Reactions of Alkanes: Bond-Dissociation Energies, Radical Halogenation, and Relative Reactivity

 

We will skip this chapter until the end of the quarter when we will cover only a very small part of the material.

 

3-1      Strength of Alkane Bonds: Radicals

Two different ways in which bonds break:

Heterolytic-both electrons go to one of the atoms;

Homolytic-one electron goes to each of the atoms.

C–H bonds differ in strength:

Methane stronger than 1, 1 stronger than 2, 2 stronger than 3

3-2        Structure of Alkyl Radicals: Hyperconjugation

Carbanions are pyramidal, carbocations and (most) radicals are planar

Hyperconjugation explains the greater stability of radicals with carbon substituents compared to hydrogen atoms

Hyperconjuation involves the overlap of adjacent orbitals, contributing electron density to the radical carbon

3-3      Conversion of Petroleum: Pyrolysis (omit, but read)

3-4   Chlorination of Methane: The Radical Chain Mechanism

        Chlorination involves two distinct steps:

          1) generation of a Cl radical with light or heat (initiation step);

          2) removal of a hydrogen atom by a Cl radical followed by reaction of the carbon radical with Cl2 (propagation steps).

          There are other reactions involved, including those replacing additional hydrogen atoms to make poly chlorinated products and termination steps that combine two radicals.

3-5   Other Radical Halogenations of Methane

        Iodination is endothermic and therefore, not practical.

          Fluorination is very exothermic and difficult to control.

          Bromination is practical but bromine is relatively very expensive and used less often than is chlorine.

3-6   Chlorination of Higher Alkanes: Relative Reactivity and Selectivity

        Primary C—H bonds are stronger than are secondary, and tertiary are weakest of all.

          In complex hydrocarbons, there are typically more primary than secondary and more secondary than tertiary hydrogens.

          The stronger bonds are counterbalanced by statistical favoring of Primary and secondary hydrogens.

3-7   Selectivity in Radical Halogenation with Fluorine and Bromine

          Selectivity in radical halogenation increases in the order:

          F<Cl<Br.

          Radical abstraction of a hydrogen by a Br radical is endothermic and thus the transition state is late, resembling the radical. Thus, energetic differences in the radicals generated will translate into significant differences in transition state energies. The resulting differences in activation energies will increase the relative rate of removing a hydrogen from more substituted carbons, leading to more stable radicals.

          In contrast, hydrogen abstraction by a Cl (and F) radical is exothermic with a transition state where little bond cleavage has occurred. Energetic differences in product radicals will have little effect on transition state energies and rates for generating differently substituted radicals will be similar.

3-8   Synthetic Radical Halogenation

3-9   Synthetic Chlorine Compounds and the Stratospheric Ozone Layer

3-10  Combustion and the Relative Stabilities of Alkanes

          Relative stabilities for isomeric compounds can be obtained by comparing heats of combustion. The more heat produced upon combustion, the less stable is the isomer. The more highly branched isomer will generally be more stable.