haloalkanes Flashcards
classification
primary-the c atom bonded to the halogen is only attached to one other c
secondary - the c atom bonded to the halogen is only attached to two other c
tertiary …
what is the essential condition for free radical substitution
essential condition - uv light, provides the energy required to break the halogen-halogen bond and start the reaction
definition for free radical
a species with an unpaired electron
overall reaction for free-radical substitution
overall reaction: CH4+ Cl2 -> CH3Cl + HCl
initiation for free-radical substitution
1-initiation:Cl2 -> 2Cl*
The halogen molecule splits into two free-radicals
Breaking the halogen-halogen bond is called homolytic fission
propagation for free-radical substitution
2 – Propagation
1:Cl*+CH4 -> *CH3 + HCl
The free-radical produced during initiation takes a H from the alkane
This turns the alkane into a free-radical and also produces a hydrogen halide
2:CH3+Cl2 ->CH3Cl+ Cl
The alkyl radical produced in step 1 of propagation takes a halogen atoms from a halogen molecule (not all of the halogen molecules were split in Initiation)
This forms a haloalkane and reproduces the halogen free-radical that started propagation step
termination for free-radical substitution
3 – Termination
CH3 + Cl -> CH3Cl
2Cl* -> Cl2
2*CH3 -> CH3CH3
Two free-radicals combine
how to reduce the amount of further substitution
use excess alkane
the ozone layer
ozone-a pale blue poisonous gas which forms 12-50kkm above the earths surface and acts as a barrier to the harmful UV-B radiation
radiation can cause skin cancer and cataracts
ozone can be broken down by a reaction with chlorine radicals which is released by CFC’s
CFC’s
haloalkanes that contain both chlorine and fluorine
these break down the ozone layer
CFC’s are now banned
write three equations to show how trichlorofluoromethane is involved in damaging the ozone layer
1-initiation CCl3F -> Cl* + CCl2F
Note: In the initiation step for ozone breakdown, you simply need to produce a chlorine radical from whatever molecule you are given.
2-propagation
Cl + O3 ⟶ ClO* + O2
ClO* + O3 ⟶ 2O2 + Cl*
why are CFC’S so damaging to the ozone layer
as the breakdown of ozone can be caused by a single chlorine free-radical
as cl* is regenerated in the final propagation step and causes a chain reaction in the decomposition of ozone
why do things that arent CFC’s work in the decomposition of the ozone layer
does not contain cl/release cl
c-f bonds are strong and dont break
rules for curly arrows
-double headed arrow represents the movement of a pair of electrons
-must start from a lone pair or from a bond
-must end at an atom or a point where a bond is being made
definition of a nucleophile
and electron pair donor
rates of substitution(hydrolysis)
the halide ion lost from the haloalkane can be identified by using aqueous silver nitrate in ethanol eg Ag+(aq) + Cl-(aq)->AgCl(s)
method
-place a measured sample of each halogenalkane into separate test tubes and place into a water bath at 50c
-in another test tube place a solution of ethanol, water and aqueous silver nitrate and place in the same water bath
-once at equal temps add equal vol of ethanol mixture and haloalkane solutions together
-time how long it takes for each precipitate to form
results of rates of substitution
chloropropane - white ppt AgCl - in dilute ammonia=colourless , longest time to appear
bromopropane - cream ppt AgBr, cream in dilute ammonia, colourless in concentrated ammonia
iodopropane - yellow ppt AgI , yellow in dilute and concentrated , shortest time to appear
Fluoropropane - no visible change
reason for the differing rates of hydrolysis
the stronger the bond the harder it is to break and the slower the rate of reaction will be more energy is required to break it
the weaker the bond the faster the rate will be
