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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