Textbook: Bruice 6th Ed. (2010)

Chapter 12: Radicals • Reactions of Alkanes

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Individual Problems

Individual Problems

Problem # 346

For the reaction below, draw the structures of the radical intermediate and the final product.

Solution

This is a free radical halogenation. The hint is that you see light (hν) in the reaction arrow, indicating radicals are involved.

The more substituted the carbon atom, the more stable the corresponding radical, so the major product of this reaction is the 3º alkyl halide.

Problem Details

MendelSet practice problem # 346 submitted by Matt on June 7, 2011.

Subject: Organic Chemistry

Subject Topic(s): Radicals (Stability, Halogenation)

Question Type: Reactions (predict the product, show the starting material..)

Keyword(s): free radical halogenation

Problem # 324

Using curved hooks, draw a mechanism for the free radical bromination reaction shown below.

Solution

Note that the radical will form on the 3° carbon only, as this is the most stable radical.

Problem Details

MendelSet practice problem # 324 submitted by Matt on June 7, 2011.

Subject: Organic Chemistry

Subject Topic(s): Radicals (Stability, Halogenation)

Question Type: Mechanism (show a mechanism using curved arrows..)

Keyword(s): free radical halogenation, radical mechanism

Problem # 1280

The structure of 2-methylbutane is shown below.

a) Draw the structures of all possible monochloro products resulting from the free-radical chlorination of 2-methylbutane.

b) Based on statistics alone, what do you expect the major product to be? Is this the same structure as the expected major product? Explain.

c) How would the relative yield of the products differ if bromine was used instead of chlorine?

Solution

a) There are five different carbons in 2-methylbutane, so hypothetically, you should get five different products, 20% A, 20% B, 20% C, 20% D, and 20% E.

But notice that A and B are the same product! So only four products are possible.

b) Based on statistics alone, we would expect product A (or B) to be the major product, encompassing 40% of all the products (20% A + 20% B).

But this is not what happens in real life because A is primary, and for free-radical halogenation reactions, more substituted products are favored. So the major product will be C, which is tertiary.

This happens because the intermediate is a radical, and radical stability goes 3° > 2° > 1°.

c) Chlorine reacts faster than bromine, and so is less selective than bromine.

For example, we know that the 3° halide will be the major product. But with chlorine, the proportion might turn out to be ~70% 3° , 20% 2°, and 10% 1°.

But since bromine reacts more slowly, you will get better selectivity. The proportion of products will might be something like ~95% 3° , 4% 2°, and 1% 1°.

While on this topic of reactivity, it's interesting to note that iodine reacts too slowly to be useful, and fluorine reacts so violently it's dangerous!