Problem 1 - A Complete Resource Book in Chemistry for JEE Main

Question

C-X bond is strongest in (a) \(\mathrm{CH}_{3} \mathrm{Br}\) (b) \(\mathrm{CH}_{3} \mathrm{Cl}\) (c) \(\mathrm{CH}_{3} \mathrm{I}\) (d) \(\mathrm{CH}_{3} \mathrm{~F}\)

Step-by-Step Solution

Answer

The C-F bond in \(\mathrm{CH}_{3} \mathrm{~F}\) is the strongest.

Step 1: Understand the Question

We are asked to determine which methyl halide compound has the strongest carbon (C) to halogen (X) bond. The options given are different halides of methyl: \(\mathrm{CH}_{3} \mathrm{Br}\), \(\mathrm{CH}_{3} \mathrm{Cl}\), \(\mathrm{CH}_{3} \mathrm{I}\), and \(\mathrm{CH}_{3} \mathrm{~F}\).

Step 2: Consider Bond Strength Factors

C-X bond strength generally depends on two main factors: bond energy and the atomic size of the halogen. Stronger bonds typically have higher bond energies and involve smaller atoms, as large atoms form longer, weaker bonds.

Step 3: Evaluate Atomic Sizes

Among the halogens \(F\), \(Cl\), \(Br\), and \(I\), fluorine (\(F\)) has the smallest atomic size and iodine (\(I\)) has the largest. Smaller atoms like \(F\) share electrons more effectively with carbon than larger ones.

Step 4: Compare Bond Energies

The C-F bond, given the small size of fluorine, has a very high bond energy due to the strong overlap between the small fluorine atom and carbon. Other bonds (C-Cl, C-Br, and C-I) have lower bond energies due to less effective overlap.

Step 5: Conclusion

Given the analysis of both atomic size and bond energy, the strongest C-X bond is the one formed between carbon and fluorine in \(\mathrm{CH}_{3} \mathrm{~F}\). This is due to the small size and high electronegativity of fluorine.

Key Concepts

Atomic Size

Atomic size, also called atomic radius, refers to the distance from the nucleus of an atom to its outermost electron shell. In the periodic table, atomic size generally decreases across a period and increases down a group. For halogens, which include fluorine (F), chlorine (Cl), bromine (Br), and iodine (I), this means:

• Fluorine has the smallest atomic size.
• Iodine has the largest atomic size.

The size of these atoms significantly affects how they form bonds with carbon in methyl halides. Smaller atomic size allows the atom to get closer to the carbon atom, leading to a stronger bond. With smaller atoms like fluorine, electrons can be shared more effectively with carbon, making the C-F bond stronger compared to C-I, where iodine's large size results in a longer and weaker bond. Understanding atomic size helps us predict how strong these molecular bonds will be.

Bond Energy

Bond energy is the measure of bond strength in a chemical bond. It is defined as the amount of energy needed to break one mole of a bond in a molecule in the gas phase into its constituent atoms. Bond energy is closely related to bond length, which is influenced by the atomic size we discussed earlier.
In methyl halides, the bond energy decreases as the size of the halogen atom increases. This means:

• The C-F bond has the highest bond energy due to fluorine's tiny atomic size, leading to short and strong bonds.
• The C-I bond, with iodine being the largest halogen, has the lowest bond energy.

The concept of bond energy is important as it directly correlates with the stability of the bond. Stronger bonds, like the C-F bond, are harder to break and contribute to the compound's stability. Therefore, by assessing the bond energy, we understand why some bonds are more robust than others.

Methyl Halides

Methyl halides are organic compounds where a methyl group is attached to a halogen atom. These are commonly referred to as \(\text{CH}_3X \) compounds, where X can be fluorine (F), chlorine (Cl), bromine (Br), or iodine (I). Methyl halides are important in both industrial and laboratory settings due to their reactivity and usefulness in various chemical reactions.
The strength of the C-X bond in methyl halides is critical because it determines the compound's reactivity and applications. For example:

• Methyl fluoride (\(\text{CH}_3\text{F} \)) is less reactive due to its strong C-F bond, making it stable for many uses.
• Methyl iodide (\(\text{CH}_3\text{I} \)), on the other hand, has a weaker C-I bond, which makes it more reactive and suitable for specific chemical synthesis processes.

By understanding the bond strength in these compounds, scientists and chemists can select the appropriate methyl halide for their specific needs, ensuring the desired reactivity and performance in their applications.