3.3.3 Halogenoalkanes Flashcards
what are halogenoalkanes?
an alkane with a halogen atom attached to it
why are most carbon-halogen bonds polar
halogens are more elctronegative than carbon
what are some of the uses of a halogenoalaknes?
-refrigerants
-solevents used in pharmacetuicals
why has the use of halogenoalkanes been restricted
due to the effect of chlorofluorocarbons on thee atmosphere
what is a nucleophile
an electron-pair donor
why is the carbon atom delta positive
it doesnt have enough electrons
what does the nucleophile do
donates an electron pair to somewhere which doesnt have enough electrons
e.g. carbons
what does the nucleophile attack
the delta positive carbon
what are the 3 nucleophiles that undergo subsitiution reactions
- CN-
- NH3
- OH-
name the 3 nucleophiles that undergo substitution reaactions
-cyanide ion
-ammonia
-hydroxide ion
what is meant by nucleophilic substitution
the nucleophile attacks the cabon which is less dense compared to the halogen.
it replaces the halogen group
the halogen group becomes the product of the reaction
what are the conditions for the nucleophilic substituition of the hydroxide ions
-warm aqueous sodium hydroxide
-under reflux
what reagent is used for the nucelophilic substitution of the hydroxide ions
WARM aqueous sodium hydroxide
cheekyy
where does the 2nd arrow go from during the nucleophilic substituition
from the bond to the halogen
what conditions are needed for the nucleopilic substituitoin of cyanide ions
- WARM ethanolic potassium cyanide
-under REFLUX
what reagent is used in the nucleophilic substituition of cyanide ions
-ethanolic potaassium cyanide
what does the reaction with cyaninde ions produce
nitriles
whaat does the reaction with hydroxides produce
alcohols
what are conditions for the nucleophilic substituition of ammonia
Heated with ethanolic ammonia
in EXCESS
why is there excess ethanolic ammonia
so it cant as a base
for additional reactiosn
why does the excess ammonia act as a base
cause in the intermediate the nitrogen has 4 bonds and this needs to be removes
-so that it will have 3 bonds
what does the reaction with ammonia produce
amines
what direction does the arrow go when the intermediate is attacked by the base
from the bond to the nitogrn
explain the trend in bond enthalpies for the C-halogen bond
as the bond enthalpy decreases it bcomes more reactive
out of all the C-halogen bonds what is the most reactive element
C-I
why is C-I the most reactive
it has the lowest bond enthalpy
what are the products of the elimination reaction
an alkene
what are the conditions in the elimination reaction
warm ethanolic sodium hydroxide
under reflux
where does the arrow go to from the hydroxides in the elimination reactions
to the adjacent hydrogen off the carbon with the halogen
how doe we know wether the hydroxide is going to undergo a elimination or substitution reacttion
the solevent
explain the difference the hydroxides in the elimaination and substiution reactions
elimination- ethanolic sodium hydroxides
substitution- dilute sodium hydroxide
what are chloroflurocarbons used for
solvents
refridgenats- fridges
propellants- deodarants
what do chloroflurocarbons do
they break down ozone (O3) in the atmosphere
State the benefit to life on Earth of ozone in the upper atmosphere.
absorbs harmful ultraviolet rays
that eq u did
State the benefit to life on Earth of ozone in the upper atmosphere.
-absorbs infrared radiation
- due to molecules having polar bonds
explain what enables CFs to break the ozone
because of the UV rays in the atmosphere
which breaks the C-Cl bonds
and forms free radicals
what do the free radicals do when they are broken by UV radiations
they catalyse the break down of ozone
.Cl
explain why the C-Cl bonds break the easiets compared to the C-F bonds
- bonds are easier to break because it has the lowest bond enthalpy compared to the C-F bonds
outline the direction of the arrows in the elimination reaction
the hydroxides attack the adjacent hydrogen that has the halogen bonded to it
the bond breaks from that hydrogen towards the 2 carbons
bond breaks between C and X
what do the chlorine free radicals do
catalyses the decomposition ozone
what are the products of the elimination reaction
water
and the halogen
what type of bond fission is present in halogenoalkanes
hetrolytic
what must you not forrget on the ammonia
the lone pair
what is thee geeneeral formula for ammonia
NH3
what is meant by condition
either ethanolic or aqueous
explain why the reactants are heated in a sealed container? (2 marks)
heated to increase ROR
sealed tube to prevent loss of ammonia
what is thee role of hydroxide ions in the elimination reaction
they act as a base
Suggest why the reactions of halogenoalkanes with potassium cyanide is useful in industrial synthesis
it increases thee length of the carbon chain
