ORGANIC CHEMISTRY - CHAPTER 3: REACTIONS OF ALKANES

IMPORTANT CONCEPTS

1. The ΔH^o of bond homolysis is defined as the bond-dissociation energy, DH^o. Bond homolysis gives radicals or free atoms.

2. The C–H bond strengths in the alkanes decrease in the order

3. Catalysts speed up the establishment of an equilibrium between starting materials and products.

4. Alkanes react with halogens (except iodine) by a radical chain mechanism to give haloalkanes. The mechanism consists of initiation to create a halogen atom, two propagation steps, and various termination steps.

5. In the first propagation step, the slower of the two, a hydrogen atom is abstracted from the alkane chain, a reaction resulting in an alkyl radical and HX. Hence, reactivity increases from I₂ to F₂. Selectivity decreases along the same series, as well as with increasing temperature.

6. The Hammond postulate states that fast, exothermic reactions are typically characterized by early transition states, which are similar in structure to the starting materials. On the other hand, slow, endothermic processes usually have late (productlike) transition states.

7. The ΔH^o for a reaction may be calculated from the DH^o values of the bonds affected in the process as follows:

               ΔH^o = ∑DH^o_{bonds broken} - ∑DH^o_{bonds formed}

8. The ΔH^o for a radical halogenation process equals the sum of the ΔH^o values for the propagation steps.

9. The relative reactivities of the various types of alkane C – H bonds in halogenations can be estimated by factoring out statistical contributions. They are roughly constant under identical conditions and follow the order

The reactivity differences between these types of CH bonds are greatest for bromination, making it the most selective radical halogenation process. Chlorination is much less selective, and fluorination shows very little selectivity.

10. The ΔH^o of the combustion of an alkane is called the heat of combustion, ΔH^o_comb. The heats of combustion of isomeric compounds provide an experimental measure of their relative stabilities.

From "Organic Chemistry" Textbook of VOLLHARDT & SCHORE