Problem 18 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

Which of the following order is correct for dipole moment? (a) \(\mathrm{CH}_{3} \mathrm{~F}>\mathrm{CH}_{3} \mathrm{Cl}>\mathrm{CH}_{3} \mathrm{Br}>\mathrm{CH}_{3} \mathrm{I}\) (b) \(\mathrm{CH}_{3} \mathrm{Cl}>\mathrm{CH}_{3} \mathrm{Br}>\mathrm{CH}_{3} \mathrm{~F}>\mathrm{CH}_{3} \mathrm{I}\) (c) \(\mathrm{CH}_{3} \mathrm{Br}>\mathrm{CH}_{3} \mathrm{Cl}>\mathrm{CH}_{3} \mathrm{I}>\mathrm{CH}_{3} \mathrm{~F}\) (d) \(\mathrm{CH}_{3} \mathrm{Cl}>\mathrm{CH}_{3} \mathrm{~F}>\mathrm{CH}_{3} \mathrm{Br}>\mathrm{CH}_{3} \mathrm{I}\)

Step-by-Step Solution

Answer

(d) \( \mathrm{CH}_{3} \mathrm{Cl} > \mathrm{CH}_{3} \mathrm{~F} > \mathrm{CH}_{3} \mathrm{Br} > \mathrm{CH}_{3} \mathrm{I} \)

Step 1: Understanding Dipole Moment

The dipole moment is a measure of the separation of positive and negative charges in a molecule. It depends on two factors: the difference in electronegativity between the bonded atoms and the distance between them.

Step 2: Electronegativity Consideration

Considering the electronegativity, Fluorine (F) has the highest value followed by Chlorine (Cl), Bromine (Br), and then Iodine (I). Higher electronegativity generally indicates a higher ability to attract electrons, contributing to dipole moment.

Step 3: Bond Length Evaluation

The C-X bond length increases as we move from F to I (F

Step 4: Combining Factors

Despite the high electronegativity of F, the shorter bond length means CH3F may not have the highest dipole moment. Typically, CH3Cl has the highest dipole moment due to a combination of strong electronegativity and optimal bond distance, compared to other halogens.

Step 5: Correct Order Identification

By understanding the balance between electronegativity and bond length, the order of dipole moment is: CH3Cl > CH3F > CH3Br > CH3I. Therefore, the correct choice is option (d).

Key Concepts

Electronegativity

Electronegativity is a fundamental concept in chemistry that describes how strongly atoms attract electrons in a bond. In a molecule, the difference in electronegativity between atoms can lead to a polar bond, where one atom has a partial negative charge while the other has a partial positive charge. This charge separation is what contributes to the dipole moment.
In the context of halogen derivatives like methyl halides (CH_3F, CH_3Cl, CH_3Br, CH_3I), the electronegativity of the halogen atom plays a crucial role in determining the dipole moment. Fluorine is the most electronegative element and commonly imparts strong polar character to bonds it forms.

• Fluorine (F): 3.98
• Chlorine (Cl): 3.16
• Bromine (Br): 2.96
• Iodine (I): 2.66

As you can see, fluorine tops the list, indicating its high capacity to attract electrons, thus influencing the polarity and the resulting dipole moment of a molecule.

Bond Length

Bond length is another critical factor influencing the dipole moment of a molecule. The bond length refers to the distance between the nuclei of two bonded atoms. In terms of dipole moments, a longer bond might reduce the effective strength of the dipole across the molecule.
In halogen derivatives, as we move from CF to CI, the atomic size and consequently the bond length increase:

• C-F: 1.39 Angstroms
• C-Cl: 1.77 Angstroms
• C-Br: 1.94 Angstroms
• C-I: 2.14 Angstroms

While fluorine has the highest electronegativity, its short bond length with carbon often means that the dipole moment could be lower compared to other halogens like chlorine. This is because, even though fluorine attracts electrons strongly, the dipole's effectiveness decreases with the compact bond distance.

Halogen Derivatives

Halogen derivatives in organic chemistry largely refer to compounds where one or more hydrogen atoms have been replaced by halogen atoms like F, Cl, Br, or I in a hydrocarbon framework. These substitutions drastically change the molecule's properties, especially its polarity and dipole moment.
In compounds such as methylhalides (CH_3X), the presence of different halogens changes the dipole moment. The order of dipole moments observed in these compounds is based on a balance between electronegativity and bond length. For example, CH_3Cl generally has a higher dipole moment than CH_3Br or CH_3I because the chlorine's balance of bond length and electronegativity strikes an optimal configuration for producing a strong dipole moment.
The differences between these derivations are significant in fields like materials science, pharmaceuticals, and environmental chemistry, where dipole interactions can influence the behavior of molecules in different environments.

Organic Chemistry Competition Questions

Competitive organic chemistry exams often test understanding of core concepts like dipole moments, asking students to rank molecules based on these criteria. Such questions assess students’ ability to apply their knowledge of electronegativity and bond length in predicting molecular properties.
To excel in these questions, it helps to:

• Recognize trends in bond lengths and electronegativity across the periodic table.
• Visualize the molecular geometry and how these properties translate into dipole moments.
• Remember that while electronegativity dictates charge separation, bond length influences the dipole's result across the molecular structure.

A common mistake is assuming the strongest dipole moment correlates directly with the most electronegative element, ignoring bond length considerations. Hence, practice with a variety of molecules is key to mastering these competition questions. With consistent study and analysis of problem sets, students can deepen their understanding and improve their performance in these exams.

Previous

Problem 17

Next

Problem 25

Other exercises in this chapter

Problem 15

Which will give white ppt. with \(\mathrm{AgNO}_{3}+\mathrm{NH}_{4} \mathrm{OH}\) ?

View solution

Problem 17

Which of the following gives turbidity immediately with Lucas-reagent (anhy. \(\left.\mathrm{ZnCl}_{2}+\mathrm{HCI}\right) ?\)

View solution

Problem 25

1-Propanol can be prepared from propene by (a) \(\mathrm{HOH} / \mathrm{H}^{\oplus}\) (b) \(\mathrm{Hg}(\mathrm{OAc})_{2} / \mathrm{H}_{2} \mathrm{O}\) and \(\m

View solution

Problem 27

The decreasing order of reactivity of methyl alcohol (i), isopropyl alcohol (ii), tertiary butyl alcohol (iii) and ethyl alcohol (iv) for esterification will be

View solution

Key Concepts

Electronegativity

Electronegativity is a fundamental concept in chemistry that describes how strongly atoms attract electrons in a bond. In a molecule, the difference in electronegativity between atoms can lead to a polar bond, where one atom has a partial negative charge while the other has a partial positive charge. This charge separation is what contributes to the dipole moment.
In the context of halogen derivatives like methyl halides (CH_3F, CH_3Cl, CH_3Br, CH_3I), the electronegativity of the halogen atom plays a crucial role in determining the dipole moment. Fluorine is the most electronegative element and commonly imparts strong polar character to bonds it forms.

• Fluorine (F): 3.98
• Chlorine (Cl): 3.16
• Bromine (Br): 2.96
• Iodine (I): 2.66

As you can see, fluorine tops the list, indicating its high capacity to attract electrons, thus influencing the polarity and the resulting dipole moment of a molecule.

Bond Length

Bond length is another critical factor influencing the dipole moment of a molecule. The bond length refers to the distance between the nuclei of two bonded atoms. In terms of dipole moments, a longer bond might reduce the effective strength of the dipole across the molecule.
In halogen derivatives, as we move from CF to CI, the atomic size and consequently the bond length increase:

• C-F: 1.39 Angstroms
• C-Cl: 1.77 Angstroms
• C-Br: 1.94 Angstroms
• C-I: 2.14 Angstroms

While fluorine has the highest electronegativity, its short bond length with carbon often means that the dipole moment could be lower compared to other halogens like chlorine. This is because, even though fluorine attracts electrons strongly, the dipole's effectiveness decreases with the compact bond distance.

Halogen Derivatives

Halogen derivatives in organic chemistry largely refer to compounds where one or more hydrogen atoms have been replaced by halogen atoms like F, Cl, Br, or I in a hydrocarbon framework. These substitutions drastically change the molecule's properties, especially its polarity and dipole moment.
In compounds such as methylhalides (CH_3X), the presence of different halogens changes the dipole moment. The order of dipole moments observed in these compounds is based on a balance between electronegativity and bond length. For example, CH_3Cl generally has a higher dipole moment than CH_3Br or CH_3I because the chlorine's balance of bond length and electronegativity strikes an optimal configuration for producing a strong dipole moment.
The differences between these derivations are significant in fields like materials science, pharmaceuticals, and environmental chemistry, where dipole interactions can influence the behavior of molecules in different environments.

Organic Chemistry Competition Questions

Competitive organic chemistry exams often test understanding of core concepts like dipole moments, asking students to rank molecules based on these criteria. Such questions assess students’ ability to apply their knowledge of electronegativity and bond length in predicting molecular properties.
To excel in these questions, it helps to:

• Recognize trends in bond lengths and electronegativity across the periodic table.
• Visualize the molecular geometry and how these properties translate into dipole moments.
• Remember that while electronegativity dictates charge separation, bond length influences the dipole's result across the molecular structure.

A common mistake is assuming the strongest dipole moment correlates directly with the most electronegative element, ignoring bond length considerations. Hence, practice with a variety of molecules is key to mastering these competition questions. With consistent study and analysis of problem sets, students can deepen their understanding and improve their performance in these exams.