haloalkanes

• haloalkanes are a homologous series of saturated carbon compounds containing one or more halogen atoms
• they're regarded as substituted alkanes because one or more hydrogen atoms has been replaced by a halogen atom
• haloalkanes have many uses including:

  • anaesthetics
  • pharmaceuticals
  • flame retardants
  • solvents
• in the naming of haloalkanes, the halogens are treated as branches and the naming is done in the same way for branched alkanes
• monohaloalkanes is where one hydrogen atom is substituted for one halogen atom
• monohaloalkanes can have three different structural types which are primary, secondary and tertiary. This is determined by the number of alkyl groups attached to the carbon atom that is directly attached to the halogen atom
• due to the polar nature of the carbon-halogen bond, haloalkanes are susceptible to nucleophilic attack
• due to the presence of the slight positive charge on the carbon atom, the monohaloalkanes act as electrophiles
• the nucleophile donates a pair of electrons forming a covalent bond with the carbon atom of the C-X bond
• at the same time, the halogen atom is thrown out as a halide ion and is replaced or substituted by the nucleophile
• monohaloalkanes undergo nucleophilic substitution reactions with:

  • aqueous alkalis to form alcohols (a solution of aqueous potassium hydroxide or sodium hydroxide is used and the nucleophile is the OH-)
  • alcoholic alkoxides to form ethers (alkoxides are formed when an alkali metal is added to an alcohol and the methoxide ion is the nucleophile)
  • ethanoic potassium or sodium cyanide to form nitrates (the cyanide ion is the nucleophile, CN-)
• the end nitrile formed in the reaction of ethanoic potassium or sodium cyanide to form nitrates contains one more carbon atom than the original monohaloalkane which makes the reaction very simple in synthetic organic chemistry because its a way to increase the chain length of an organic compound

  the nitric can also be converted into the corresponding carboxylic acid by acid hydrolysis i.e. reaction with water catalysed by hydrogen ions from the acid
• haloalkanes are heated under reflux with a strong base like ethanoic potassium or sodium hydroxide to form alkenes
• the elimination of hydrogen halides from some monohaloalkanes can result in the formation of two alkenes

  the resulting alkene can be tested using bromine water which will rapidly decolourise in the presence of an unsaturated hydrocarbon
• the presence of halogen ions can be tested by reacting the substance with silver nitrate solution

  the silver halide is precepted due to them being insoluble, the principate colour is what tells you the halide ion was originally present in the substance
• a monohaloalkane will undergo nucleophilic substitution by one of two different reaction mechanisms: sn1 or sn2
• sn1 is a two-step process
• in the first step of the sn1 mechanism, the carbon-halogen bond breaks heterolytically to form a negative halogen atom and a trigonal planar carbocation intermediate which has 3 carbon groups attached
• the first step of the sn1 mechanism is the 'slow step' and is called the rate determining step
• the second step of the sn1 process is fast so it isnt included in the rate order
• the second step of the sn1 mechanism is when the nucleophile is added
• the reaction is overall first order, and is most likely for tertiary monohaloalkanes and least likely for primary monohaloalkanes

  this is because tertiary haloalkanes produce the most stable carbocations and also due to steric hindrance in the sn2 route
• in the sn2 reaction, the nucleophile forms a bond with the carbon attached to the halogen atom

  at the same time the carbon-halogen bond breaks which results in a trigonal bipyramidal transition state which has 5 groups attached to the carbon atoms
• as two molecules are involved in the rate determining step, the sn2 reaction is in second order overall
• primary haloalkanes usually follow sn2 mechanism because a less stable carbocation is formed
• when a nucleophile attacks a primary haloalkane it approaches the positive carbon from the side away from the halogen atom
• alkyl groups are said to have a positive indictive effect which means they're electron donating and can push electrons onto the positively charged carbon atom which stabilises the carbocation
• the size of the alkyl groups is very important and is known as steric hindrance effect
• in the sn2 mechanism, the nucleophile attacks the carbon atom in the C-X bond from the side opposite to the halogen atom

  in a tertiary haloalkane, attack from that side is likely to be hindered because 3 alkyl groups will limit access to the C+ atom

  primary haloalkanes have no more than 1 alkyl group attached to the halogen bearing carbon atom so the access to the C+ atom will be much easier
• tertiary carbocations with their 3 alkyl groups are the most stable, and primary carbocations with just 1 alkyl groups are the least stable species