Hi everyone this is Maverick Puah the Chemistry Guru. Now sometimes in Organic Chemistry
when we are doing the elimination reaction of alcohols or
elimination reaction of halogenoalkanes to form alkenes we
do encounter situations where I've two or more possible
alkenes that can be formed. So what we want to discuss today in this video
is to make use of Saytzeff Rule to predict the major
product for elimination reactions of halogenoalkanes and alcohols
now Saytzeff Rule says that if you can form more than one possible alkene
from an elimination reaction then the more substituted alkene
will be the major product. So let's take a look at this example now what
we have here is this compound CH3-CHBr-CH2-CH3
this is my 2-bromobutane. Now if you want to do an elimination reaction
of halogenoalkane the reagents and conditions is using KOH in ethanol reflux
and elimination reaction will take away a H-Br and it forms our
alkene. So if we look at this compound, we notice this Br is on my
second carbon and you notice we actually have 2 different H that can be eliminated
together with this Br. Now this C-1 is a
methyl group, then this C-3 is my
CH2 group so C-1 and C-3
they are different so if I eliminate this H with my Br
I'll form one alkene. If I remove the other H from Br
I'll form another alkene, we get two different products. So let's try to
draw these two products. Now the first product that can be formed is
if the Br is eliminated together with my H
in position 1, so I box this up and when I take
away this HBr then later the double bond will be between C-1 and C-2
so my product would look something like this. My C-1
will be here then I have a double
bond between carbons 1 and 2. This will be a CH
C-3 will be my CH2
and C-4 will be CH3
So this is my position 1, 2, 3 and 4
So if I take away this HBr then my product will look something like this
the double bonds will be between carbons 1 and 2
Now if I remove the H from C-3, so let's say if I take away
this H together with Br, then the double bond will be
between carbons 2 and 3. So again let us draw my
products out. I will have a CH3
so this is my C-1 then
C-2 this is my CH with double bond then
C-3 is a CH, C-4 will be CH3
again if we number off that makes it easier for us to compare the two so
this is my position 1, this is my carbon number 2, 3 and 4
so this will be the alkene that is being formed if I take away the H
from position 3. Now the next thing we want to do is we want to
compare the two products. Now Saytzeff Rule says that the more
substituted alkene is my major product. So how do we determine
the degree of substitution. It's fairly straightforward we just box up our
alkene and we go and count the number of non-H groups
that is attached to my alkene. So if we look at the first compound
on my left hand side, I just box up my
alkene and I consider the number of R groups attached to my
alkene. So this is a H it's not a R group, this is a
H, this is a H, so we have one R group here
so we know that this alkene on my left hand side
has 1 R group
So the degree of substitution is one. Then how about the other alkene
Now the other alkene on my right hand side, if I box up my
double bond and I go and count the number of R groups, this is
one R group, H is not a R group and H is not a R group
this is my second R group, so this
alkene on my right hand side actually has 2 R
groups. So if I compare the two, again back to Saytzeff Rule, Saytzeff Rule
says that more substituted alkene is my major product so the
one on the left hand side there is only 1 R group, the one on the right hand side we have
2 R groups. So the compound on the right is more substituted
so this will be my major product
Now remember Saytzeff Rule also applies for alcohols so if you have an elimination
reaction of alcohol to form alkene and if there are two different products that can be formed then
the same idea applies, the more substituted alkene will be the major product
alright I hope that from this video we understand Saytzeff Rule a bit better and we
know how to use Saytzeff Rule to apply to elimination reactions of halogenoalkanes
and alcohols to form our alkenes
Now if you find this video involving Saytzeff Rule useful, please give me the thumbs up, like this
video and subscribe to my YouTube Channel for more weekly video lessons on A Level Chemistry
If you have any concepts or questions that you want to clarify please drop them at the comment section
below so that I can address them in future video lessons.
That's all we have for this video, I'll see you next time
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