Problem 3 - General Chemistry: Principles and Modern Applications

Question

Supply a structural formula for each of the following compounds. (a) 1,3,5 -trimethylbenzene; (b) \(p\) -nitrophenol; (c) 3 -amino- 2,5 -dichlorobenzoic acid (a plant-growth regulator).

Step-by-Step Solution

Answer

The structural formulas of these three compounds would look as follows: \n 1,3,5-trimethylbenzene: A benzene ring with CH3 groups attached to positions 1, 3, and 5. \n \(p\)-nitrophenol: A benzene ring with an OH group in position 1 and a \(NO_2\) group in position 4. \n 3-amino-2,5-dichlorobenzoic acid: A benzene ring with a COOH group at position 1, a NH2 group at position 3, and Cl atoms at positions 2 and 5.

Step 1: Creating the structural formula for 1,3,5-trimethylbenzene

Firstly, it's important to know that the benzene ring is a six-membered ring with alternating double bonds. '1,3,5-trimethylbenzene' means that there are methyl groups CH3 at positions 1,3 and 5 of the benzene ring. The positions of the methyl groups are relative to where the numbers 1,3,5 are assigned to the carbon atoms of the benzene ring.

Step 2: Creating the structural formula for \(p\)-nitrophenol

The term \(p\)-nitrophenol refers to the placement of a nitro (\(NO_2\)) group and a hydroxyl (\(OH\)) group on a benzene ring, with the nitro group at the para- position, which means it is opposite the hydroxyl group. Phenol is a benzene ring with an hydroxyl (OH) group at position 1. The nitro (NO2) group will be placed at position 4, directly opposite the OH group for the \(p\)- prefix to apply.

Step 3: Creating the structural formula for 3-amino-2,5-dichlorobenzoic acid

3-amino-2,5-dichlorobenzoic acid has three substituents on the benzene ring. 'Benzoic acid' implies a COOH group at position 1 on the ring. '3-amino' refers to an NH2 group at position 3, and '2,5-dichloro' means there are chlorine atoms at positions 2 and 5 of the benzene ring.

Key Concepts

Understanding the Benzene Ring

The benzene ring is a keystone in organic chemistry, foundational in the structure of numerous organic compounds. This six-membered ring is composed of six carbon atoms bonded in a hexagonal planar shape, with alternating single and double bonds — a pattern known as conjugation. This conjugation contributes to the ring's exceptional stability and unique chemical properties, such as its reluctance to participate in addition reactions that would break the delocalized pi-electron system.
A benzene ring can be represented by two equivalent resonance structures, demonstrating the electron delocalization across the ring. To denote the delocalization in structural formulas, you may often see a circle inside the hexagonal arrangement of carbons. This simplicity hides the true complexity of the electron interactions within the ring.
Furthermore, benzene is a hydrocarbon with the molecular formula C6H6, where each carbon atom is also bonded to a hydrogen atom. The arrangement offers a symmetrical structure, which also contributes to benzene's chemical stability. Understanding the benzene ring is essential for grasping the architecture of more complex aromatic compounds described in organic nomenclature.

Navigating Substituent Groups on Benzene Rings

Substituent groups are atoms or groups of atoms that are attached to the central structure of a molecule - in this case, the benzene ring. They are the variances that make each benzene-containing compound unique, like decorations on a basic structure that alter its aesthetics and function. In organic chemistry, these groups are critical as they influence the chemical behavior of the molecule.
Typically, substituents are classified based on their electron-withdrawing or electron-donating effects. They can significantly affect the reactivity and physical properties of benzene derivatives, such as boiling points, solubility, and density. Substituents like the methyl group (CH3) are electron-donating, which activates the benzene ring toward electrophilic substitution reactions while groups like the nitro group (NO2) are electron-withdrawing and deactivate the ring.
The position of a substituent on a benzene ring is indicated using numerical locants or terms such as ortho (adjacent carbon atoms), meta (separated by one carbon), or para (opposite sides of the ring). Understanding how to identify and name these substituents in relation to their position on the benzene ring is essential for mastering organic compound nomenclature.

Decoding Organic Compounds Nomenclature

Organic compound nomenclature is a systematic method of naming organic chemical compounds as recommended by the International Union of Pure and Applied Chemistry (IUPAC). This nomenclature allows chemists to communicate complex structures unambiguously. A compound's name can reveal its structure, which is crucial when discussing compounds with multiple functional groups or substituents.
In the IUPAC system, compounds are named based on the longest continuous carbon chain, with the parent hydrocarbon name providing the suffix (e.g., -ane, -ene, -yne). For benzene-containing compounds, the term phenyl is used when benzene is a substituent, while the term benzene is retained as the parent compound in compounds where it's the most significant structure. For example, in 1,3,5-trimethylbenzene, the prefix 'tri-' signifies the presence of three methyl groups, and their positions on the ring are identified by the numbers 1,3,5.
The understanding of prefixes (indicating the number and types of substituents), infixes (denoting the main carbon chain's nature), and suffixes (indicating the primary functional group) is necessary to decode the complete name of an organic compound. This intricate naming process helps maintain consistency in the vast and growing field of organic chemistry.