AQA A Level Chemistry - Haloalkanes

what are the 3 ‘classifications’ of haloalkanes?

• primary

• secondary

• tertiary

what do the classifications of haloalkanes mean?

• primary - carbon bonded to halogen is bonded to 1 other carbon

• secondary - carbon bonding to halogen is bonded to 2 other carbons

• tertiary - carbon bonded to halogen is bonded to 3 other carbons

what is the order of reaction rate of different classifications of haloalkanes?

• tertiary - react fastest

• secondary - middle

• primary - react slowest

why do tertiary haloalkanes react faster than primary haloalkanes?

• tertiary haloalkanes react with Sn1 mechanism

• primary react with Sn2 mechaism

how does the polarity of the C-X bond change down group 7?

• as you go down group 7, polarity decreases

• this is because electronegativity decrease down the group

what is the order of reaction rate for different C-X bonds?

• C-I → fastest

• C-Br → middle

• C-Cl → slowest

• C-F → doesn’t react

what are the two factors that affect rate of reaction of different C-X bonds/haloalkanes?

• bond polarity (much less significant)

• bond enthalpy (v. significant)

how does bond polarity affect rate of breaking of C-X bond?

• increasing bond polarity means bond is more likely to break

• this is because bond attracts nucleophiles more

• relatively insignificant

how does bond enthalpy change down group 7?

• as you go down group 7, bond enthalpy increases

• this is because the X atoms get larger, and have a greater Ar

• so the C and X atoms cannot get as close together

• so there is more distance between the nucleus of 1 atom and the outer e- of the other

how does boiling point change across the haloalkanes?

• increases with increasing chain length

• increases going down g7

why does the boiling point increase with increasing chain length?

• longer chain means greater Mr

• also means more points of contact

• VDW forces increase, so require more energy to overcome

why does the boiling point increase down group 7?

• permanent dipole-dipole interactions increase as bond gets more polar (insignificant)

• Ar of halogen increases, increasing Mr of molecule so increasing VDW forces increasing boiling point

describe the solubility of haloalkanes

• although polar, they are insoluble or very slightly soluble in water

• soluble in organic solvents so used for dry cleaning fluid/degreasing agents

what is another term for breaking covalent bonds?

fission

what are the two types of fission?

• homolytic

• heterolytic

what is homolytic fission?

• both electrons go to one atom

• creates 2 ions

• uses curly arrow

what is heterolytic fission?

• one electron goes to each atom

• always forms 2x free radicals

• uses fish hook arrow

what does a curly arrow represent?

movement of a pair of electrons

what is free radical substitution?

• reaction of chlorine and alkane in atmosphere to form chloroalkane and HCl gas

• requires presence of UV light

give the mechanism for free radical substitution of methane

what is the ozone layer and what does it do?

• ozone = O3

• it absorbs UV light, preventing some of it from reaching earth

• reduces sunburn, skin cancer, cataracts

what is ozone depletion caused by?

• CFCs

• break down into Cl radicals

• (F radicals not formed)

what were CFCs used for and why?

• used in fridges, propellants in aerosols, flame retardants

• non toxic

• non flammable

• inert

what is a better alternative to CFCs?

• HCFs

• no chlorine = no chlorine radicals

• F free radicals don’t form

what is the initiation step for the mechanism of CFC ozone depletion?

what is the propagation step for CFC ozone depletion?

what is the termination step for CFC ozone depletion?

what is the overall equation for CFC ozone depletion?

2O₃ → 3O₂

what is the other radical that can cause ozone depletion?

nitrogen monoxide

give the mechanism for ozone depletion by the nitrogen monoxide radical

why were CFCs banned?

• research by different groups in scientific community provided legislation to ban CFCs

• 200 countries have pledged to reduce use in Montreal Protocol

• alternatives have been synthesized

what is a nucleophile?

electron pair donor

what are the 3 nucleophiles that react with haloalkanes?

• hydroxide

• cyanide

• ammonia

what are the conditions for the reaction of a hydroxide nucleophile with a haloalkane?

• uses AQUEOUS NaOH or KOH

• reflux to increase temp to increase rate

• haloalkane dissolved in a little ethanol (ethanolic)

what is the general equation for the reaction of a hydroxide with a haloalkane?

haloalkane + sodium hydroxide → alcohol + sodium halide

give the mechanism for the reaction of bromoethane with hydroxide

what are the conditions required for nucleophilic substitution of cyanide?

• aqueous solution of KCN

• ethanolic haloalkane

• reflux gently

what is the general equation for the reaction of a haloalkane with cyanide?

• haloalkane + potassium cyanide → nitrile + potassium halide

• R-CH2X + KCN → R-CH2CN + KX

what is the mechanism for nucleophilic substitution of cyanide?

what are the conditions required for nucleophilic substitution of ammonia?

• concentrated ammonia solution

• ethanolic haloalkane

• sealed container under pressure

give the general equation for the reaction of ammonia with a haloalkane

• haloalkane + ammonia → primary amine + ammonium halide

• R-CH2X + 2NH3 → R-CH2NH2 + NH4X

give the mechanism for the nucleophilic substitution of ammonia to bromoethane

describe the conditions required for elimination of a halogen

• ethanolic NaOH/KOH (NOT AQUEOUS)

  • this causes the OH- ion to act as a base and accept a proton to become water

• hot, reflux

give the general equation for elimination of a haloalkane

• haloalkane + potassium hydroxide → alkene + potassium halide + water

• R-CH2CH2X + KOH → R-CH=CH2 + H2O + KX

give the mechanism for elimination of bromoethane

• C-Br bond breaks as enthalpy < C-H

how does isomerism of the alkenes produced occur through elimination?

• on 4+ carbon chain haloalkanes, isomers can be produced

• depends on which H the hydroxide targets - either side of the Br

describe how you would carry out a practical to compare the rates of hydrolysis of haloalkanes

• add 1 cm³ ethanol to 3 drops haloalkane

• stand test tubes in 60C water bath

• add test tube of silver nitrate to water bath

• add 1cm³ of silver nitrate to each tube and time each until precipitate forms

  • chloride - white ppt

  • bromide - cream ppt

  • iodide - yellow ppt