Lesson 3 of 13 · 6 min
The carbon-halogen bond
NCERT §6.3
The bottles differ in one bond: C–Cl, C–Br or, in the next lab over, C–I. Kavya wants to know why the iodides on the next shelf are the ones that react fastest.
The lesson in notes
In short
A halogen is more electronegative than carbon, so the C–X bond is polar: carbon carries a partial positive charge (δ+) and the halogen a partial negative charge (δ−).
Down the group the halogen atom grows (F smallest, I largest), so the C–X bond lengthens from C–F to C–I.
Bond lengths in the methyl halides: CH₃–F 139 pm, CH₃–Cl 178 pm, CH₃–Br 193 pm, CH₃–I 214 pm.
C–X bond enthalpies fall as the bond lengthens: 452 (C–F), 351 (C–Cl), 293 (C–Br) and 234 (C–I) kJ mol⁻¹.
Dipole moments: CH₃F 1.847 D, CH₃Cl 1.860 D, CH₃Br 1.830 D, CH₃I 1.636 D. The largest is for CH₃Cl, not CH₃F: F has the bigger charge separation but the far shorter bond.
The δ+ carbon is the site a nucleophile attacks, and the weaker, longer C–I bond breaks most easily; this explains the reactivity order R–I > R–Br > R–Cl > R–F met later.