15.1 - Haloalkanes
naming - Primary = halogen - carbon 1 alkyl group
- Secondary – halogen – carbon + 2 alkyl groups
- Tertiary – halogen – carbon + 3 alkyl groups
reactivity - Haloalkanes have H + Halogen.
- Halogen is more electronegative than carbon atoms so e- pair in bond is closer to H atom = polar.
- Carbon is slightly +ve and can attract lone pair, species donating lone pair = nucleophiles.
- Nucleophiles – atom or group attracted e- deficient carbon to donate 2 e- to form covalent bond.
1) OH- (2 lone) 2) H2O (2 lone) 3)NH3 (2 lone)
- When haloalkane + nucleophile -> nucleophile replaces halogen = new func group (substitution)
Hydrolysis - Substitution = molecule being split into 2 products where halogen atom is replaced by OH-.
Nucleophilic 1)OH approaches opposite side of C atom with halogen. (arrow from OH with lone pairs and another arrow
substitution at C-halogen with delta charges)
2) direction minimises repulsion between OH and delta –ve halogen.
3) lone pair of e- on OH is attracted and donated to delta +ve atom.
4) new bond is formed between O in OH and C = C-halogen breaks by heterolytic fission -> alcohol + halide.
- Haloalkanes can be converted to alcohols with NaOH (aq) at room temp and is slow but heated
under reflux to obtain good yield.
- Carbon-halogen bond is broken and OH- group replaces halogen in haloalkane – rate of hydrolysis
depends on strength of bond.
- Graph with bond enthalpy;
1) lowest bond enthalpy = faster reaction
2) 3rd lowest = react faster than 2nd.
2) highest = unreactive as large amount of E required to overcome bond.
Measuring - Carried out in presence of AgNO3 – so halide ions are produced which react with Ag+ to form
rate of precipitate of silver halide.
hydrolysis - Nucleophile is present in AgNO3(aq) - usually H2O = haloalkanes insoluble in H2O = reaction carried
out in presence of ethanol solvent
- Ethanol = allows water and haloalkane to mix and produce one product.
1) 3 test tubes with 1cm3 of ethanol and 2 drops of haloalkane
2) stand the test tubes in water bath at 60C
3) Place test tube with 0.1mol dm-3 of AgNO3 in water bath and allow all tubes to reach a constant
temperature.
4) add 1cm3 of AgNO3 to each test tube and measure time taken for precipitate (max 5 mins)
- Has most precipitate = rate of hydrolysis increases as strength of carbon-halogen bond decreases.
- Least precipitate = slower rate = strongest = as more E required for precipitate to form.
Hydrolysis in - Primary = slowest rate of reaction
haloalkanes - Tertiary = fastest rate of reaction
- Due to reaction mechanism – primary reacts by 1 step but tertiary has 2 step
- 1st step = carbon – halogen bond of tertiary haloalkane breaks by heterolytic fission = tertiary
carbocation and halide ion
- 2nd step = OH ion attacks carbocation to form organic product
- Increased rate due to increased stability of tertiary carbocation > primary.
15.2 - Organo halogen compounds in environment
uses - Molecules with halogen in carbon chain – used in many pesticides.
- Rarely found in nature -> not easily broken down naturally in environment.
- General solvents, dry cleaning solvents, making polymers, flame retardants and refrigerants.
Ozone - Found at outer edge of stratosphere with make up of tiny fraction of gases – absorb biologically damaging
layer UV radiation from sun rays = only some pass through to earth.
- UV related to sunburn as it has harmful effects
- Continued depletion of ozone = more UV to pass through to earth = damaging to living organisms =
increase in genetic damage and risk of skin cancer.
- Ozone is continuously being formed and broken down by UV.
- O2 ->2O (using high energy UV to break into radicals)
- O2 + O -> O3 (O2 and radicals which ozone forms and breaks down where rate of form and break is same)
CFCs + - Human activity at CFC disrupts the equilibrium.
ozone - Was used as refrigerants due to being very stable because of its strength of C-halogen bond.
- CFCs remain stable until they reach the stratosphere - CFC break down forming Cl radicals which catalyse
breakdown of ozone layer
naming - Primary = halogen - carbon 1 alkyl group
- Secondary – halogen – carbon + 2 alkyl groups
- Tertiary – halogen – carbon + 3 alkyl groups
reactivity - Haloalkanes have H + Halogen.
- Halogen is more electronegative than carbon atoms so e- pair in bond is closer to H atom = polar.
- Carbon is slightly +ve and can attract lone pair, species donating lone pair = nucleophiles.
- Nucleophiles – atom or group attracted e- deficient carbon to donate 2 e- to form covalent bond.
1) OH- (2 lone) 2) H2O (2 lone) 3)NH3 (2 lone)
- When haloalkane + nucleophile -> nucleophile replaces halogen = new func group (substitution)
Hydrolysis - Substitution = molecule being split into 2 products where halogen atom is replaced by OH-.
Nucleophilic 1)OH approaches opposite side of C atom with halogen. (arrow from OH with lone pairs and another arrow
substitution at C-halogen with delta charges)
2) direction minimises repulsion between OH and delta –ve halogen.
3) lone pair of e- on OH is attracted and donated to delta +ve atom.
4) new bond is formed between O in OH and C = C-halogen breaks by heterolytic fission -> alcohol + halide.
- Haloalkanes can be converted to alcohols with NaOH (aq) at room temp and is slow but heated
under reflux to obtain good yield.
- Carbon-halogen bond is broken and OH- group replaces halogen in haloalkane – rate of hydrolysis
depends on strength of bond.
- Graph with bond enthalpy;
1) lowest bond enthalpy = faster reaction
2) 3rd lowest = react faster than 2nd.
2) highest = unreactive as large amount of E required to overcome bond.
Measuring - Carried out in presence of AgNO3 – so halide ions are produced which react with Ag+ to form
rate of precipitate of silver halide.
hydrolysis - Nucleophile is present in AgNO3(aq) - usually H2O = haloalkanes insoluble in H2O = reaction carried
out in presence of ethanol solvent
- Ethanol = allows water and haloalkane to mix and produce one product.
1) 3 test tubes with 1cm3 of ethanol and 2 drops of haloalkane
2) stand the test tubes in water bath at 60C
3) Place test tube with 0.1mol dm-3 of AgNO3 in water bath and allow all tubes to reach a constant
temperature.
4) add 1cm3 of AgNO3 to each test tube and measure time taken for precipitate (max 5 mins)
- Has most precipitate = rate of hydrolysis increases as strength of carbon-halogen bond decreases.
- Least precipitate = slower rate = strongest = as more E required for precipitate to form.
Hydrolysis in - Primary = slowest rate of reaction
haloalkanes - Tertiary = fastest rate of reaction
- Due to reaction mechanism – primary reacts by 1 step but tertiary has 2 step
- 1st step = carbon – halogen bond of tertiary haloalkane breaks by heterolytic fission = tertiary
carbocation and halide ion
- 2nd step = OH ion attacks carbocation to form organic product
- Increased rate due to increased stability of tertiary carbocation > primary.
15.2 - Organo halogen compounds in environment
uses - Molecules with halogen in carbon chain – used in many pesticides.
- Rarely found in nature -> not easily broken down naturally in environment.
- General solvents, dry cleaning solvents, making polymers, flame retardants and refrigerants.
Ozone - Found at outer edge of stratosphere with make up of tiny fraction of gases – absorb biologically damaging
layer UV radiation from sun rays = only some pass through to earth.
- UV related to sunburn as it has harmful effects
- Continued depletion of ozone = more UV to pass through to earth = damaging to living organisms =
increase in genetic damage and risk of skin cancer.
- Ozone is continuously being formed and broken down by UV.
- O2 ->2O (using high energy UV to break into radicals)
- O2 + O -> O3 (O2 and radicals which ozone forms and breaks down where rate of form and break is same)
CFCs + - Human activity at CFC disrupts the equilibrium.
ozone - Was used as refrigerants due to being very stable because of its strength of C-halogen bond.
- CFCs remain stable until they reach the stratosphere - CFC break down forming Cl radicals which catalyse
breakdown of ozone layer
