Chapter 9: Seeing Double: The Alkenes

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Welcome to the Deep Dive, where the show that takes a pile of information digs through it and gives you the core insights you need.

Today, we're jumping into a really versatile part of these molecules with those carbon double bonds.

They're kind of foundational if you want to build almost anything chemically.

We're drawing heavily today from chapter 9 of Organic Chemistry MA for Dummies, second edition.

It's a great resource for getting a solid handle on these compounds.

Our goal here isn't just to list facts.

We want to pull out the really useful stuff, the tips, the aha moments, and why understanding this matters for actually doing organic chemistry.

Exactly.

What's really fascinating about alkenes is they're not just another box to check on the functional group list.

They act like these central hubs, these molecular crossroads.

Think of them like connection flights.

Maybe your target molecule, molecule C, doesn't have a double bond at all, but the best way to get there from molecule A might involve transforming it into an alkene.

Molecule B first.

There are these crucial intermediates that let chemists precisely control transformations, and that's why you find them everywhere from simple plastics to really complex pharmaceuticals.

Okay, so let's nail down the basics first.

What is an alken, fundamentally?

It's a compound with at least one carbon -carbon double bond.

That double bond changes everything compared to, say, alkenes with only single bonds.

Now, anyone who studied organic chemistry knows the reaction list can feel huge.

Our source really hammers home one point for success.

Practice, practice, practice.

It's not just memorizing.

Oh, absolutely.

You can't just read about organic chemistry.

You have to do it.

It's like learning piano.

You don't just read the cheap music, right?

You play the scales over and over.

The chapter mentions some great tools.

Reaction note cards.

Basically, flashcards for reactions are super helpful for just drilling the reagents and products.

And then there are reaction schemes.

These are like flow charts for functional groups.

They show how you can convert one type into another.

Really helps you see the reagents.

Okay, good.

Can you apply that reaction to a totally new molecule?

That's the next level.

You start with one -step problems, but eventually you get to multi -step synthesis.

That's figuring out the whole sequence A to B to C.

That takes serious dedication, like becoming a master painter, really.

Okay, building on that structure, let's talk about degrees of unsaturation.

This is a neat concept.

So alkenes are saturated hydrocarbons, right?

Max hydrogens, formula CNH2N plus two.

But stick a double bond in there, and you lose two hydrogens compared to the alkane version.

That loss gives you one degree of unsaturation.

Sometimes it's called a double bond equivalent, or index of hydrogen deficiency.

An alkene with one double bond usually fits the formula CNH2N.

And this simple idea is, well, it's surprisingly powerful for figuring out structures.

If someone just hands you a molecular formula, C something, H something, calculating the degrees of unsaturation tells you immediately if you should be looking for double bonds, or maybe rings, or even triple bonds.

It's like the first clue, and it's additive, which is key.

One double bond is one degree.

A ring structure also counts as one degree, because forming the ring also means losing two hydrogens compared to the open chain.

A triple bond counts as two degrees of unsaturation, since it's like having two double bonds in one spot.

So, you know, for a three carbon molecule, C3H8 is propane, zero degrees.

C3H6 could be propane, one degree double bond, or cyclopropane, one degree ring.

And C3H4 is propane, two degrees, triple bond.

If you had a molecule with, say, one ring, one double bond, and one triple bond, that's one plus one plus two equals four degrees of unsaturation total.

Right.

And you can figure this out just from the formula.

There's an equation.

Yep.

For simple hydrocarbons, it's pretty straightforward.

Degrees of unsaturation, number of carbons X2 plus two, number of hydrogens two.

Okay.

But what about molecules with other atoms, like oxygen or nitrogen or halogens?

Good point.

You just need to adjust the hydrogen count before plugging it into the formula.

For halogens F, Cl, Br, you treat each one as if it's a hydrogen.

So you add one to the hydrogen count for each halogen.

For nitrogen, you do the opposite.

You subtract one hydrogen for each nitrogen atom.

And atoms like oxygen or sulfur, you can just ignore them for this calculation.

They don't affect the hydrogen count in the same way.

Let's try that example.

C8H6F3NO2.

Okay.

Three fluorines means add three hydrogens.

One nitrogen means subtract one hydrogen.

Ignore the two oxygens.

So C8H6F3NO2 becomes, for calculation purposes, C8H6 plus three one, which is C8H8.

Oh, okay.

C8H8.

Right.

Now plug that into the formula.

882 plus 282.

That's 16 plus 282, which is 1882.

So 10.

Five degrees of unsaturation.

Exactly.

Five degrees.

That tells you a lot.

It could be rings, double bonds, triple bonds, or some combination adding up to five.

It really helps narrow things down, especially when you combine it with other data, like from mass spectrometry, which often gives you that initial formula.

That is a powerful little trick.

Okay.

So we know they have double bonds.

We know about unsaturation.

How do we actually name these things?

Nomenclature time.

Thankfully, it builds on alkane naming, which is nice.

The main change is the ending, right?

Yep.

Dead simple.

Alkane ends in topotane.

Alkene ends in tokenane.

So ethane becomes ethane.

Propane becomes propene.

Cyclohexane becomes cyclohexane if it has a double bond in the ring.

But what if the double bond could be in different places, like in a five carbon chain?

Right.

Then you need a number.

You number the main carbon chain to give the double bond the lowest possible starting number.

So if the double bond starts at carbon one, it's one pentene.

If it starts at carbon two, it's two pentene.

And that double bond numbering takes priority, gets the lowest number, even if that means a substituent group gets a higher number.

The alkene rule trumps the substituent rule.

Okay.

Prioritize the double bond number.

Got it.

Also, the main chain you choose must include the double bond, even if you could find a slightly longer chain of carbons that doesn't include it.

The double bond has to be part of that parent name.

For rings, it's similar.

You number through the double bond, starting with one and two for the double bonded carbons, and continue around to give any substituents the lowest possible numbers after that.

What about molecules with multiple double bonds?

They use prefixes, ID for two, tri for three, tetra for four, and so on.

And you put numbers at the start to show where each double bond begins.

For example,

143 -butadiene has two double bonds, starting at carbons one and three.

Makes sense.

Are there common names we need to know too, like non -systematic ones?

Definitely.

IUPAC is the official system, but some old common names just stick.

You really need to know them.

Ethene is almost always called ethylene.

Propene is usually propylene.

Another important one is styrene.

That's a benzene ring with a vinyl group, a CHCH2 group attached.

It's the building block for polystyrene plastic.

You encounter that stuff everywhere.

Ah, okay.

Good to know their shortcuts.

Now, here's something that makes alkenes really distinct.

Their shape, their stereochemistry.

Single bonds, CC bonds, they can spin freely, like little propellers, but double bonds, CC, they're locked in place.

Rigid.

Exactly.

That rigidity is fundamental.

Because they can't rotate freely, you can have different spatial arrangements of the atoms connected to that double bond.

These are called stereosomers.

Same atoms, same connections, but different 3D shapes.

And the classic example is cis and trans.

Right.

The cis -trans system works well when you have two identical groups attached across the double bond, often hydrogens, but could be other groups too.

If the identical groups are on the imaginary line drawn through the double bond, that's the cis isomer.

Think cis equals same side.

If the identical groups are on opposite sides, that's the trans isomer.

And because they have different shapes, cis and trans isomers are actually different compounds with different properties.

Melting points, boiling points, reactivity sometimes.

And this only happens because the double bond can't rotate.

Single bonds don't have cis -trans isomers.

Precisely.

That free rotation scrambles everything too quickly.

Okay.

But cis -trans works for identical groups.

What if all four groups attached to the double bond carbons are different?

Ah, good question.

Then cis -trans doesn't really apply cleanly.

For that, we use the EZ system.

It's more general.

E comes from the German and Gaggen meaning opposite, and Z comes from Zusama meaning together.

Okay.

E for opposite, Z for together.

But opposite or together relative to what?

Relative to priority.

You have to assign priorities high or low to the groups attached to each carbon of the double bond.

You do this using the conning gold prelog rules.

Once you assign priorities on each side, you look at the two high priority groups.

If the two high priority groups are on the same side of the double bond, it's the Z isomer.

Zusama and equal together.

If the two high priority groups are on opposite sides, it's the E isomer and Gaggen E col is opposite.

Got it.

So it all hinges on assigning those priorities correctly using the conning gold prelog rules.

How does that work?

What are the rules?

Rule number one is atomic number.

Look at the atom directly attached to the double bond carbon.

The higher the atomic number, the higher the priority.

So iodine beats bromine, bromine beats chlorine, chlorine beats fluorine, fluorine beats oxygen, oxygen beats nitrogen, nitrogen beats carbon, carbon beats hydrogen.

That's the basic sequence.

Okay.

Higher atomic number wins.

What if there's a tie?

Like both groups start with a carbon atom.

Then you go to the next atoms out the chain.

You compare the atomic numbers of the atoms attached to those first atoms.

You find the first point of difference.

So like an ethyl group, CH2CH3, would have higher priority than a methyl group, CH3.

Because of the second position, the ethyl group has a carbon while the methyl just has hydrogens.

Carbon beats hydrogen.

You keep going until the tie is broken.

Right.

Find the first point difference.

And what about double or triple bonds within the substituent groups themselves?

Ah, there's a specific rule for that.

You treat multiple bonds as if they were an equivalent number of single bonds.

So a CO group is treated as if the carbon is singly bonded to two oxygens.

A CN group is treated as if the carbon is singly bonded to three nitrogens.

So if you were comparing CO to CN, the CO group would actually get higher priority.

Why?

Because the first atom attached in the CO case is oxygen, atomic number eight.

Well, in CN, it's nitrogen, atomic number seven.

Oxygen beats nitrogen.

It's a very systematic process.

Okay.

That's quite detailed, but it gives a precise way to name any alkene stereoisomer.

Now, shifting slightly, why do reactions sometimes favor making one isomer over another?

Say, more E than Z or more trans than cis.

Yeah, that selectivity is common and it almost always boils down to stability.

The universe tends to favor lower energy states and more stable molecules are lower in energy.

The biggest factor for alkene stability is substitution.

How many non -hydrogen groups are attached directly to the double bond carbons?

Generally speaking, the more alkyl groups or other non -H groups attached to the CUC unit, the more stable the alkene is.

So a tetrasubstituted alkene for alkyl groups is usually the most stable, followed by trisubstituted, then desubstituted, and monosubstituted is the least stable among those.

More stuff stuck onto the double bond carbons makes it more stable.

Why is that?

It's often explained by hyperconjugation, a stabilizing interaction, where electrons from adjacent CH single bonds can kind of leak into the pi system of the double bond.

More adjacent groups means more potential for the stabilizing effect.

Substitution matters.

What about cis versus trans?

You mentioned that earlier.

Right.

Within a pair of desubstituted isomers, the trans isomer is almost always more stable than the cis isomer.

Why is that?

It really comes down to steric hindrance.

Think of it like atoms needing personal space.

In a cis isomer, the two bulkier substituent groups are forced onto the same side of the double bond.

They're crowded.

Their electron clouds repel each other, which is destabilizing.

In the trans isomer, those bulky groups are on opposite sides, farther apart, less propulsion, more space, more stability.

The personal space analogy works well.

Okay, so we know what they are, how to name them, their shapes, their stability.

How do we actually make alkenes?

What are the key reactions?

The source highlights three main ways chemists make alkenes.

Dehydrophilogenation, dehydration, and the Wittig reaction.

Let's take those one by one.

Dehydrophilogenation.

Sounds complex.

It just means removing HX, like removing HGL or HBR.

You start with an alkyl halate that is an alkane skeleton with a halogen atom, Cl, Br, or I attached.

You react to that alkyl halate with a strong base.

The base pulls off a proton, H +, from a carbon next to the carbon holding the halogen.

At the same time, the halogen leaves as an anion, like Br, and the electrons shuffle around to form the new double bond between those two carbons.

It's an elimination reaction.

Okay, base removes H from one carbon, halogen leaves from the next carbon, double bond forms between them.

Got it.

Exactly.

And the exact way this happens, the mechanism can be E1 or E2.

Different step -by -step processes.

We don't need to fully detail now, but they both lead to the alkene.

And dehydration.

Sounds like removing water.

Precisely.

You start with an alcohol, heat it up with a strong acid catalyst, like sulfuric acid.

The alcohol molecule loses H2O and H from one carbon and the OH group from the adjacent carbon, and again, you form a double bond between those carbons.

Very similar concept to dehydrophilogenation, just losing water instead of HX.

Typically goes by an E1 mechanism, too.

Okay, two elimination methods.

What's the Wittig reaction?

That one sounds different.

Oh, the Wittig is really cool and super important.

Jorg Wittig actually won the Nobel Prize for discovering it.

It's a powerful way to make a carbon double bond,

very specifically.

Basically, you take an aldehyde or a ketone, something with a CO group, a carbonyl, and you react it with a special region called a phosphorine, or sometimes a Wittig reagent.

This phosphorine has a phosphorus double bonded to a carbon, usually written like PS3PCR2.

That PS3P is just triphenylphosphine, a common carrier group.

The magic is that the oxygen atom of the carbonyl group gets swapped out for the part of the phosphorine.

So you're literally swapping the CO for a CC.

How does that even work and where does the phosphorine come from?

Good questions.

You usually make the phosphorine first.

It's often a two -step process.

Step one, react triphenylphosphine, pH3P, with an alkyl halide.

This makes something called a phosphonium salt.

Step two, treat that salt with a very strong base.

The base rips a proton off the carbon next to the phosphorus, creating the phosphorine, which is actually a resonance hybrid, but we often draw it as PH3PCR2.

Okay, so you make the special reagent, then react it with the aldehyde or ketone.

What happens then, the mechanism?

The mechanism is thought to go like this.

The carbon end of the phosphorine attacks the carbonyl carbon.

This forms an intermediate called a betaine.

Then the negatively charged oxygen of the betaine quickly attacks the positively charged phosphorus, forming a four -membered ring intermediate called an oxyphosphatane.

This ring is unstable and collapses.

The driving force is the formation of a very strong phosphorus -oxygen double bond in triphenylphosphine oxide, PH3PO, which is a byproduct.

When that PO bond forms, the other two carbons are kicked out, joined together as your desired alkene.

That strong PO bond formation really drives the whole thing forward.

It's quite elegant.

Wow, that is elegant.

A Nobel Prize well deserved.

Okay, so we've covered a huge amount today.

We started with the basics, what alkenes are, that CEC bond, talked about degrees of unsaturation as a structural clue, went through naming rules, the N -ending numbering strands, EZ stereochemistry, then stability, why more substituted and trans isomers are generally favored, and finally, how to make them.

Elimination reactions like dehydroflagination and dehydration and the really clever Wittig reaction.

Yeah, it really brings home that waypoint idea.

They're central to so many transformations.

So for you listening, grasping these concepts, the structure, the naming, the stability, how they're made, it really builds that core understanding.

You need to tackle more organic chemistry reactions down the line.

And maybe a final thought to chew on.

Think about that interplay between the rigidity of the double bond, the slight stability differences between isomers like cis and trans or E and Z.

That subtle tuning of structure and stability is exactly what allows chemists to use alkenes to build such an incredible range of molecules.

Everything from everyday plastics to incredibly complex, life -saving drugs.

How does knowing this molecular level design change how you see the materials and medicines all around you?

It's kind of amazing when you think about it.

It really is a great point to end on connecting the dots from the molecule to the world.

Thanks for joining us for this deep dive into alkenes.

Keep practicing those reactions, stay curious, and we'll catch you on the next deep dive.