Problem 96 - Chemistry The Central Science In Si Units

Question

An unknown organic compound is found on elemental analysis to contain \(72.0 \%\) carbon, \(12.0 \%\) hydrogen, and \(16.0 \%\) oxygen by mass. It has a molecular weight of \(100 \mathrm{~g} / \mathrm{mol}\) and is slightly soluble in water. Upon careful oxidation it is converted into a ketone which contains \(73.4 \%\) carbon, \(10.3 \%\) hydrogen, and \(16.3 \%\) oxygen by mass. Indicate two or more reasonable structures for the unknown.

Step-by-Step Solution

Answer

The compound is likely an alcohol; possible structures include 3-hydroxyhexane or 4-methylpentan-2-ol.

Step 1: Determine Empirical Formula of the Unknown Compound

To find the empirical formula, use the percentages given for carbon (C), hydrogen (H), and oxygen (O). Assume 100 g of the compound, so you have 72 g of C, 12 g of H, and 16 g of O. Convert these to moles:- For carbon: \( \frac{72.0 \, \text{g}}{12.01 \, \text{g/mol}} = 6.00 \, \text{mol} \)- For hydrogen: \( \frac{12.0 \, \text{g}}{1.008 \, \text{g/mol}} = 11.9 \, \text{mol} \)- For oxygen: \( \frac{16.0 \, \text{g}}{16.00 \, \text{g/mol}} = 1.00 \, \text{mol} \)The ratio of C:H:O is approximately 6:12:1, which simplifies to 6:12:1 or \( \text{C}_6\text{H}_{12}\text{O} \), the empirical formula.

Step 2: Verify with Molecular Weight

The empirical formula \( \text{C}_6\text{H}_{12}\text{O} \) has a molar mass of\[ 6 \times 12.01 + 12 \times 1.008 + 16.00 = 100 \, \text{g/mol} \]which matches the given molecular weight of the compound, confirming it is also the molecular formula.

Step 3: Analyze Oxidation Product

The oxidation product, a ketone, has 73.4% C, 10.3% H, and 16.3% O. Using the same method as before, assume 100 g: calculate moles and find the empirical formula. You get approximately C:H:O as 6:10:1, close to \( \text{C}_6\text{H}_{10}\text{O} \), suggesting loss of water or a dehydrogenation process.

Step 4: Propose Structures

Given the molecular formula \( \text{C}_6\text{H}_{12}\text{O} \) and its conversion to a ketone, reasonable structures include ones capable of forming ketones upon oxidation. Two possible structures conforming to molecular formula include:1. 3-Hydroxyhexane (\( \text{CH}_3\text{CH}_2\text{CH(OH)CH}_2\text{CH}_2\text{CH}_3 \))2. 4-Methylpentan-2-ol (\( \text{CH}_3\text{CH}_2\text{C(OH)(CH}_3)\text{CH}_2\text{CH}_3 \))Both structures have the requisite functional groups to form ketones.

Key Concepts

Molecular Weight

Molecular weight is an important concept in chemistry that refers to the mass of a molecule. It is usually expressed in units of grams per mole (g/mol).

The molecular weight is calculated by summing up the atomic weights of all the atoms present in a molecule. For example, if you have a molecule of water (H₂O), the molecular weight is calculated by adding the atomic weights of hydrogen and oxygen. Thus, the molecular weight of water is approximately:

• Hydrogen: 2 atoms × 1.008 g/mol = 2.016 g/mol
• Oxygen: 1 atom × 16.00 g/mol = 16.00 g/mol

So, the total molecular weight of water is 18.016 g/mol. Understanding the molecular weight is crucial in chemical reactions and stoichiometry calculations as it allows chemists to measure the necessary amounts of substances required for reactions precisely. In the case of the organic compound in our problem, the given molecular weight helped confirm the empirical formula as the actual molecular formula.

Organic Compound Analysis

Organic compound analysis involves examining and determining the composition of organic molecules. This type of analysis is often done through techniques such as elemental analysis, which assesses the types and percentages of elements in a compound.

In the original problem, the organic compound was analyzed and found to contain specific percentages of carbon, hydrogen, and oxygen. Such information is crucial in determining the empirical formula of the compound. By knowing the empirical formula, we can start deducing the molecular structure. Moreover, organic compound analysis provides insights into possible chemical reactions the compound might undergo. For instance, understanding the organic compound’s behavior upon oxidation led to further analysis of its structure change, forming a ketone as an oxidation product.