Free Radical Reactions

This paragraph introduces the concept of radicals, defined as atoms or species with an unpaired electron, and explains their formation through two types of bond cleavage: homolytic and heterolytic. Homolytic cleavage results in two radicals with equal electron distribution, exemplified by the breakage of a bond between two bromine atoms. Heterolytic cleavage, in contrast, involves an unequal distribution of electrons, as seen in the cleavage of carbon-bromine and carbon-hydrogen bonds, leading to the formation of ions with distinct charges. The paragraph emphasizes the role of electronegativity in determining the direction of electron flow during bond cleavage.

The paragraph delves into the specifics of free radical reactions, outlining the three key steps: initiation, propagation, and termination. Initiation occurs when a neutral molecule forms two radicals, propagation involves a single radical on each side of the reaction, and termination happens when two radicals combine, ceasing the chain reaction. The paragraph provides examples of each step and encourages viewers to practice identifying them. It also touches on the chlorination of methane, a free radical substitution reaction that produces methyl chloride and hydrochloric acid, setting the stage for a discussion on the reaction mechanism.

This section describes the detailed mechanism of methane chlorination, a free radical reaction initiated by heat or ultraviolet light, which generates chlorine radicals. The propagation phase involves the chlorine radical reacting with methane to form a methyl radical and hydrochloric acid. Subsequently, the methyl radical reacts with molecular chlorine to produce methyl chloride and regenerate chlorine radicals, allowing the reaction to continue. The paragraph also discusses possible termination steps, such as the recombination of two chlorine radicals into chlorine gas or the formation of ethane from two methyl radicals, highlighting the complexity of free radical reactions.

The paragraph compares the reactivity and selectivity of chlorine and bromine in their reactions with alkanes, such as propane. It explains that chlorine, being more reactive, is less selective and can abstract both primary and secondary hydrogens with ease, leading to a mixture of products. Bromine, however, is more selective, preferring to replace secondary hydrogens over primary ones due to its lower reactivity. The discussion also includes the concept of radical stability, which influences the selectivity of the halogenation reaction, with tertiary radicals being more stable than primary radicals.

This paragraph explores the relationship between activation energies and the selectivity of halogenation reactions. It uses the Hammond postulate to explain why chlorine, with its small difference in activation energy for abstracting primary versus secondary hydrogens, is less selective compared to bromine, which has a large difference in activation energy and thus selectively abstracts tertiary hydrogens. The paragraph also presents the relative reactivity rates of chlorine and bromine with different types of hydrogens on alkanes, further illustrating the difference in selectivity between the two halogens.

The final paragraph focuses on calculating the percent yield of products in the halogenation of propane with chlorine, using the previously discussed relative reactivity rates. It demonstrates how to determine the major and minor products based on the number of each type of hydrogen atom available for reaction and their respective reactivity rates. The example provided calculates the percent yield of 2-chloropropane and 1-chloropropane, showing that 2-chloropropane is the major product due to the higher reactivity of secondary hydrogens with chlorine radicals.