Question

Consider the following haloalkanes
$(1)$ $C{H}_{3}F$
$(2)$ $C{H}_{3}Cl$
$(3)$ $C{H}_{3}Br$
$(4)$ $C{H}_{3}I$
The correct sequence of increasing order of dipole moments is

• A. 1 < 2 < 3 < 4
• B. 4 < 3 < 2 < 1
• C. 4 < 3 < 1 < 2
• D. 3 < 4 < 1 < 2

Answer 1

Answer: (C)

$\underset{1.64D}{C{H}_{3}I}<\underset{1.79D}{C{H}_{3}Br}<\underset{1.84D}{C{H}_{3}F}<\underset{1.94D}{C{H}_{3}Cl}$
The charge separation in $C-X$ bond decreases in the order
$C-F>C-Cl>C-Br>C-I,$ due to decrease in the electronegativity as we more from $F\to Cl\to Br\to I$ .
$C{H}_{3}Cl$ has higher dipole moment than $C{H}_{3}F$ , because of larger bond length of $C-Cl$ bond. The larger $C-Cl$ bond length causes dipole moment $(\mu )=q\times d$ to be larger for $C{H}_{3}Cl$ than $C{H}_{3}F$ . In case of other haloalkanes, the effect of larger $C-X$ bond length cannot outweigh the effect of decreased charge separation.