# Dipole Moment of p-dibromobenzene

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).

• Who is "Sir" ? A teacher ? Why should p-dibromobenzene have a dipole moment ? – Maurice Jun 16 '20 at 12:13
• Hmmm. This source mentions a $\mu$ of 0.00 but PubChem mentions a $\mu$ of 1.43 in gas state. – Aniruddha Deb Jun 16 '20 at 12:25

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).

• Could you provide an explanation for the non-zero dipole moment in 1,4-diiodobenzene? On the basis of symmetry, I'd expect the bond moments to cancel each other out, similar to other 1,4-dihalobenzenes. Also will it be possible to determine the direction of the dipole moment vector? – user14250 Jun 16 '20 at 14:33
• @Guru Vishnu: 1,4-diiodobenzene evidently has shown some value. I guess it's due to high polarizability of large atom. But, you should ask theoretical chemist to have an explanation. – Mathew Mahindaratne Jun 16 '20 at 14:38
• Thanks for your reply! I've asked the same question separately - Why does 1,4-diiodobenzene have a non-zero dipole moment? as I don't know any theoretical chemists other than users here. – user14250 Jun 18 '20 at 13:26