Which of the following has the strongest C–X bond?

\(CH_3F\)

The correct answer is option 4. \(CH_3F\).

The strength of a carbon-halogen bond \((C–X)\) is influenced by several factors, primarily bond length and bond dissociation energy. These factors depend on the size and electronegativity of the halogen atom \((X)\).

Factors Affecting C–X Bond Strength:

Bond Length: The bond length increases as you move down the halogen group in the periodic table because the size of the halogen atoms increases. A shorter bond length generally corresponds to a stronger bond because the bonding electrons are closer to both nuclei, resulting in a stronger attractive force.

Bond Dissociation Energy: Bond dissociation energy is the energy required to break a bond. Higher bond dissociation energy indicates a stronger bond.

Analysis of the Given Compounds:

1. \(CH_3Br\) (Methyl Bromide):

Bromine (Br) is larger than chlorine, leading to an even longer \(C–Br\) bond. The bond dissociation energy is lower than that of \(C–Cl\), making the bond weaker.

2. \(CH_3Cl\) (Methyl Chloride):

Chlorine (Cl) is larger than fluorine, resulting in a longer \(C–Cl\) bond. Although chlorine is still quite electronegative, the bond is weaker than the \(C–F\) bond due to the increased bond length and slightly lower bond dissociation energy.

3. \(CH_3I\) (Methyl Iodide):

Iodine (I) is the largest halogen, resulting in the longest \(C–I\) bond. The \(C–I\) bond has the lowest bond dissociation energy among the given options, making it the weakest bond.

4. \(CH_3F\) (Methyl Fluoride):

Fluorine (F) is the smallest and most electronegative halogen. The \(C–F\) bond is very short due to the small size of fluorine. The high electronegativity of fluorine leads to a strong attraction between the carbon and fluorine atoms. The bond dissociation energy is the highest among the C–X bonds, making it the strongest bond.

Comparison of Bond Strengths:

The bond strength decreases as the size of the halogen atom increases: \(C–F > C–Cl > C–Br > C–I\). This trend is due to the increasing bond length and decreasing bond dissociation energy as you move down the halogen group.

Conclusion:

\(CH_3F\) (methyl fluoride) has the strongest C–X bond because fluorine is the smallest halogen with the highest electronegativity, resulting in the shortest bond length and the highest bond dissociation energy. This makes the \(C–F\) bond the strongest among the given options.