12.2 Chemical Reactions of Alkanes

Question 1

Do alkanes generally have a high or low reactivity?

Answer 1

low

Question 2

Why do alkanes have a low reactivity?

Answer 2

• CーC + CーH σ-BONDS STRONG
• NON-POLAR CーC
• NON-POLAR CーH

Question 3

Which are weaker, non-polar or polar bonds?

Answer 3

polar

(covalent bonds break heterolytically when shared pair of electrons are taken by 1 of the bonded atoms. In polar bonds, the shared pair is already closer to one of the bonded atoms. So, polar bonds are closer to breaking than the non-polar and therefore weaker.)

Question 4

What is the general word equation for the combustion of an alkane?

Answer 4

Alkane + Oxygen ⟶ Carbon Dioxide + Water

Question 5

What is the general word equation for the incomplete combustion of an alkane?

Answer 5

alkane + oxygen ⟶ carbon monoxide/carbon + water

Question 6

What are the products of complete combustion of an alkane?

Answer 6

carbon dioxide + water

Question 7

What are the products of incomplete combustion of an alkane?

Answer 7

carbon dioxide and/or carbon

+ water

(carbon is formed in even lower levels of oxygen)

Question 8

What are the conditions of combustion?

Answer 8

High temperature

High pressure

Question 9

How do the conditions of complete and incomplete combustion differ?

Answer 9

complete combustion = plentiful O₂

incomplete combustion = limited supply of O₂, not enough for reactants to form carbon dioxide

Question 10

What is the general equation for the reaction of alkanes with halogen?

Answer 10

ALKANE + HALOGEN ⟶ HALOALKANE + HYDROGEN HALIDE

(using UV light)

Question 11

Under what conditions do alkanes react with halogens?

Answer 11

UV light

Question 12

What is the reaction mechanism for the reaction for the halogenation of alkanes?

Answer 12

radical substitution

Question 13

Methane + Bromine ⟶

Answer 13

Bromomethane + Hydrogen Bromide

Question 14

Outline the reaction mechanism for the following reaction & label the steps:

CH₄ + Br₂ ⟶ CH₃Br + HBr

Answer 14

INITIATION:

Br₂ ⟶ 2Br•

using UV light

PROPAGATION:

Br• + CH₄ ⟶ •CH₃ + HBr

•CH₃ + Br₂ ⟶ CH₃Br + Br•

TERMINATION:

2Br• ⟶ Br₂

2•CH₃ ⟶ C₂H₆

Question 15

What are termination reactions in radical substitution?

Answer 15

reactions which take radicals out of system

Question 16

The propagation step in the halogenation of alkanes is described as a “chain reaction”. Why?

Answer 16

the Br radical reacts to form new radicals,

which allows for further/subsequent reactions to take place.

Question 17

The propagation step in the halogenation of alkanes can carry on producing products until…

Answer 17

all radicals are taken out of the system.

Question 18

What is the limitation of using radical substitution to produce a particular isomer haloalkane?

Answer 18

Br radical can be substituted at different points on the chain

forming different isomers.

or multiple substitutions can occur.

Question 19

Outline the mechanism for which C₃H₆Cl₂ is converted to C₃H₅Cl₃:

Answer 19

RADICAL SUBSTITUTION

INITIATION: Cl₂ ⟶ 2Cl•

PROPAGATION:

C₃H₆Cl₂ + Cl• ⟶ •C₃H₅Cl₂ + HCl

•C₃H₅Cl₂ + Cl₂ ⟶ C₃H₅Cl₃ + Cl•

TERMINATION:

2Cl• ⟶ Cl₂

2•C₃H₅Cl₂ ⟶ C₆H₁₀Cl₄

•C₃H₅Cl₂ + Cl• ⟶ C₃H₅Cl₃