Problem 20 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following are arranged in the decreasing order of dipole moment? (a) \(\mathrm{CH}_{3} \mathrm{Cl}, \mathrm{CH}_{3} \mathrm{Br}, \mathrm{CH}_{3} \mathrm{~F}\) (b) \(\mathrm{CH}_{3} \mathrm{Cl}, \mathrm{CH}_{3} \mathrm{~F}, \mathrm{CH}_{3} \mathrm{Br}\) (c) \(\mathrm{CH}_{3} \mathrm{Br}, \mathrm{CH}_{3} \mathrm{Cl}, \mathrm{CH}_{3} \mathrm{~F}\) (d) \(\mathrm{CH}_{3} \mathrm{Br}, \mathrm{CH}_{3} \mathrm{~F}, \mathrm{CH}_{3} \mathrm{Cl}\)

Step-by-Step Solution

Answer

None of the options correctly reflect the decreasing order of dipole moment \(\mathrm{CH}_{3}\mathrm{F} > \mathrm{CH}_{3}\mathrm{Cl} > \mathrm{CH}_{3}\mathrm{Br}\).

Step 1: Understanding Dipole Moment

The dipole moment is a measure of the polarity of a chemical bond within a molecule. It is determined by the product of the charge difference between atoms and the distance between them. Molecules with high electronegativity differences and bond distances generally have larger dipole moments.

Step 2: Analyzing the Molecules

Consider each molecule:- In \(\mathrm{CH}_{3}\mathrm{F}\), fluorine has the highest electronegativity, resulting in a strong dipole moment.- \(\mathrm{CH}_{3}\mathrm{Cl}\) has a strong dipole moment, but slightly less than \(\mathrm{CH}_{3}\mathrm{F}\), due to chlorine's lower electronegativity compared to fluorine.- \(\mathrm{CH}_{3}\mathrm{Br}\) has the lowest dipole moment since bromine is the least electronegative of the three halogens here.

Step 3: Arranging by Dipole Moment

Based on electronegativity, the correct order from highest to lowest dipole moment is \(\mathrm{CH}_{3}\mathrm{F} > \mathrm{CH}_{3}\mathrm{Cl} > \mathrm{CH}_{3}\mathrm{Br}\). This will guide us in selecting the correct option.

Step 4: Comparing with Given Options

Review the options given:- Option (a): \(\mathrm{CH}_{3} \mathrm{Cl}, \mathrm{CH}_{3} \mathrm{Br}, \mathrm{CH}_{3}\mathrm{~F}\)- Option (b): \(\mathrm{CH}_{3} \mathrm{Cl}, \mathrm{CH}_{3}\mathrm{~F}, \mathrm{CH}_{3}\mathrm{~Br}\)- Option (c): \(\mathrm{CH}_{3} \mathrm{Br}, \mathrm{CH}_{3} \mathrm{Cl}, \mathrm{CH}_{3}\mathrm{~F}\)- Option (d): \(\mathrm{CH}_{3} \mathrm{Br}, \mathrm{CH}_{3}\mathrm{~F}, \mathrm{CH}_{3}\mathrm{Cl}\)None of the options reflects the arrangement \(\mathrm{CH}_{3}\mathrm{F} > \mathrm{CH}_{3}\mathrm{Cl} > \mathrm{CH}_{3}\mathrm{Br}\). Double-check the options and dipole moment order to ensure accuracy.

Step 5: Conclusion

After reviewing all options again, consider if there are possible examination errors or misinterpretations. If time permits, request clarification from your instructor. There may be inaccuracies in the problem setup. Rely on the known dipole moment order from step 3 for accuracy.

Key Concepts

Electronegativity

Electronegativity is a key concept in understanding the chemical behavior of molecules. It refers to an atom's tendency to attract and hold onto electrons in a chemical bond. Different atoms have different electronegativities, which affects how electrons are shared in bonds. When one atom has a higher electronegativity than another, it pulls the shared electrons closer to itself.
For example, in the molecules

• \(\mathrm{CH}_{3}\mathrm{F}\), fluorine is highly electronegative, and it pulls electrons more strongly than hydrogen and carbon.
• \(\mathrm{CH}_{3}\mathrm{Cl}\), chlorine also attracts electrons but less so compared to fluorine.
• \(\mathrm{CH}_{3}\mathrm{Br}\), bromine has a lower electronegativity compared to both fluorine and chlorine.

Understanding electronegativity helps in predicting how atoms interact and form molecules with specific properties, such as dipole moments.

Chemical Bond Polarity

Chemical bond polarity refers to the distribution of electrical charge over the atoms joined by the bond. A bond is polar if the electrons are not equally shared due to differences in electronegativity between the two atoms. The greater the electronegativity difference, the more polar the bond becomes.
For example, in \(\mathrm{CH}_{3}\) molecules linked to different halogens:

• In \(\mathrm{CH}_{3}\mathrm{F}\), the bond between carbon and fluorine is highly polar. Fluorine's strong attraction for electrons makes the bond exhibit a noticeable charge difference.
• In \(\mathrm{CH}_{3}\mathrm{Cl}\), the carbon-chlorine bond is less polar than the carbon-fluorine bond due to chlorine's lower electronegativity.
• In \(\mathrm{CH}_{3}\mathrm{Br}\), the carbon-bromine bond is the least polar, resulting in the smallest dipole moment.

Recognizing these differences in bond polarity is crucial for understanding which molecules have higher or lower dipole moments.

Molecular Structure Analysis

To analyze molecular structures and their effects on chemical properties, one must consider how atoms within a molecule are arranged. This arrangement determines the molecule's shape and affects its overall dipole moment. The dipole moment is a vector quantity, meaning both the magnitude and direction of electron distribution are important.
The vectors within a molecule add up to form the net dipole moment:

• In \(\mathrm{CH}_{3}\) compounds, the three hydrogen atoms are bonded to carbon symmetrically.
• Anything that causes asymmetry in electron distribution, like a highly electronegative atom such as fluorine, forms a significant dipole moment.
• Molecule's orientation also affects how vectors add up. Thus influencing its dipole moment.

In conclusion, analyzing molecular structure provides insight into how different elements within a molecule contribute to its properties, such as dipole moments and interactions with other substances.