Sciencemadness Discussion Board

Dipole moment in arenes....

sankalpmittal - 12-1-2013 at 07:29

If we take a benzene ring and attach two hydroxyl groups at its para (1,4) position , then its dipole moment is not zero.
Similarly , if we take another benzene ring and attach two alkanoate (COOR) groups at its para position and then also , its dipole moment is non zero...


Why is it so ?

smaerd - 12-1-2013 at 08:26

Think about the bond angles on the hydroxyl groups. The O-H bonds will create a dipole moment. O has an EN value of ~3.5 and H ~ 2.1 (pauling scale).The O-H bond is not planar think back to VSEPR: 4 substituents 2 bonds meaning two lone pairs, gives a bent molecular shape and a tetrahedral geometry.

sankalpmittal - 12-1-2013 at 09:00

Quote: Originally posted by smaerd  
Think about the bond angles on the hydroxyl groups. The O-H bonds will create a dipole moment. O has an EN value of ~3.5 and H ~ 2.1 (pauling scale).The O-H bond is not planar think back to VSEPR: 4 substituents 2 bonds meaning two lone pairs, gives a bent molecular shape and a tetrahedral geometry.


Well shouldn't O-H be just linear structure like CO ? If we assign bond angle to O-H , then we are doing hybridization on it. In hybridization , ......

Wait !!

Oh , you are finding hybridization of OH attached to the benzene ring !!

Anyways , should it not imply that carbon is the central atom in junction : C-OH , where C is a carbon of benzene ring. Carbon will be sp2 hybridized.

Please can you explain how you assigned bent structure to O-H ?

That will only be possible if we take oxygen atom as central atom , and apply the concept of hybridization on it.

smaerd - 12-1-2013 at 11:38

you must be able to shift your focus. There is no real 'central' atom. The central atom in butane is whatever atom we are focusing on. So yes from the nearest carbon to the hydroxyl moeity the carbon is indeed planar and Sp2 hybridized, trigonal planar, whatever way you understand it.

Now move your focus from that carbon atom and make the oxygen your 'central' atom. What is the oxygen attached too? A carbon(Sp2 hybridized like we said above), and a hydrogen. Does it have any lone pairs? Yes. How many? 2 lone pairs or less importantly 4 electrons. What do we call in VSEPR a structure with two sigma(single) bonded attachments and two lone pairs of electrons? "Bent", just like water. this bending is not going to make the hydroxyl hydrogen linear. Notice even on some skeletal structures people draw hydrogens as bent, just like methoxy groups.
Phenol

Anisole


Hopefully this is an ah-ha moment for you.



[Edited on 12-1-2013 by smaerd]

sankalpmittal - 12-1-2013 at 20:57

Ok , so O-H in benzene ring is angular bent.

But there are two hydroxyl groups attached at the para position. How will that cause dipole moment in the ring ?

The two OH group will be angular bent. Oh , understood... There will be net dipole due to O-H junction..

O in OH have lone pair. Does that contribute to dipole moment also ?

sankalpmittal - 13-1-2013 at 23:01

Ok , I meant that my teacher told that oxygen in O-H has a lone pair electrons which will repel the electrons in the benzene ring and cause dipole moment. I was asking in my previous post , whether this would also be the factor for dipole moment , in addition to the bond angle in O-H bond ?

smaerd - 14-1-2013 at 06:06

That's also a possibility. The delocalized pi bonds of the arene ring could interfere with the lone-pairs on the oxygen (electron pair repulsion leads to distortion). I would imagine most of the effect would be on the oxygen though due to the stability of aromaticity then again that's just a guess I haven't seen an electrostatic map of the molecule.

sankalpmittal - 14-1-2013 at 07:11

Quote: Originally posted by smaerd  
That's also a possibility. The delocalized pi bonds of the arene ring could interfere with the lone-pairs on the oxygen (electron pair repulsion leads to distortion). I would imagine most of the effect would be on the oxygen though due to the stability of aromaticity then again that's just a guess I haven't seen an electrostatic map of the molecule.


Thanks for your reply... :)