Problem 191 - A Complete Resource Book in Chemistry for JEE Main

Question

A compound with molecular formula \(\mathrm{C}_{6} \mathrm{H}_{14} \mathrm{O}_{5}\) is converted by the action of acetyl chloride to a compound with molecular mass 334 . The number of OH groups in the compound is

Step-by-Step Solution

Answer

The compound has 4 OH groups.

Step 1: Analyze the molecular changes

The molecular formula of the compound is given as \( \mathrm{C}_{6} \mathrm{H}_{14} \mathrm{O}_{5} \). When treated with acetyl chloride, the hydroxyl (OH) groups are acetylated, and each OH group reacts with one acetyl chloride molecule to form an ester linkage (-O-COCH3). Each acetyl group added contributes 42 units to the molecular mass (the mass of one acetyl group is 43, subtracting 1 for the replaced hydrogen). Determine the number of OH groups by analyzing how much the molecular mass has increased.

Step 2: Calculate the initial molecular mass

Calculate the initial molecular mass of the compound with formula \( \mathrm{C}_{6} \mathrm{H}_{14} \mathrm{O}_{5} \). Using atomic masses: C = 12, H = 1, and O = 16. Compute as follows:\[ 6(12) + 14(1) + 5(16) = 72 + 14 + 80 = 166 \]

Step 3: Determine the mass increase

The final molecular mass is given as 334. Find the mass increase during the conversion by subtracting the initial mass from the final mass:\[ 334 - 166 = 168 \]

Step 4: Calculate the number of OH groups

Each acetyl group adds 42 in mass. To find the number of OH groups, divide the total mass increase by the mass contribution of each acetyl group:\[ \frac{168}{42} = 4 \]This means there are 4 OH groups in the compound.

Key Concepts

Hydroxyl group

In organic chemistry, hydroxyl groups are quite significant. The hydroxyl group, denoted as \(-OH\), is a functional group consisting of an oxygen atom bonded to a hydrogen atom. These groups are commonly found in alcohols and are key in determining the chemical properties of the compound.

• They are polar due to the electronegativity difference between oxygen and hydrogen.
• This polarity makes compounds containing hydroxyl groups generally soluble in water.
• The hydrogen bonding ability of the hydroxyl group contributes to higher boiling and melting points of compounds.

When it comes to reactivity, hydroxyl groups can undergo various chemical reactions, including dehydration and esterification, like the acetylation reaction you see in this exercise.

Acetylation reaction

The acetylation reaction is a fundamental process in organic chemistry wherein an acetyl group is introduced into a molecule. This process often involves the use of acetyl chloride.
In this context, hydroxyl groups in the compound react with acetyl chloride to form esters. Specifically, the \(-OH\) group from the compound and the \(-Cl\) from acetyl chloride interact, resulting in the formation of an ester linkage \(\mathrm{-O-COCH_3}\).

• The acetyl group \(\mathrm{(CH_3CO)}\) is added in place of the hydroxyl hydrogen.
• This reaction adds mass to the compound—42 grams per mole for each \(-OH\) converted—due to the extra atomic mass from the acetyl group.

This step can significantly alter the properties of the original compound, such as its molecular mass, which is key to solving the exercise's problem.

Molecular mass calculation

Calculating molecular mass is a critical skill in chemistry that helps identify compounds and predict their behaviors. The molecular mass is determined by summing the atomic masses of all atoms in a molecule.
For example, the molecular mass of the compound in the initial step is calculated as follows:

• Carbon (\(\mathrm{C}\)): 12 amu times 6 = 72 amu.
• Hydrogen (\(\mathrm{H}\)): 1 amu times 14 = 14 amu.
• Oxygen (\(\mathrm{O}\)): 16 amu times 5 = 80 amu.

Thus, the molecular mass of \(\mathrm{C}_{6} \mathrm{H}_{14} \mathrm{O}_{5}\) is 166 amu initially. After acetylation, when the molecular mass increases to 334 amu, it helps determine the number of hydroxyl groups by calculating the increase in molecular mass, as shown in the solution.

Functional groups in chemistry

In organic chemistry, functional groups are specific groups of atoms within molecules that have distinct chemical properties. They are the reactive part of the molecule and dictate how the molecule will react in chemical reactions.

• The hydroxyl group, \(-OH\), is just one example of a functional group.
• Each functional group has its own characteristic reactions and significantly affects the physical properties of the compound.
• They provide sites for chemical reactions, such as the esterification and acetylation reactions in this exercise.

By understanding functional groups, chemists can predict and manipulate the reactivity and properties of organic compounds, making them indispensable when tackling problems in chemistry, such as determining changes in molecular mass after a reaction.