What is the dipole moment of p-dibromobenzene?
I know that p-dichlorobenzene has zero dipole moment, but my teacher said that this is not the case for bromine. I am still confused and feel its dipole moment as zero. My teacher said that it does have some dipole moment (supported through experimental data).
PubChem gives dipole moment ($\mu$) of 1,4-dibromobenzene (p-dibromobenzene) as $\pu{1.43 D}$ in gas phase and $\pu{1.87 D}$ in liquid phase at $\pu{20 ^\circ C}$. It has given 1987 version of Handbook of Organic Chemistry (J. A. Dean, Ed.) as the reference. PubChem also gave $7.77$ as dielectric constant at $\pu{10 ^\circ C}$ and $6.7$ at $\pu{40 ^\circ C}$ with the same reference. However, I afraid to tell you that all of these values are incorrect. The 1999 version of Handbook of Organic Chemistry (same reference in different edition) listed followings for 1,4-dibromobenzene and its isomers (also included 1,4-diiodobenzene and its isomers for comparison):
$$ \begin{array}{c|ccc} \hline \text{Compound} & \text{Dipole moment, }\mu & \text{Dielectric constant, }\epsilon & \text{reference page}\\ \hline \color{blue}{\text{1,4-Dibrmobenzene}} & \color{blue}{\pu{0.0 D}} \ (-) & 2.57 \ (\pu{95 ^\circ C}) & \text{p. 5.111}\\ \text{1,3-Dibrmobenzene} & \pu{1.5 D} \ (\pu{20 ^\circ C}\text{ in benzene}) & 4.21 \ (\pu{20 ^\circ C}) & \text{p. 5.111}\\ \text{1,2-Dibrmobenzene} & \pu{2.13 D} \ (\pu{20 ^\circ C}\text{ in benzene}) & 7.86 \ (\pu{20 ^\circ C}) & \text{p. 5.111}\\ \color{red}{\text{1,4-Diiodobenzene}} & \color{red}{\pu{0.19 D} \ (\pu{20 ^\circ C}\text{ in benzene})} & 2.88 \ (\pu{120 ^\circ C}) & \text{p. 5.113}\\ \text{1,3-Diiodobenzene} & \pu{1.22 D} \ (\pu{20 ^\circ C}\text{ in benzene}) & 4.11 \ (\pu{50 ^\circ C}) & \text{p. 5.113}\\ \text{1,2-Diiodobenzene} & \pu{1.70 D} \ (\pu{20 ^\circ C}\text{ in benzene}) & 5.41 \ (\pu{50 ^\circ C}) & \text{p. 5.113}\\ \hline \end{array} $$
Accordingly, 1,4-diiodobenzene actually has a dipole moment, but, every other 1,4-dihalobenzenes do not have a dipole moment. The values given in PubChem must be corrected during 15th edition of the book.
Reference:
John A. Dean, Editor, Lange's Handbook of Organic Chemistry, Fifteenth Edition; McGraw-Hill, Inc.: New York, NY, 1999, p. 5.111-5.113 (ISBN: 0-07-016384-7).
