Chapter 12 Alkanes

Question 1

What are alkanes, and where are they found?

Answer 1

Saturated hydrocarbons which are often used in crude oil and natural gas.

Question 2

What is the general formula for alkanes?

Answer 2

Aliphatic= CnH2n+2
Alicyclic= CnH2n

Question 3

What type of bonds do alkanes contain, and how are they formed?

Answer 3

Sigma Bonds only
Formed via the head on overlap of orbitals between the bonding atoms
They can rotate freely

Question 4

What is the shape and bond angle of alkanes and why?

Answer 4

109.5 degrees, Tetrahedral
Around the central carbon atoms, there are 4 bonding regions and 0 lone pairs.
The 4 bonding regions repel equally, giving this bond angle and shape to minimise repulsion the most

Question 5

What is fractional distillation and why can it be carried out?

Answer 5

Fractional Distillation is separating alkanes into fractions, having similar chain lengths, thus having different specific uses.
This is possible as different chain lengths of alkanes have different boiling points

Question 6

How does chain length affect boiling point?

Answer 6

Longer chain lengths give larger boiling points.
Longer chain lengths have a greater surface area of contact between molecules. This increases the strength of the London Forces between these longer alkanes.
More energy is needed to overcome these stronger intermolecular forces, giving longer alkanes with longer chain lengths a larger boiling point.

Question 7

How does branching affecting boiling?

Answer 7

The more branching, the lower the boiling point.
More branching decreases the surface area of contact between molecules. This means the London Forces are weaker.
Less energy is needed to overcome these weaker intermolecular forces, giving more branched alkanes a lower boiling point.

Question 8

Why are alkanes generally unreactive?

Answer 8

The C-C and C-H bonds are strong, they have high bond enthalpies
The C-C bonds is not polar as the atoms have the same electronegativity.
The C-H is effectively not polar as the difference in electronegativity between C and H is too low

Question 9

What type of reactions, in what conditions, can alkanes undergo with oxygen? What are the products?

Answer 9

Complete combustion in a plentiful supply of oxygen. Produce only CO2 and Water
Incomplete combustion in a limited supply of oxygen. Produces CO, Water, and potentially carbon (soot)

Question 10

What is the name and condition of the reaction of alkanes with halogens?

Answer 10

Free Radical Substitution
Under UV light with either Bromine or Chlorine
(Iodine too unreactive, Fluorine would likely explode)

Question 11

What is the first step of the Methane and Chlorine? What is it called?

Answer 11

Initiation
Cl2 → 2Cl*

Question 12

What are the second steps of the reaction of Methane and Chlorine? What is it called?

Answer 12

Propagation
Cl* + CH4 → CH3 + HCl
CH3 + Cl2 → CH3Cl + Cl*

Question 13

Why is propagation a cycle?

Answer 13

The radical is regenerated in the second stage. Radicals are constantly produced which will keep on reacting, creating a cycle.

Question 14

What is the final steps of the reaction of Methane and Chlorine? What is it called?

Answer 14

Termination
Cl+Cl→ Cl2
CH3+CH3→ C2H6
Cl+CH3→CH3Cl

Question 15

Why are aliphatic alkanes often converted to cyclic/branched alkanes?

Answer 15

Cycloalkanes promote efficient combustion, burn easier

Question 16

Why does cyclopropane react with chlorine without UV light?

Answer 16

The bond angle of cyclopropane is 60 degrees, but is normally 109.5
There is lots of angle strain in cyclopropane, with more repulsion than there is regularily
It is most stable if the ring is broken, allowing addition reactions to be undergone.

Question 17

What are the limitations of free radical substitution of alkanes?

Answer 17

Further substitution: Halogen radicals will continue to substitute hydrogens on the alkane, producing a mixture of mono, di, tri … substituted haloalkanes
Substitution at different points in the carbon chain: The hydrogen replaced is different. A mixture of products will be formed, including isomers of each other. More isomers are possible when larger carbon compounds are formed. This makes the process undesirable for one specific compound.