Chemistry - Halogenoalkanes

• Define halogenoalkanes

  Part of a homologous series where one or more hydrogen atoms in the alkane have been replaced by a halogen atom

  Commonly bromine, chlorine and iodine

  Carbon halogen bond is polar
• Why is the carbon halogen bond polar?

  Halogens are more electron and carbon is more susceptible to nucleophilic attack
• Define nucleophilic attack

  When an electron rich species ( nucleophile ) attacks an electron deficiency species ( electrophile )
• What is the elimination reaction?

  The loss of a small molecule to produce a double bond

  halogenalkanes undergo elimination to create an alkene

  Requires ethanolic sodium hydroxide ( sodium hydroxide dissolved in ethanol )
• How does unsymmetrical halogenoalkanes affect affect elimination reactions?

  Makes more than one Alkene

  Double bond can fall on either side of the eliminated halogen group
• What is the test for halogenalkenes

  Add aqueous Sodium hydroxide

  Neutralise with aqueous silver nitrate

  If present a precipitate will form

  Test with ammonium to prove halogen

  Cl - white - dissolves in dilute ammonia

  Br - cream - dissolves in concentrated ammonia

  I - yellow - does not dissolve in ammonia
• what happens during nucleophilic substitution

  an OH- replaces a halogen group

  creates an alcohol and halogen ion
• what must be shown in the exam for a nucleophilic substitution

  partial charges

  lone electrons of nucleophile

  curly arrows to show movement of electrons

  second curly arrows from centre of carbon-halogen bond to halogen
• how is nucleophilic substitution prepare alcohols

  adding OH- nucleophile by aqueous sodium hydroxide

  
  

  through reflux ( continuous evaporation and condensation )

  used hot plate / heating mantel to prevent alcohol igniting

  water filtered from bottom of reflux tube for water to fill upwards

  
  

  can also be classed as hydrolysis as the OH- can be obtained by teh addition of water
• what two factors are effected by changing a halogen dependant on whether chloro , bromo or iodo is used

  electronegativity

  bond strength
• how is electronegativty effected by changing a halogen in halogenalkanes

  decreases as halogen size increase

  theoretically = C-Cl is most polar so would break the easiest
• how is bond strength effected by changing a halogen in halogenalkanes

  strongest bond = C-Cl so therefore is the most difficult to break

  fastest to break = iodine
• how can the rate of reaction be detected once a halogen has been changed

  after hydrolysis a halide ion is formed in the solution

  detected by adding ammonia and timing how long it takes for a precipitate to form ( disappearing cross or colorimeter )
• what are the 3 uses of halogenalkanes

  solvents

  anaesthetics

  refrigerants
• describe the use of halogenalkanes - solvents

  the polar and non-polar sections of halogenaklanes are able to mix with polar and non-polar sections of organic substances ( water ) and therefore become solvents

  non-flammability - used for dry cleaning
• describe the use of halogenalkanes - anaesthetics

  chloroform - first anaesthetic - revolutionised surgical procedures

  ' knock out ' drug
• describe the use of halogenalkanes - refrigerants

  prenement dipole -dipole attraction presense means boiling points are very close

  liquids can evaporate easily ( vise versa )

  CFCs = heated needed to change state is removed from the fridge to cool

  non-flammable

  non-toxic
• what are the regulations for uses of halogenalkanes

  limited use due to statutory regulations

  polycholoroalkanes = toxic

  CFCs = damage the ozone layer and allows UV radiation to reach the earth's surface and cause skin cancer

  cause a radical chain reaction

  alternative - HCFs = radicals are not formed by exposed to UV radiation