PowerPoint to accompany

Chapter 21

Alkanes: Scaffolds for

Organic Chemistry

Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia

Reactions of Alkanes   Because of its structural role, alkanes are

relatively unreactive.   They make excellent nonpolar solvents.

  Forcing conditions are required for a reaction   Combustion.   Free-radical reactions.

  In order to be able to predict the product of a reaction, it is important to be able to distinguish between carbon (and hydrogen) types in an alkane.   Primary, secondary, tertiary and quaternary carbon

atoms.   Primary, secondary and tertiary hydrogens.

Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia

Reactions of Alkanes

The five most important organic reaction types are:

  Substitution of one atom or group of atoms by another atom or group of atoms (substitution reaction).

  Addition of one molecule to another to give a new molecule (addition reaction). Addition normally takes place across a double or triple bond.

  Elimination of two atoms or group of atoms from a molecule (elimination reaction). This reaction is the reverse of the addition reaction.

  Conversion of one structure to an isomeric struc- ture (rearrangement reaction).

  Oxidation and reduction are reactions that involve the loss or gain of electron density by a carbon respectively.

Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia

Reactions of Alkanes

Figure 21.25

  In alkenes, hydrogen and carbon atoms are classified as follows:

Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia

Reactions of Alkanes

Figure 21.23

Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia

Combustion

  The complete combustion of any alkane yields CO2 and H2O as the only products.

  Combustion reactions are exothermic.

2C2H6(g) + 7O2(g) 4CO2(g) + 6H2O(l) ΔH = -2855 kJ

Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia

Combustion - Napalm

  The commonly quoted composition is 21% benzene, 33% gasoline, and 46% polystyrene. This mixture is difficult to ignite.

  Movie – Vietnam.

Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia

Free-Radical Reactions

  Consider the following halogenation reactions: Reaction 1

Reaction 2

CH4 + Cl2 no reaction

CH4 + Cl2 CH3Cl + CH2Cl2 + CHCl3 + CCl4

dark

light

This type of reaction is called a substitution reaction.

Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia

Free-Radical Reactions

  The free-radical halogenation is also an example of a radical chain reaction.

  Such reactions are characterised by three steps   Initiation   Propagation   Termination

  Initiation produces the first free-radicals by a homolytic bond cleavage.

Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia

Free-Radical Reactions

  Propagation involves a series of new reactions which generate other radicals, e.g.

Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia

Free-Radical Reactions

  Termination steps stop the formation of free-radicals forming neutral molecular species, e.g.

Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia

Free-Radical Reactions

  Given the opportunity, free-radical reactions are regiospecific.

  More so for brominations than chlorinations.