The following are all substitution reactions, two of which we study in later chapters. With no knowledge of mechanism, what would you expect the ratio of products to be for each reaction, based on a random statistical distribution?
(b) Replacing a hydrogen (H) with bromine (Br):

Verified step by step guidance

1. Step 1: Identify the type of reaction. This is a substitution reaction where a hydrogen atom on the benzene ring is replaced by bromine (Br). The reaction is expected to follow a random statistical distribution since no specific mechanism is provided.
2. Step 2: Analyze the structure of the molecule. The benzene ring has three distinct positions for substitution: ortho (adjacent to the methyl group), meta (one position away from the methyl group), and para (opposite to the methyl group). These positions are labeled as A, B, and C in the image.
3. Step 3: Count the number of hydrogens available for substitution at each position. The ortho positions (A) have two hydrogens, the meta positions (B) have two hydrogens, and the para position (C) has one hydrogen.
4. Step 4: Calculate the statistical ratio of products based on the number of hydrogens available for substitution. Since there are 2 hydrogens at ortho, 2 at meta, and 1 at para, the ratio of products will be proportional to these numbers: 2:2:1.
5. Step 5: Conclude that the expected ratio of products (A:B:C) based on random statistical distribution is 2:2:1, assuming no preference for any specific position due to electronic or steric effects.

Key Concepts

Substitution Reactions

Substitution reactions involve the replacement of one atom or group in a molecule with another atom or group. In organic chemistry, these reactions are common in aromatic compounds, where a hydrogen atom can be replaced by a halogen, such as bromine or chlorine. Understanding the nature of these reactions is crucial for predicting product ratios based on the stability and reactivity of the intermediates formed.

Statistical Distribution of Products

In the context of substitution reactions, statistical distribution refers to the likelihood of forming different products based on the number of available reactive sites and the symmetry of the molecule. For example, if a molecule has multiple equivalent positions for substitution, the products will form in a ratio that reflects the number of ways each product can be generated, assuming no preference for any specific site.

Mechanism of Bromination

The mechanism of bromination in aromatic compounds typically involves the formation of a bromonium ion intermediate, which can lead to different substitution products depending on the orientation of the substituents already present on the aromatic ring. Understanding this mechanism helps predict the distribution of products, as certain positions may be more favorable for substitution due to electronic effects or steric hindrance.