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Video transcript

many times in chemistry we'll see different molecules that have the same constituent atoms for example these two molecules here they both have four carbons 1 2 3 4 1 2 3 4 so if I were to write their chemical formula BC for and then they both have 1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 10 hydrogen's so both of them both of them have the chemical formula c4h10 c4h10 but they're still fundamentally different molecules and you can see that because they have different bonding for example over here we have a carbon that is bonded to three other carbons in a hydrogen over here I can't find any carbon that's bonded to three other carbons I can find ones that are bonded to two other carbons but not one that's bonded to three other carbons so how we've put the atoms together is actually different they're bonded to different things and so when we have this situation where you have the same constituent atoms where you have the same chemical formula but you're still dealing with different different molecules because of either what their how their how their bonds are made or how their what their shape is we call those isomers so an isomer isomer you have the same chemical formula same chemical formula formula but you could have different bonding but different different bonding bonding or shape bonding shape or orientation or orientation so over here you have just different bonding and this type of isomer is called a structural isomer so these characters are structural isomers same constituent atoms but different bonding structural isomers so that's structural isomers right right over there now when you look at this pair this pair you see why those will look like structural isomers ality they have the same constituents both of these for example have four carbons four carbons and they both have one two three four five six one two three four five six seven eight and they both have eight hydrogen's so we could both these are both c4h8 h8 it looks like they're bonded similarly for example I mean the left-hand side here these look identical and on the right-hand side you have a carbon bonded to another carbon then it's bonded to three hydrogen's carbon bonded to another carbon that's bonded to three hydrogen's carbon bonded to hydrogen carbon bonded bonded to a hydrogen so it looks like the struck the bonding that everything is bonded to the same things but you might notice a difference over here on the right-hand side this ch3 is on the bottom right while over here it's on the it's on the top right and you might say okay well we know what's the big deal there these you know all these molecules are all moving around maybe they're rotating with respect to each other and these things could this thing could have rotated down to become what we have up here if this was a single bond a single bond would allow for that type of rotation it would allow for these things to rotate around each other for the thing the molecule to rotate around that bond but a double bond does not allow that rotation so this fixes these two things this fixes these two things in place and because of that these are actually two different molecules over here on the top you have the ch3 groups they're both they're both on there both I guess you could say facing down or they're both on the same side of the double bond while over here they are on different sides they are on different sides of the double bond and so this type of isomerism where you have the same constituent and you even have the same bonding this is called stereoisomerism so we're here we're carrying much more about how things sit in three dimensions not we don't just care about what's bonded to water the constituents and actually this one is we'll see this is also stereoisomer because this carbon is bonded to the same things in either case so these are both these are both situations these are both stereo isomers stereo stereo isomers and this particular variation stereoisomer it's called a sis trans isomer sis is when you have the two groups on the same side sis and trans is when you have the two groups on the opposite sides of the double bonds sis trans isomers sis trans isomers isomers and these are often called geometric isomers geometric geometric isomers so that's a subset so when I'm talking about sis trans or geometric I'm talking about these two characters over here they are a subset of stereo isomers now what's going on over here have no double bond I'm not talking about sis and trans the carbon as I just said is bonded to fluorine chlorine bromine and hydrogen fluorine bromine chlorine and hydrogen how are these two things different and the way that they're different is if you were to actually try to superimpose them on each other you will see that it is impossible they are mirror images of each other and because there's four different constituents here you can actually not you cannot superimpose this molecule onto this molecule over here and actually because of that they actually have different chemical properties and so this over here these these two characters which is a subset of stereo isomers stereo isomers are concerned with how things are are positioned in three dimensions not just their how their bonding is different but this subset where you have these mirror images that cannot be superimposed we call these enantiomers so these two characters these are enantiomers enantiomers and an and show comes from Greek the Greek word or the Greek root opposite so these are opposites of each other and they cannot be superimposed they're mirror they're mirror images so all of these are different variations of isomers and once again you might say okay these are clearly two different molecules I have different bonding but even sis trans isomers will have different molecular will have different chemical properties these two in particular they aren't that different but they do have different chemical properties but sometimes they're so different that one might be able to exist in a biological system while the other is not one might be okay for your health and the other might not be okay for your health same thing for enantiomers one might be biologically active in certain way and the other one might not be biology biologically active in that same way
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