Cis and Trans Isomers: Physical Properties, Polarity, and Identification

in this video i'm going to talk about cis and trans isomers

so what are cis and trans isomers how would you describe them so

the cis isomer has two groups on the same side

so on the left we have the cis isomer and the trans isomer has two groups

on opposite sides now these two isomers they have different physical properties

in fact the isomer on the left has a dipole moment

of approximately three dbi and the one on the right is zero

the one on the right is nonpolar the one on the left is polar

the carbon chlorine bond is relatively polar and so if you draw the dipole moment

notice that on the right the arrows they cancel the arrow has to point towards the more electronegative chlorine atom

on the left they don't cancel in fact there's a net dipole moment that goes upward

if you add these two vectors and so that's why this molecule is polar

so because it's polar the boiling point of the cis isomer is greater than that of the trans isomer

so for the cis isomer the boiling point is about 60 degrees celsius and for the trans isomer it's less it's

about 47.5 so polar molecules typically have a higher boiling point than non-polar

molecules as you can see the cysts and trans isomers have different physical

properties they have different boiling points and different diaper moments

and this is due to the fact that the double bond lacks free rotation in order for the trans isomer to convert

to the cis isomer you have to break the pi bond and so this hydrogen cannot easily switch

positions with the chlorine and that's where you have such different physical properties in the case of

ethane you do have free rotation among the carbon carbon single bond the energy barrier is very low it's like

12 kilojoules per mole in order to switch between the staggered and the eclipse conformation

but in the case of cis and trans isomers the energy barrier is very very high so it takes a lot of energy to break that

pi bond and so these are two distinct isomers with different physical properties so it

takes a lot of energy to go from the transform into the cis form and so there's no

free rotation with uh cis and trans isomers it takes a lot of energy to convert from one form

to another now there's different ways to represent cis and trans isomers

you can also represent them using rings in this example

we're going to have two hydroxyl functional groups on the wedge

so that is is going out of the page and on the right we're going to have one on the dash

which is going into the page and one on the wedge so on the left

we have the cis isomer because both hydroxyl groups are going in the

same direction out of the page on the right we have the trans isomer one is going into the page the other one is

going out to the page so one is going up the others going down and so they're in opposite directions

and so that's another way in which you could represent cis and trans isomers now consider these four molecules

so what i want you to do is determine which of these alkenes can show

cis and trans isomerism so which of these molecules can you draw cis and trans isomers for

so let's look at the first example on the left can we draw the cis and trans isomer for

this alkane it turns out that we can't because on the carbon at the left

it has two atoms that are the same so if the two groups are the same you can't show cis and trans isomerism

for this one you could this is the trans-isomer so this carbon

is attached to two different groups it's attached to a hydrogen and a methyl group

and this carbon is attached to two different groups a hydrogen and an ethyl group

so what we have here is the trans isomer so if we want to draw the cis isomer we

can draw like this and so notice that the hydrogen atoms are facing the same side

so that's the cis isomer for this particular alkene now for the third example we can't show

any cis or trans-isomers because this carbon is attached to the same group

it doesn't have two different groups attached to it the same is true for the last one

this carbon that's part of the double bond is attached to two methyl groups so because these two methyl groups are

identical to each other this alkene doesn't have any cis or trans isomers

so the only one that has it is this example so that's how you can tell if an alkene

will have cis and trans isomers look at each carbon that is part of the double bond

and analyze them see if they're attached to two different groups or if they're attached to two

of the same groups you