Problem 6 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Which of the following pairs of compounds are positional isomers (a) CCCC(C)=O CCC(=O)CC and (b) \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{CHO}\) and CCCC(C)(C)O (c) CCCC(C)=O and CC(C)CC=O (d) CCC(=O)CC and CC(C)CC=O

Step-by-Step Solution

Answer

Pairs (a) and (c) are positional isomers.

Step 1: Understanding Positional Isomers

Positional isomers are compounds that have the same molecular formula but different placement of functional groups or substituents on the parent chain. The total number of atoms in each isomer is the same, but the position of a functional group is different.

Step 2: Analyzing Pair (a)

The compounds are 2-pentanone (CCCC(C)=O) and 3-pentanone (CCC(=O)CC). They have the same molecular formula, C5H10O, and both are ketones, but the position of the carbonyl group (C=O) differs. In the first compound, the carbonyl group is on the second carbon atom, while in the second compound, it is on the third carbon atom.

Step 3: Analyzing Pair (b)

The compounds are pentanal (CH3CH2CH2CH2CHO) and 2-methyl-2-butanol (CCCC(C)(C)O). They have different molecular formulas (pentanal is C5H10O and 2-methyl-2-butanol is also C5H12O), indicating they are not the same class of compounds, as one is an aldehyde and the other is an alcohol. They are not positional isomers.

Step 4: Analyzing Pair (c)

The compounds are 2-pentanone (CCCC(C)=O) and 3-pentanone (CC(C)CC=O). They have the same molecular formula, C5H10O, and both are ketones, but differ in the position of the carbonyl group, as explained before. These compounds are positional isomers.

Step 5: Analyzing Pair (d)

The compounds are 3-pentanone (CCC(=O)CC) and 3-pentanone (CC(C)CC=O). They essentially describe the same ketone compound with the carbonyl group on the central chain, confirming that they are the same structure rather than positional isomers.

Key Concepts

Molecular Formula

The molecular formula of a compound provides a complete count of each type of atom present. It does not indicate how these atoms are arranged or bonded within the compound. For example, many different compounds can share the same molecular formula but differ in their structural arrangement, leading to different chemical properties.
Consider a molecular formula like C\(_5\)H\(_{10}\)O, which can correspond to several structures like ketones or aldehydes. By only looking at the molecular formula, we know the types and number of atoms involved, but not their specific arrangement. This is why knowing the molecular formula is just the starting point in identifying isomers.

• The same molecular formula can yield different configurations (isomers).
• It tells us the types and numbers of atoms, but not the structure.

Functional Group

A functional group is a specific group of atoms within a molecule that is responsible for a characteristic reaction of that molecule. It defines the class of the molecule and often dictates how it reacts chemically. For example, hydroxyl groups (-OH) make a compound an alcohol, while carbonyl groups (C=O) are key in ketones and aldehydes.
In positional isomerism, the molecular formula remains unchanged, but the functional group occupies different positions within the molecule, thus creating unique compounds with distinct properties. Importantly, functional groups are often the reactive sites in molecules, significantly influencing the molecule's chemical behavior.

• Functional groups define the nature and class of the molecule.
• They are crucial sites for chemical reactions.

Ketones

Ketones are a type of organic compound characterized by the presence of a carbonyl group (C=O) bonded to two carbon atoms. They often have a general formula of C\(_n\)H\(_{2n}\)O and are significant in many biological processes.
Ketones are versatile compounds that occur naturally in the body and are involved in many chemical reactions. The position of the carbonyl group along the carbon chain in ketones leads to different positional isomers. For example, 2-pentanone and 3-pentanone are positional isomers with the carbonyl group placed differently – on the second and third carbon atoms, respectively.

• Ketones have a carbonyl group bonded to two carbons.
• They play diverse roles in chemical and biological processes.

Aldehydes

Aldehydes feature a carbonyl group (C=O) bonded directly to at least one hydrogen atom. This distinguishes them from ketones where the carbonyl group is flanked by carbon atoms.
Aldehydes are often found at the end of a carbon chain, making them more reactive due to the hydrogen attached to the carbonyl carbon. Their general formula can be written as RCHO, where R is a hydrocarbon chain. Of note, aldehydes participate in a broad range of reactions, including oxidation and reductions.

• Aldehydes have a carbonyl group bonded to at least one hydrogen.
• They often occur at the end of a carbon chain.

Carbonyl Group

The carbonyl group is a functional group characterized by a carbon atom double-bonded to an oxygen atom (C=O). This group is a defining feature in both ketones and aldehydes and is central to their chemical reactivity.
In ketones, the carbonyl group is sandwiched between two carbon atoms, which defines their properties and reactivity distinct from aldehydes. Conversely, in aldehydes, the carbonyl group is terminal and bound to at least one hydrogen atom. This arrangement makes aldehydes typically more reactive than ketones. The position of the carbonyl group is also crucial in determining the isomer type, such as positional isomers where its placement will change but the overall atom composition remains constant.

• The carbonyl group consists of a carbon double-bonded to oxygen.
• It is crucial in defining both physical properties and reactivity.