Module 4.1.2 - Alkanes

Question 1

Are alkanes polar or non-polar molecules?

Answer 1

Non-polar

Question 2

What type of hydrocarbon are alkanes?

Answer 2

Saturated hydrocarbons

Question 3

What type of bonds occur between C-H and C-C?

Answer 3

Sigma bonds

Question 4

What is a sigma bond?

Answer 4

Formed between two carbon atoms by the direct single overlap of orbitals directly between bonding atoms, which allows free rotation of the sigma bond

Question 5

What is the bonding in alkanes? (3)

Answer 5

• saturated hydrocarbons
• C-C and C-H made up of sigma bonds
• by the direct single overlap of orbitals directly between bonding atoms

Question 6

For what energy needed to boil a liquid alkane?

Answer 6

Energy is needed to break the induced dipole-dipole attraction forces between the molecules

Question 7

What happens to the boiling point as the carbon chain length increases? (4 points)

Answer 7

Boiling point increases,
as more surface contact between molecules (and more electrons),
so there are more induced dipole-dipole interactions between the molecules,
which need more energy to overcome

Question 8

What happens to the boiling point of a branched isomer?

Answer 8

A branched isomer has a lower boiling point than the unbranched isomer
In a more branched alkane, less surface contact between molecules meaning fewer induced dipole-dipole interactions,
therefore need less energy to break the weaker London forces between molecules

Question 9

What is the trend in boiling point as the number of branches increase?

Answer 9

As the number of branches increases, the boiling point decreases

Question 10

How do you name alkanes?

Answer 10

• Find longest carbon chain (parent chain), name the stem
• Name any alkyl groups attached to parent chain, put in before stem
• Number alkyl groups from the end to give the lowest numbers possible
• With multiple alkyl groups, list in alphabetical ordered in the name
• Cyclic alkanes have the prefix -cyclo

Question 11

What shape and bond angle do alkanes have?

Answer 11

• Tetrahedral
• 109.5 degrees
• 4 bond pairs around the C atom repel each other equally and get as far apart as possible

Question 12

Why are alkanes relatively unreactive?

Answer 12

C-C and C-H sigma bonds are non-polar and strong (and similar electronegativities)

Question 13

What two chemical reactions do alkanes undergo?

Answer 13

• Combustion
• Halogenation

Question 14

What is combustion of alkanes?

Answer 14

An exothermic reaction as heat energy is released to the surroundings (alkanes good as fuels)

Question 15

What is complete combustion of alkanes?

Answer 15

In plentiful supply of oxygen, alkanes burn completely to produce CO2 and H2O

Question 16

What is incomplete combustion of alkanes?

Answer 16

In limited supply of oxygen, alkanes burn incompletely to produce CO and H2O
(CO is toxic gas and prevents haemoglobin binding with oxygen)

Question 17

What reaction mechanism does halogenation of alkanes undergo?

Answer 17

Free radical substitution

Question 18

What are the reagents of halogenation of alkanes?

Answer 18

• alkane
• halogen (F2, Cl2, Br2, I2)

Question 19

What are the conditions for the halogenation of alkanes?

Answer 19

UV light/300 degrees celsius (+alkane in excess)

Question 20

What happens in the initiation stage of free radical substitution (in halogenation of alkanes)?

Answer 20

The halogen splits into two radicals by homolytic fission

Question 21

What must you show in free radical substitution reactions?

Answer 21

The species which has the radical, represented by using a dot

Question 22

What occurs in the propagation stage of free radical substitution (of halogenation of alkanes)?

Answer 22

1st step: Halogen (X) radical attacks the alkane molecule and the alkane forms a radical as hydrogen from alkane forms HX (one less hydrogen atom in alkane molecules)
2nd step: diatomic halogen and alkane radical form the haloalkane and the halogen radical regenerates

Question 23

What happens in termination stage of free radical substitution (of halogenation of alkanes)?

Answer 23

The reaction ends when two radicals combine forming a new molecule (add the radicals together)

Question 24

What are the disadvantages of free radical substitution? (3)

Answer 24

• Low % yield of desired haloalkane
• Separation from the product mixture, by fractional distillation, is costly
• Excess of alkane needs to be bought in bulk

Question 25

What are the limitations of free radical substitution? (3)

Answer 25

• Leads to formation of a mixture of products
• Further substitution means mixture of haloalkanes is produced (until all halogen atoms replaced)
• Reactions at different positions in carbon chain (different structural isomers - name examples in an exam of different isomers that can form)