Question

Propose structures for aromatic hydrocarbons that meet the following descriptions: (a) \(\mathrm{C}_{9} \mathrm{H}_{12}\); gives only one \(\mathrm{C}_{9} \mathrm{H}_{11} \mathrm{Br}\) product on substitution with bromine (b) \(\mathrm{C}_{10} \mathrm{H}_{14}\); gives only one \(\mathrm{C}_{10} \mathrm{H}_{13} \mathrm{Cl}\) product on substitution with chlorine (c) \(\mathrm{C}_{8} \mathrm{H}_{10}\); gives three \(\mathrm{C}_{8} \mathrm{H}_{9} \mathrm{Br}\) products on substitution with bromine (d) \(\mathrm{C}_{10} \mathrm{H}_{14}\); gives two \(\mathrm{C}_{10} \mathrm{H}_{13} \mathrm{Cl}\) products on substitution with chlorine

Short answer

(a) Propylbenzene (cumene), (b) tert-butylbenzene, (c) meta-xylene, (d) ethylbenzene.

Step-by-step solution

Understanding the question

We need to propose structures for aromatic hydrocarbons based on molecular formulas and the amount of different substitution products formed with halogens. Each formula represents a unique hydrocarbon which, when treated with a halogen, will produce one or more unique substitution products.

Analyzing the structure for (a)

Given that (a) has a molecular formula of \(\mathrm{C}_{9} \mathrm{H}_{12}\), an aromatic compound with only one type of substitution product suggests that there is a high degree of symmetry. The likely structure is propylbenzene or cumene, \(\mathrm{C}_{6}\mathrm{H}_5\mathrm{C}_3\mathrm{H}_7\), since any hydrogen atom replaced by bromine in the propyl chain will yield the same product, \(\mathrm{C}_{9}\mathrm{H}_{11}\mathrm{Br}\).

Analyzing the structure for (b)

For (b), \(\mathrm{C}_{10} \mathrm{H}_{14}\), and the fact it gives only one \(\mathrm{C}_{10} \mathrm{H}_{13} \mathrm{Cl}\) product suggests an aromatic compound with symmetrical substitution positions. The structure likely is tert-butylbenzene, \(\mathrm{C}_{6}\mathrm{H}_5\mathrm{C}_4\mathrm{H}_9\), because any of the hydrogens in the tert-butyl group can be replaced by chlorine, producing the same compound.

Analyzing the structure for (c)

(c) \(\mathrm{C}_{8} \mathrm{H}_{10}\); producing three different \(\mathrm{C}_{8} \mathrm{H}_{9} \mathrm{Br}\) products infers multiple unique substitution sites. The structure is likely xylene isomers; specifically, meta-xylene \(\mathrm{C}_{6}\mathrm{H}_{4}(\mathrm{CH}_3)_2\), where there are three different positions for substitution.

Analyzing the structure for (d)

For (d), \(\mathrm{C}_{10} \mathrm{H}_{14}\) giving two substitution products implies a flexible but limited structural variance. The structure is likely ethylbenzene, \(\mathrm{C}_{6}\mathrm{H}_5\mathrm{C}_2\mathrm{H}_5\), or ortho-xylene where two non-identical positions can lead to different substitution outcomes.

Key concepts

Molecular Formula

The molecular formula is a fundamental concept in chemistry. It represents the actual number of atoms of each element in a compound. For aromatic hydrocarbons, the molecular formula helps us determine the number and type of atoms involved. This is crucial for understanding the structure and behavior of the compound.
Aromatic hydrocarbons, like benzene, have a base formula \(C_6H_6\). By adding different alkyl groups, we can derive various aromatic compounds with distinct molecular formulas.
For example, in the exercise, we examine compounds like \(C_9H_{12}\), \(C_{10}H_{14}\), and \(C_8H_{10}\). These formulas suggest the compound's composition and possible isomers or structures they represent. Knowing the molecular formula allows chemists to predict several characteristics, including reactivity and substitution behavior.

• Every molecular formula is a tool to identify the unique combination of atoms.
• It provides clues about the possible isomeric structures.
• The formula indicates the potential for forming various substitution products.

Substitution Reactions

Substitution reactions are pivotal in organic chemistry. They involve replacing an atom or group in a molecule with another atom or group. In aromatic hydrocarbons, substitution often happens with halogens like bromine or chlorine.
Consider propylbenzene from the given exercise. When bromine substitutes a hydrogen atom in the propyl group, the new compound remains largely similar. This is because all hydrogen atoms in the propyl group are equivalent, demonstrating symmetry.

• Substitution is a way to modify organic molecules by replacing specific atoms or groups.
• In aromatic compounds, substitution usually preserves the aromatic ring structure.
• The number of substitution products formed indicates the symmetry and potential unique positions within the molecule.

Substitution reactions help chemists understand and predict the behavior of aromatic hydrocarbons. They illuminate where in a molecule changes are most likely to occur. This is essential for designing reactions and creating specific compounds.

Symmetry in Chemistry

Symmetry plays a critical role in determining the reactivity and properties of chemical compounds. In aromatic hydrocarbons, symmetry can simplify the number of products in substitution reactions.
When we say a compound is symmetrical, it means that parts of the molecule can be interchanged without affecting its structure. This happens in cases like tertiary-butylbenzene, where replacement of any hydrogen in the butyl group results in the same product.

• Symmetry can lead to equivalence of atoms or groups, reducing the number of possible substitution products.
• It often results in compounds having fewer isomers.
• Compounds with high symmetry are less complicated to analyze in reactions due to reduced variability in substitution outcomes.

Understanding symmetry in a compound allows chemists to predict its behavior in chemical reactions. It simplifies the conceptualization of reaction pathways and potential products, making it a valuable tool in organic synthesis.