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Organic chemistry
Halogenation and ozonolysis of alkynes
Reaction of an alkene with a diatomic halogen (halogenation) or with ozone (ozonolysis). Created by Jay.
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- During ozonolysis of terminal alkyne it looks more obvious to get HCOOH as product instead of CO2. Am i correct?(5 votes)
- Yes, indeed. But the HCOOH is further oxidized by the O3 to form HOCOOH (H2CO3 or carbonic acid), which then decomposes into CO2 and H2O.(18 votes)
- how come for the second example of ozonolysis ithe second reaction is co2 instead and not o-doublebond-c with an oh and an h on it?(9 votes)
- In terminal alkynes the terminal carbon is only bonded to an H atom, so it can't form a second carboxylic acid when the triple bond is cleaved by the reaction. Instead it picks up two O atoms and forms CO2.(0 votes)
- Do u need the CCl4 as a solvent? would the reaction proceed without CCl4?(7 votes)
- At, why is "the mechanism of halogenation of alkynes not completely understood"? It seems to imply scientists understand previously covered mechanisms on this website. If so, what is the general steps scientists take to understand how a mechanism work? 0:40(4 votes)
- Mechanisms are usually determined by trying to isolating or clearly identifying the intermediates of a reaction. For example the reaction could be performed in an NMR tube or IR spec machine, and a real time information collected on the reaction mixture.
In some cases the intermediates are so short lived (i.e Transition states) that much more complex experiments need to be carried out, or the mechanism predicted through computer modelling techniques like 'Density Functional Theory'. The modelling techniques predict the energy of a wide range of intermediates and predict which is the most favourable.
Hope this helps!
Disclaimer: I have never done mechanistic studies, but I am studying Chemistry at University.(3 votes)
- according to my kaplan book an ozonolysis of an alkene gives the product of two aldehydes. why does the ozonolysis of an alkyne give you two carboxylic acids instead of two aldehydes? makes no sense.(1 vote)
- It's difficult to understand because the mechanism is not completely known. But the ozonolysis of alkynes in the absence of water generally gives an anhydride:
RC≡CR' + O₃ → RCO-O-COR'
If the reaction is performed in the presence of water, the anhydride hydrolyzes to form two carboxylic acids:
RCO-O-OCOR' + H₂O →RCOOH + R'COOH(7 votes)
- How does ozonolysis work in an alkyne? If you try to do the mechanism like explained for alkenes, it doesn't work.(3 votes)
- What if instead of H2O , H3O+ is used? Which would be the products of a terminal alkyne ozonolysis?(2 votes)
- Acid will cause the ozonide to hydrolyze faster. A terminal alkyne will form carbonic acid as one of the products.(2 votes)
- If water is present during halogenation can a halohydrin form?(2 votes)
- Why do we require ccl4 as a solvent in the halogenation reaction ?(1 vote)
- Cl₂ is a gas, so using the CCl₄ as a solvent brings the molecules closer together to react.
Br₂ is a liquid. You add it dropwise to a solution of the alkene in CCl₄ keep the reaction from going too fast.(3 votes)
- Hi there. I haven't found a good place to ask this, so I decided to post this at the end of the last video. How would one do a permanganate oxidation for an alkyne? I couldn't find a video that explained this.(1 vote)
- Have you looked at these videos?
https://www.youtube.com/watch?v=nlfhRL8Mj-Q
https://www.youtube.com/watch?v=XVZEzdAluEA(2 votes)
Video transcript
Let's look at two more
reactions of alkynes, and we'll start with the
halogenation of alkynes. So I start with my
alkyne over here, and I add to my alkyne one more
equivalent of a halogen, so x2. My solvent is going to
be carbon tetrachloride. And I'm going to add
those two halogen atoms across my triple
bond in an anti addition, so those two halogens end up on
opposite sides from each other. So this is an anti addition
of my halogens, like that. Now, I could add two molar
equivalents of my halogen. And if that happens,
each of these carbons is going to end up with two
bonds to halogens, like that, for my product. Now, the mechanism of the
halogenation of alkynes is not completely understood,
so because of that, we're just going to move on to
one practice example instead of showing the mechanism. So let's look at an alkyne. So we'll go ahead and
draw an alkyne over here. So here I have my carbon triple
bonded to another carbon. I'm going to put a
methyl group on one side here, and let's
write a CH3 in here. And I'll put an ethyl group
on this side, so CH2CH3. So to that alkyne, I'm
going to add bromine. And I'm going to use carbon
tetrachloride as my solvent, and I'm going to say one
more equivalent of my bromine is added. So when you do
your stoichiometry, just one more
equivalent like that. So I'm going to add my two
bromines on anti to each other, right? So let's go ahead and
show my triple bond became a double bond. And my two bromines are going
to add on anti to each other. So they're going to
add on opposite sides of my double bond, like that. And I still have a
methyl group over here on the connection to
the carbon on the left. So I'll go ahead and put in
my methyl group, like that. And the carbon on
the right still has an ethyl group
attached to it, so CH2CH3. So that would be the
result of the halogenation of this alkyne. Let's take a look at one
more reaction of alkynes. Let's look at ozonolysis,
the ozonolysis of alkynes. Let me go ahead and write
ozonolysis right here. And we've seen this reaction
before, similar reaction, when we did this with alkenes. So this time we're going
to do it with alkynes. Let's take a look at an
alkyne, so there's an alkyne. I'm going to say it's an
internal alkyne, meaning the triple bond is found in
the interior of the molecule. It's not on the end
of the molecule. So let's look at ozonolysis
of internal alkynes first. And when you're
doing ozonolysis, you're adding ozone to the
molecule in the first step. So we went in a very,
very detailed mechanism for the ozonolysis
of alkenes, and you go back and watch that video. For this video we're
not going to go through any kind of a mechanism. We're just going to
go for the products. So we add ozone in the first
step, and in the second step, we're going to add water. And what this does, is this
cleaves your triple bond and gives you carboxylic
acids as your products, so two of them. So let's go ahead and draw
those two carboxylic acids, and then we'll try to point out
where everything comes from. So here's one of the
carboxylic acids, and then here is going to be
the carboxylic acid that's going to result
on the right side. So I'll make this
R prime over here. So let's go ahead and point out
which carbons are what here. So let's show that
this carbon over here is bonded to an R group. So that's this carbon
bonded to an R group. And then over here on
the right, this carbon is the one bonded to
an R prime, right? This carbon is the one
bonded to an R prime. So you cleave your
triple bond, right? You break your triple bond. And you're going to create two
separate molecules from this, so you get carboxylic
acids from this reaction. Let's look at ozonolysis
of terminal alkynes now. Instead of the triple
bond being in the interior of the molecule,
now the triple bond is on the end of the molecule. So that makes this a
hydrogen, right here. So let's add, once again,
ozone in my first step and water in my second step. And on the left
side, the left side is going to give us the same
product as before, right? So let's go ahead
and identify that in blue, this carbon
and this R group. We saw before that's going
to give us a carboxylic acid, like that. So let's go ahead
and draw that again. So it's the same. That portion of the
molecule gives us the same product as before. So we get a carboxylic acid. And once again, the carbons--
this is the carbon in blue, and that R group in blue--
those are the same ones on the left side
of your reaction. Now, on the right
side of my reaction-- just go ahead and put
that hydrogen in there. On the right side of the
reaction, the terminal alkyne portion, you're actually
going to get carbon dioxides. So now you have only one
carbon to think about. This is the only carbon
you have right here. So you're going to
get CO2 out of it. So let's go ahead and draw CO2
as your product, like that. Remember, it's a
linear molecule. And so this reaction
was used decades ago for structure
determination, right? If a terminal alkyne was present
and you reacted it with ozone, then you would get
some carbon dioxide. And you could also
analyze the molecule by the carboxylic
acids that you get. For example, you
could see how many carbons are in your R group. And then that would give you
an idea about the structure and all those kinds of things. So this used to be a
very important reaction in organic chemistry. Now, with all spectroscopy stuff
that organic chemistry has, this reaction isn't really
used as much anymore. So let's look at one example
of an ozonolysis reaction. So let's look at this one
right here, like that. So we're going to take
that terminal alkyne. We're going to add ozone
to it in the first step, and then we're going to add
water to it in the second step. And let's see how many carbons
we're dealing with here. Sometimes that's what
confuses students, right? So there's one carbon, two
carbons, three carbons. And on this side is
our fourth carbon. And that carbon on the far
right is bonded to a hydrogen, making this a terminal alkyne. So I have four carbons
to worry about. And when this undergoes
alkyne cleavage, it's going to cleave
the molecule here. It's going to break
that triple bond. And so you're going
to get two products. You're going to get one product
with three carbons, right, over here on the left. And then one product with
one carbon on the right. So the product with
three carbons on the left is going to be a
carboxylic acid. So all you have to do is draw
a three carbon carboxylic acid. So let's go ahead and do that. So here's our three
carbon carboxylic acid. That's just one of our products. And the terminal
alkynes will give us CO2, which takes care
of the other carbon on the right over here. So we're going to get CO2 as the
other product of this reaction. So that sums up all of
our reactions of alkynes. In the next video, we're
going to take a look at some synthesis problems,
using all the stuff that we've learned in the first
semester of organic chemistry, especially including some
of the alkyne reactions.