Organic Halogen Compounds

• Organic Halogen compounds contain one or more halogen atoms (F, Cl, Br or I) attached to a hydrocarbon chain.
• Organic halogen compounds don’t occur naturally.
• The halogen atom modifies the un-reactive hydrocarbon chain, making it reactive.
• halogenoalkanes are the simplest form of organic halogen compounds (halogen atom attached to an alkane chain).
• The halogenoalkanes are named after their parent alkanes, using the same rules as learnt in Developing fuels:
• The properties of the halogenoalkanes depend upon which halogen atom they contain.

Physical properties

• The carbon-halogen bond is polar, but it doesn’t make a big difference to the physical properties of the compounds. All halogenoalkanes are immiscible with water, but dissolve in organic compounds.
• Their boiling points depend upon the size and the number of halogen atoms present: an increase in the size of the halogen atom and the more of them there are, the higher the boiling point.
• The influence of halogen atoms on the boiling point is important when it comes to designing compounds for particular purposes (i.e. refrigerants).
• If you want a halogenalkane with a high boiling point, you would add a larger halogen atom (i.e. one from lower down the group).
• However this causes a problem, as the heavier ones cause environmental damage.
• These factors have to be considered when looking for a replacement for CFC’s.

Reactions with halogenoalkanes

• Reactions of the halogenoalkanes involve the breaking of any carbon-halogen bond, which can break both homolytically or heterolytically.
• Homolytic fission forms free radicals. It occurs when the correct frequency of radiation is absorbed by the halogenalkane:
  CH₃Cl + hv CH₃^. + C^.
  
• This reaction occurs in the stratosphere where the halogenoalkanes are exposed to UV radiation.
• This reaction forms chlorine free radicals which catalyse the decomposition of the ozone layer.
• Heterolytic fission is common under ambient conditions in the troposphere; the reactions tend to be carried out in a solution.
• The carbon-halogen bond is already polar and it can break when enough energy is provided forming a negative halide ion and a carbocation (positively charged carbon atom):
  CH₃Cl CH₃⁺ + Cl^-
  
• These ions don’t last for long, as they react with the other substances in the solution.

Reactivity of the halogenoalkanes

• All reactions of halogenoalkanes involve the breaking of the carbon-halogen bond.
• The stronger the C-Hal bond is, the harder it is to break.
• The higher the bond energy, the stronger the bond. The bond energies are stronger higher up the group, as the halogens are smaller.
• The high bond energy of C-F bond makes it hard to break; this is why fluoro compounds are very un-reactive.
• As you move down the group seven elements, the C-Hal bond gets weaker, thus the compounds get more reactive.
• Chloro compounds are un-reactive and so stay in the environment for a long time.
• This means that substances such as CFC’s can last long enough to make it into the stratosphere, where they catalyse the destruction of the ozone layer.
• Bromo and Iodo compounds are fairly reactive, which makes them useful as intermediates in organic compound synthesis.

Substitution Reactions

• Substitution reactions are common with the halogenoalkanes.
• An example of a substitution reaction is when a halogenalkane and hydroxide ions are reacted together. The halogen alkane is hydrolysed to form an alcohol:
  CH₃-CH₂-CH₂-CH₂-Br + OH^- CH₃-CH₂-CH₂-CH₂-OH + Br^-
  
• This substitution reaction occurs, because the C-Br bond is polar and the oxygen atom within the hydroxide group is negatively charged.
• The partial positive charge on the carbon atom attracts the negatively charged oxygen of the hydroxide ion. A lone pair of electrons on the oxygen atom forms a bond with the C atom as the C-Br bond breaks.
• The reaction involves heterolytic fission as ions have been formed rather than free radicals.
• A carbocation is not formed as the OH^- ion reacts at the same time as the C-Br bond breaks.
• A full pair of electrons has moved in the reaction, rather than just one electron during homolytic fission.
• halogenoalkanes can react with many other reagents other than hydroxide ions. All that is needed is a group carrying a pair of electrons to start forming a bond to the carbon atom.
• These attacking groups which donate a pair of electrons to a positively charged carbon atom to form a covalent bond are called nucleophiles.
• These nucleophiles can either be negative ions like OH or molecules with a lone pair of electrons (H₂O and NH₃ etc.)
• The table to the right shows some common nucleophiles.
• The carbon atom that is attacked by the nucleophile can either be part of a carbocation (carrying a full positive charge) or it may be part of a neutral molecule (such as in the bromobutane reaction previously) carrying a partial charge due to bond polarisation.
• The general nucleophilic substitution can be described by the reaction below (X- represents the nucleophile):
• When a nucleophile reacts with a halogen alkane, the following reaction occurs:
  R-Hal + X^- R-X + Hal^-
  

Water acting as a nucleophile

• A neutral molecule can act as a nucleophile in addition to just negatively charged molecules.
• Provided that the molecule has a lone pair of electrons it can act as a nucleophile.
• Water molecules have a lone pair of electrons on the oxygen atom (see table above).
• Water can act as a nucleophile, attacking a halogen alkane molecule.
• The reaction occurs in two stages:

Ammonia acting as a nucleophile

• As ammonia has a lone pair of electrons it can also act as a nucleophile:
• The lone pair of electrons on the N atom attacks the halogenalkane. The product is an amine with an NH_{2 group.
  R-Hal + NH3 R-NH3⁺ + Hal^- R-NH2 + H⁺ + Hal^-}
  

Nucleophilic substitution to make halogenoalkanes

• When a halogenalkane reacts with OH^- ions, a nucleophile reaction occurs and an alcohol is formed.
• The reverse of this reaction can be used to form a halogenalkane from an alcohol, providing the conditions are correct.
• It is another nucleophilic reaction; however it is the Hal- that is the nucleophile.
• The overall reaction:
  CH₃CH₂CH₂CH₂OH + H⁺ + Br^- CH₃CH₂CH₂CH₂Br + H₂O

Useful books for revision:

Revise AS Chemistry for Salters (Written by experienced examiners and teachers of Salter's chemistry)
Revise AS Chemistry for Salters (OCR) (Salters Advanced Chemistry)

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