4.2.2 Haloalkanes
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describe the trend in bp as the halogen in the haloalkane goes down the group
• higher bp • as you go down the group, the atom inc size + more e- so London Forces increase in strength > therefore bp increases
why are haloalkanes more reactive than alkanes
• the carbon-halogen bond is polar > halogen more electronegative than C so electron pair in the C-X bond is closer to X than C > C-X bond is polar
what is a nucleophile
• an electron pair donor > negative charge / electron rich > use lone pair e- to form new bond
why do haloalkanes attract nucleophiles
• the C atom has slightly positive charge + attracts species containing lone pair of electrons
what are some common nucleophiles
• hydroxide ions OH • water molecules H2O • ammonia molecules NH3
what happens when a haloalkane reacts with a nucleophile + name of mechanism
• the nucleophile replaces the halogen in a substitution reaction > a new compound is produced containing a diff functional group • nucleophilic substitution
what is hydrolysis
• chemical reaction involving H2O or aqueous solution pf hydroxide causing the breaking of a bond in a molecule
what happens in hydrolysis of haloalkanes
• the halogen atom is replaced by an OH group > nucleophilic substitution
why do haloalkanes become more reactive down the group
• because bond enthalpies decrease > so the C-X bond is weaker + less energy needed to break the bond
how can you measure the rate of hydrolysis in haloalkanes
• haloalkane in presence of aqueous AgNO3 > halide ions react with Ag ions to form silver halide precipitate > nucleophile is H2O in aqueous silver nitrate • haloalkanes insoluble in water so carried out in ethanol solvent • diff colour precipitates are formed > yellow formed first due to iodine as C-I bond weakest so broken first
what happens to the rate of hydrolysis in primary to tertiary haloalkanes
• rate of reaction increases
what are organohalogen compounds
• molecules containing at least 1 halogen atom joined to a carbon chain > found many practical uses > rarely found in nature as they’re not broken down naturally by the environment
what are some general uses of organohalogen compounds
• general solvents • dry cleaning solvents • making polymers • flame retardants • refrigerants
what is the ozone layer
• layer of gas found in upper limit of stratosphere > made of ozone, O3 • thin layer making small fraction of whole stratosphere
why is the ozone layer very effective
• can absorb damaging UV radiation (UV-B) > radiation most commonly leading to sunburn + skin cancer in extreme cases
what is ozone
• O3 > constantly being formed + broken down by UV radiation
how is O3 formed
• O2 molecules broken down by UV light into O radicals • O radicals + O2 molecules form O3
what are CFCs
• chloro-fluoro-carbons > haloalkanes with C + F • in past most commonly used chemical in refrigerants + air conditioning
why are CFC’s harmful
• CFC’s remain stable until they reach stratosphere > they begin to break down here forming CL radicals why catalyse the breakdown of the ozone layer
why are CFC stable
• due to the strong carbon-halogen bonds, making them last long
give the step by step by step process as to how CFC’s deplete the ozone layer
• UV radiation in stratosphere provides sufficient energy to break C-X bond in CFCs by homolytic fission to form radicals > radiation initiating the breakdown is called photodissociation • Cl radical formed reacts with Ozone molecule breaking ozone down into oxygen > breakdown happens in 2 steps
give the simple equation of the breakdown of the ozone layer
• initiation: CF2Cl2 —> (uv) CF2Cl. + Cl. • propagation 1: Cl. + O3 —> ClO. + O2 • propagation 2: ClO. + O —> Cl. + O2 • overall equation: O3 + O —> 2O2
give the equation of nitrogen oxide breaking down the ozone layer
NO. + O3 —> NO2. + O2 NO2. + O —> NO. + O2 O3 + O —> 2O2