4.2.2 Haloalkanes Flashcards
(23 cards)
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
