4.2.2 Haloalkanes
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Define nucleophile
• atom or group of atoms which is** attracted to an electron deficient carbon atom ** • where it donates a pair of electrons to form a new covalent bond
Give three nucleophiles
hydroxide ions, water molecules, ammonia molecules
Define nucleophillic substitution
nuceophille** attacks a carbon atom which has a partial positive charge **- primary haloalkanes undergo a variety of reactions
Why do haloalkanes undergo nucleophillic substitution?
• carbon-halogen bond is polar • partially positive carbon is electron deficient • so it can be attacked by a nucleophile
Describe how how haloalkanes can be hydrolysed into alcohols - what type of reaction is it? What conditions are needed?
nucleophillic substitution -needs aqueous alkali 1. nucleophile approaches carbon atom attached to the halogen 2. moves to the opposite side which minimises repulsion between nucleophile and partially negative halogen ion 3. lone pair of electrons on OH- ion is donated to slight positive carbon ion 4. bond forms between oxygen and carbon 5. carbon-hydrogen bond breaks via heterolytic fission - an alcohol and halide ion form
How does hydrolysis with water compare to using an aqueous alkali?
much slower as water is a weak nucleophile
What factor influences rate of hydrolysis of haloalkanes?
strength of the carbon-halogen bond
How does rate of hydrolysis vary in the haloalkanes?
• iodoalkanes have the** weakest bond enthalpy** so less energy is required to break these bonds compared to other bonds so… • iodoalkanes react faster than bromoalkanes • bromoalkanes react faster than chloroalkanes • fluoroalkanes are unreactive as the bond enthalpy is too high
How can you measure the rate of hydrolysis in primary alkanes?
• carry out the reaction in the presence of aqueous silver nitrate and ethanol solvent (haloalkanes are insoluble in water) • halide ions react with silver ions to form a white/cream/yellow precipitate • record time taken for precipitate to form (iodobutane would form a precipitate faster than chlorobutane due to the weak C-I bond)
Describe the main uses of organohalogen compounds
pesticides - this is an issue as the compounds are not broken down naturally
How is ozone formed?
• UV breaks oxygen molecules into oxygen radicals • ozone forms from oxygen and oxygen radicals
How do CFC’s deplete the ozone layer?
• UV radiation breaks the carbon-halogen bonds (in CFC molecules) by homolytic fission to form radicals • chlorine radical is a very reactive intermediate • propogation 1 -chlorine radical reacts with ozone - breaks it down into oxygen • propogation 2 - chlorine radical is regenerated - attacks and removes another molecule of ozone
How do nitrogen radicals catalyse the breakdown of ozone?
Two step propogation
What is the overall reaction for ozone depletion?
O3 + O = 2O2