Two isomers of 1,2-dichloroethene are known. One has a dipole ...

Question

Two isomers of 1,2-dichloroethene are known. One has a dipole moment of 2.4 D; the other has zero dipole moment. Draw the two isomers, and explain why one has zero dipole moment.

CHCl=CHCl 1,2-dichloroethene

Accepted Answer

Hey everyone, let's do this. It says for each of the following molecules Ch three cl CCL four and C two H five oh H. Draw the appropriate lewis structure. Show how the bond dipole moments and dipole moments created by non bonding pairs of electrons contribute to the molecular dipole moment and estimate the magnitude of molecular dipole moments. Okay, well the first thing it says, draw the appropriate lewis structure. So when we draw the lewis structure, we have to first count the total number a valence electrons that we're going to add to our structure and we do that by adding up the individual number of valence electrons for each atom in the compound. Right then once we have that we can draw our rough draft sigma bond framework and once we draw the sigma bonds, then we can add the remaining electrons to complete the octet by adding them as lone pairs. Then the problem mentions that we have to show the diaper moments and the dipole moments created by non bonding pairs of electrons. So too is step two, is drawing the dipole moments and recall that we draw dipole moments from less electro negative atoms to the more electro negative atoms with an arrow. And this has a partial positive and a partial negative charge. And it talks about how this contributes to the molecular dipole moment. The molecular dipole moment. So that really depends on the direction of the di pole and the placement in respect to the other disciple moments. So are they opposite of each other? Are they symmetrical? These are the questions we need to ask you guys. And lastly I guess I'll put c here it asks about the magnitude. Well you're not going to take out a ruler or protractor and measure the dipole moment, right? But we can assume that the magnitude is it correlates to the polarity of the bond, which is also correlating to the difference in electro negativity. So the greater the polarity or the difference in electro negativity of the bond between two atoms than the greater the magnitude of that dipole moment. Does that make sense? Alright, let's tackle this problem. So first We have CH three cl and we need to total our number of valence electrons. So we have four plus three plus seven which is 14. So let's draw our structure. So carbon is going to be my central atom And then I'll have a chlorine and then three hydrogen ins so that's 2468 electrons. So I need some more electrons. So let's add them to chlorine. I see chlorine is bare 10, 12, 14. There we go. And that is it for lewis structure. So now let's draw our di pole moments. So do I see any electro negative atoms on the structure that chlorine sticks out like a sore thumb. Right, so chlorine is more electro negative than carbon. So we're going to draw die pole pointing towards the chlorine from the carbon. Okay and then the carbon hydrogen bonds are all non polar. So those don't have any dye polls. So what does this mean for the overall di pole? The molecular dipole moment, right? Because the molecular dipole moment for the whole molecule is different than just one single dipole moment for a bond. Well, since this is the only disciple moment in the molecule, it is going to reflect the net die pole that will be pointing upwards in that same direction. And we need to comment on the magnitude of the dipole moment. Well what is the difference in electro negativity between carbon and chlorine? Is it pretty significant? Yeah. So this will have a large molecular diaper moment pointing up in the direction of that chlorine. Okay, cool, so let's do the next one. So we have CC 04 And this is 4-plus 28 electrons which is total valence electrons. So we're going to have our central carbon chlorine on all sides all directions in our tetrahedron structure. Right? So that's it for a lewis structure because well technically all these chlorine have those lone pairs, right? That will give us our 32 electrons. Okay, now our di pole moments, well similar to part a carbon to chlorine will have a dipole moment and we have four of those. So let's draw all four of those. And what does this mean for the molecular dipole moment. Well we wanna again look at the direction and placement of these dipole moments, they are symmetrical, right? They have this one at the top is across from this one at the bottom. This one on the left is opposite of the one on the right, so that's symmetrical and by opposite, I actually meant the direction, right? So they are pointing away from each other because if they were symmetrical but they were pointing in the same direction while they would not cancel out, but these are pointing all in the opposite direction. So they are actually going to all cancel out and give a zero molecular dipole moment. I love these. It's so satisfying and just cancel them all out and get a zero. Does that make sense? Cool. So last one, we have C two H five oh H. And that gives us two times four is eight plus five times one is five plus six plus one. That is 89 10 11 12 13 14 15 16 17 1920. Sorry guys, I'm bad at mental math. And let's draw our structure. So oxygen is going to be sort of like our central atom here and we'll have hydrogen and then there's no polar bonds in that C two H five group. So we can just write them all together condensed. We don't have to draw all of them out because we know there's no die poles there And then oxygen is going to have two lone pairs. Okay, so oxygen creates some polar bonds, right? There are there's a polar bond from the oxygen to the hydrogen pointing towards the oxygen. There is a polar bond between the oxygen and the carbon also pointing towards the oxygen. And we have our lone pairs. And the problem said dipole moments created by non bonding pairs of electrons. How did those contribute? Well, those electrons are going to create a di pole pointing up. So we see that these die polls are kind of symmetrical, right? But unlike part B, they are not in the opposite direction. So they won't cancel out. But instead they will sort of add up to each other to create a net die pole moving upwards because they all point generally upwards. Right? And because we can use the vector addition to add up those dipole moments, this will have a large molecular diaper moment. Mhm. Okay, so that is it for this problem. And I hope this video was helpful. Thank you guys for watching.

Two isomers of 1,2-dichloroethene are known. One has a dipole moment of 2.4 D; the other has zero dipole moment. Draw the two isomers, and explain why one has zero dipole moment.
CHCl=CHCl 1,2-dichloroethene

Key Concepts

Isomerism

Dipole Moment

Cis-Trans Isomerism

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