15) Haloalkanes Flashcards
Haloalkanes can be classified as…?
primary, secondary or tertiary
What can you infer from the fact that halogen atoms are more electronegative than carbon atoms?
the carbon-halogen bond is polar
the e- pair is closer to the halogen
Define nucleophile
electron pair donor
an atom/group of atoms that is attracted to an electron-deficient centre or atom, where it donates a pair of e- to form a new covalent bond
What does a nucleophile do when reacting with a haloalkane?
replaces the halogen in a substitution reaction - producing a new compound with a different functional group
Name 3 possible nucleophiles
:OH-
H2O:
:NH3
Define nucleophilic substitution
a reaction in which a nucleophile is attracted to an electron-deficient carbon atom, and replaces an atom/group of atoms on the carbon atom
Define hydrolysis
a chemical reaction involving water or an aqueous solution of a hydroxide that causes the breaking of a bond in a molecule - resulting in the molecule being split into two products
Haloalkane -> alcohol
NaOH(aq)
heat under reflux
hydrolysis
Where are the 2 curly arrows in the nucleophilic substitution mechanism of NaOH(aq) with a haloalkane?
From the :O(-)H to the partially positive carbon
From the C-halogen bond to the partially negative halogen atom
What does the rate of hydrolysis of a haloalkane depend upon?
the strength of the carbon-halogen bond in the haloalkane (bond enthalpy)
How can the rate of hydrolysis be measured?
carry out the reaction in the presence of aqueous silver nitrate and recording the time taken for the silver halide precipitate to form:
Cl - white ppt slowly
Br - cream ppt
I - yellow ppt rapidly
an ethanol solvent is used as haloalkanes are insoluble in water
C-F C-Cl C-Br C-I
Bond enthalpy _ ->
Rate of hydrolysis _ ->
decreases
increases
Define organohalogen compounds
molecules that contain at least one halogen atom joined to a carbon chain
What are organohalogen compounds often used in?
pesticides
Are organohalogen compounds broken down naturally in the environment?
no
Where is the ozone layer found?
at the outer edge of the stratosphere
What does ozone do to UV-B from the sun’s rays
absorbs most of it, allowing only a small amount to reach the Earth’s surface
What is UV-B?
a form of biologically damaging ultraviolet radiation which can lead to genetic damage and an increased risk of skin cancer
Give two equations to understand the breakdown and regeneration of ozone
O2 -UV-> 2O
O2 + O -> O3
What does O represent in an equation?
an oxygen radical
What were CFCs and HCFCs commonly used in/as?
air-conditioning units
refrigerants
aerosol propellants
Why are CFCs stable compounds?
carbon-halogen bond
CFCs are stable until…?
reaching the stratosphere where they break down forming chlorine radicals
What are chlorine radicals thought to do?
catalyse the breakdown of ozone
Name the process by which a CFC absorbs UV and is broken down into radicals
photodissociation
Give the initiation, propagation (2) and overall equations for the breakdown of CF2Cl2 by UV
What is important to note about the 2nd propagation step?
initiation: CF2Cl2 -UV-> CF2Cl. + Cl.
propagation: Cl. + O3 -> ClO. + O2
ClO. + O -> Cl. + O2
overall: O3 + O -> 2O2
The chlorine radical is regenerated which can undergo the 1st propagation step again
Name another radical, other than Cl. , which catalyses the breakdown of ozone and how it is formed
nitrogen oxide radical
formed naturally during lightning strikes and as a result of aircraft travel
Give the 2 propagation steps for the breakdown of ozone by NO.
NO. + O3 -> NO2. + O2
NO2. + O -> NO. + O2
