Haloalkanes Flashcards
What is the general formula of haloalkanes
C2n H2n+1 X
What is a haloalkane
An alkane with at least one halogen atom in place of a hydrogen atom
Why is C-X polar
Because the halogen is more electronegative than the carbon atom
What is a nucleophile
An electrophile pair donor
What are examples of nucleophiles
OH-
CN-
NH3
Water is a nucleophile but reacts more slowly
What are primary secondary and tertiary haloalkanes
It depends on the position of the halogen in the carbon chain. Primary has one alkyl group secondary has 2 and tertiary has 3
What is the reaction for nucleophilic substiution
Haloalkane—> Alcohol
What are the conditions for nucleophilic substitution
Warm/hot
What are the reagents and type of reaction for nucleophilic substitution
NaOH (aqueous)
Type- Substitution
What is the conversion for nucleophilic substitution in CN
Haloalkane- Nitride ( R-C(triple) N)
What are the conditions for the reaction with CN
In ethanol
What are reagents and type of reaction for CN
KCN/NaCN
Substitution
How does the alcohol form during nucleophilic substitution
The OH- behaves as a nucleophile and has a lone pair of electrons and these are attracted and donated to the positive carbon.
Leads to the formation of a dative covalent bond between the carbon atom and the halogen atom in the OH
How do the products form
The C-H bond breaks by heterolytic fission and both electrons from the bond move to the halogen forming a halide ion
What is hydrolysis
A reaction with water or hydroxide ions that breaks a bond in a compound turning it into 2 compounds
What is the hydrolysis reaction with OH-
CH3CH2CH2CH3X + OH- —–> CH3CH2CH2CH2OH+ X-
What is the hydrolysis reaction with H20
CH3CH2CH2CH2X+ H20 ——> CH3CH2CH2CH2OH + HX
What is the hydrolysis reaction for the halides
Add ethanol as the solvent is water and organic as its imissible so dont mix so the alovent allows it to mix so the reaction can occur
Why do you add AgNO3 for the reaction
To form the precipitate with halide ions then heat in a water bath for a constant temperature
How do you compare the reactivities of the halides
Set up 3 test tubes each containing a different haloalkane, ethanol as a solvent and silver nitrate solution
Warm the tubes in a water bath (50-60 degrees)
Time how long it takes for ppt to form
A pale yellow precipitate quickly forms with 2-iodopropane so iodoalkanes are the most reactive. Bromo alkanes react slower than iodoalkanes to form a cream precipitate and chloroalkanes form a white precipitate the slowest
Why was a water bath used
To ensure all experiments are performed at the same temperature
How can the rate of hydrolysis be found
By calculating 1 divided by time taken for ppt to form
Why does it take longer for hydrolysis to take place
As the rate of hydrolysis increases the strength of the carbon-hydrogen bond decreases. Higher bond enthalpies mean there are stronger bonds so the energy required to break the C-X bond is greater so takes longer for hydrolysis to take place
How do you determine the rate of hydrolysis of a haloalkane
As you go down group 7 the strength of the C-X bond decreases so the lower the bond enthalpy the faster the rate of hydrolysis
How many oxygen atoms are bonded to eachother for the ozone
3
Why is the ozone layer useful to humans
It absorbs harmful UV radiation which could cause skin cancer from the sun and converts it into heat
What are the equations with oxygen
O2——>2O
O2+O—–>O3 (+heat)
O3——–>O2+O
How is the concentration of ozone maintained in the ozone layer
It is kept at equilibrium as a reversible reaction
O2+O —–> O3
The rate of formation = the rate of decomposition
What are CFCS
Well known haloalkanes which only contain carbon fluorine and chlorine as all the halogens have been replaced
What were CFCS used as
Aerosols and cooling devices
Why were CFCS used a refrigerants
They are non toxic
Non flammable
Very stable
Why do CFCS only break down high in the atompshere
they form chlorine radicals which catalyse the breakdown of ozone
What are 2 alternatives to CFCS
HCFC AND HFC
How would the mass of a haloalkane compare with the mass of an alkane of the same carbon chain
Greater as the mass of a halogen is greater than the mass of a hydrogen
How do CFS break down the ozone layer using radical substitution (initiation)
CFCl3———> Cl+C*FCl2
How do CFS break down the ozone layer using radical substitution (propagation)
Cl* + O3——-> ClO + O2
ClO+ O——-> Cl+ O2
What is the overral equation for the catalysation
O3+ O————> 2O2 this is the same for all ozone degradation reactions
How are nitrogen oxides produced
Car and aircraft engines and thunderstorms
What are the 2 propagation steps for nitrogen oxide
NO* + O3———–> NO2* + O2
NI2* + O————-> NO* + O2
What is the overral equation for nitrogen oxide
O3+O———>2O2
Why are all of these reactions chain reactions
The radicals are used in step 1 and regenerated in step 2
