Hydrocarbons

Question 1

List the general properties of alkanes

Answer 1

• saturated hydrocarbon: all single bonds
• General formula: C_nH_2n+2
• sp³ carbon hybridisation (only sigma σ bonds)
• tetrahedral shape around C (109.5^o bond angle)
• only chain isomerism possible
• Van der Waals’ forces only
• unreactive as non-polar and C-C/C-H bonds are strong, no pi π bonds
• 1st four are gases, next 12 are liquids, rest are solids (at r.t.p.)
• volatility increases with M_r, with branched molecules more volatile
• insoluble in water as non-polar and no ions, soluble in organic solvents e.g. CCl₄

Question 2

Outline the combustion of alkanes

Question 3

Describe the problems cause by burning hydrocarbons

Answer 3

• Incomplete combustion produces carbon monoxide CO which is toxic and bonds with haemoglobin in blood
• Unburnt hydrocarbons can be released into the atmosphere; some are carcinogens
• Catalytic converters are used to oxidise CO to CO2, unburnt hydrocarbons to CO2 and H2O, and reduce nitrogen oxides to N2
• CO2 is a greenhouse gas, causing global warming

Question 4

Define

free radical substitution

and give an example

Answer 4

The reaction in which halogen atoms subtitute for hydrogen atoms in alkanes, involving the three mechanism steps of initiation, propagation and termination.

Example: methane + chlorine can give chloromethane, dichloromethane, trichloromethane and/or tetrachloromethane

This is a photochemical chain reaction requiring UV radiation to provide energy for the formation of free radicals.

Question 5

Describe the mechanism of free radical substitution in alkanes

Answer 5

1. Initiation: homolytic fission of halogen bond to form two halogen free radicals
2. Propagation: requires 1 free radical and 1 neutral molecule
   
   1. homolytic fission of C-H bond to form alkyl free radical and hydrogen free radical which reacts with halogen free radical
   2. homolytic fission of halogen bond to form halogen free radical, one of which reacts with alkyl free radical
3. Termination: two free radicals react, removing free radicals so reactions stop

Question 7

What is cracking and how is it useful?

Answer 7

This is a reaction in which long-chain alkane molecules are broken down to shorter molecules of lower M_r containing the C=C double bond using the following steps:

1. The long-chain hydrocarbon is heated to vaporise it
2. The vapour is passed over a hot catalyst
3. Thermal decomposition takes place

Cracking produces a shorter alkane and an alkene that is more reactive due to the weaker π bond; both can have more uses than the original long-chain hydrocarbon.

Question 9

Describe the addition reaction of alkenes with hydrogen

Answer 9

• forms an alkane
• requires finely divided nickel catalyst at 140^oC
• use: convert liquid oils to saturated solid fats

Mechanism (electrophilic addition):

1. induced dipole on H₂ due to high electron density of π bond, δ⁺ attracted to π bond
2. heterolytic fission of H-H to form ions, π bond e’s used in dative covalent bond with H⁺
3. H^- has lone pair used in dative covalent bond to carbocation

Question 10

Describe the addition reaction of alkenes with steam

Answer 10

• forms an alcohol
• requires H₃PO₄, 330^oC at 6 MPa

Mechanism (electrophilic addition):

1. H-OH is polar, δ⁺ attracted to π bond due to high electron density of π bond
2. heterolytic fission of H-OH to form ions, π bond e’s used in dative covalent bond with H⁺
3. OH^- has lone pair used in dative covalent bond to carbocation
   
   • for asymmetric alkenes, OH is added to C that has the most H’s (as intermediate carbocation is more stable)

Question 11

Describe the addition reaction of alkenes with hydrogen halides (HX)

Answer 11

• forms haloalkane
• at r.t.p.

Mechanism (electrophilic addition):

1. H-X is polar, δ⁺ attracted to π bond due to high electron density of π bond
2. heterolytic fission of H-X to form ions, π bond e’s used in dative covalent bond with H⁺
3. X^- has lone pair used in dative covalent bond to carbocation
   
   • for asymmetric alkenes, X is added to C that has the most H’s (as intermediate carbocation is more stable)

Question 12

Describe the addition reaction of alkenes with halogens (X₂)

Answer 12

• forms dihaloalkane
• requires organic solvent e.g. CCl₄

Mechanism (electrophilic addition):

1. induced dipole on X₂ due to high electron density of π bond, δ⁺ attracted to π bond
2. heterolytic fission of X-X to form ions, π bond e’s used in dative covalent bond with X⁺
3. X^- has lone pair used in dative covalent bond to carbocation

Question 13

State and explain Markovnikov’s rule

Answer 13

For addition reactions with an asymmetric alkene,