Haloalkanes

• Haloalkanes are compounds containing the elements carbon, hydrogen and at least one halogen
• when naming haloalkanes:

  • prefix is added to the name of the longest chain to indicate the identity of the halogen
  • fluoro
  • chloro
  • bromo
  • iodo
  • when 2 or more halogens are present in a structure - listed in alphabetical order
• can be classed as primary secondary and tertiary
• Reactivity of the haloalkanes:

  • have a carbon-halogen bond in their structure
  • halogen atoms are more electronegative than carbon atoms
  • electron pair in the carbon-halogen bond is therefore closer to the halogen atom than the carbon atom
  • bond is polar
• • in haloalkanes the carbon atom has a slightly positive charge and can attract species containing a lone pair of electrons
  • species that donate a lone pair of electrons are known as nucleophiles
• a nucleophile is an atom or group of atoms that is attracted to an electron deficient carbon atom, where it donates a pair of electrons to form a new covalent bond
• nucleophiles include:

  • hydroxide ion
  • water molecules
  • ammonia molecules
• When a haloalkane reacts with a nucleophile, the nuclephile replaces the halogen in a substitution reaction

  • a new compound is produced containing a different functional group
  • nucleophilic substitution
• nucleophilic substitution in haloalkanes:

  • primary haloalkanes undergo nucleophilic substitution reactions with a variety of different nucleophiles to produce a wide range of different compounds
  • substitution is a reaction in which one atom or group of atoms is replaced by another atom or group of atoms
• Hydrolysis is a chemical reaction involving water or an aqueous solution of a hydroxide that causes the breaking of a bond in a molecule - results in the molecule being split into 2 products
• in the hydrolysis of a haloalkane, the halogen atom is replaced by an -OH group - an example of a nucleophilic substitution reaction
• 1. the nucleophile OH- approached the carbon atom attached to the halogen on the opposite side of the molecule from the halogen atom
  2. this direction of attack by the OH- ion minimises repulsion between the nucleophile and the delta- halogen atom
  3. a lone pair of electrons on the hydroxide ion is attracted and donated to the delta+ carbon atom
  4. a new bond is formed between the oxygen atom of the hydroxide ion and the carbon chain
  5. the carbon-halogen bond breaks by heterolytic fission
  6. the new organic product is an alcohol
  7. a halide ion is also formed
• haloalkanes can be converted to alcohols using aqueous sodium hydroxide:

  • the reaction is very slow at room temp
  • so mixture heated under reflux to obtain a good yield of product
• Hydrolysis and carbon-halogen bond strength:

  • in hydrolysis, the carbon-halogen bond is broken and the -OH group replaces the halogen in the haloalkane
  • the rate of hydrolysis depends on the carbon-halogen bond in the haloalkane
• • C-F bond is the strongest
  • C-I bond is the weakest
  • so less energy is required to break the C-I bond than other carbon-halogen bonds
• • iodoalkanes react faster than bromoalkanes
  • bromoalkanes react faster than chloroalkanes
  • fluoroalkanes are unreactive as a large quantity of energy is required to break the C-F bond
• Measuring the rate of hydrolysis of primary haloalkanes:

  • compare the rate of hydrolysis of 1-chlorobutane, 1-bromobutane, 1-iodobutane
  • CH3CH2CH2CH2X + H2O = CH3CH2CH2CH2OH + H+ + X-
  • rate of reaction can be followed by carrying out the reaction in the presence of aqueous silver nitrate
  • as the reaction takes place, halide ions (X-) are produced which react with Ag+ (aq) ions to form a ppt of silver halide
  • Ag+ (aq) + X- (aq) = AgX (s)
• • the nucleophile in the reaction is water - present in the aq silver nitrate
  • haloalkanes are insoluble in water and the reaction is carried out in the presence of an ethanol solvent
  • ethanol allows water and the haloalkane to mix and produce a single-solvent rather than 2 layers
• 1. set up 3 test tubes:
  2. add 1cm3 ethanol and 2 drops of either 1-chlorobutane, 1-bromobutane or 1-iodobutane
  3. stand the test tubes in water bath - 60 degrees
  4. place test tube containing 0.1 moldm-3 silver nitrate in water bath - allow all tubes to reach constant temp
  5. add 1cm3 silver nitrate to each test tubes - start stop watch
  6. observe test tubes for 5 min and record time taken for ppt to form
• 1-chlrobutane - white ppt forms very slowly

  1-bromobutane - cream ppt forms slower than with 1-iodobutane but faster than with 1-chlorobutane

  1-iodobutane - yellow ppt forms rapidly
• observations are explained by considering bond enthalpies of the carbon-halogen bonds - compound with the slowest rate of reaction is the one that has the strongest carbon-halogen bond:

  • 1-chlorobutane reacts slowest = C-CL bond is strongest
  • 1-iodobutane reacts fastest = C-I bond is weakest
• • strength of the carbon-halogen bond is not the only factor that influences the rate of hydrolysis - primary, secondary, tertiary
  • tertiary haloalkane is hydrolysed fastest
  • hydrolysis of the primary alkane is the slowest
  • main reason lies within reaction mechanism
• • primary alkane will react by a one-step mechanism
  • tertiary alkane will react by a two step mechanism
  • in the first step the carbon-halogen bond of the tertiary alkane breaks by heterolytic fission, forming a tertiary carbocation and halide ion
  • second step, hydroxide ion attacks the carbocation to form the organic product
• • increased rate and different reaction rates can be explained by the increased stability of the tertiary carbocation compared to that of the primary carbocation