3.3 Haloalkanes

Question 1

What happened in free radical substitution reactions

Answer 1

—> H atoms are replaced by halogen atoms on (halogen)alkanes
For every H replaced, a (halogen)^2 is used and a H(halogen) is made
- For every H replaced by a halogen,
- One eg.Cl2 is used and one HCl is made

This is the overall equation
Eg.
For methane = dichloromethane
CH4 + 2Cl2 = CH2Cl2 + 2HCl

Question 2

Whats initiation?

Answer 2

Upon exposure to UV light, a halogen molecule breaks apart
Into two halogen atom free radicals (eg. F•)
—> light provides energy to break covalent bond

Free radicals are a species with a odd number of electrons
They’re very reactive

Eg. F2 = 2F•

Question 3

What’s propagation

Answer 3

For every H that’s replaced, the overall equation
is divided into two propagation equations

1. (Halogen)alkane reacts with 1 halogen atom free radical (removing H from alkane)
   
   • this produces H(halogen) and a carbon-based radical
2. C based radical reacts with a halogen molecule (to put one halogen atom onto C based radical)
   
   • producing another halogen radical and a halogenalkane
     The chain reaction repeats in another step 1

Eg.
CH4 + Cl• —> •CH3 + HCl
•CH3 + Cl2 —> CH3Cl + Cl•

Question 4

What’s termination

Answer 4

If two free radicals collide a molecule will form,
Stopping the chain reaction

Eg. For CH2Cl-CCl3
CH2Cl +CCl3 (both free radicals) = CH3Cl-CCl3

Question 5

What are physical properties of halogenalkanes

Answer 5

Contain the functional group CX (x is halogen)
General formula of homologous series is CnH2n+1X
—
—Nature of the C—X bond..
- polar due to halogen being more electronegative than H
- electronegativity decreases down G7, becoming less polar as you go down

— boiling point
- two types of intermolecular forces between haloalkane molecules
Are vanderwaals or permanent dipole-dipole interactions

If chain length increases
- vanderwaals increases as relative molecular mass increases
- as more electrons are present
- hence bp increases

If halogen atom changes
- although the dipole dipole interactions are stronger the more polar the bond
- changing vdws has a greater effect on bp
Hence bp of an iodoalkane is higher than bromo

— solubility
- despite polarity, are insoluble or only slightly soluble in water
- soluble in organic solvents due to ability to mix with hydrocarbons
(Used as dry cleaning fluids / degreasing agents)

Question 6

What free radical substitutions happen in the ozone layer

Answer 6

Ozone, O3, is an allotrope of oxygen
The highest levels of ozone are int the stratosphere, known as the ozone layer
Ozone enhances absorption of UV by O2 and N2
Preventing most of the harmful UV rays from reaching earth
— reaching earth causes sunburn, but necessary for vitamin D
— small wavelengths cause cancer, cataracts, plant tissue damage

.

Over past few decades, ozone layer has decreased in thickness
Due to photochemical chain reactions by halogen free radicals
(Source of these are halogenalkanes)
—> solvents, flame retardants, anaesthetics

Chlorine radicals cause most damage (from CFCs)
Don’t degrade in lower atmosphere, so diffuse upwards
Into ozone layer, where UV cause homolysis of C-Cl bond
— R-CF2-Cl (cfc) —> RCF2• + Cl•
— Cl• + O3 —> ClO• + O2 (Cl reacts with ozone, decomposing)
— ClO• + O3 —> 2O2 + Cl• (chlorine radical reformed reacting further with ozone)

In this sense, Cl radical is a catalyst for ozone decomposition
Contributing to the hole.
Overall, 2O3 —> 3O2

Question 7

What’s being done to reduce rate of ozone depletion

Answer 7

Over 200 countries have pledged to phase out
the production of zone depleting agents

Chemists have developed and synthesised alternative
Cl-free compounds with low toxicity, wch dont deplete ozone
—>like HFCs, CHF3 (hydrofluorocarbons, trifluoromethane)