Alkene nomenclature
Alkene intro and stability
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- Lets take a look at the simplest alkene so that would just be
- 2 carbons that are double bonded to each other and then I put the hydrogens around each carbon
- like that.lets comapare that to a 2 carbon alkene.so a 2 carbon alkene will then have
- 6 hydrogens around it like that so when we did our alkene nomenclature we called a 2 carbon
- alkane we call this molecule in the right ethane so this over here is ethane.
- So you can see ane ending for an alkane.and eth ofcourse means there are 2 carbons
- so if we look at the 2 carbon alkene over here on the left i know that the root is going to be eth
- so i go ahead and write eth the ending is alkene not alkane so instead of an ane ending
- its going to get an ene ending.so this molecule over here is ethene.
- the other name for this molecule is ethelene so ethene or ethelene either one is fine.
- So lets look at the molecular formulas for these molecule.
- So on the right ethane has C2H6 like that so had the general formula if we have n carbons you have 2n+2 hydrogens like that.C2H2n+2
- on the left has the molecular formula of C2H4 and therefore its general formula would be
- if you have n carbon CnH2n hydrogens like that.So we can see that ethene doesn't
- have as many hydrogens as ethane does.Ethane has as many hydrogens as possible as for these carbon atoms
- so ethane is said to be saturated with hydrogen so lets go ahead and write that this one over here molecule
- as "saturated" with hydrogens it can't get any more hydrogens.
- Ethene over here only has 4 hydrogens.so it could actually add 2 more so that some chemistery we havn't gone into yet.
- But right now is not completely saturated with hydrogen so it is said to be "unsaturated".
- So we have "unsaturated" VS "saturated".since we have 4 hydrogens over here
- and 6 hydrogens over here.so in future video we will see how these saturated unsaturated relate to fact that everyone heard before.
- so look at these molecules more lets look at the hybridization states of the carbon atoms.
- so lets look at hybridization state of this carbon atom .there are only single
- bonds around that carbon.therefore that must be sp3 hybridized carbon like that and exact same situation
- for this carbon so each sp3 hybridized carbon is going to exhibit tetrahedral
- geometry.what about the carbons over here on the left.
- well this carbon has a double bond so therefore must be sp2 hybridization
- so sp2 hybridized this carbon over here also sp2 hybridized.sp2 hybridized carbon are
- trigonal planar in terms of there geometry.
- so molecule on the left is flat and the molecule on right is tetrahedral
- all single bonds in the molecule on the right so if i look at all these bonds all single bonds
- they are all sigma bonds so the bond between my 2 carbon atom is a sigma bond.
- and sigma bonds allow free rotation so on the right there is free rotation between the
- 2 carbon atoms.and since there is a free rotation between those 2 carbon atoms you can get several different
- conformation for this ethane molecule.so we saw in earlier video about conformations
- for ethane.so single bonds allow free rotations
- on the right in this double bond one of these bond is sigma bond.and other bond is pi bond.
- so the pi bond helps stabilize that bond and it prevents any free rotation.
- so there is no free rotation over here on left for double bond.
- so that's a very important difference.double bonds and no free rotation
- single bonds in the right do have free rotation.
- let's look at some more alkenes.over there in left we can see
- that we have all hydrogens bonded to my double bond like that.
- Lets take off one of those hydrogens and lets put in R group on there so
- i still have double bond I still have hydrogens attached to my carbon.
- I am gonna take of one of those hydrogens and put in R group.
- so I have some sort of rest of the molecule over here so it substitute one time
- so I am gonna call it mono substituted alkene.
- Lets take of another hydrogen and lets put on another R group.So I have H and H and R and I will make this R prime
- so this is a different R group.So this is a di substituted alkene.
- So mono substitute alkene di substituted alkene.
- and you can see where I am going with this Keep this hydrogen keep this R group
- make this R prime (R') I will make this R double prime (R")So this is a tri substituted alkene
- and then I have one more spot where I can substitute this is R ,this is R" double prime
- and then finally this is R triple prime (R''') So I have 4 different substituent
- so it is tetra substituted.So I have a tetra substituted alkene over here.
- Now the question is out of these four alkene which one is the most stable.
- So it turns out that the tetra substituted alkene is the most stable
- so the more substituents more alkule groups you have the more stable that makes your molecule.
- so tetra substituted is more stable followed by tri substituted alkene followed by di substituted alkene
- followed by mono substituted alkene.
- is the least stable out the once that we have talked about. So these alkule groups
- allow for delocalization of electronic density throughout the molecule and that
- helps to stabilize the molecule.so the actual explanation to this is fairly complicated
- and it involves quantum mechanics .Its very similar to the effect called hyperconjugation which
- we will discuss in a later video. The video on carbocat.So this sort of explained in later video
- SO lets take a look at 2 examples and lets try to identify the degree of substitution and identify the stability
- of these molecules.
- so lets look at these alkenes here so lets take this alkene and then lets take
- this alkene and lets compare these 2 in terms of their stability
- so first if I look at the molecule on the left I need to think what degree of substitution
- is this.is it mono substituted,di substituted,tri substituted or tetra substituted.
- and the way to do it is to focus on your double bond.
- and look at what else is in these molecules.
- well I know that there are hydrogen here and hydrogen here on that side so
- there are 2 alcule groups coming off of my double bond so I think it obvious it becomes
- di substituted so this is a di substituted alkene once you draw your
- hydrogen it is much easier to see.
- On the right over here if I look at my double bonds well I know that this
- must be a hydrogen over here so how many
- different alcule groups I have.
- I have 3 different alcule groups coming off of my double bond.
- So on the right this one is tri substituted .So I have a di substituted alkene on the left
- and Tri sunstituted alkene on the right.
- The more substituted the alkene is the more stable it is.
- So this molecule over here on the right is more stable of the two
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