Alkanes

Question 1

Alkane to Halogenoalkane : reagents and conditions
- reaction type

Answer 1

Reagents : Halogen (X2)
Conditions : UV light
- free radical substitution

Question 2

3 steps in free radical substitution

Answer 2

1. Initiation
2. Propagation
3. Termination

Question 3

What is a free radical ?

Answer 3

A species with an unpaired electron (very reactive)

Question 4

What occurs during initiation during free radical substitution ?

Answer 4

Free radicals are produced
UV light used to break X-X bond in halogen by heterolytic fission, giving two •X •X radicals
(Where X represents a halogen)

Question 5

What occurs during propagation in free radical substitution?

Answer 5

• Halogen free radical reacts with alkane
• Forming an alkyl radical
  -which then reacts with a halogen, forming a halogenoalkane and reforming a halogen free radical

Question 6

What occurs during termination in free radical substitution?

Answer 6

• Removal of free radicals as they react with one another to form stable compounds

Question 7

Why does free radical substitution often produce a low yield of Halogenoalkane

Answer 7

• The desired Halogenoalkane often undergoes further substitution, as radicals are so reactive that we cannot control what they react with

Question 8

How to remove desired Halogenoalkane from reaction mixture

Answer 8

• often requires distillation, as further substituted Halogenoalkanes (CH2Cl2 , CHCl3 , CCl4) are in solution

Question 9

What is cracking

Answer 9

Process by which larger (often not very useful) alkanes are broken down into smaller alkanes or alkenes

Question 10

Three types of cracking

Answer 10

• thermal
  -catalytic : zeolite catalyst
  -steam (lots of alkenes)

Question 11

What is reforming?

Answer 11

The process by which straight chain hydrocarbons are turned into branched and cyclic hydrocarbons (that combust better)

Question 12

Summarise fractional distillation

Answer 12

-crude oil vaporised
- Vapor passed into fractionating column with temperature gradient : Bottom is hottest, top is coolest
- Hydrocarbon condenses and is collected at its boiling point
-short hydrocarbons (low boiling point - less London forces) condense at cool top and vice versa

Question 13

Incomplete combustion harmful products / combustion harmful products

Answer 13

• CO - poisonous gas
• C(s) - carbon particulates (soot) - global dimming - respiratory issues
  -SO2 - impurities in oil - acid rain
• NOX - high temperatures in engine - acid rain - asthma

Question 14

Reforming conditions and reagents

Answer 14

Reagents : platinum catalyst (Al2O3 )
Temperature : 500 degrees

Question 15

How are free radicals formed

Answer 15

• heterolytic fission of a covalent bond
• for free radical substitution UV breaks the bond between the halogen atoms

Question 16

What is a saturated hydrocarbon ?

Answer 16

• a hydrocarbon in which each carbon forms four single bonds
• there are no double bonds presents whatsoever, only single bonds