haloalkanes Flashcards
What is the general formula for haloalkanes?
CnH2n+1 X
X is a halogen
What is a primary haloalkane?
Halogen bonded to a C that’s bonded to another C
H
C-C-X
H
What is a secondary haloalkane?
The halogen is bonded to a C that is bonded to 2 other Cs
H
C-C-C
X
What is a tertiary haloalkane?
Halogen bonded to a C that’s bonded to 3 other Cs
C
C-C-C
X
How are haloalkane synthesised?
(by alkanes + halogen)
They can be synthesised from alkanes by reaction with halogens in a free radical substitution reactions
How are haloalkanes synthesised ?
(electrophilic addition with alkenes)
They can be synthesised in electrolytic addition reactions with alkenes by reaction with hydrogen halides and to form dihaloalkenes with halogens
How are haloalkanes synthesised?
(alcohols + halide ions)
By reacting alcohols with halide ions.
The OH group of the alcohol is substituted for a halide ion
What is the general equation for the reactions between an alcohol and halide ion?
Alcohol + Hydrogen Halide -> Haloalkane + water
ROH + HX —> RX + H2O
What do we use for the synthesis of a chloroalkane?
Use NaCl and sulfuric acid
The Cl replaces the OH group where its positioned
The Na and OH make sodium hydroxide
What do we use for a bromoalkane?
Sodium bromide NaBr in the presence of concentrated H2SO4
The Br replaces the OH group
Sodium hydroxide is a product
What is used for iodoalkanes?
Sodium iodide - NaI
and phosphoric acid catalyst because iodide ions react with sulphuric acid
Iodine replaces the OH on the alcohol
What is heterolytic fission?
Forming a bond between :Y- and X
:Y- is the nucleophile
Pair of electrons in the bond move to one atom - ions formed
What is homolytic fission
each electron in the bond move to a different atom - radicals formed (2 half headed curly arrows)
What is nucleophilic substitution ?
halogen more electronegative than C
—> polar bond
electrons in the CX bond are attracted towards the halogen atom so the C is delta positive and halogen slightly negative
The electron definitely carbon atom is readily attacked by a nucleophile
The nucleophile donates its lone pair to carbon forming a bond
the carbon halogen bond then breaks by heterolytic fission and a halide ion is released
What is a nucleophile?
A lone pair donor
General mechanism for nucleophilic substitution
1) lone pair on nucleophile is donated to the carbon where X is bonded
2) electron pair for CX bond given to X
Arrow is drawn from the bond to X
3) Nucelophile replaces X
4) X becomes a halide ion
how to draw a nucleophile
:Nu-
Hydrolysis reaction with hydroxide ions (:OH-)
Forming alcohols
Nucleophilic sub
reagent - NaOH (aq)
Conditions: warm,aqueous
when haloalkanes are warmed with aqueous NaOH or KOH alcohols are formed
NaOH and KOH are ionic compounds containing the nucleophile
Nucleophilic sub
Forming nitriles :CN-
Reagent - KCN dissolved in ethanol
When haloalkane are warmed with an ethanol’s solution of potassium cyanide, KCN nitriles are formed
This adds an extra C to the chain
The nucleophile is KCN :CN- (cyanide ion)
Nucleophilic sub
Excess ammonia (:NH3) - forming primary amines
Reagent - NH3 dissolved in ethanol (ethanolic ammonia )
Conditions - excess NH3
When haloalkanes are warmed with an excess of NH3 in a sealed container, primary amines are formed (NH2)
Nucleophile = NH3
—> nitrogen had the lone pair of electrons
overall equation for forming primary amines
example:
CH3CH2Br + 2NH3 -> CH3CH2NH2 + NH4Br
Why do we use ethanol and not water as a solvent for reactions between haloalkanes and ammonia or KCN
if water is present there will be hydroxide ions present and these are nucleophile and this could lead to a reduced yield of desired product as OH- ions would form alcohols
what are chlorofluorocarbons
CFCs
Haloalkanes containing both chlorine and fluorine atoms but no hydrogen
short chain ones are gases eg in fridges
longer chain ones are used for dry cleaning
CFC gases are unreactive but when they eventually end up in the atmosphere they decompose to give chlorine free radicals
why is the ozone layer important
it absorbs the harmful UV radiation from the sun that causes skin cancer
Ozone in made from O3
stages for the reaction for chlorine radical
1) R-Cl bond breaks in the presence of UV radiation to produce Cl•
R-Cl—> R• + Cl•
2) Cl• then decompose ozone
Cl• + O3 —> ClO + O2
3) the free radicals (ClO•) attack more ozone and regenerate Cl•
ClO• + O3 -> 2O2 + Cl•
Overall equation :
2O3 —> 3O2
Cl• isn’t destroyed and is regenerated so it acts as a catalyst in the breakdown of ozone to oxygen
how does nitrogen monoxide NO
decompose ozone
•NO + O3 -> •NO2 + O2
•NO2 + O -> •NO + O2
overall equation:
O3 + O —> 2O2
