Primary, Secondary, and Tertiary Hydrogens

Classification and Reactivity of Alkane Hydrogens

Alkane hydrogens are classified based on the carbon to which they are attached: primary (1°), secondary (2°), and tertiary (3°). This classification is crucial in understanding their reactivity in free radical halogenation reactions.

• Primary (1°) hydrogen: Attached to a carbon bonded to one other carbon.

• Secondary (2°) hydrogen: Attached to a carbon bonded to two other carbons.

• Tertiary (3°) hydrogen: Attached to a carbon bonded to three other carbons.

• In propane, there are six 1° hydrogens and only two 2° hydrogens, yet the major product results from replacement at a 2° hydrogen.

• When a chlorine free radical (Cl·) reacts with propane, abstraction of a hydrogen atom can give either a 1° radical or a 2° radical.

• The structure of the radical formed in this step determines the structure of the observed product, either 1-chloropropane or 2-chloropropane.

• The product ratio shows that the 2° radical is formed preferentially.

Mechanism of Chlorination of Propane

Stepwise Free Radical Chain Mechanism

Chlorination of propane proceeds via a free radical chain mechanism, involving initiation, propagation, and termination steps.

• Initiation: Splitting of the chlorine molecule to form two chlorine radicals.

• First propagation step: Abstraction (removal) of a primary or secondary hydrogen from propane by a chlorine radical. (secondary radical) (primary radical)

• Second propagation step: Reaction of the alkyl radical with chlorine to form the alkyl chloride. (2-chloropropane) (1-chloropropane)

Stability of Free Radicals

Substitution and Radical Stability

The stability of free radicals increases with the degree of substitution at the radical center. This affects the product distribution in halogenation reactions.

• The energy required to form a methyl free radical is greatest, and it decreases for a 1°, 2°, and 3° free radical.

• The more highly substituted the carbon atom, the less energy is required to form the free radical.

• Free radicals are more stable if they are more highly substituted:

• In chlorination of propane, the 2° hydrogen atom is abstracted more often because the 2° radical and its transition state are lower in energy than the 1° radical and its transition state.

• Abstraction of the 2° hydrogen is 13 kJ/mol more exothermic than abstraction of the 1° hydrogen.

Bond Dissociation Energies for the Formation of Free Radicals

Energetics of Radical Formation

Bond dissociation energies (BDEs) provide insight into the stability of free radicals formed during halogenation.

Note: More highly substituted free radicals are more stable than less highly substituted ones.

Energy Diagram for Propane Chlorination

Activation Energies and Reaction Pathways

The energy diagram for the first propagation step in propane chlorination illustrates the difference in activation energies for formation of 1° and 2° radicals.

• Formation of the 2° radical has a lower activation energy than formation of the 1° radical.

• Energy required to break the CH3CH2CH3-H bond: kJ/mol ( kcal/mol)

• Energy released in forming the H-Cl bond: kJ/mol ( kcal/mol)

• Total energy for reaction at the primary position: kJ/mol ( kcal/mol)

• Energy required to break the CH3CHCH3-H bond: kJ/mol ( kcal/mol)

• Energy released in forming the H-Cl bond: kJ/mol ( kcal/mol)

• Total energy for reaction at the secondary position: kJ/mol ( kcal/mol)

The activation energy to form the 2° radical is slightly lower, so the 2° radical is formed faster than the 1° radical.

Bromination of Propane: Rate of Substitution

Free Radical Bromination and Selectivity

Bromination of propane is a free-radical reaction that requires both heat and light. It is much more selective than chlorination.

• The 2-bromopropane (2°) is favored by a 97:3 product ratio.

• From this product ratio, the two 2° hydrogens are each 97 times as reactive as one of the 1° hydrogens.

• Bromination is more selective than chlorination because the major reaction is favored by a larger amount.

• To explain this enhanced selectivity, we must consider the transition states and activation energies for the rate-limiting step.

Energy Differences Between Chlorination and Bromination

Bond Dissociation and Reaction Energetics

The energy differences between chlorination and bromination result from the difference in the bond-dissociation enthalpies of H-Cl (431 kJ) and H-Br (368 kJ).

• Energy required to break the CH3CH2CH3-H bond: kJ/mol ( kcal/mol)

• Energy released in forming the H-Br bond: kJ/mol ( kcal/mol)

• Total energy for reaction at the secondary position: kJ/mol ( kcal/mol)

• The HBr bond is weaker, and abstraction of a hydrogen atom by Br· is endothermic.

• This endothermic step explains why bromination is much slower than chlorination, but it still does not explain the enhanced selectivity observed with bromination.

• Although the difference in values of ΔH between abstraction of a 1° hydrogen and a 2° hydrogen is still 13 kJ/mol, the energy diagram for bromination shows a much larger difference in activation energies for abstraction of the 1° and 2° hydrogens than for chlorination.

Summary Table: Reactivity and Selectivity in Halogenation

Additional info: The selectivity of halogenation increases with the stability of the radical intermediate and the endothermicity of the hydrogen abstraction step. Bromination is slower but much more selective than chlorination due to the nature of the transition state and activation energy differences.