Calculating Radical Yields: Videos & Practice Problems

In radical halogenation reactions, understanding the stability of radicals is crucial for predicting the major product. The general trend indicates that tertiary radicals are more stable than secondary radicals, which in turn are more stable than primary radicals. This stability hierarchy allows for the prediction of major products based on the type of radical formed during the reaction.

To calculate the exact percentages of major and minor products, one must consider the relative rates of halogenation at a specific temperature, typically 25 degrees Celsius. Chlorination and bromination exhibit different selectivity patterns due to their relative rates. Chlorination is known for its non-selectivity, as it prefers to halogenate tertiary carbons only slightly more than primary and secondary carbons. Specifically, chlorination favors tertiary radicals about 5 times more than primary ones, leading to a wide variety of products.

In contrast, bromination is highly selective, favoring tertiary carbons approximately 1600 times more than primary carbons. This significant difference in selectivity explains why bromination is often preferred in synthetic applications where a specific product is desired. The selectivity of these reactions can be quantified using a table of relative rates, which provides a clear comparison of how each halogenation method behaves under standard conditions.

It is important to note that these ratios are temperature-dependent. As the temperature increases, the selectivity of the reactions diminishes, leading to a more uniform distribution of products. This is due to the increased energy in the system, which allows for more random interactions among the reactants. Therefore, at elevated temperatures, even bromination can become less selective.

When studying these concepts, one may encounter variations in the numerical values for the relative rates in different textbooks. While the specific numbers may differ, the overall trends and principles remain consistent. It is advisable to adhere to the values provided by your instructor if they differ from those discussed here, as the fundamental concepts will still apply regardless of the specific figures used.

In the study of organic chemistry, understanding the process of monochlorination of butane is essential. Butane, a four-carbon alkane, can undergo chlorination, resulting in different products based on the type of hydrogen atoms (protons) present. In butane, there are two types of protons: primary and secondary. Primary protons are those attached to the terminal carbon atoms, while secondary protons are attached to the internal carbon atoms.

For butane, the structure reveals 6 primary hydrogen atoms and 4 secondary hydrogen atoms. The relative reactivity of these protons during chlorination varies, with primary protons having a relative rate of 1 and secondary protons having a relative rate of 4. This means that secondary protons are more likely to react with chlorine, leading to a higher yield of secondary chlorination products.

To calculate the percentage yields of each product, we first determine the total relative reactivity. The calculations are as follows:

1. For primary chlorination:
Number of primary protons = 6
Relative rate = 1
Contribution = 6 × 1 = 6

2. For secondary chlorination:
Number of secondary protons = 4
Relative rate = 4
Contribution = 4 × 4 = 16

Next, we sum these contributions to find the total relative reactivity:
Total = 6 + 16 = 22

Now, we can express the yields as fractions of the total:

Percentage yield of primary product = \(\frac{6}{22} \approx 27.3\%\)
Percentage yield of secondary product = \(\frac{16}{22} \approx 72.7\%\)

This indicates that while the chlorination process shows some selectivity, with 72.7% of the product being the secondary chlorination product, the overall yield is still considered low in the context of chemical reactions. A more selective reaction would yield a greater difference between the two products, making the process more efficient.

Understanding these concepts is crucial for mastering organic reactions and their mechanisms. As you practice, aim to perform these calculations without relying on structured layouts, reinforcing your grasp of the underlying principles.