Question

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

Short answer

Question: Arrange the following methyl halides in the correct order of dipole moments: CH3F, CH3Cl, CH3Br, and CH3I. Answer: The correct order of dipole moments is CH3F > CH3Cl > CH3Br > CH3I.

Step-by-step solution

Understanding the concept of dipole moment

In a molecule, a dipole moment occurs when there is a separation of charges due to differences in electronegativity between bonded atoms. In our case, we are looking at the C-X bond in methyl halides, where X represents a halogen (F, Cl, Br, or I). The dipole moment can be defined as \(\mu = \delta\times d\), where \(\mu\) is the dipole moment, \(\delta\) is the difference in electronegativity, and \(d\) is the distance between the charges. Since the bond lengths do not vary significantly between the methyl halides, we can focus on the electronegativity differences to determine the dipole moments comparatively.

Comparing electronegativities of different halogens

To decide on the order of dipole moments, we need to examine the electronegativities of the halogens in the periodic table. The electronegativity values for elements decrease as we go down a group, so the order of electronegativity is F > Cl > Br > I.

Determining the order of the dipole moments

Since the dipole moment is determined by the difference in electronegativity, we can now determine the order: 1. CH3F: C-F bond has the largest electronegativity difference because F is the most electronegative halogen. 2. CH3Cl: C-Cl bond has the next largest electronegativity difference since Cl is the second most electronegative halogen. 3. CH3Br: C-Br bond has a smaller electronegativity difference due to Br being less electronegative than both F and Cl. 4. CH3I: C-I bond has the smallest electronegativity difference because I is the least electronegative halogen among the options.

Identifying the correct answer

Now, by comparing the given answer options, we can see that the correct order of dipole moments matches the order given in option (a) \(\mathrm{CH}_{3} \mathrm{F}>\mathrm{CH}_{3} \mathrm{Cl}>\mathrm{CH}_{3}\mathrm{Br}>\mathrm{CH}_{3} \mathrm{I}\). Therefore, option (a) is the correct answer.

Key concepts

Methyl Halides

Methyl halides are a group of organic compounds that consist of a methyl group (CH₃-) bonded to a halogen atom (like fluorine, chlorine, bromine, or iodine). These compounds are significant in chemistry due to their varied applications and interesting chemical properties.
Understanding the structure of methyl halides can help us explore their reactivity and physical properties. The methyl group is bonded to the halogen through a covalent bond, forming the general formula CH₃X, where X represents a halogen.

• Examples include methyl fluoride (CH₃F), methyl chloride (CH₃Cl), methyl bromide (CH₃Br), and methyl iodide (CH₃I).
• These compounds differ in their chemical behavior mainly due to the nature of the halogen they contain.

The presence of the halogen atom significantly influences the compound's characteristics due to differences in electronegativity, which leads to varying dipole moments.

Electronegativity

In chemistry, electronegativity refers to the ability of an atom to attract shared electrons in a chemical bond. It plays a crucial role in determining the bond characteristics of molecules.
Electronegativity is a concept that helps explain why some atoms pull electrons more strongly towards themselves than others.

• Fluorine is the most electronegative element, followed by chlorine, bromine, and iodine, in that order.
• This trend is due to the decrease in electronegativity as you move down the periodic table.

Electronegativity differences between atoms are essential in deciding the polarity of bonds within a molecule. In methyl halides, the disparity in electronegativities between carbon and the halogen atom creates a dipole moment, with the more electronegative atom pulling the shared electrons closer to itself.

C-X Bond

The C-X bond refers to the bond between the carbon atom in the methyl group and the halogen atom (X) in methyl halides. This bond is a key focus when studying these compounds since it largely defines their chemical behavior.
Bond polarity, influenced by the electronegativity difference, is a critical attribute of the C-X bond.

• The polarity of the C-X bond increases with greater differences in electronegativity between carbon and the halogen.
• The length of the C-X bond also varies with the size of the halogen atom; larger halogens create longer bonds.

These factors affect the dipole moment of the entire compound. As electronegativity differences decrease and bond lengths increase, the resulting dipole moment tends to decrease, which is the case as you move from CH₃F to CH₃I.

Molecular Polarity

Molecular polarity is an essential concept in understanding the physical properties and reactivity of compounds. It arises due to differences in electronegativity between atoms within a molecule, resulting in positive and negative poles.
A polar molecule, like many methyl halides, has regions of partial positive and negative charge due to uneven electron distribution.

• The extent of molecular polarity depends largely on the overall dipole moment, resulting from both electronegativity differences and molecular geometry.
• In methyl halides, the dipole moment influences properties such as solubility, boiling, and melting points.

The order of electronegativity (F > Cl > Br > I) in methyl halides like CH₃F and CH₃Cl enhances their polarity. Consequently, CH₃F has a greater dipole moment than other methyl halides, significantly impacting its physical and chemical characteristics.