Halogenoalkanes Flashcards
how do alkanes react with halogens
in the presence of UV light
what is formed when alkanes and halogens react together in the presence of UV light
a halogenoalkane and hydrogen halide
what is the reaction for the formation of halogenoalkanes and why
a substitution reaction
a hydrogen in the alkane is replaced by the halogen
The hydrogen lost from the alkane will join with the second half of the diatomic
halogen molecule to form HX
what’s the mechanism for the formation of halogenoalkanes
free radical substitution
what’s a free radical
a reactive species due the presence of an unpaired electron
what are the 3 stages in free radical substitution
1) initiation
2) propagation 1 and 2
3) termination
explain the initiation step in free radical
substitution for formation of halogenoalkanes
ultraviolet light causes the covalent bond between halogen atoms to break forming 2 halogen free radicals
what happens in 1st propagation for formation of halogenoalkanes
the halogen radical reacts with the alkane
the halogen radical removes a hydrogen from alkane
creating a hydrogen halide and alkyl radical
what happens in 2nd propagation for formation of halogenoalkanes
the alkyl radical reacts with halogen molecule and
creates halogenoalkane and halogen radical
what does the halogen radical created in the end of 2nd propagation do after
the halogen radical created at the end of propagation 2 goes on to react with another alkane molecule and 1st propagation is repeated
there will be a chain reaction until no more halogen molecules or alkane molecules
what happens in termination step for formation of halogenoalkanes
2 radicals react to form neutral molecules
what are chlorofluorocarbons (CFC’s)
they are halogenoalkanes containing both chlorine and fluorine but no hydrogen
describe short chain CFCS and what they were used for
they are gasses
they were used in refrigerators
what were long chain CFCS used for
they are used for degreasing solvents
and dry cleaning
how are CFC gases like under normal conditions
very unreactive
what happens when CFC gasses reach the atmosphere
they decompose to produce chlorine free radicals
why is the ozone layer important
it absorbs harmful UV radiation from sun that causes skin cancer by damaging DNA in skin cells
what layer is ozone layer in
stratosphere
what do chlorine radicals do to ozone
chlorine radicals decompose ozone in the stratosphere creating a hole in the earths ozone layer
name all the steps in the decomposition of ozone
1) the c-cl bond is broken in the presence of uv radiation to produce chlorine radical
R-Cl —> R* + Cl*
2) the chlorine radical decomposes ozone in the stratosphere
Cl* + O3 —> ClO* +O2
3) the resulting ClO3 radicals attack more ozone molecules and regenerate Cl*
ClO* + O3 —> 2O2 * + Cl*
what can we call the Cl* in regards to the breakdown of O3
as the Cl* is not destroyed and is regenerated we can say it acts as a catalyst in the breakdown of ozone to oxygen
what’s the overall equation for decomposition of ozone
2O3 —> 3O2
what are the safer alternatives to CFCS
chemists have developed safer chlorine free compounds which is primarily used in air conditioning and refrigerators
why are the C-X bonds polar
because the halogen atoms are more electronegative than carbon
the electrons in the C-X bond are more attracted to the halogen leaving it delta negative and the carbon delta positive
what is the delta positive carbon atom attacked by
nucleophile
what’s a nucleophile
electron pair donor
name the 3 types of nucleophillic substitution with halogenoalkanes
1) NaOH/KOH (:OH ions) to form alcohols
2) KCN ( :CN- ions) to form nitriles
3) with excess ammonia (:NH3) to form primary amines
how do we form alcohols from halogenoalkanes
Reagent: NaOH / KOH
Conditions : Warm and aqueous
When Halogenoalkanes are warmed with aqueous sodium hydroxide (NaOH) or potassium hydroxide (KOH), alcohols are formed. NaOH and KOH are ionic compounds containing the nucleophile: OH.
how do we form nitriles from halogenoalkanes
Reagent: KCN (:CN- ions)
Conditions: warmed in aqueous ethanol
When Halogenoalkanes are warmed with an aqueous ethanol solution of potassium cyanide (KCN), nitriles are formed. This adds an extra carbon to the chain. The nucleophile in KCN is :CN-
how are primary amines formed from halogenoalkanes
reagent: NH3
conditions: warmed, excess ammonia, sealed container
When Halogenoalkanes are warmed with an excess of ammonia in a sealed container, primary amines are formed (2 hydrogen atoms attached to the nitrogen). The nucleophile is ammonia, in which nitrogen atom has the lone pair of electrons:
what mechanism is alkene formed from halogenoalkanes
elimination mechanism
describe reagents and conditions elimination mechanism
describe the different roles of hydroxide ions
reagents: NaOH
conditions: hot in ethanol
In the reaction between aqueous sodium hydroxide and a Halogenalkane, the hydroxide ions act as a nucleophile (electron pair donor) and an alcohol is formed by nucleophilic substitution.
• However, the hydroxide ions can also act as a base (accept H*), so an alternative elimination reaction also takes place in which one of the hydrogen atom and the halogen atom are eliminated from the Halogenalkane to give an alkene.
describe reagents and conditions elimination mechanism
describe the different roles of hydroxide ions
reagents: NaOH
conditions: hot in ethanol
In the reaction between aqueous sodium hydroxide and a Halogenalkane, the hydroxide ions act as a nucleophile (electron pair donor) and an alcohol is formed by nucleophilic substitution.
• However, the hydroxide ions can also act as a base (accept H*), so an alternative elimination reaction also takes place in which one of the hydrogen atom and the halogen atom are eliminated from the Halogenalkane to give an alkene.
what are the steps in elimination reaction
Stage 1: The Hydroxide ion removes a hydrogen atom from an adjacent carbon to the halogen atom.
Stage 2: The electrons in the carbon-hydrogen bond move onto the carbon-carbon bond to create a double bond.
Stage 3: the electrons in the carbon-halogen bond move onto the halogen atoms, breaking the bond and creating a halide ion.
