Alkenes are an important type of molecule in organic chemistry that we’re going to see a lot more of in this series. But before we can really get into the many cool reactions alkenes do, we need to go over some of the basics. In this episode of Crash Course Organic Chemistry, we’ll review and build on our knowledge of alkene nomenclature, revisit our friend the carbocation, and learn Markovnikov’s Rule: an important tool that will help us predict the products of addition reactions involving alkenes.
Series Sources:
Brown, W. H., Iverson, B. L., Ansyln, E. V., Foote, C., Organic Chemistry; 8th ed.; Cengage Learning, Boston, 2018.
Bruice, P. Y., Organic Chemistry, 7th ed.; Pearson Education, Inc., United States, 2014.
Clayden, J., Greeves, N., Warren., S., Organic Chemistry, 2nd ed.; Oxford University Press, New York, 2012.
Jones Jr., M.; Fleming, S. A., Organic Chemistry, 5th ed.; W. W. Norton & Company, New York, 2014.
Klein., D., Organic Chemistry; 1st ed.; John Wiley & Sons, United States, 2012.
Louden M., Organic Chemistry; 5th ed.; Roberts and Company Publishers, Colorado, 2009.
McMurry, J., Organic Chemistry, 9th ed.; Cengage Learning, Boston, 2016.
Smith, J. G., Organic chemistry; 6th ed.; McGraw-Hill Education, New York, 2020.
Wade., L. G., Organic Chemistry; 8th ed.; Pearson Education, Inc., United States, 2013.
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to me markovnikov's rule to this day is still: "the poor will be poorer and the rich will be richer" kind of mindset
everything said in video part 1:
0:10
The Australian Blue Mountains and the American Blue Ridge Mountains both get their colorful names from a blue haze that blankets them on hot summer days.
0:19
This haze comes from small molecules that scatter sunlight, specifically small-wavelength blue light.
0:24
One of these small molecules is isoprene, a volatile, biogenic organic compound…
0:29
or, in simpler terms, an organic chemical made by living things, in this case, trees, that readily evaporates into the air.
0:37
Like many alkenes, isoprene's pi electrons in double bonds make it a reactive molecule.
0:42
It reacts with ozone, nitrogen dioxides, and other atmospheric pollutants — not always in good ways.
0:49
And isoprene polymerizes to make a major component of natural rubber.
0:53
The reactions we’ll learn over the next few episodes will let us add different things to alkenes.
0:58
But first, we have to revisit some alkene nomenclature and another familiar friend:
1:02
carbocations, those molecules with positively charged carbon atoms.
1:07
[Theme Music]
1:17
An alkene is a molecule containing carbon-carbon double bonds.
1:21
Unlike single bonds, double bonds are rigid, they can’t easily rotate because the pi bond would need to break first and that costs energy.
1:29
In episode 2, we talked briefly about cis-and trans-isomers of alkenes.
1:34
As a refresher, this naming system helps us describe two different geometric isomers around a double bond.
1:39
For example, let's look at pent-2-ene.
1:41
If the methyl and ethyl groups are on opposite sides around the double bond, we call it trans-pent-2-ene.
1:47
And if the methyl and ethyl groups are on the same side around the double bond, we call it cis-pent-2-ene.
1:52
But, in the context of alkenes, cis- and trans- is an old school naming system that only works when the double-bond carbons are attached to two hydrogens and two R-groups.
2:01
If we consider something like 2-chloropent-2-ene, the cis- and trans- system fails to help us accurately communicate where the groups are.
2:09
So we need a better way!
2:11
Thankfully, organic chemists have us covered.
2:13
Similar to assigning R and S enantiomers of molecules, we can prioritize the groups on each carbon of the double bond, using the rule that higher atomic number means higher priority.
2:23
In this first isomer of 2-chloropent-2-ene, the carbon on the left side of the double bond is attached to an ethyl group and a hydrogen atom.
2:31
The ethyl wins priority, because its carbon has a higher atomic number than hydrogen.
2:35
The carbon on the right side of the double bond is attached to a chlorine and a methyl group.
2:40
Here the chlorine wins priority.
2:42
If we mark our priority winners and losers, we can see that high priority groups on each double-bonded carbon are on the same side of the double bond.
2:50
So this is the Z isomer, which comes from the German word zusammen for together.
2:55
Or as I remember it: on ze Zame Zide.
2:59
So we have (Z)-2-chloropent-2-ene.
3:01
The other isomer of 2-chloropent-2-ene has high priority groups on opposite sides with respect to the double-bonded carbon atoms.
3:09
So this is the E isomer, derived from the German word entgegen or opposite.
3:14
This isn’t quite as clever, but I remember it as Ecross for across.
3:19
So it's (E)-2-chloropent-2-ene.
3:22
Assigning priorities isn't always so easy, so if we have a tie, we need to keep hopping along until one group wins.
3:27
For example, in this molecule, the left-hand carbon of the double bond is straightforward.
3:32
Bromine wins priority over the methyl group.
3:35
But on the right-hand carbon, the first position gives us the same thing: a carbon with two hydrogens attached.
3:41
A tie!
3:42
So we have to keep going and compare the next two atoms.
3:45
There, we can see that the triple-bonded carbon wins priority, because it's like 3 carbons at once compared to 1 carbon on the other side.
3:52
Now we can see our priority winners are on the Zame Zide, so it's a Z-isomer, and we can call this guy (Z)-5-bromo-4-ethylhex-4-en-1-yne.
4:02
Returning to our first example, trans-pent-2-ene can be more precisely called E-pent-2-ene, and the cis isomer is the Z-alkene.
4:09
And if one side of the alkene has two of the same group, like 2-methylpent-2-ene here, we don’t need to use E and Z.
4:16
So now we know how to name alkenes a little bit better, and we can start playing around with some reaction chemistry.
4:21
Many of the chemical reactions associated with alkenes are addition reactions,
4:26
which means the pi electrons are attracted to electrophiles, and groups get added to the carbons on each side of the double bond.
4:32
In episode 12, we saw what happens when we mix hydrogen bromide and cis-but-2-ene.
4:37
It's a pretty straightforward nucleophilic attack where the alkene double bond attacks a proton, and one of the previously double-bonded carbons gets a bromine.
4:45
No matter which carbon bonds to the bromine, the product is the same.
4:49
But things aren't usually so simple.
4:51
For instance, if we add hydrogen bromide to 2-methylbut-2-ene, which isn't symmetrical, we can potentially make two different products.
4:58
Here's the catch though — when we do this reaction in a lab, we only make one of these two products.
5:05
This seemingly-mysterious observation actually has a perfectly good explanation.
5:08
It's because of the stability of different carbocations, which are positively charged carbon atoms.
5:13
To see where carbocations come in, we have to break this reaction into steps.
5:18
Remember that hydrogen bromide is a strong acid, so it's fully dissociated into H+ and Br- ions.
5:24
Most textbooks show an attack on an undissociated molecule of hydrogen bromide, so that’s how we’ll show it in this series.
5:30
So first, the alkene initiates a nucleophilic attack on the proton formed by the dissociation of hydrogen bromide —
5:36
donating a pair of electrons, snagging the proton, and leaving the remaining carbon of the double bond short two electrons.
5:44
This creates a positive charge: a carbocation.
5:47
There are two possible carbocations that can form here.
5:51
One where the positive charge ends up on a carbon surrounded by 3 other carbons, called a tertiary carbocation.
5:58
And one where the positive charge is on a carbon surrounded by 2 carbons and a hydrogen, called a secondary carbocation.
Thank you for the amazing visuals. Organic chemistry is a visual subject.
The E isomer is like waving ello from across the table
The way I remember E isomers is that they are enemies, so of course they are on opposite sides.
Honestly the part when she says, "okay, deep breath" is so real.
thank you!!!!
Where are the dashes and wedges :’))
9:45 I love this part lol, it's so cool, I'm actually shocked, never been taught in high school
Where is the difference to the Wagner Meerwein Shift ?
So I’m actually a PhD chemist who mainly does organic chemistry. I watched this in the hope to see if E and Z isomers have different reactivity to HBr. That topic was unfortunately not mentioned….
But when I listen to this, I’m no longer surprised that so many students tell me that organic chemistry is confusing. If this video here is a good representation on how it’s taught then I would probably also get confused.
There is no need to introduce a major principle on addition reactions but then spending more time on special cases than on the most common one. And the foundation needs more detail. 1. It’s always like a magnet: negative charge goes towards positive charge to form an intermediate and the reaction takes place if that can easily stabilize into something more favorable. When I start explaining reactions in org chem then I first talk about electronegativity. 2. You need to first explain at length why certain carbocations are more stable than others, practice that a little bit and then apply this to addition reactions. Markovnikov derives from there automatically. Proton shifts can happen occasionally, while methyl shifts are much more rare. If you talk about that then you also need to explain why a methyl group can actually move while most other groups don’t. Better would be not to teach that at all though.
wait you ate this
You lost me with the shift stuff ! I hate chemistry
how do you know the 1,2-shift will happen or happens?
For those confused by later episodes come back to this one and pay attention to markovnikovs rule
In 4:01, why din't you name it as an alkene?
This video was pretty good. Understood some bits and pieces of my lecture today, but this video connected all the dots. Thank you!
I read that you have to have IDENTICAL groups on each side of the double bond for it to be cis or trans, but that's not what you are showing here.
This is definitely one of my favorites of crash course.
3:59 is it supposed to be (Z)-4-bromo-3-ethylpent-3-en-1-yne?
At this point, it feels like I am watching a sci-fi movie with many futuristic terms
Not "on ze zame zide" 😆 I will remember that for the rest of my life!
Excellent!
Thanks ma'am 😇😇😇😇🌸🌸🌸🌸
Markovnikov’s rule states that the electrophile attaches to the most stable part of the carbocation which consists the “least hydrogen”. Can someone back me up…
I'm from the Blue Mountains in Australia! So cool to see them featured here 😃
This series is really really helpful!
Seeing Organic Chemistry in a whole different view!
Love that I am dutch and that there are easier way to memorize german. Entgegen –> tegen(over) ; zuzamen –> samen. Respectively against and together
YouTube recommended me this 1 day before chemistry exam. Damn, even YouTube cares more about me than I do.
Absolutely amazing video, the animations help SOOO much and the concepts were explained so well! Made for a great review. Thank you CC for once again carrying my OChem grade.
WE NEED MATERIALS SCIENCE COURSE
Hey, CrashCourse, love the videos. I'm not sure if it's been discussed already, but are there any plans for a mathematics playlist? I'd like to see that.
Could you do videos on animal science???
Interesting and useful . Great job on animations of hyperconjugation 👍
because your explaination is very awesome
I hope you can update the next episode quickly
great crash course of organic chemistry
i thank the creators.
lol thank you!