In this leah4sci video, we're going to look at a summary of organic chemistry 1 reactions
so that you know what to work on going into organic chemistry 2
or as a review for your organic chemistry 1 final.
We'll start with Alkene reactions where an alkene is a molecule that has a double bond between 2
carbon atoms and in this reaction we break the pi bond and add something across those two carbons.
In catalytic hydrogenation, we react with H2 in the presence of metal catalyst to break the pi
bond and add hydrogen on both carbon atoms. This is a syn addition in that both hydrogens add to
the same face of the former pi bond. The next set of reactions are called electrophilic addition
where the pi bond acts as the nucleophile and the thing we're adding is an electrophile. The
mechanisms follow a similar pattern where the pi bond reaches for the electrophile and then we add
across the pi bond starting with hydrohalogenation for example H-Cl. We break the pi bond and add the
hydrogen to the less substituted carbon, chlorine or the other halogen to the more
substituted carbon following markovnikov's rule given that we have a carbocation intermediate.
look out for a potential hydride shift in this reaction. For halogenation we add X2 for example
Cl2 to give us a chlorine or a halogen on both carbons of the pi bond. This is an anti addition
where the second halogen adds opposite the first due to the halonium or bridged intermediate.
Halohydrin Formation looks like a halogenation except that the reaction takes place in a polar
protic solvent for example water so that we get one halogen and one OH adding to the molecule.
The Halogen adds less substituted and the Hydroxide more substituted following
Markovnikov's Rule but they add opposite to each other for an anti addition reaction.
We have three types of hydration reactions. Acid Catalyzed Hydration, Oxymercuration Reduction,
and Hydroboration Oxidation. Notice that I move the pi bond over to account for a potential
carbocation rearrangement. In acid catalyzed hydration we use acid in the presence of water
to give us a carbocation intermediate, the alcohol winds up on the tertiary carbon due
to the carbocation intermediate and hydride shift following Markovnikov's Rule. For Oxymercuration
Reduction we have mercury catalyst in the first step that acts as a carbocation baby-sitter
and therefore the hydroxide will add to the more substituted carbon without the
potential for a carbocation rearrangement. This reaction follows anti addition.
In Hydroboration Oxidation, BH3 grabs on to the pi bond so that hydrogen goes to the more substituted
carbon and boron ultimately replaced with a hydroxide goes to the less substituted carbon.
This reaction is anti-markovnikov because OH is on the less substituted carbon
but Hydroxide and Hydrogen add to the same face of the pi bond giving me a syn addition. The
following Alkene reactions are less common but still worth knowing starting with epoxidation.
Epoxidation uses a peracid which has an extra oxygen in what looks like a carboxylic acid. This
adds across the pi bond so that oxygen grabs on to both carbon atoms giving us a 3-membered ring
called an epoxide. Given that Oxygen is holding both carbons at the same time,
this is a syn addition. We have 3 reactions for dihydroxylation where we're adding 2
hydroxy or OH groups across the pi bond. OsO4 will give us 2 OH groups that are
syn to each other as will KMnO4 in a cold dilute solution but not in a hot concentrated solution.
Going back to the epoxidation reaction, if we use a peracid such as MCPBA, we’ll get the epoxide.
But if we follow it up with H3O+ that epoxide will break open to give us another hydroxy group
but this time as an anti addition where the hydroxy groups are opposite from each other.
Oxidative cleavage is when we break not only the
pi bond but also the sigma bond meaning we're breaking the carbon skeleton.
And for this we once again have 3 reactions, the trick for all of this is to simply cut your pi
bond in half and add Oxygens on either side. What else you add depends on the specific reaction.
KMnO4 in a hot concentrated solution will give us maximum oxidation where the secondary
carbon gives us a ketone but the primary carbon gives us a carboxylic acid. We have 2 types of
Ozonolysis Reactions, the first under reductive conditions using O3 followed by DMS or (CH3)2S,
this may be the only one you've seen in class where once again the secondary carbon will give
us a ketone but the primary carbon will give us an aldehyde rather than a carboxylic acid.
But O3 in Oxidative conditions using peroxide will give us a similar product to KMnO4
where a secondary gives us a ketone and primary gives us another Carboxylic acid.
In Cyclopropanation we break the pi bond to form a 3-membered ring with carbon
a cyclopropane. The first is the haloform with a carbene intermediate
where I react a haloform such as CHCl3 in the presence of tert-butanol to get my 3-membered
ring with Chlorine on that new carbon. In the Simmons-Smith Reaction we see a carbenoid
where CH2I2 in a Zinc Copper catalyst gives us the cyclopropane. For an in-depth
review on alkene reactions and mechanisms visit my website leah4sci.com/alkene.
Before we continue on to Alkyne Reactions be sure to give this video a thumbs up, subscribe to my
channel and hit that bell notification icon so you don't miss out on any new videos. And with that,
let's move on to alkyne reactions which are very similar to alkene but don't forget to
account for that extra pi bond and understand how that influences the mechanisms and the products.
You can learn more about these reactions on my website at leah4sci.com/alkyne. When it comes
to Alkyne reduction we have 3 reactions. The first is similar to alkenes where we react H2
in a metal catalyst for a full reduction meaning we break all the pi bonds to give us an alkane.
For the next reaction we have a partial reduction called dissolving metal reduction using non-ionic
or radical sodium or lithium in that liquid ammonia to give us a trans alkene. But if we
react the alkyne in H2 in a Lindlar's catalyst we get another partial reduction for a cis alkene. As
with alkenes we have multiple hydration reactions with the difference is visible only in a terminal
alkyne. For oxymercuration I use a slightly different catalyst HgSO4 in H2SO4 to break
just one pi bond and get a markovnikov or more substituted hydroxy. But since this intermediate
is an unstable enol it will automatically tautomerize undergoing keto-enol tautomerization
to give me a ketone. For hydroboration I can use some version of boron, here I have
Disiamylborane to once again break just one pi bond to get an OH group this time added to the
less substituted or anti-markovnikov position, once again giving me an unstable enol that will
tautomerize through keto-enol tautomerization this time giving me a terminal carbonyl or an aldehyde.
When adding halogens to an alkyne, the halogens will continue to add
and so you show the reagent in excess. For hydrohalogenation we add excess HX such as HBr
to break both pi bonds and have 2 bromines added at the more substituted or Markovnikov position.
If we use X2 for halogenation both pi bonds break and now we add
4 halogens 2 to each carbon that used to have a triple bond.
Oxidative Cleavage is a lot more exciting for Alkynes starting with Ozonolysis where we use
the same trick. Slice it down the pi bonds and put an Oxygen everywhere you split a pi bond,
that means we add 4 Oxygen atoms to give us 2 Carboxylic Acids for the internal alkyne and
carboxylic acid plus carbon dioxide if we have a terminal alkyne. If we react an alkyne with KMnO4,
we have two options. If we react an alkyne with KMnO4 under mild conditions H2O and neutral. We
only break this pi bond to give us one alcohol on each carbon for each pi bond that we break.
That means 2 alcohols for the first pi and 2 for the second which is so unstable that
it will react to give us a ketone in place of the 2 alcohols for a Diketone product.
If we react the Alkyne with KMnO4 in a hot basic solution, we do the same thing as with Ozonolysis,
cut it right down the middle for carboxylic acids or carbon dioxide for a terminal carbon.
Last we have chain elongation where if you react a terminal alkyne with NaNH2 in liquid ammonia
followed by an alkyl halide, the alkyne will add to the alkyl halide
to give us a longer carbon chain. Phew! That was a lot! So onto Radical Reactions.
Radical Reactions will be covered in either Orgo 1 or Orgo 2 but we'll start with the
basics that you're more likely to see in Orgo 1 and that is the 3 steps of a radical reaction.
Initiation is when you have a molecule that undergoes homolytic cleavage where the sigma bond
breaks evenly so that each atom gets one electron rather than heterolytic cleavage where one atom
gets two electrons and the other gets none. These lone electrons are your very unstable radicals. In
step 2 the most common step of a radical reaction is Propagation and that is where one radical
will attack a non-radical to do a chain reaction giving us a brand new radical. The last step
Termination is quite uncommon and this is where 2 radicals will come together to form a bond
making the radicals disappear. To see this in detail visit my website leah4sci.com/radical.
The two radical reactions you may have seen are radical reactions of alkanes and radical
reactions of alkene. The first common radical reaction where an alkane will react with a halogen
in the presence of heat or light where this is H nu for a wavelength of light
causing the halogen to turn into a radical attacking the alkane giving us a haloalkane.
And the second is radical halogenation of alkenes where you react an alkene with
HX in this case HCl in the presence of peroxides like ROOR, this breaks the pi bond on the alkene
and gives us a similar product to the non-radical hydrohalogenation
except that the halogen now adds to the less substituted or anti-markovnikov position.
And now for the most exciting, I mean most difficult Orgo 1 reaction set,
substitution and elimination reactions. Let's start with nucleophilic substitution,
the SN1 reaction that stands for Substitution Nucleophilic Unimolecular.
In this reaction we replace the leaving group with the nucleophile in a unimolecular meaning
one step at a time reaction. This is a very slow reaction with a carbocation intermediate.
Where the more substituted the carbocation the faster or should I say, less slow the reaction.
This is often a solvolysis reaction where the solvent in this case methanol acts as the solution
and as the nucleophile or the attacker. In this case giving us OCH3 in place of the leaving group.
The rate of an SN1 reaction is equal to some constant K times the concentration of
the Alkyl group or the substrate. SN2 stands for Substitution Nucleophilic
Bimolecular. Where once again if we start with the reactant that has a leaving group for example
chlorine and react it with a strong nucleophile in the presence of a polar aprotic solvent,
the product will show the nucleophile having replaced the leaving group where if we have
a chiral starting molecule, the product also chiral will show an inversion of configuration.
Since we're reacting with a strong negative nucleophile, this is a very fast reaction and
that means no carbocation. The stronger the attacker the faster reaction giving
me a rate that is equal to K times the substrate meaning the alkyl group times the nucleophile.
E1 stands for Beta Elimination Unimolecular and may look very
similar to SN1 because the two reactions are always in competition with each other
except that in this reaction we remove the leaving group and a nearby hydrogen or beta hydrogen
to give us a pi bond between the alpha and beta carbon. You may see heat added to elimination
reactions because heat favors the formation of a pi bond. In SN1 E1 you still have competition,
this is more relevant in E2. This is a slow reaction with a carbocation intermediate
where the more substituted the carbocation, the faster or once again less slow this reaction.
Where the reaction rate for E1 is equal to K times the substrate meaning the Alkyl group.
If you have more than one beta hydrogen, you have potential to get 2 different products.
Here we drew the major product but we can also form a pi bond to the less substituted carbon
giving us the minor product. This follows Zaitsev’s Rule which tells us that the more
substituted the pi bond the more stable it is and the more likely it is to form.
E2 stands for Beta Elimination Bimolecular where once again we remove the leaving group and
beta hydrogen to give us a pi bond but this time it's Bimolecular meaning both molecules react at
the same time for a fast reaction. E2 reactions require a strong base in this case we have a CH3O-
dissolved in its own conjugate solvent in this case CH3OH a polar protic solvent for
a very fast reaction. Once again no carbocation intermediate. You’ll often see heat added to help
stabilize the formation of a pi bond favoring E2 over SN2. Once again if you have multiple
beta hydrogens, eliminating the more substituted beta hydrogen gives us the major product following
Zaitsev's Rule and the less substituted beta hydrogen will give us the minor product.
The one exception to this rule is if we have a big bulky base such as tert-butoxide
that is so big it can only reach the less substituted hydrogen to give us the non-Zaitsev
or Hofmann product as the major product. For even more on substitution elimination
visit my website leah4sci.com/elimination or for even more on all the orgo 1 reactions,
mechanisms, cheat sheets, practice problems and more, go to leah4sci.com/syllabus linked below
so that you can use my resources to follow along with your Organic Chemistry syllabus.
If you're using this summary to pre-study for the upcoming semester, check out my organic chemistry
pre-study guide at leah4sci.com/prestudy. The link again, leah4sci.com/prestudy.