Problem 70 - General Chemistry: Principles and Modern Applications

Question

Write the name and structure of the benzene derivatives described below. (a) Formula: \(\mathrm{C}_{8} \mathrm{H}_{10} ;\) forms three monochlorination products when treated with \(\mathrm{Cl}_{2}\) and \(\mathrm{FeCl}_{3}\) (b) Formula: \(\mathrm{C}_{9} \mathrm{H}_{12}\); forms one monochlorination product when treated with \(\mathrm{Cl}_{2}\) and \(\mathrm{FeCl}_{3}\) (c) Formula: \(\mathrm{C}_{9} \mathrm{H}_{12}\); forms four monochlorination products when treated with \(\mathrm{Cl}_{2}\) and \(\mathrm{FeCl}_{3}\)

Step-by-Step Solution

Answer

The benzene derivatives are (a) Ethyl benzene, (b) Isopropyl benzene or Cumene, and (c) Tert-butyl benzene.

Step 1: Identify The Structure

Start by identifying the substituents on the benzene ring from the given molecular formula. For example, in compound (a) with the molecular formula \(C_8H_{10}\), subtract the benzene's formula \(C_6H_6\) from this to find the remaining carbon and hydrogen atoms. This gives you \(C_2H_{4}\), which indicates that there are two extra carbons attached to the benzene ring.

Step 2: Draw The Possible Structures

Next, draw the possible structures where the substituents can be attached. For compound (a), the two carbons could be attached to the benzene ring in a few ways - they could form an ethyl group (\(C_2H_{5}\)), or form two separate methyl groups (\(CH_3\)). Furthermore, these groups can be attached to different positions on the benzene ring which can result in formation of different monochlorination products.

Step 3: Identify The Number of Monochlorination Products

The key to solve this is considering the number of monochlorination products. In compound (a), we are told that it forms three monochlorination products. This implies that we have three unique positions on the benzene ring where a chlorination reaction can occur - typically this means we have an ethyl group attached, as different positions of chlorination on the ethyl group result in different compounds, in addition to chlorination of the ring itself. Hence the structure of compound (a) is ethyl benzene.

Step 4: Apply the Same Process to the other compounds

Repeat the same process for compounds (b) and (c). For example, compound (b) has three extra carbons and forms only one monochlorination product, indicating they are attached in such a way to the benzene ring that only one unique chlorination product can form. This must be an isopropyl group, hence the structure is isopropyl benzene or cumene. Compound (c) with thus be tert-butyl benzene as it also has three extra carbons but forms four unique monochlorination products.

Key Concepts

Monochlorination of Benzene

Monochlorination of benzene involves substituting one hydrogen atom on the benzene ring with a chlorine atom. This chemical reaction follows an electrophilic aromatic substitution mechanism, where the benzene ring acts as an electron-rich site, attracting electrophiles like chlorine. The process typically utilizes chlorine gas ($Cl\_2$) and a Lewis acid catalyst such as iron(III) chloride ($FeCl\_3$) to facilitate the reaction.

During monochlorination, different isomers may form depending on the position where chlorine is added to the ring. In the textbook exercise, the number of monochlorination products provides clues to the identity and structure of the benzene derivative. For instance, compound (a) with the formula $C\_8H\_\{10\}$ forms three different products, suggesting that chlorine can attach to three distinct positions, indicating substituents that create symmetrical or asymmetrical possibilities for chlorination on the ring.

Understanding the outcome of monochlorination reactions is essential for organic chemists, as it helps determine the structure of complex molecules and tailor synthesis for desired products.

Organic Chemistry

Organic chemistry is the study of carbon-containing compounds and their properties, reactions, and synthesis. It covers a broad range of substances, including hydrocarbons and their derivatives, like the various benzene-related compounds discussed in this exercise. It's a foundational subject for many scientific fields, including pharmacology, biochemistry, and materials science.

In organic chemistry, understanding the behavior of molecules during reactions is key to identifying their structure and potential applications. The exercise focuses on how the molecular formula informs us about the number of carbon and hydrogen atoms and how these atoms are arranged (structure). This knowledge can be applied to anticipate the types of reactions these compounds may undergo, such as monochlorination, which is a representative transformation in organic chemistry.

Molecular Structure Identification

Identifying the molecular structure of organic compounds is a crucial skill in chemistry. In the given exercise, we use deductive reasoning to reverse-engineer the structure of a compound based on its molecular formula and reactivity, specifically monochlorination product count.

To determine the structure, one subtracts the formula of benzene ($C\_6H\_6$) from the given molecular formula. The remainder suggests possible substituents and their arrangement on the benzene ring. For example, $C\_8H\_\{10\}$ implies two additional carbons, suggesting an ethyl group or two methyl groups. The number of unique monochlorination products gives further insights into the substituent's positions on the ring, as demonstrated in the ubiquitous problem-solving approach in organic chemistry. Identifying the possible molecular structures is akin to a puzzle, with each clue narrowing down the possibilities until the correct structure is determined.

Substituent Effect on Benzene

The substituent effect on benzene rings is a fascinating aspect of organic chemistry that influences the reactivity and orientation of substitution reactions. Substituents already attached to the benzene ring can be activating or deactivating and dictate the position where new substituents will add—known as the directing effect.

In the exercise context, this effect explains why different benzene derivatives yield varying numbers of monochlorination products. The nature and position of a substituent impact whether the chlorine atom adds ortho, meta, or para to the existing group. Compounds (a), (b), and (c) exhibit different behavior during chlorination, highlighting the diverse range of possible outcomes due to substituents' influence on the benzene ring. Understanding these principles help chemists to predict the products of reactions and design synthesis with high specificity.