Write the abbreviated structural formulae and name all the structural isomers of C6H14.

Warm-up 11/19

10.2 AlkanesReactivity and Reactions

General Formula CnH2n+2

Low Reactivity◦ Be able to explain why.◦ Saturated hydrocarbons◦ Strong C-C and C-H bonds

Relatively high bond enthalpies◦ Low polarity

Why?

Combustion reactions with oxygen

Substitution reactions with halogens

10.2.1 Alkanes

Alkanes are flammable Highly exothermic One of the most common

energy sources CnH 2n+2 + O2 CO2 +

H2O Balancing not difficult if

you use the “shortcut”

10.2.2 Combustion Reactions

1. All carbon atoms are converted into Carbon Dioxide

-fixes the carbon dioxide coefficient

2. All hydrogen atoms are converted to water -fixes the water coefficient3. Lastly balance the oxygen molecules

Balancing Combustion Reactions

C8H18 + O2 CO2 + H2O

First balance the carbons:

C8H18 + O2 8 CO2 + H2O

Example: Complete Combustion of Octane

Next balance water: C8H18 + O2 8 CO2 + 9 H2O

Balance Oxygen:

C8H18 + 25/2 O2 8 CO2 + 9 H2O

2 C8H18 + 25 O2 16 CO2 + 18 H2O

If there is not enough oxygen available, incomplete combustion occurs and CO and C are also produced as products

CO—colorless, odorless, toxic gas

C—black smoke These products along with

lead additives in gasoline are a major source of pollution

10.2.2 Incomplete Combustion

CH4 + O2 CO + H2O

Balanced:

CH4 + 3/2 O2 CO + 2 H2O

2CH4 + 3 O2 2 CO + 4 H2O

Incomplete Combustion

Alkanes do not react with halogens in the dark at room temperature, but will react in the presence of sunlight: C6H14 (g) + Br2 (g) → C6H13Br (l) + HBr (g)

Reactions of Alkanes: with Halogens

Many organic molecules undergo substitution reactions.

In a substitution reaction one atom or group of atoms is removed from a molecule and replaced with a different atom or group.

Example: Cl2 + CH4 CH3Cl + HCl

Free Radical Substitutions

11

The ultraviolet light is a source of energy that causes the chlorine molecule to break apart into 2 chlorine atoms, each of which has an unpaired electron

The energies in UV are exactly right to break the bonds in chlorine molecules to produce chlorine atoms.

Free Radical Mechanism-The Initiation Step

12

Free radicals are formed if a bond splits evenly - each atom getting one of the two electrons. The name given to this is homolytic fission.

Homolytic Fission

13

The productive collision happens if a chlorine radical hits a methane molecule.

The chlorine radical removes a hydrogen atom from the methane. That hydrogen atom only needs to bring one electron with it to form a new bond to the chlorine, and so one electron is left behind on the carbon atom. A new free radical is formed - this time a methyl radical, CH3 .

Free Radical Propagation

14

If a methyl radical collides with a chlorine molecule the following occurs:

CH3.  +  Cl2 CH3Cl  +  Cl.

The methyl radical takes one of the chlorine atoms to form chloromethane

In the process generates another chlorine free radical.

This new chlorine radical can now go through the whole sequence again, It will produce yet another chlorine radical - and so on and so on.

Free Radical Propagation II

15

The free radical propagation does not go on for ever.

If two free radicals collide the reaction is terminated.

2Cl. Cl2

CH3.  +  Cl . CH3Cl

CH3 .  +  CH3

. CH3CH3

Termination Steps

16

Write the steps in the free radical mechanism for the reaction of chlorine with methyl benzene. The overall reaction is shown below. The methyl group is the part of methyl benzene that undergoes attack.

Exercise

17

Initiation Cl2 2Cl. Propagation

Solution

Termination 2Cl. Cl2

18

Substitution of an alkane with a halogen has 3 steps:

1. Initiation2. Propagation3. Termination

Rate of reaction: Cl2 > Br2 > I2 … Why???

Reactions of Alkanes: with Halogens

1. Explain why alkanes have such low reactivity. 2. What types of reactions do alkanes readily

undergo? Give an example. 3. Use an equation to describe the incomplete

combustion of alkanes. 4. Describe, using equations, the reactions of

methane and ethane with chlorine and bromine. 5. Explain the reactions of methane and ethane

with chlorine and bromine in terms of a free-radical mechanism. Reference should be made to homolytic fission and the reaction steps of initiation, propagation and termination.

Homework

1. When propane undergoes complete combustion in air, for each mole of propane burnt, how many moles of oxygen are consumed and how many moles of water are formed?

Warm-up 11/20