers due to large difference in melting points

  3. Iodination reactions require an oxidising agent

  4. Fluoro compounds not prepared by this method due to high reactivity of fluorine
• Preparation of Haloarenes from amines by Sandmeyer's reaction


  1. Treat primary aromatic amine with sodium nitrite to form diazonium salt

  2. Mix diazonium salt with cuprous chloride or cuprous bromide to replace diazonium group with -Cl or -Br

  3. Replace diazonium group with iodine by shaking with potassium iodide
• Alkyl halides


  • Colourless when pure
  • Bromides and iodides develop colour when exposed to light
  • Many volatile halogen compounds have sweet smell
• Write structures of different dihalogen derivatives of propane
• Among the isomeric alkanes of molecular formula C5H12, identify the one that on photochemical chlorination yields: (i) A single monochloride, (ii) Three isomeric monochlorides, (iii) Four isomeric monochlorides
• Melting and boiling points


  • Methyl chloride, methyl bromide, ethyl chloride and some chlorofluoromethanes are gases at room temperature
  • Higher members are liquids or solids
  • Halogen derivatives have higher boiling points than parent hydrocarbons due to greater polarity and molecular mass
  • Boiling points decrease in the order: RI> RBr> RCl> RF
  • Boiling points decrease with increase in branching
  • Melting points of para-isomers of dihalobenzenes are higher than ortho- and meta-isomers
• Solubility


  • Haloalkanes are very slightly soluble in water
  • Haloalkanes tend to dissolve in organic solvents
• Density of haloalkanes increases with increase in number of carbon atoms, halogen atoms and atomic mass of halogen atoms
• Nucleophilic substitution reactions


  • Nucleophile replaces existing nucleophile in a molecule
  • Haloalkanes are the substrate
  • Nucleophile attacks the partially positive carbon atom bonded to halogen
  • Halogen atom departs as halide ion
• KCN forms alkyl cyanides as main product while AgCN forms isocyanides as the chief product
• SN2 mechanism


  • Bimolecular nucleophilic substitution
  • Incoming nucleophile interacts with alkyl halide causing C-halide bond to break and new C-nucleophile bond to form
  • Occurs in a single step with no intermediate
  • Configuration of carbon atom inverts
• SN1 mechanism


  • Unimolecular nucleophilic substitution
  • Occurs in two steps: (1) Slow cleavage of C-halide bond to form carbocation, (2) Nucleophile attacks carbocation
  • Rate depends on concentration of alkyl halide, not nucleophile
  • More stable carbocations form faster
• Order of reactivity: Primary halides > Secondary halides > Tertiary halides for both SN1 and SN2 reactions
• Step I is the slowest and reversible step
• Rate of reaction depends on the slowest step, i.e. concentration of alkyl halide and not on concentration of hydroxide ion
• Greater the stability of carbocation, greater will be its ease of formation from alkyl halide and faster will be the rate of reaction
• Order of reactivity of alkyl halides towards SN1 and SN2 reactions


  • 3° alkyl halides undergo SN1 reaction very fast
  • Allylic and benzylic halides show high reactivity towards the SN1 reaction
• Pairs of halogen compounds

  • CH3CH2CH2CH2Br vs (CH3)2CHCH2Br
  • CH3CH2CH2Br vs CH3CH2CH2I
• Primary halide undergoes SN2 reaction faster
• Iodine is a better leaving group than bromine, so it will be released at a faster rate in the presence of incoming nucleophile
• Order of reactivity of alkyl halides in SN1 and SN2 reactions

  • R-I > R-Br > R-Cl >> R-F
• Optical activity


  Rotation of plane polarised light produced by passing ordinary light through certain compounds
• Dextrorotatory (d-form)


  Compound that rotates the plane of plane polarised light to the right (clockwise direction)
• Laevorotatory (l-form)


  Compound that rotates the plane of plane polarised light to the left (anticlockwise direction)
• Optical isomers


  Stereoisomers that are non-superimposable mirror images
• Asymmetric carbon


  Carbon atom with four different substituents attached, resulting in non-superimposable mirror images