Alkanes

Question 1

What are alkanes?

Answer 1

• straight branched hydrocarbon chains
• single covalent bonds between carbon atoms
• homologous series: successive compounds in the alkane series differ by 1 carbon and 2 hydrogens (CnH2n + 2 (n= number of carbons))

Question 2

Sources of alkanes

Answer 2

natural gas
- anaerobic decay plant/animal matter via microbes

Petroleum

• decomposition of plants/animals over millions of years
• mixture of hydrocarbons with smaller amounts sulphur and nitrogen containing organic compounds: separated by fractional distillation

Question 3

What are the uses of alkanes

Answer 3

• gas
• petroleum
• fuel

Question 4

Alkanes in biology

Answer 4

• floral scents
• animal pheromones
• plant waxy cuticle

Question 5

Naming system of alkanes

Answer 5

Prefix (where located) - parent (how many carbons) - suffix (what family does it belong to)

• alkanes suffix = ‘ane’
• parent = longest continuous carbon chain
• name each branch of alkyl group and position on parent chain ( e.g. 2-methyl = methyl group on carbon 2)
• name alkyl groups alphabetically (if branches the same indicate with prefix e.g. di, tri)

Question 6

Chemical formula of methyl

Answer 6

CH3

Question 7

Chemical formula ethyl

Answer 7

CH2CH3

Question 8

Chemical formula propyl

Answer 8

CH2CH2CH3

Question 9

isomers

Answer 9

= 2 molecules with the same molecular formula but with different 3D structures

Question 10

Constitutional (structural) isomers

Answer 10

= 2 molecules with the same molecular formula but different arrangements of atoms

Question 11

Branched alkanes properties

Answer 11

• smaller surface area = dont pack as well together

- fewer intermolecular interactions so lower boiling/melting points than straight chain

Question 12

Straight chain alkane properties

Answer 12

At room temperature:

• small molecules = gasses
• larger molecules = liquids/solids

The longer the chain the more interactions (van der waals) = takes more energy to break it apart: higher melting/boiling point than branched chains

Question 13

Conformation

Answer 13

The orientation of atoms in a molecule

Question 14

What are the 2 readily available conformations of alkanes (when 2+ bond)

Answer 14

Eclipsed (the hydrogens almost align)

Staggered ( the hydrogens dont align and fill alternate spaces)

Question 15

The parent name for alkanes (up to 6 carbons)

Answer 15

1 = meth
2 = eth
3 = prop
4 = but
5 = pent
6 = hex

Question 16

Give the prefixes of alkanes (first 4)

Answer 16

Methyl
Ethyl
Propyl
Butyl

Question 17

What prefix is used for hydrocarbon ring molecules

Answer 17

Cyclo

Question 18

Stereoisomerism

Answer 18

When 2+ compounds have the same structural formula but differ in arrangement of the bonds in space (how rotations are formed around single bonds)

Question 19

What are the 3 different types of stereoisomerism

Answer 19

• conformational
• geometric
• optical

Question 20

When chlorine’s are added to an alkane what kind of conformations can be formed?

Answer 20

• fully eclipsed
• gauche (skewed)
• eclipsed
• anti (staggered)

Lowest energy when chlorine’s facing completely away from eachother

Question 21

Cycloalkanes

Answer 21

• hydrocarbons form ring structure
• formula: CnH2n with sp3 hybridised carbons
• restricted rotation of c-c bonds = cyclic structures
  2 configurations
• chair (all 6 conformations gauche = more energetically favourable)
  -boat ( 4 gauche and 2 eclipsed conformations)

Question 22

Cis isomers

Answer 22

In cycloalkanes when 2+ substituents are pointing the same direction

Question 23

Trans isomers

Answer 23

In cycloalkanes when 2+ substituents are pointing opposite directions

Question 24

What are the 3 main reactions of alkanes

Answer 24

• combustion
• cracking
• dehydrogenation
• halogenation

Question 25

Combustion

Answer 25

Alkanes react with oxygen to create carbon dioxide and water
- releases heat and have large negative enthalpies of combustion

Question 26

Cracking

Answer 26

• large alkanes break into small alkanes and alkanes

- more useful when small

Question 27

Dehydrogenation

Answer 27

• takes hydrogen molecule away fom alkane and to make an alkene

Question 28

Halogenation

Answer 28

• replacing a hydrogen in an alkane with a halogen (happens in bright light) and forms a halogenoalkane = radical reaction

Starts chain reaction (3 stages: initiation, propagation, termination)

Question 29

Explain the initiation stage of halogenation

Answer 29

• break the Cl-Cl bond = 2 Chlorine atoms
• is a homolytic break = one electron goes to each
• creates 2 free radicals ( very reactive)

Question 30

Explain the propagation stage of halogenation

Answer 30

• Chlorine free radical takes hydrogen from methan to form hydrogen chloride
• creates methyl free radical
• methyl free radical reacts with a chloride = another free radical Coloring and molecule of chloromethane

This step can repeat over and over again

Question 31

Explain the termination stage of halogenation

Answer 31

Free radicals are removed by either:

• 2 chlorine free radicals reacting to create chlorine
• 2 methyl free radicals reacting tom create ethane
• 1 chlorine free radical and methyl free radical react together to give chloromethane

Question 32

What is a primary carbon

Answer 32

A carbon bonded to 1 other carbon

Question 33

What is a secondary carbon

Answer 33

A carbon bonded to 2 other carbons

Question 34

What is a tertiary carbon

Answer 34

A carbon bonded to 3 other carbons

Question 35

What is the relative reactivity of primary : secondary : tertiary and why?

Answer 35

1 : 4 : 5

Caused by hyper conjugation
- forms a radical which is stabilised by hyperconjugation
- carbon centre changes from sp3 to sp2
- delocalisation of bonding electrons from carbon next door overlap and partially fill p orbital = net stabilising effect
The more binding pairs (prim, sec, tert) the H can contribute their electronegativity and make more stable