Halogenoalkanes

• What is the general formula of a halogenoalkane

  R - X
• Describe the bond polarity of halogenoalkanes

  - they have a polar bond

  - because the halogens are more electronegative than the carbon
• Explain the polarity of the C-Br bond in 2-bromopropane (1)

  - Br is more electronegative than carbon so Br has a slight negative charge and carbon has a slight positive charge
• Describe the solubility of a halogenoalkane

  - not soluble in water - the polar bond is not polar enough

  - main intermolecular forces are dipole-dipole attractions and van der waals forces

  - halogenoalkanes mix with hydrocarbons so they can be used as dry cleaning fluid
• Describe the boiling point of halogenoalkanes

  - depends on the number of carbon atoms and halogen atoms

  - boiling point increases with increased chain length

  - boiling point increases going down the halogen group
• Explain the trend in boiling points: fluoromethane (-78) Iodomethane (4)

  - the molecules can pack less tightly together

  - due to branched structure

  - the van der waals forces increases so the boiling point increases so requires more energy to overcome
• Describe ozone


  - 3 oxygen atoms- O3

  - decomposes into oxygen
• Describe the problems of ozone at ground level

  - causes lung irritation

  - causes degradation of plastics
• Why is the ozone layer important

  - absorbs UV radiation

  - too much UV radiation causes skin cancer by damaging DNA
• How do chlorine free radicals form?

  - C-Cl bond breaks homolytically

  - in presence of UV radiation (causes skin cancer)
• State the propagation reactions

  Cl. + O3 ———> ClO. + O2

  ClO. + O3 ———> 2O2 + Cl.

  - resulting free radicals get regenerated

  - chlorine free radicals acts as a catalyst in the breakdown of ozone, causing a hole in the ozone layer
• State the overall reaction

  2O3 ———> 3O2
• Physical properties of halogenoalkanes

  - solubility

  - boiling points

  - branching - more branches lowers the boiling point - atoms cant pack closely together
• Describe solubility of halogenoalkanes

  - not soluble in water because polar bonds aren't polar enough

  - IMF are van der waals and dipole dipole attractions
• What determines how the C-X bond reacts?

  - the bond polarity

  - the bond enthalpy
• Describe the bond polarity of halogenoalkanes

  - halogens are more electronegative than carbon

  - so nucleophiles can attach to slightly positive carbon atom
• Describe the bond enthalpy of halogenoalkanes

  - the C-X bond gets weaker going down the group

  - fluorine is the smallest halogen and the shared electron in the bond is more attracted to fluorine's nucleus

  - this makes it a strong bond, but going down group 7, the shared electron is further away and so attraction is weaker

  - reactivity increases going down the group
• Define nucleophile

  - an electron pair donor

  - either a negative charged ions

  - has a lone pair of electrons - used to form a covalent bond

  - lone pair is on a electronegative atom
• Define substitution

  - a replacement of the halogen atom by the nucleophile
• Give a suitable reagent that can be added separately to each compound to distinguish between them - describe what you would see

  - AgBr + AgI

  AgBr:

  -reagent: concentrated ammonia

  -observation: cream precipitate dissolves

  AgI:

  - insoluble in concentrated ammonia
• Describe nucleophillic substitution

  It is a mechanism where the nucleophile will replace the halogen in a halogenoalkane

  Uses hydroxide ion, ammonia, or cyanide ion as a nucleophile

  
• describe the conditions and reagents needed during nucleophillic substitution with an hydroxide ion

  conditions:

  • aqueous
  • warm

  reagents:

  • potassium hydroxide or sodium hydroxide
• describe the conditions and reagents needed during nucleophillic substitution with a cyanide ion

  conditions:

  • aqueous
  • heat under reflux

  Reagents:

  • potassium cyanide
• describe the conditions and reagents needed during nucleophillic substitution with ammonia

  conditions:

  • excess ammonia - to prevent further substitution from occurring
  • heat under reflux
  • heat under pressure

  reagents:

  • ammonia
• Why is does the solutions need to be aqueous during nucleophillic substitution?

  If the solvent is changed to just ethanol then an elimination reaction occurs
• What is the product of elimination?

  Product is an alkene
• what are the conditions and reagents of elimination?

  conditions:

  • NaOH/KOH is dissolved in ethanol, ethanolic solvent
  • the mixture is heated

  reagents:

  ethanolic solvent
• describe the test to show whether a solvent involves an alkene

  ethene decolourises a bromine solution

  showing it has a carbon carbon double bond
• What determines if the reaction will go through nucleophillic substitution or elimination?

  Depends on:

  the conditions of the reaction (aqueous/ethanolic solution)

  hydroxide ions at room temperature dissolved in water favour substitution

  hydroxide ions at room temperature dissolved in ethanol favour elimination
• Describe how primary, secondary and tertiary halogenoalkanes react

  primary halogenoalkanes react by substitution only

  secondary halogenoalkanes react by substitution and elimination

  tertiary halogenoalkanes react by elimination only
• Halogenoalkanes undergo which 2 types of mechanisms?

  nucleophillic substitution

  elimination
• What does the rate of substitution reactions depend on?

  The strength of the C-X bond

  The weaker the bond, the easier it is to break and so the reaction is faster

  so fluoroalkanes are very unreactive
• Describe water in nucleophillic substitution reactions

  It is a poor nucleophile

  It can react slowly with halogenoalkanes in a substitution reaction
• Describe a test for halides

  Aqueous silver nitrate is added to a halogenoalkane

  The halide leaving group combine with a silver ion to form a silver halide precipitate

  The precipitate only forms when the halide ion has left the halogenoalkane and so the rate of formation of precipitate can be used to compare the reactivity of the different haloalkanes

  The quicker the precipitate formed, the faster the substitution reaction and the more reactive the halogenoalkane
• what is the change in functional group in a nucleophillic substituition reaction with aqueous hydroxide ions?

  Halogenoalkane ——> alcohol

  Type of reagent: nucleophile (hydroxide ions)
• what is the change of functional group in a nucleophillic substitution reaction with cyanide ions?

  halogenoalkanes——>nitrile

  type of reagent: nucleophile (cyanide ions)
• what is the change of functional group in a nucleophillic substitution reaction with ammonia?

  halogenoalkane——>amine

  type of reagent: nucleophile (ammonia ion)
• why do we need excess ammonia?

  to prevent further substitution from occurring

  otherwise it leads to a lower yield of the amine
• define elimination

  removal of a small molecule (often water) from the organic molecule
• what is the change of functional group in an elimination reaction with alcoholic hydroxide ions?

  halogenoalkane ——> alkene

  type of reagent: base (hydroxide ion)