Problem 45 - Chemistry: The Central Science in SI Units, Global Edition

Question

Draw the molecular structure for \((\mathbf{a})\) a carboxylic acid that is an isomer of ethyl ethanoate, \((\mathbf{b})\) an alcohol that is an isomer of dimethylether.

Step-by-Step Solution

Answer

The required molecular structures are: 1. Butanoic acid (\(CH_3CH_2CH_2COOH\)) as an isomer of ethyl ethanoate: ``` H H H H | | | | H - C - C - C - C - O - H | | | H H OH ``` 2. Ethanol (\(CH_3CH_2OH\)) as an isomer of dimethyl ether: ``` H H | | H - C - C - O - H | H ```

Step 1: Calculate the molecular formula for ethyl ethanoate

Ethyl ethanoate (ethyl acetate) has an ethyl group (C2H5) attached to an ethanoate group (CH3COO). Thus, the molecular formula for ethyl ethanoate is \(C_4H_8O_2\).

Step 2: Calculate the molecular formula for dimethyl ether

Dimethyl ether has two methyl groups (CH3) attached to an oxygen atom. Therefore, the molecular formula for dimethyl ether is \(C_2H_6O\).

Step 3: Determine the carboxylic acid isomer of ethyl ethanoate

Now, we need to find a carboxylic acid with the molecular formula \(C_4H_8O_2\). The simplest carboxylic acid with this formula is butanoic acid. It consists of a 4-carbon chain with a carboxylic acid group (-COOH) at one end. Its structure is: \(CH_3CH_2CH_2COOH\).

Step 4: Draw the molecular structure for butanoic acid

The molecular structure for butanoic acid is: ``` H H H H | | | | H - C - C - C - C - O - H | | | H H OH ```

Step 5: Determine the alcohol isomer of dimethyl ether

Next, we need to find an alcohol with the molecular formula \(C_2H_6O\). The simplest alcohol with this formula is ethanol. It consists of a 2-carbon chain with a hydroxyl group (-OH) at one end. Its structure is: \(CH_3CH_2OH\).

Step 6: Draw the molecular structure for ethanol

The molecular structure for ethanol is: ``` H H | | H - C - C - O - H | H ``` So the required structures are: 1. Butanoic acid (\(CH_3CH_2CH_2COOH\)) as an isomer of ethyl ethanoate. 2. Ethanol (\(CH_3CH_2OH\)) as an isomer of dimethyl ether.

Key Concepts

Isomerism

Isomerism is a fascinating concept in chemistry, where compounds with the same molecular formula have different structural arrangements. This leads to the existence of isomers. These isomers can display very different chemical and physical properties. Understanding isomerism is crucial because it explains how molecules can be structurally rearranged to form new substances.

There are several types of isomerism, but the two main categories are:

• Structural Isomerism: This occurs when molecules differ in the connectivity of atoms. For example, butanoic acid and ethyl ethanoate both have the formula \(C_4H_8O_2\), but their atoms are arranged differently, making these substances structural isomers.
• Stereoisomerism: This type involves isomers with the same atom connectivity, but different spatial orientations. This is more common in complex organic molecules.

Recognizing isomers requires analyzing the molecular structure and considering how molecules can be rearranged without altering the underlying molecular formula. Understanding isomerism allows chemists to redesign molecules for pharmaceuticals, materials, and more.

Molecular Structure

The molecular structure of a compound determines its properties and reactivity. It refers to the arrangement of atoms within a molecule and the bonds between them. Organic chemists use molecular structure diagrams to visualize and understand these arrangements.

When drawing the molecular structure, the following concepts are crucial:

• Bonds: Molecules are held together by chemical bonds, primarily covalent bonds in organic compounds. The type and length of these bonds affect the molecule's stability.
• Functional Groups: Groups of atoms that impart specific properties to a molecule. For example, the carboxyl group \(-COOH\) is characteristic of carboxylic acids.
• 3D Shapes: Though often drawn on a 2D plane, molecules are three-dimensional. VSEPR theory (Valence Shell Electron Pair Repulsion theory) helps predict molecular geometry based on electron pair repulsion.

To draw molecules like butanoic acid and ethanol, one must accurately depict atom positions, connectivities, and functional groups. Mastering this skill is essential in grasping organic reactions and mechanisms.

Carboxylic Acids

Carboxylic acids are an essential class of organic compounds characterized by the presence of a carboxyl group \(-COOH\). They play a significant role in both biochemistry and industrial chemistry. Their typical structure involves a carbon backbone with one or more carboxyl groups attached.

Key features of carboxylic acids include:

• Acidity: The carboxyl group can donate a proton \(H^+\), making these compounds acidic. This acidity is due to the stability of the carboxylate ion \(-COO^-\) formed after losing the proton.
• Solubility: Lower molecular weight carboxylic acids are generally soluble in water due to hydrogen bonding. As the carbon chain length increases, solubility decreases.
• Applications: These compounds are widely used, from acetic acid in food preservation to more complex acids in pharmaceuticals and synthetic chemistry.

A clear understanding of carboxylic acids' properties and reactions is vital in organic chemistry. Their ability to participate in various reactions, like esterification and reduction, makes them versatile building blocks in synthesis.