Question

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

Short answer

(a) Mesitylene, (b) Cumene, (c) Ortho-xylene, (d) P-xylene.

Step-by-step solution

Understanding Aromatic Hydrocarbons

Aromatic hydrocarbons are compounds that include one or more aromatic rings, typically benzene or derivates. The molecular formula for aromatic hydrocarbons is expressed as C_nH_{n+k}, where n indicates the number of carbon atoms, and k is variable based on the structure.

Analyze Part (a): Single Product for C9H12

The molecule C9H12 is expected to give only one C9H11Br product upon bromination, which suggests it should have a symmetrical structure. Only compounds like mesitylene (1,3,5-trimethylbenzene) meet this criterion, as bromination at any aromatic hydrogen leads to the same product.

Derive Structure (a) for C9H12

Propose mesitylene, also known as 1,3,5-trimethylbenzene, which is symmetric. Its symmetrical layout ensures that bromination results in the same substituted benzene, leading to only one possible product.

Analyze Part (b): Single Product for C10H14

The molecule C10H14 should yield one product upon chlorination, which implies it should be symmetrical as an aromatic system. Cumene (isopropylbenzene) fits this requirement due to symmetry around the aromatic ring.

Derive Structure (b) for C10H14

Propose the compound cumene or isopropylbenzene. Chlorination of the aromatic hydrogen atom will result in a single possible chlorinated product.

Analyze Part (c): Three Products for C8H10

Given C8H10 producing three brominated products implies partial symmetry. The structure to fit this is xylene, specifically ortho-xylene (1,2-dimethylbenzene), where substitution at different positions on the ring leads to three distinct bromination outcomes.

Derive Structure (c) for C8H10

Propose ortho-xylene (1,2-dimethylbenzene) as the structure. This compound allows for three different brominated derivatives due to the positions on the aromatic ring.

Analyze Part (d): Two Products for C10H14

C10H14 generating two chloro derivatives indicates two types of hydrogens on the aromatic ring, suggesting unequal substituents' positions like in p-xylene.

Derive Structure (d) for C10H14

Propose p-xylene (1,4-dimethylbenzene). This structure results in two different chlorinated products due to the symmetrical positioning of the methyl groups on the benzene ring.

Key concepts

Benzene Derivatives

Benzene derivatives are hydrocarbon compounds where hydrogen atoms on a benzene ring are replaced or substituted by other atoms or groups of atoms. Typically, this substitution involves alkyl groups, halogens, or other functional groups. Benzene itself is a planar, cyclic molecule composed of six carbon atoms, each with a hydrogen atom, forming a structure known as an aromatic ring. This aromaticity grants benzene its unique chemical properties.

Benzene derivatives can vary widely, offering diversity by changing the number and type of substituent groups. For example, if three hydrogen atoms in benzene are substituted with methyl groups in symmetrical locations, the result is mesitylene, or 1,3,5-trimethylbenzene. Such derivatives are essential in understanding reactions like bromination and chlorination, which involve substitution of these hydrogen or alkyl groups with halogen atoms, leading to different derivative products. Exploring benzene derivatives helps illustrate how molecular symmetry affects chemical reactivity and product formation.

Bromination

Bromination is a chemical reaction that involves the addition of a bromine atom to the organic compound, often an aromatic hydrocarbon. In the context of benzene and its derivatives, bromination typically occurs via an electrophilic aromatic substitution reaction. An electrophile, in this case, bromine ( abla Br_2 abla), targets the electron-rich aromatic ring, replacing a hydrogen atom with a bromine atom.

This reaction requires a catalyst, often iron bromide ( abla FeBr_3 abla), to successfully introduce the bromine onto the aromatic ring. The nature and location of substituents on a benzene derivative significantly influence bromination outcomes. For instance, in a compound like mesitylene, the symmetric arrangement of methyl groups ensures that bromination yields only a single type of product, as every aromatic hydrogen in available position yields identical substitution results. On the other hand, an asymmetrical molecule like ortho-xylene permits bromine to attach at multiple distinct sites, creating several different brominated products.

Chlorination

Chlorination in organic chemistry is similar to bromination but involves the introduction of chlorine atoms. This process often utilizes abla Cl_2 abla and a catalyst like ferric chloride ( abla FeCl_3 abla) to effect electrophilic aromatic substitution in the benzene ring. The process leads to the replacement of one hydrogen with a chlorine atom at an available position on the ring.

Just like bromination, chlorination's effectiveness and yield depend on the molecular symmetry and type of substituents present on the benzene derivative. For example, in isopropylbenzene (cumene), the symmetrical arrangement dictates that chlorination yields a single chlorinated product. In contrast, a compound like p-xylene, with its two distinct methyl groups positioned on the ring, may result in two different chlorinated products depending on which hydrogen atom is replaced.

Symmetry in Organic Molecules

Symmetry in organic molecules profoundly influences their chemical properties and reactions. When discussing aromatic hydrocarbons, symmetry can predict the number of possible reaction products. Symmetrical molecules, such as mesitylene, have substituents positioned such that chemical reactions like halogenation yield only one type of product because all substitution sites are equivalent.

In unsymmetrical molecules, such as ortho-xylene, different positions on the benzene ring are not entirely equivalent. This results in multiple reaction outcomes, as bromine or chlorine can replace hydrogens in different positions, leading to distinct products. Recognizing and understanding symmetry is pivotal for accurately predicting and understanding reaction outcomes for both synthesis and analysis of organic compounds. Thus, symmetry is not just a geometric concept but a fundamental aspect of chemical reactivity and product identification.