4A - Alkanes

Question 1

General formula of:

1. Alkanes
2. Cycloalkanes
3. Alkynes

Answer 1

1. Alkanes = C_nH_2n+2
2. Cycloalkanes = C_nH_2n
3. Alkynes = C_nH_2n-2

Question 2

A venn diagram to show the relationship between aliphatic, alicyclic and aromatic compounds?

Answer 2

Question 3

Give the 3 ways that structural isomerism can occur.

What happens to the hydrocarbon chain? What else is different?

Answer 3

Chain isomers = Hydrocarbon chain is branched or unbranched.

Position isomers = Hydrocarbon chain stays the same, but functional group is found in different positions along the chain.

Functional group isomers = Functional group is differnt.

Question 4

Alkanes like CH₄, C₂H₆… are known as “straight chain” alkanes. Explain why the carbon atoms are not arranged in a straight line, but a zigzag instead.

Answer 4

The other 3 electrons on the carbon atom repel against each other which causes the carbon-carbon bond to be at an angle.

Question 5

The trend in boiling point in relation to the number of carbons in alkanes?

Explain why this is?

Answer 5

As the number of carbon atoms in the alkane increase, the boiling point increases.

There’s more electrons present in the molecule so there’s stronger london forces. More energy is required to break the bonds so the boiling point increases.

Question 6

Why are alkanes relatively unreactive? (2)

Answer 6

Alkanes only contain C-C single bonds and C-H bonds. The C-C and C-H onds are very strong/stable so they’re difficult to break.

C-C bond is also non-polar (since the electronegativity of carbon and hydrogen are so similar) so it’s difficult for alkanes to attach other molecules.

Question 7

Word equation for:

1. Complete combustion of alkanes.
2. Incomplete combustion fo alkanes

Answer 7

1. Alkane + O₂ → CO₂ + H₂O
2. Alkane + O₂ → CO + H₂O

Question 8

Diesel cars traditionally produced aa lot more soot than petrol cars. Suggest why this might be expected.

Answer 8

Diesel contains longer chains of hydrocarbons than petrol

Question 9

What is a free radical substitution reaction?

Answer 9

The reaction of an alkane and a halogen

Question 10

Name the mechanism and the type of bond fission that occurs when you have: alkane + halogen

Give any conditions required for this reaction. Why is it needed?

Answer 10

Mechanism = Radical substitution

Bond fission = Homolytic fission

Conditions = UV light - Only UV light provides enough energy to break the bonds in the halogen.

Question 11

What are the 3 stages of a radical substitution reaction? It’s relation to the number of radicals made or used?

Answer 11

Initiation (makes 2 radicals)

Propagation (uses 1 and makes 1)

Termination (uses 2 to make a compound)

Question 12

Give the equations for the mechanism of:

Butane + Bromine → ?

Also give the full quation of the reaction.

Answer 12

Full equation: C₄H₁₀ + Br₂ → C₄H₉Br + HBr

Initiation:

Br₂ → Br• + Br•

Propagation:

C₄H₁₀ + Br• → C₄H₉• + HBr

C₄H₉• + Br₂ → C₄H₉Br + Br•

Termination:

Br• + Br• → Br₂

C₄H₉• + Br• → C₄H₉Br

C₄H₉• + C₄H₉• → C₈H₁₈

Question 13

Limitations of using radical substitution in organic synthesis? (3)

Answer 13

• Substitution can occur anywhere on the hydrocarbon chain.
• Further substitution can occur. This makes it hard to make one desired product.
• Carbon chain may change in length (e.g. it can double in the termination stage)

Question 14

Explain the increase in boiling points of the cycloalkanes shown in the table

Answer 14

Boiling point increases as the number of carbons in the cycloalkane increase.

There’s stronger intermolecular forces, more london forces so more energy is required to break the bonds so boiling point increases.