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Current time:0:00Total duration:5:24

Video transcript

let's say we were asked to name the molecule on the top left we would start by numbering our carbons so this would be carbon 1 2 3 & 4 notice we have a double bond starting at carbon 2 so the name of this molecule would be 2 butene 2 because we have our double bond starting at carbon 2 buuuut because we have 4 carbons and n because we have a double bond present in the molecule what about naming the molecule on the right we number our carbons 1 2 3 & 4 and once again we have a double bond starting at carbon 2 so the name of this molecule would be 2 butene however these are two different molecules and the reason why is because there's no free rotation around a double bond single bonds have free rotation but double bonds don't so you couldn't rotate the molecule on the left to look like the molecule on the right therefore they must be isomers of each other and we need a way to distinguish between our isomers and so one way to do that is to use sis trans terminology so if we look at the molecule on the left we can see we have two methyl groups and those two methyl groups are on the same side of our double bonds so if I draw a line in here it's easier to see those two methyl groups are on the same side and we call that the sis I summer so we put cysts in front of our name Here I am attempting to write it in italics so this would be sis 2 butene on the right when we look at those methyl groups these two methyl groups are on opposite sides of the double bond so I draw a line in here to make it easier to see those two methyl groups are on opposite sides and we call that trans so this is the trans isomer I'm going to write trans here in italics attempt two anyway so we have sis 2 butene and trans 2-butene these are different molecules with different properties if you want to use sis trans terminology you're looking for two identical groups and you are comparing them so let's look at these next two examples here and figure out which one is cysts which one is trans we're looking for identical groups so over here we have an ethyl group attached to our double bond and on the right we have an ethyl group attached to our double bond those two ethyl groups are on the same side of our double bond so this must be the sis isomer on the right we have this ethyl group and this fo group are on opposite sides of our double bond so that must be the trans isomer alright let's do some more examples so I'll go down to here on the Left we have the cinnamaldehyde molecule we're looking for two identical groups so we can use sis or trans you can also use hydrogen's right you don't have to use a methyl group or an ethyl group so if we look at our double bond we know there's a hydrogen attached to this carbon and we know there's a hydrogen attached to this carbon and those two hydrogens are on opposite sides of our double bonds I'm drawn a line here to make it easier to see alright these two hydrogen's are on opposite side so we're talking about trans here those hydrogen's are across from each other what about the tetra substituted alkene on the right we need two identical groups to use our cysts trans and here we have an ethyl group and here we have an ethyl group right over here we have a methyl group and an isopropyl group but the two methyl groups are on the same side of our double bond so I draw a draw a line in here and we see that these two groups are on the same side therefore therefore we're talking about sis here so this double bond has a sis configuration let's compare the drawing on the left to the drawing on the right the first time you look at these two drawings you might think these are two isomers and I could use sis trans terminology to distinguish between them however you can't because these are just two ways to represent the same molecule if you picked up this molecule on the left and you flipped it up you would get the drawing on the right so they're not isomers of each other this is the same molecule and a fast way to figure that out is to look at this carbon and you can see you have two identical groups bonded to that carbon so you can't use sis trans terminology that's different from the example we did a minute ago we had two identical groups write these two ethyl groups here however those two ethyl groups weren't bonded to the same carbon those two ethyl groups are bonded to different carbons so this ethyl group is bonded to this carbon and this ethyl group is bonded to this carbon so we were able to use sis trans terminology so we looked at our double bond and we said those two ethyl groups are on the same side of our double bond so this represents a sis configuration of the double bond so we can't do that up here because we while we do have two identical groups those identical groups are bonded to the same carbon