The correct sequence of bond enthalpy of ‘C–X’ bond is :

The correct answer is: CH₃ – F > CH₃ – Cl > CH₃ – Br > CH₃ – I.

Step-by-Step Explanation:

To determine the correct sequence of bond enthalpy for the carbon-halogen (C–X) bond in methyl halides, we analyze the atomic properties of the halogens down Group 17:

1. Variation in Atomic Size:
As we go down the halogen group in the periodic table from fluorine (F) to iodine (I), the atomic size of the halogen atom increases because of the addition of new electronic shells:
Size of F < Cl < Br < I

2. Effect on Bond Length:
As the atomic size of the halogen increases, the distance between the carbon atom and the halogen atom increases. This leads to less effective orbital overlap between the carbon orbital and the larger, more diffuse valence orbitals of the halogen (2p orbital of F, 3p of Cl, 4p of Br, and 5p of I). Consequently, the C–X bond length increases in the order:
C–F < C–Cl < C–Br < C–I

3. Bond Enthalpy Trend:
Bond enthalpy (the energy required to break the bond) is inversely proportional to bond length. Shorter bonds have stronger orbital overlap and are harder to break, resulting in higher bond enthalpy. Because the C–F bond is the shortest, it is the strongest and has the highest bond enthalpy, while the C–I bond is the longest, weakest, and has the lowest bond enthalpy.

Therefore, the C–X bond enthalpy decreases down the group in the following sequence:
CH₃ – F > CH₃ – Cl > CH₃ – Br > CH₃ – I