Alkanes

Question 1

What is a hydrocarbon?

Answer 1

A molecule that only contains carbon and hydrogen
And is thus organic

Question 2

Alkane

Answer 2

A homologous series of hydrocarbon that does not have a functional group
Represented as being saturated

Question 3

General formula for alkanes

Answer 3

CnH2n+2

Question 4

Properties of alkanes

Answer 4

Saturated
Non polar
Increasing boiling and melting points up the homologous series with increasing carbon chain
Increasing viscosity with increasing carbon chain
Decreasing volatility with increasing carbon chain
types: cycloalkanes, branched alkanes and linear alkanes

Question 5

Saturated meaning

Answer 5

Alkanes contain no double bonds and thus the maximum possible number of hydrogen atoms

Question 6

How does the boiling/melting point in an alkane change?

Answer 6

Increases up the homologous series so more energy required to melt/boil it with a longer carbon chain
Because larger molecules means larger surface area for more London forces
Thus requires more energy to overcome

Question 7

How does branching affect the melting and boiling points?

Answer 7

Decreases it because branching causes less points of contact between molecules, so less London forces due to less induced dipoles
Therefore lowers bp/mp

Question 8

What do alkanes reacting with a halogen form?

Answer 8

A halogenoalkane

Question 9

How do alkanes and halogens react?

Answer 9

In photochemical reactions ie under the presence of UV light

Question 10

Why does the reaction between a halogen and an alkane require UV light?

Answer 10

Because UV light provides energy to break the bond between the halogens in the dihalogen molecule to form 2 radicals

Question 11

What is the name for halogen + alkane reaction?

Answer 11

Radical substitution
Because a hydrogen atom on the alkane is substituted for the radical halogen (a halogen atom with an unpaired electron)

Question 12

3 stages in formation of halogenoalkanes

Answer 12

Initiation
Propagation
Termination

Question 13

Initiation: summary

Answer 13

The formation of halogen radicals where the bond between halogen molecule is broken in presence of UV (photochemical reaction)

Question 14

Initiation: the bond breaks between halogen molecules details?

Answer 14

Named homolytic fission
Because the bond (shared pair of 2 electrons) is broken by one electron going to one halogen atom and the other going to other halogen atom = 2 radicals

Question 15

Initiation: how is the homolytic fission shown in a mechanism?

Answer 15

There are 2 curly arrows from the bond to each chlorine atom. With only half an arrow head to show 1 electron moved
Then arrow to show how in presence of UV light, 2 radicals are formed with an unpaired electron

Question 16

Propagation: summary

Answer 16

Radicals are used up only to produce more radicals

Question 17

Propagation: alkane + halogen radical

Answer 17

Forms a hydrogen halide because the hydrogen from the alkane is removed to form hydrogen halide because it reacts with halogen radical
And thus the remaining alkane becomes a radical because it lost the hydrogen

Question 18

Propagation: alkane radical + halogen

Answer 18

One of the halogen atoms react with alkane radical to form the halogenoalkane and the other halogen atom remains a radical

Question 19

Propagation: too much halogen issue

Answer 19

If there is too much of the halogen:
The desired halogenoalkane can react further with the halogen radical (because too much of it) to form a dihalogenoalkane by substituting it from another hydrogen
Can repeat to form trihalogenoalkanes etc

Question 20

Propagation: when does this stop?

Answer 20

When all reactant molecule have been wiped so only radicals remain

Question 21

Termination: summary

Answer 21

The removal of radicals by reacting radicals together to form stable molecules

Question 22

Termination: What are the 3 possible reactions?

Answer 22

Halogen radical + halogen radical = halogen diatomic molecule
Halogen radical + alkane radical = halogenoalkane (desired product)
Alkane radical + alkane radical = long chain alkane molecule

Question 23

Main problems with radical substitution

Answer 23

You form a mixture of products which is not the desired product
You can form a mixture of structural isomers

Question 24

How can a mixture of products be formed?

Answer 24

Having excess halogens
So the halogen radicals react with desired product (halogenoalkane) by displacing a hydrogen from it or form a dihalogenoalkane
Then repeated = trihalogenoalkane, tetrahalogenoalkane which is repeated till no more hydrogen left to be displaced

Question 25

How to reduce the problem of having a mixture of products

Answer 25

Reacting with excess alkane
So a halogen radical has greater chance of colliding with excess alkane molecule than the desired product

Question 26

How can a mixture of structural isomers be formed?

Answer 26

If you start with a long chain alkane eg propane then the halogen radical can substitute any of the hydrogen along the chain to form a mixture of structural isomers eg 1-chloropropane or 2-chloropropane