Reactions of Alkanes | Organic Chemistry 1

Reactions of alkanes are studied in this chapter: the important concepts of saturated hydrocarbons (equivalent atoms, primary, secondary, tertiary, quaternary atoms in chemistry, stability of radicals), the two types of reactivity in organic chemistry (heterolytic vs. homolytic), the radical chain reactions, the halogenation of alkanes (mechanism, products and selectivity).

Chemically Equivalent Atoms

2 atoms are chemically equivalent when they have an identical environment. For example, the six hydrogen atoms of ethane are considered to be chemically equivalent since they are attached to a carbon atom that is linked to 2 other hydrogens and one methyl group. They have the same environment.

 

ethane: all the H are equivalent

1,1,4,4-tetramethylcyclohexane: 2 groups of equivalent H

 

 

 This concept is very important in spectroscopy (NMR) and in reactions (e.g. redical halogenation of alkanes - see below).

Primary, Secondary, Tertiary and Quaternary in Chemistry

For carbon atoms:

- Primary = a carbon attached to only ONE other carbon atom
- Secondary = a carbon attached to TWO other carbon atoms
- Tertiary = a carbon attached to THREE other carbon atoms
- Quaternary = a carbon attached to FOUR other carbon atoms

 

For hydrogen atoms, chemical functions or radicals:

- Primary = a hydrogen/chemical function/radical on a carbon attached to only ONE other carbon atom
- Secondary = a hydrogen/chemical function/radical on a carbon attached to TWO other carbon atoms
- Tertiary = a hydrogen/chemical function/radical on a carbon attached to THREE other carbon atoms

 

Heterolytic vs. Homolytic Cleavage

Heterolytic Cleavage: Bonding Electrons Move as Pair
(a normal double-barbed arrow shows the movement of a pair of electrons).

Homolytic Cleavage: Bonding Electrons Separate
(a single-barbed arrow shows the movement of a single electron).

 

Radicals

Radicals are formed by homolytic cleavage.
Radical stability increases along the series from primary to secondary to tertiary (hyperconjugation); consequently, the energy required to create them decreases.

 

Radical Chain Mechanism

Radical chain reactions proceed in 3 stages: initiation, propagation and termination.
 

Chlorination of Methane:

Initiation (At the beginning of the reaction)
Homolytic cleavage of the Cl-Cl bond:


Propagation (Steps 1 and 2 several times in a row)
Step 1 - Abstraction of an H atom by Cl:


Step 2 - Abstraction of a Cl atom by R:


Chain Termination (At the end of the reaction)
Radical-radical combination:

Determining Products of Mono-halogenation of Alkane

1) Identify all the equivalent hydrogen groups in the starting alkane.
2) Replace one hydrogen atom from a equivalent hydrogen group.

The number of products is equal to the number of equivalent hydrogen groups

 

Give all the possible products resulting from the mono-chlorination of propane:

Selectivity

Tertiary radicals are more stable and form faster than secondary and primary radicals.
Therefore, their relative ratios are higher than the statistical ratios.
On the contrary, the relative ratios of primary radicals are lower than the statistical ones.