Question

Draw the structure of (a) benzoic acid, \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COOH}\) (b) 2 -chloro-3-methylpentanoic acid; (c) hexanoic acid; (d) propenoic acid.

Short answer

The structures are as follows: (a) a benzene ring with an attached carboxyl group; (b) a five-carbon chain with a chlorine on the second carbon and a methyl group on the third carbon; (c) a six-carbon chain with a carboxyl group at one end; (d) a three-carbon chain with a double bond between the first two carbons and a carboxyl group at one end.

Step-by-step solution

Structure of Benzoic Acid

Benzoic acid consists of a benzene ring (\(C_6H_5\)) attached to a carboxylic acid group (\(COOH\)). Begin by drawing a hexagon to represent the benzene ring, with alternating double and single bonds between carbon atoms. Then, draw a carboxyl group (\(COOH\)) attached to one of the carbons in the ring.

Structure of 2-Chloro-3-methylpentanoic Acid

The parent chain is pentanoic acid, a five-carbon carboxylic acid chain. Starting with the carboxylic acid group, draw a five-carbon chain. On the second carbon, add a chlorine atom (\(Cl\) substituent). On the third carbon, add a methyl group (\(CH_3\) substituent).

Structure of Hexanoic Acid

Hexanoic acid is a saturated six-carbon carboxylic acid. Draw a straight chain of six carbon atoms (\(C_6H_{12}O_2\) without side groups) with a carboxylic acid group (\(COOH\)) at one end.

Structure of Propenoic Acid

Propenoic acid is a three-carbon carboxylic acid with a double bond between the first and second carbons. Starting from the carboxylic acid group (\(COOH\)), draw a three-carbon chain. Place a double bond between the first and second carbon atoms.

Key concepts

Benzoic Acid Structure

Benzoic acid is a classical example of an aromatic carboxylic acid, where a benzene ring is directly attached to a carboxylic acid group, denoted as \(COOH\). The chemistry of benzoic acid is foundational, given its role in organic synthesis and its prevalence in many natural products.

To represent its structure, imagine a hexagon symbolizing the benzene ring - this hexagon reflects the stable arrangement of six carbon atoms connected through alternate single and double bonds. Each carbon atom in the ring shares one hydrogen atom, except for one, which connects to the carboxylic acid group. Together, they form a planar and cyclic molecule that is both simple and vital in the realm of organic chemistry.

When drawing the benzene ring, it's customary to illustrate the alternating double bonds, and at the spot where the carboxylic acid group attaches, you draw a 'T' shape. This 'T' shape is actually a shorthand for the \(COOH\) group, with the vertical line of the 'T' representing the carbon atom and the three branches indicating the oxygen and hydrogen atoms.

Chlorinated Carboxylic Acids

Chlorinated carboxylic acids are carboxylic acids that contain one or more chlorine atoms attached to the carbon chain. These halogenated compounds have distinct properties and are important in various chemical processes, from organic synthesis to pharmaceuticals.

To draw them, start with the basic structure of the carboxylic acid – the familiar \(COOH\) group. With that as your anchor, extend the carbon chain to the specified length. Once you've established the carbon skeleton, add the chlorine atoms at the designated positions, using the locant numbers as the guide for placement. For example, '2-chloro-' indicates that a chlorine atom should be present on the second carbon of the chain, counting from the carboxylic acid end.

Locant Numbers and Substituents

It's crucial to pay attention to these numbers as they dictate where the chloro substituent should be located on the carbon chain. Incorrect placement of chlorine can lead to an entirely different molecule with potentially different chemical behavior and properties.

Carboxylic Acid Nomenclature

The nomenclature of carboxylic acids is governed by the IUPAC rules, offering a systematic way to name these compounds based on the length of their carbon chain and the presence of any substituents.

Starting with the longest continuous carbon chain that includes the carboxylic acid group, the parent name of the molecule is derived from the corresponding alkane name with the '-e' replaced by '-oic acid'. Any substituents attached to the carbon chain, such as chlorine (\(Cl\)) or methyl groups (\(CH_3\)), are noted with prefixes along with locant numbers indicating their position on the chain.

Example

Consider 'hexanoic acid' as an example. The name signifies a six-carbon chain ('hex-') with a carboxylic acid attached ('-anoic acid'). If a chlorine atom is attached to the second carbon, it becomes '2-chlorohexanoic acid'. This language, once mastered, allows chemists to convey complex molecular structures with precise and understandable terminology.

Drawing Organic Molecules

Drawing organic molecules is a fundamental skill in chemistry that helps students and chemists visually understand and communicate molecular structures.

There are a few guidelines to follow: the lines represent bonds, with single lines for single bonds and double lines for double bonds. Carbon atoms typically form four bonds in neutral compounds and are often not labeled in structures because their presence is implied at the junctions of lines. Hydrogen atoms bonded to carbon are also often not shown for simplicity's sake unless they carry significance for the structure or reaction being depicted. For exercises like drawing propenoic acid, recognizing the double bond's position is key – it drastically affects the molecule's reactivity and properties.

Practice, consistency, and an understanding of bonding and molecular geometry are the pillars of skillfully drawing organic molecules. Each structure drawn is an interplay of these concepts that are central to the world of organic chemistry.