Assignment And Quiz Corrections

Question 1

Why was this wrong

Answer 1

This question asks you to rank the compounds in order of decreasing melting point. The melting point of a compound is influenced by factors such as molecular structure, symmetry, and the type of intermolecular forces present. Here’s a breakdown of why you might have chosen the wrong answer:
1. Compound I: This is a straight-chain alkane, which has stronger intermolecular forces (due to better packing) compared to branched alkanes.
2. Compound II: This is a branched-chain alkane. Branching disrupts packing efficiency, which lowers the melting point.
3. Compound III: This is the most symmetric molecule of the three (tertiary structure). Symmetry allows for more efficient packing in the solid phase, which typically increases the melting point.

Correct Answer: D (III > I > II)

Explanation:
• III has the highest melting point due to its high symmetry and efficient packing.
• I comes next because it is a straight-chain alkane, allowing for stronger van der Waals forces.
• II has the lowest melting point because branching reduces packing efficiency.

If you selected C, you might have underestimated the effect of symmetry on the melting point of III.

Question 2

The difference between symmetry and branching lies in how they affect molecular structure and properties such as melting point, boiling point, and packing efficiency:

Answer 2

1. Symmetry• Definition: Symmetry refers to the uniform distribution of atoms or groups in a molecule, creating a balanced and organized structure.
   • Effect on Melting Point:
   • Highly symmetrical molecules (e.g., Compound III in your question) pack more efficiently in a crystalline solid.
   • Better packing leads to stronger intermolecular forces in the solid state, resulting in a higher melting point.
   • Example: Neopentane (highly symmetrical) has a higher melting point compared to less symmetrical alkanes with the same molecular formula.
2. Branching• Definition: Branching occurs when a molecule has side chains or substituents attached to the main carbon chain, reducing its linearity.
   • Effect on Melting Point:
   • Branching disrupts the molecule’s ability to pack efficiently into a solid lattice.
   • Poor packing decreases the intermolecular forces in the solid state, lowering the melting point.
   • Example: Isobutane (branched) has a lower melting point than n-butane (straight-chain) due to less efficient packing.

Key Difference

•	Symmetry enhances melting point because it promotes efficient packing in the solid state.
•	Branching lowers melting point because it disrupts packing and reduces the strength of intermolecular forces.

Practical Application in Your Question

•	Compound III has the highest symmetry, leading to the highest melting point.
•	Compound II has significant branching, leading to poor packing and the lowest melting point.
•	Compound I is a straight chain, so it packs better than a branched chain but not as well as a highly symmetrical molecule.

Question 3

Why was this wrong

Answer 3

This question asks you to identify the compound with the lowest boiling point. Here’s how boiling points are influenced and why the correct answer is D (IV):

Factors Affecting Boiling Point:

1.	Hydrogen Bonding:
•	Molecules capable of hydrogen bonding (e.g., alcohols like Compound I) have significantly higher boiling points.
•	Compound I contains an -OH group, so it will have the highest boiling point due to strong intermolecular hydrogen bonds.
2.	Dipole-Dipole Interactions:
•	Molecules with polar bonds (like ethers, e.g., Compound III) have higher boiling points than nonpolar molecules, though lower than alcohols.
3.	Van der Waals Forces:
•	Nonpolar molecules (alkanes, like Compounds II and IV) rely on weak van der Waals forces.
•	Larger surface area and less branching lead to stronger van der Waals forces and a higher boiling point.
4.	Branching:
•	Branched alkanes have lower boiling points than straight-chain alkanes due to reduced surface area and weaker van der Waals forces.

Analysis of the Compounds:

•	Compound I: Contains an -OH group (alcohol), so it has the highest boiling point due to hydrogen bonding.
•	Compound II: A straight-chain alkane with relatively higher boiling point than branched alkanes.
•	Compound III: An ether, which has a moderate boiling point due to dipole-dipole interactions but is higher than nonpolar alkanes.
•	Compound IV: A branched alkane. Branching lowers its surface area, making van der Waals forces weaker, and thus, it has the lowest boiling point.

Correct Answer: D (Compound IV)

You likely selected B (Compound II), but this is a straight-chain alkane, which has stronger van der Waals forces than the branched alkane Compound IV. Hence, Compound IV has the lowest boiling point.

Question 4

Why was I wrong

Answer 4