Question 4.29

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Annah Khan 1B
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Joined: Fri Jul 22, 2016 3:00 am

Question 4.29

Postby Annah Khan 1B » Sat Jul 22, 2017 6:47 pm

"There are three different isomers of dichlorobenzene, C6H4Cl2, which differ in the relative positions of the chlorine atoms on the benzene ring. (a) Which of the three forms are polar? (b) Which has the largest dipole moment?"

I was having trouble figuring this question out. How do I know which of the three forms are polar or nonpolar? And how do dipoles relate? Thanks in advance!

Kyle Sheu 1C
Posts: 87
Joined: Fri Jun 23, 2017 11:39 am

Re: Question 4.29

Postby Kyle Sheu 1C » Sun Jul 23, 2017 12:20 am

A) 1 and 2 are polar.
A polar covalent bond is formed when one atom attracts electrons better than the other (directly related to electronegativity). The Cl is bonded with a C, and of the two, Cl is much more electronegative and holds a partial negative charge, while carbon has a partial positive charge, causing the probability of an e- near the Cl to be higher than in normal covalent bonds.

We can think of polar covalent bonds as a vector (think magnitude and direction), where the "tail" is at the partial positive charge, and the "tip" is at the partial negative charge. So for Figures 1 and 2, our 2 drawn vectors (see diagrom below please) result in an overall movement to the top and right, whereas for Figure 3 the two vectors directly cancel each other (therefore, nonpolar).

B) 1 should have the largest dipole moment. Since we are looking for the largest dipole moment (and because dipoles can be represented by vectors), we need to find the one where the vectors interfere the least. In 3, the vectors directly cancel. In 1 and 2, the vectors only partially interfere, and in 1, the vectors "go more in the same direction" (for lack of a more scientific description). Hope this is helpful!
Screen Shot 2017-07-23 at 12.20.05 AM.png

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