[12.2] chemical reactions of alkanes

Question 1

why are alkanes unreactive with most common reagents?

Answer 1

• C-C and C-H sigma bonds are strong (so difficult to lose electrons?)
• C-C bonds are non-polar (only have london forces)
• the electronegativity of carbon and hydrogen is so similar that the C-H bond can be considered to be non-polar

Question 2

what is a combustion reaction?

Answer 2

when alkanes react with a plentiful supply of oxygen to produce carbon dioxide and water
- gives out heat

Question 3

what is the equation for balancing a complete combustion reaction of any alkane?

Answer 3

CₓHᵧ + (x + y/4)O₂ -> xCO₂ + 0.5yH₂O

Question 4

what happens during incomplete combustion?

Answer 4

• when oxygen is limited in combustion
• hydrogen is always oxidised to water
• combustion of carbon is incomplete, so forms soot (C) or carbon monoxide (CO) instead

Question 5

how do alkanes react with halogens?

Answer 5

• in the presence of sunlight, alkanes react with halogens. the high-energy ultraviolet (UV) radiation present in sunlight provides the initial energy for a reaction to take place
• this can only happen with chlorine and bromine, as fluorine is too reactive and iodine is not reactive enough

Question 6

what is free radical substitution?

Answer 6

when H atoms on alkanes or halogenalkanes are replace by halogen atoms

Question 7

what are the 3 stages of free radical substitution?

Answer 7

1. initiation
2. propagation
3. termination

Question 8

what occurs during the initiation stage?

Answer 8

• reaction starts when the covalent bond in a halide molecule is broken by homolytic fission
• two halogen atom free radicals are produced
• energy for this bond fission is provided by UV radiation
• eg. F₂ -> 2F●

Question 9

what occurs during the propagation stage?

Answer 9

(molecule + radical -> molecule + new radical)
- for every H that is replaced, there is one pair of propagation reactions
1. the alkane / halogenalkane reaction with the halide radical (eg. F●). this removed a H atom from the alkane / halogenalkane, which produces HF (eg.) and a C based free radical
2. the C based radical from step 1 reactions with F₂ (eg.) to put an atom of halide onto the C based radical. this also produces another halogen free radical (eg. F●) to continue the chain reaction in another step 1

Question 10

what occurs during the termination stage?

Answer 10

(2 radicals -> molecule)
- if two free radicals collide, they will form a molecule and stop the chain reaction
- any two free radicals involved in the mechanism could collide in this way

Question 11

example overall equation (methane + chlorine)

Answer 11

CH₄ + Cl₂ -> CH₃Cl +HCl

Question 12

example of propagation reactions (methane + chlorine)

Answer 12

1. CH₄ + Cl● -> ●CH₃ + HCl
2. ●CH₃ + Cl₂ -> CH₃Cl + Cl●

Question 13

example of termination reactions (methane + chlorine)

Answer 13

• Cl● + ●Cl -> Cl₂
• ●CH₃ + ●CH₃ -> CH₃-CH₃
• ●CH₃ + ●Cl -> CH₃Cl

Question 14

limitations of free radical substitution

Answer 14

• further substitution results in a mixture of products eg. mono, di, tri, and tetra substituted products
• if the carbon chain is longer, we will get a mixture of monosubstituted isomers by substitution at different positions in the carbon chain (eg. pentane could form 3 monosubstituted isomers)