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Alkene structure and classification

Video transcript

let's compare the structures of ethane and ethane ethane is an alkane with an A and E ending and it has the molecular formula c2h6 Essien is an alkene with an e and e ending and it has the molecular formula c2 h4 for 2 carbons 6 hydrogen's is the maximum number that you can have so we say that ethane is completely saturated with hydrogen's if we look at a scene we only have 4 hydrogen's for 2 carbons so we say that ethene is unsaturated so for two carbons it's possible to have more so this is unsaturated next let's look at the carbons present in both molecules they will start with ethane this carbon is sp3-hybridized and so is this one so SN contains two sp3 hybridized carbons and we know the geometry around an sp3 hybridized carbon is tetrahedral so we should have tetrahedral geometry around both of these carbons for ethene let me use a different color here so this carbon and this carbon are both sp2 hybridized so sp2 hybridized carbons and we know the geometry around an sp2 hybridized carbon is trigonal planar so there's planar geometry around both of these carbons finally let's look at the bonding between the two carbons so for ethane this Sigma bond between the two carbons has some free rotation so different confirmations of the SI molecule are possible so we have free rotation free rotation about the Sigma bonds between our two carbon atoms but for ethane if we look at those two carbon atoms so this one and this one there's a double bond between those two carbons and we know that there's no free rotation around a double bond so there's no free rotation so you're not going to get different different confirmations for a theme so no free rotations amount no confirmations and that affects the structure of your alkenes on the left is ethane and if we look at our two sp3 hybridized carbons we can see the tetrahedral geometry around them and for the Sigma bond between those two carbons we know there's free rotation so here I am rotating the molecule to show different confirmations of ethane for ethene or ethylene our two sp2 hybridized carbons have planar geometry around them so if I rotate this molecule to decide here you can see that it is planar and there's no free rotation because of this double bond because of the presence of a pi bond so here I am trying to rotate the model set and you can see that the molecule does not rotate you can classify alkenes according to their degrees of substitution if you take ethene and you take a hydrogen off and you add on an R group you now have a mono substituted alkene so on the right is an example of a mono substituted alkene and if I put in two hydrogen's it might be a little bit more obvious we know that this carbon has two hydrogen's and we know that this carbon has one we have an alkyl group coming off of this carbon a methyl group and so this is an example of a mono substituted alkene I want to name this alkene we saw how to name them in an earlier video we would make this carbon one this is carbon two and this is carbon three the longest carbon chain including our alkene and a 3 carbon alkene is called propene so let me write that down here so this is propene next let's look at a die substituted alkene so now we're talking about two our groups so here I've put in R and R Prime and R and R prime it might be the same or they might be different here are an R prime R on the same carbon but it's possible to have a die substituted alkene where R and R prime are bonded to different carbons and then another example of a die substituted out on the right here so our and our prime are bonded to different carbons but notice a difference these two are groups are on opposite sides of the double bond so discs and this one on opposite sides whereas in this example if I draw a line right here both our groups are on the same side of the double bonds and we know that one of these doesn't rotate to form the other because there's no free rotation around our double bond so here we have three examples of die substituted alkenes let's look at this one down here and let's name it so find our longest carbon chain that includes our double bonds and I want to give the lowest number possible to our double bonds we're going to start right here at carbon 1 and this is carbon 2 and this is carbon 3 so 3 carbon alkyne is called propane and we have a methyl group coming off carbon 2 so this would be 2 methyl propane in terms of which type of die substituted alkene this is let's go ahead and draw in our hydrogen so it's a little bit easier to see so for this carbon there are two hydrogen's bonded to it and then for the carbon on the left it has a methyl group and another methyl group so two are groups that happen to be the same so that's this example of a dive substituted alkene where both of our our groups are bonded to one carbon now let's look at a trisubstituted alkene so we have 3 our groups are our prime and our double prime and again are our prime and our double prime might be the same or they might be different so here's an example of a trisubstituted alkene let me go ahead and draw in the hydrogen on this carbon so it's easier to see that we have 3 R groups bonded to the double bonds if I want to name it I need to find the longest carbon chain that includes my double bond so this would be carbon 1 this would be carbon 2 3 4 & 5 so a five carbon alkene is called pentene so let me write that in here and our double bonds it starts at carbon 2 so this would be 2 teen and finally I have a methyl group coming off of carbon 2 so to complete the name I need 2 methyl so two methyl two pentane would be the name for this trisubstituted alkene next let's look at a tetra substituted alkene so our R prime R double Prime and our triple prime so this molecule is actually tetra substituted if we find our double bond so this carbon and this carbon so this top carbon here actually it's go ahead and name it first and then we'll look at why it's a tetra substituted alkene so this would be carbon one and then I have to follow my double bond for carbon 2 so we have a cyclohexene derivative again we saw this in an earlier video so let me go ahead and write cyclohexene down here so cyclohexene and we have a methyl group coming off of carbon 1 and a methyl group coming off of carbon 2 so this would be 1 to dimethyl so 1 to dimethyl cyclohexene would be the name and now let's look at why it's tetra substituted so let me use a different color here so for carbon 1 I have 1 1 methyl group coming off on this side and then I have this alkyl group 4 as part of the Ring coming off of that side so that carbon has to are groups bonded to it and carbon 2 also has two R groups bonded to it so a methyl group here and again I have this portion of the ring so that's why this is a tetra substituted alkene