Problem 63 - A Complete Resource Book in Chemistry for JEE Main

Question

Which has the maximum boiling point as well as melting point out of the following: CCCCC CCC(C)C CC(C)(C)C (1) (2) (3) (a) 1 in both the cases (b) 1,2 (c) 1,3 (d) 2,1

Step-by-Step Solution

Answer

(a) 1 in both cases.

Step 1: Identify Molecules from Smiles Notation

The given SMILES notations correspond to these structures: 1. CCCCC is n-pentane. 2. CCC(C)C is isobutane. 3. CC(C)(C)C is neopentane.

Step 2: Understand Boiling and Melting Points

Boiling and melting points are influenced by the structure of the molecule. Straight-chain alkanes generally have higher boiling and melting points compared to their branched counterparts because of increased surface area leading to stronger van der Waals forces.

Step 3: Analyze Each Molecule

1. **n-pentane**: Being a straight-chain alkane, it has the highest surface area and therefore the strongest van der Waals forces among the three. 2. **isobutane**: This is a branched structure, reducing its boiling and melting points compared to linear alkanes. 3. **neopentane**: This is more branched than isobutane, leading to the lowest boiling and melting points among the three.

Step 4: Compare the Properties

Order the compounds based on boiling and melting points. As n-pentane has a straight chain, it will have a higher boiling and melting point than the more branched isobutane and neopentane.

Step 5: Conclude Which Has Maximum Boiling and Melting Points

Based on structural analysis, n-pentane (1) has both the maximum boiling and melting points, making it the answer to both the melting and boiling point queries.

Key Concepts

SMILES notation

SMILES, which stands for Simplified Molecular Input Line Entry System, is a way to represent a chemical structure using text. This notation allows us to encode the structure of molecules concisely, making it easier to input and analyze molecular information using computers. It uses letters and symbols to represent atoms and bonds, forming a sequence that describes the molecular structure. For example, the SMILES notation for n-pentane is "CCCCC", which simply lists all the carbon atoms in a row, denoting a straight-chain alkane. This method is extremely helpful for identifying molecules quickly and succinctly without having to draw complex structures.

n-pentane

n-pentane is a type of alkane, which means it is a hydrocarbon with only single bonds between the carbon atoms. This molecule is made up of a straight chain of five carbon atoms, which is why we use the prefix 'n', standing for 'normal', to indicate its linear structure. Due to its unbranched nature, n-pentane has a relatively high boiling and melting point compared to its isomers. The straight-chain structure allows the molecules to pack closely together, enhancing the van der Waals forces, which are weak attractions between molecules. These stronger intermolecular forces require more energy to overcome, leading to higher phase transition points.

van der Waals forces

Van der Waals forces are weak attractions that occur between molecules due to temporary changes in charge. These forces include attractions between molecules that are not covalently bonded. In the case of alkanes like n-pentane, these forces play a significant role in determining boiling and melting points. As the molecule's surface area increases, so do these forces, since more contact points allow for greater interaction between molecules. Thus, linear alkanes like n-pentane exhibit greater van der Waals interactions compared to their branched counterparts, leading to higher melting and boiling points. This principle helps explain why the structure of organic compounds significantly impacts their physical properties.

alkanes

Alkanes are a group of hydrocarbons that consist entirely of carbon and hydrogen atoms bonded by single bonds. They are known as saturated hydrocarbons since each carbon is fully saturated with hydrogen. Alkanes can be found in two main forms: straight-chain and branched-chain. The physical properties of alkanes, such as boiling and melting points, heavily depend on their structure.

• **Straight-chain alkanes**, like n-pentane, have higher boiling and melting points due to increased surface area leading to stronger van der Waals forces.
• **Branched-chain alkanes**, like isobutane and neopentane, have lower boiling and melting points because the molecular branching reduces the surface area that can participate in van der Waals interactions.

Understanding these structural variations is crucial for predicting how alkanes will behave under different temperatures, which is essential for their practical applications in industries and everyday use.