R3.3

Question 1

radical def

Answer 1

= reactive intermediate with a single unpaired electron, formed by homolysis of a covalent bond, containing an atom that does not have an octet of electrons

Question 2

homolysis

Answer 2

homolytic fission
a covalent bond breaks evenly with each atom ending up with one of the electrons from the covalent bond

Question 3

why does energy need to be supplied for homolytic fission

Answer 3

a lot of energy is required to break bonds symmetrically to overcome bond dissociation energy

Question 4

in what form is energy supplied for homolysis

Answer 4

UV light (hv) or heat (∆)

Question 5

ozone layer depletion process

Answer 5

O3 is concentrated in a layer above the Earth’s surface ⇒ absorbs some of the ultraviolet (UV) radiation from the Sun, preventing it from reaching the Earth’s surface
CFCs (chlorofluorocarbons) are easily broken down by UV radiation to form highly reactive radicals that react with the ozone ⇒ the ozone layer is depleted

Question 6

ozone holes

Answer 6

regions where the ozone layer has been sufficiently depleted ⇒ greater levels of UV radiation reaching the Earth’s surface ⇒ skin cancers

Question 7

radical substitution

Answer 7

= chemical reaction where a radical species replaces another atom in a molecule, usually a hydrogen

Question 8

halogenation

Answer 8

radical substitution with halogens to form alkyl halides

Question 9

different halogens in halogenation

Answer 9

only useful with Cl2 or Br2
F2 ⇒ too violent
I2 ⇒ too slow

Question 10

monohalogenation

Answer 10

= a single hydrogen atom on a carbon has been replaced by a halogen
excess halogen is used ⇒ possible to replace more than one hydrogen on a carbon atom
can be achieved by adding halogen X2 to an excess of alkane

Question 11

reaction mechanism def

Answer 11

a sequence of elementary steps by which a chemical reaction occurs

Question 12

substrate

Answer 12

an organic molecule in which the reaction occurs

Question 13

unreactivity of alkanes is caused by

Answer 13

1. non-polar nature of C-C, C-H bonds ⇒ relatively symmetrical distribution of electrons within the covalent bond ⇒ limited attraction from other molecules
2. C–C and C–H bonds have a high bond enthalpy and require a lot of energy in order to break
3. sigma bonds are stronger than pi bonds ⇒ alkanes are less reactive than alkenes/alkynes

Question 14

mechanism of radical substitution

Answer 14

1. Initiation: radical formation
2. Propagation: radical species reacts with a covalent species (alkane) forming a new radical and a new covalent species (chain reaction)
3. Termination: two radicals form a single covalent species, terminating the chain reaction

Question 15

chain reaction

Answer 15

a reactive species undergoes a reaction that produces more reactive species, causing additional reactions to then occur (2+ repeating steps)