Organic Chem: Alkanes

Question 1

What are alkanes?

Answer 1

Straight or branched chain hydrocarbons with only single covalent bonds between the carbon atoms
CnH2n+2

Question 2

What do successive compounds in the alkane series differ by? What type of series is this?

Answer 2

One carbon and two hydrogen atoms

Homologous series

Question 3

What are the two main sources of alkanes?

Answer 3

Natural gas

Petroleum

Question 4

How are alkanes released as natural gas? What type is the main constituent?

Answer 4

Anaerobic decay of plant and animal matter by microorganisms

Methane (85-90%)

Question 5

How is petroleum formed and what is it composed of?

Answer 5

Formed form the decomposition of plants and animals over millions of years
Composition varies but contains a mixture of hydrocarbons with smaller amounts of sulphur and nitrogen containing organic compounds

Question 6

How is petroleum separated?

Answer 6

Fractional distillation

Question 7

How to name an alkane

Answer 7

1. Select longest continuous chain as parent, all other alkyl chains as branches
2. Number carbon atoms in parent chain starting with end closest to the first carbon with branch
3. Name each branch alkyl group and designate position on parent chain e.g. 2-methyl is methyl group attached to carbon 2
4. When several different alkyl groups attached list alphabetically
5. When same alkyl group occurs more than once indicate with prefix like di, tri, etc

Question 8

What is an isomer?

Answer 8

When two molecules have the same molecular formula but different 3D structure

Question 9

What is a constitutional/ structural isomer?

Answer 9

When molecular formulae are the same but the arrangement of atoms differ

Question 10

Compare branched alkanes to straight chain alkanes.

Answer 10

Branched chains have smaller surface area and don’t pack together as well
Branched chains have fewer molecular interactions so lower boiling and melting points compare to straight chains

Question 11

What are some physical properties of straight chains alkanes?

Answer 11

The longer the chain, the more interactions (van der Waal forces) so more energy is required to disrupt

At room temp: small molecules= gases
Larger molecules= liquids or solids

Question 12

What are cycloalkanes?

Answer 12

Hydrocarbons forming a ring structure

Not planar

Question 13

What are the two configurations of cyclic compounds?

Answer 13

Chair: all 6 conformations are gauche (skew). Energetically more favourable
Boat: 4 gauche and 2 eclipsed conformations

Question 14

What is cracking?

Answer 14

The breakdown of a large alkane into smaller alkanes and alkenes
Small alkanes and alkenes are more usefully than long ones

Question 15

What is dehydrogenation?

Answer 15

Removal of H2 to form unsaturated hydrocarbons

Question 16

What is halogenation?

Answer 16

Replacing a H with a halogen
Radical reaction
Alkyl halides are useful as intermediates for the manufacture of compounds

Question 17

What is a primary carbon in an alkane?

Answer 17

A primary carbon is bonded to one other carbon

Question 18

What is a secondary carbon in an alkane?

Answer 18

A secondary carbon has two carbons attached to it

Question 19

What is a tertiary carbon in an alkane?

Answer 19

A tertiary carbon is bonded to three other carbons

Question 20

How much more reactive are secondary carbons compared to primary carbons?

Answer 20

Secondary carbons are four times as reactive as primary carbons

Question 21

How much more reactive are tertiary carbons compared to primary carbons?

Answer 21

Tertiary carbons are five times as reactive as primary carbons

Question 22

How are radicals stabilised?

Answer 22

Hyperconjugation
As the radical forms, the carbon centre transforms from sp3 hybridised to sp2 hybridised
Delocalisation of the bonding electrons from carbon next door by overlap with partly filled p orbital
Net stabilising effect

Question 23

How do primary carbons act in hyperconjugation?

Answer 23

One bonding pair stabilise lone electron

Question 24

How do secondary carbons act in hyperconjugation?

Answer 24

Two bonding pairs stabilise lone electron

Question 25

How do tertiary carbons act in hyperconjugation?

Answer 25

Three bonding pairs stabilise lone electron

Question 26

Are primary, secondary or tertiary carbons most stable in hyperconjugation?

Answer 26

Tertiary