14. Hydrocarbons

Question 1

What are the reactions which produce alkanes?

Answer 1

Hydrogenation: addition of hydrogen to an alkene, Pt/Ni catalyst and heat
Cracking of a longer chain alkene, heat with Al2O3

Question 2

Why are alkanes generally unreactive?

Answer 2

They are made up of single C-C and C-H covalent bonds which require a lot of energy to break
They are non-polar as very small difference in electronegativity, meaning they are not very reactive with polar reagents

Question 3

Complete combustion of alkanes:

Answer 3

Occurs in excess oxygen
Produces water and carbon dioxide

Question 4

Incomplete combustion of alkanes

Answer 4

Occurs in insufficient oxygen
Produces carbon particulates, carbon monoxide and some carbon dioxide

Question 5

Why do alkanes make good fuels?

Answer 5

They release huge amounts of energy when burnt
Relatively readily available
Easy to transport

Question 6

How do alkanes react with halogens?

Answer 6

They undergo free radical substitution in the presence of UV light

Question 7

What are the stages of free radical substitution?

Answer 7

Initiation: UV light splits covalent bond in Cl2 to form two chlorine atoms each with an unpaired electron (free radicals)

Propagation: Chlorine free radicals are used up in reactions with alkanes, producing an alkyl radical which reacts with Cl2 to form more free radicals

Termination: All the free radicals are completely used up in 3 different reactions (chorine radical plus alkyl radical, chlorine plus chlorine, alkyl plus alkyl)

Question 8

Why is cracking performed?

Answer 8

Shorter alkane chains are more in demand and useful than the heavier fractions

Question 9

What are the two types of cracking and the conditions required for each?

Answer 9

Thermal cracking: high temps (1000℃) , high pressures (70atm), produces lots of alkenes

Catalytic cracking: high temps (450℃), aluminium oxide catalyst, slight pressure, produces mostly aromatic hydrocarbons and motor fuels

Question 10

How is carbon monoxide produced from incomplete combustion toxic?

Answer 10

It binds irreversibly to the haemoglobin in red blood cells, preventing oxygen from being transported around the body, causing death

Question 11

How are oxides of nitrogen formed and how are they harmful?

Answer 11

Nitrogen and oxygen from the air react in car engines due to high temperatures and pressures to produce oxides of nitrogen
This rises as gas and dissolves in rain clouds
Acid rain falls, which damages aquatic life and vegetation

Question 12

What are the reactions which produce alkenes?

Answer 12

Cracking of a vaporised longer chain alkane
Dehydration of an alcohol with heated Al2O3 catalyst
Elimination of halogenoalkane using ethanol NaOH and heat

Question 13

What is electrophilic addition?

Answer 13

The addition of an electrophile (electron pair acceptor) to a double bond
The C=C double bond is broken, and a new single bond is formed from each the two carbon atoms

Question 14

Describe the reaction of alkenes with hydrogen

Answer 14

Electrophilic addition, hydrogenation reaction to form alkanes
Conditions: excess hydrogen, Pt/Ni catalyst and 150℃
C=C double bond opens up to form covalent bonds with the 2 new hydrogen atoms

Question 15

Describe the reaction of alkenes with steam

Answer 15

Electrophilic addition, hydration reaction to form alcohols
Conditions: water vapour, phosphoric acid catalyst
-OH group joins to the carbon atom bonded to the most carbon atoms to produce major product (Markovnikov’s rule)

Question 16

Describe the reaction of alkenes with hydrogen halides

Answer 16

Electrophilic addition to form a halogenoalkane
Hydrogen halide is polar due to difference in electro negativity
Electron pair in double bond attracts hydrogen electrophile, forms C-H bond, positive carbocation intermediate
Hydrogen bonds to carbon atom bonded to the most hydrogen atoms (Markonikov’s rule)
The negative halide bonds to carbon atom bonded to most others to form primary product

Question 17

Describe the reaction of alkenes with halogens

Answer 17

Electrophilic addition to form a di-halogenoalkane
Electron dense double bond causes uneven distribution of electrons in halogen, inducing a dipole
The electron pair attracts the halogen electrophile, forming a C-X bond
This produces a positive carbocation intermediate, attracting the negative halogen

Question 18

Describe the reaction of alkenes with cold dilute acidified KMnO4

Answer 18

Oxidation reaction to form diols
KMnO4 is a strong oxidising agent, manganate (VII) ions oxidise alkenes
Purple solution decolourises

Question 19

What does Markownikoff’s rule say?

Answer 19

The hydrogen electrophile adds to the carbon atom bonded to the most hydrogen atoms to form the most stable (secondary) carbocation intermediate
This leaves the nucleophile to bond to the carbon atom bonded to the most other carbon atoms

Question 20

How can aqueous bromine be used to show the presence of a C=C bond?

Answer 20

Alkenes decolourise bromine water (orange-brown to colourless)

Question 21

Describe the inductive effects of alkyl groups in electrophilic addition

Answer 21

When a hydrogen halide bonds to an unsymmetrical alkene, there are two possible products
Carbocations with more alkyl groups are more stable (secondary/tertiary carbocations) as they gave a positive inductive effect
These are produced more quickly, therefore they are the major product