2.10 Halogenoalkanes Flashcards
halogenoalkanes can be classified into
primary
secondary
tertiary
CH3 CH2 CH2 CH2 CH2 I
1 - iodopentane
primary
CH3 CH2 CH2 CH2 CH(Br) CH3
2 - bromopentane
secondary
(CH3)2 C Cl CH2 CH3
2 - cloro - 2 - methylbutane
tertiary
why carbon–chlorine bond is polar
it has more electrons and chlorine atom is very electronegative
carbon atom is slightly positive
why halogenoalkanes attacked by nucleophiles
nucleophiles attack slightly+ C atom in the carbon — halogen bond
the halogen atom gains electrons from the sigma bond and forms halides
attract electron rich nucleophiles
reactions of halogenoalkanes
name 3
elimination
substitution ( nucleophilic )
hydrolysis
elimination reaction
hydrogen atom is removed from carbon next to the one bonded to halogen
product is ?
alkene
or a mixtures of alkene
elimination reaction
eq reagents condition product reaction type
eq: refer to notes pg 2
reagents: conc potassium / sodium hydroxide
condition: heat under reflux with ethanol
product: propene ( reactant : 1 - bromopropane )
reaction type: elimination
elimination reaction
mechanism
draw it out
refer notes at pg 2
elimination reaction
draw diagram for heat under reflux
to prepare propene from 2 - bromopropane
refer notes pg 2
elimination reaction
why need to distill the mixture
to remove volatile product as it forms
elimination reaction
why condenser is not used in heat under reflux
propene has very low boiling points
will not condense at room temp
substitution reaction
halogen is substituted by an -OH, producing an alcohol
eq reagents condition product reaction type
eq : refer to notes
reagents: dilute potassium / sodium hydroxide
condition: heat under reflux in aq solution in ethanol
product: propanol ( reactant: chloropropane )
reaction type: nucleophilic substitution / hydrolysis
substitution reaction
why halogenoalkane is dissolved in alcohol
and alkali dissolved in water?
as a solvent for both aq and halogenoalkane
to allow halogenoalkane and water to mix
elimination and substitution
compare
conditions
type of reactions
action of OH-
product
- conditions: reflux with aq solution of KOH
type of reactions: nucleophilic substituion
action of OH- : nucleophilic
product: alcohol
2.
conditions: reflux with alcoholic solution of KOH
type of reactions: elimination
action of OH- : base
product: alkene
2- chlorobutane in ethanolic aolution gives 3 diff alkenes
name and draw displayed formula
refer to notes pg 3
mechanism of nucleophilic substitution
2 types when hydroxide ions attack a halogenoalkane
SN2
SN1
the mechanism of nucleophilic substitution depends on?
primary secondary or tertiary halogenoakanes
SN2 mechanism for?
primary and secondary
SN2 mechanism
draw the mechanism
involves?
transition state
single step mechanism
refer notes pg 4
reaction profile of SN2
refer notes pg 4
when does transition step occurs ?
in SN 2
refer to notes pg 4
the rate of order of SN2
idoalkane
bromoalkane
chloroalkane
C–I bond is weakest reaction faster
SN1 mechanism for
tertiary and secondary halogenoalkanes
SN1 mechanism
draw the mechanism
involves?
carbocation intermediate
two steps
refer to notes pg 5
SN1 mechanism
reaction profile
refer notes pg 5
two reasons why SN1 is more favoured by tertiary halogenoalkane
- more stable tertiary carbocation intermediate is formed
- presence of two alkyl groups which are bulky
attached to slightly+ C of C–X bond
inhibit approach of OH-
effect is called as steric hindrance
can primary halo undergo SN1
formation of primary carbocation has higher activation energy
rate of reaction is slower
increase conc of NaOH will affect rate of which mechanism?
SN2
as high conc of nucleophile are requireed for the slowest step to speed up the reactoin
further reactions of halogenoalkanes
- reaction with alcoholic ammonia
halogen is substituted with -NH2, producing amine
eq reagents condition product reaction type mechanism
eq: refer to notes pg 6
reagents: excess ammonia in ethanol
condition: heated in a sealed tube
product: ethylamine ( reactant: chloroethane )
reaction type: nucleophilic substitution
mechanism: SN2
further reactions of halogenoalkanes
- reaction with alcoholic ammonia
draw the mechanism
refer to pg 6
further reactions of halogenoalkanes
- reaction with alcoholic ammonia
why reaction is carried out in a sealed tube rather than heat under reflux
ammonia gas would escape
it would not be condensed by reluc condenser
boiling point 33.43 degree celcius
further reactions of halogenoalkanes
- reaction with alcoholic ammonia
alternative method to heating in sealed tube
conc solution of ammonia at room temp
leave for a long time
to increase rate of reaction
further reactions of halogenoalkanes
- reaction with alcoholic ammonia
why ammonia is used in excess
there is 2 nucleophiles present
- ethylamine
- NH3
competition btwn them
so excess ammonia -to reduce the proportion of further substitution
ethylamine has line pairs of electrons can act as nucleophiles
further reactions of halogenoalkanes
- reaction with alcoholic ammonia
why ammonia chloride salt is formed instead of HCL
NH3 is in excess when react with HCL forms
ammonia chloride
further reactions of halogenoalkanes
- reaction with alcoholic ammonia
even in presence of alcoholic conc NH3
no elimination taek place only nucleophilic substitution why?
NH3 is a weak base ( slow reaction ) but a strong nucleophile and attack slightly + C in C—X bond
OH- strong base, weak nucleophile
further reactions of halogenoalkanes
- reaction with water ( hydrolysis )
to study the reactivity of halogenoalkanes
the experiment procedure?
refer notes on G7
further reactions of halogenoalkanes
- reaction with water ( hydrolysis )
same primary halogenoalkane
rate of hydrolysis
explanation
refer notes on pg 7
further reactions of halogenoalkanes
- reaction with water ( hydrolysis )
primary, secondary, tertiary alcohol
rate of hydrolysis
explanation
refer notes pg 7
test for halogenoalkanes
experiment procedure?
halo is warmed with some NaOH in ethanol and water
the mixture is acidified by adding dilute HNO3
Ag NO3 solution i added
chloroalkane
bromoalkane
iodoalkane
observation with AgNO3
and ionic eq
refer notes pg 8
test for halogenoalkane
halo is warmed with some NaOH in ethanol and water
the mixture is acidified by adding dilute HNO3
Ag NO3 solution i added
is actually difficult to distinguish the colours. what can you add to confirm each of these precipitate?
add NH3
AgCl dissolve in dilute NH3
AgBr insoluble in dilute NH3 but is soluble in conc NH3
AgI insoluble in dilute and conc NH3
test for halogenoalkane
halo is warmed with some NaOH in ethanol and water
the mixture is acidified by adding dilute HNO3
Ag NO3 solution i added
why NaOH was added
to allow nucleophilic substitution
to hydrolyse the halogenoalkanes and liberate the halides
test for halogenoalkane
halo is warmed with some NaOH in ethanol and water
the mixture is acidified by adding dilute HNO3
Ag NO3 solution i added
why the solution has to be acidic prior to addition of AgNO3?
neutralise NaOH
prevent from getting grey percipitate of AgOH from KOH when AgNO3 is added
IUPAC naming
CFC - 11
trichlorofluromethane
IUPAC naming
CFC - 123
1,1,2 - trichloro - 1,2,2 trifluroethane
properties of CFCs ( no hydrogen atom )
inert
non flammable
non toxic
odourless / tasteless
volatile
uses of CFCs
refrigerants - cooling liquid
aerosol propellants - perfume and insecticides spray
cleaning agent - solvent in dry cleaning and electronic industries
flame retardant - fire extinguisher
CClBrF2
fire retardant
non flammable
inert due to strong C–F bond
forms a protective layer prevent underlying material from igniting and exclude O2
halogenoalkanes absorbs heat from fire
absorb free radicals in combustion propagation
CCl2F2 as refrigerant
inert
low boiling point
vapourise rapidly at low temp
inertness and stability of CFCs has a disadvantge
tend to persist in the atmosphere for long periods
destroy ozone layer
structure of ozone layer
refer to note pg 10
the initiation , propagation eq when UV strikes a CFC molecule producing free radicals
refer to note pg 10
why chlorine radicals act as catalyst?
the consequences?
regenerate
so small amount can destroy thousand of ozone molecules before termination step
replacement of CFCs called hydroclorofluorocarbon
which contain hydrogen atoms
advantages?
more reactive
broken down in the lower part of atmosphere and pose a lower risk to ozone layer
replacement of CFCs called hydroclorofluorocarbon
which contain hydrogen atoms
disadvantges
greenhouse gas
flammable
formation of photochemical smog
HCFC - 22
CHClF2
naming
chlorodifluromethane
HCFC - 123
CF3CHCl2
naming
2,2 - dicloro - 1,1,1 - trifluoroethane
why ozone is bad near earth s surface
harmful to respiratory systems
why ozone is good at stratosphere
absorb Uv
reduce skin cancer
why ozone is preferred than chlorine as a disinfectant
1.
ozone involves in a reversible reaction
turns to pure o2 when unused
leaves no harmful by products
ozone oxidation faster than chlorine
2.
chlorine is acidic in water
has a taste / unpleasant odour
cars produce nitrogen oxide ( free radical )
but aircraft causes more damage why?
nitrogen oxides produce from cars are oxidised to NO2
when it rains it produced HNO3 acid rain
NO never gets a chance to reach stratosphere
aircraft fly closer to upper atmosphere, NO can reach stratosphere
NO breaks down ozone
two propagation steps
refer notes pg 11
CCl4 is a good fire retardant but pose risk to ozone
CBrF3 is not as effective but has the smallest environmental impact
why ?
- C — F in CBrF3 has the strongest bond
it takes a lot of energy to break to form radicals
C—F causes global warming
reach stratosphere
never breaks down
- Br : F
1 : 3
although C – Br can be broken, 3F is still there cannot be broken
3.
C—Br is weaker
easier to be broken before reaching the stratosphere
