Welcome to Last Minute Lecture.
This free chapter overview is designed to help students review and understand key concepts.
These summaries supplement not replaced the original textbook and may not be redistributed or resold.
For complete coverage, always consult the official text.
Welcome back to the Deep Dive.
Today we're getting right into the heart of organic chemistry.
We are.
We're looking at the compounds where carbon meets nitrogen, the amines, amides, and of course amino acids.
And our mission today is to really distill Chapter 27, not just to know what these compounds are, but to understand why they are so fundamental to everything, from our own bodies to the clothes we wear.
It's a great topic.
And the story really kicks off with a kind of happy accident back in 1856.
You have this 18 -year -old William Henry Perkin trying to make quinine.
Right, the anti -malarial drug, a very complex molecule.
And he fails.
I mean, he fails spectacularly, ends up with this black sticky mess.
But then, this is the key, he washes it with alcohol.
And out comes this incredible vibrant purple color, mauve, the world's first synthetic dye.
So what does that have to do with nitrogen?
Everything, really.
Perkin's discoveries show that the nitrogen atom is sort of the chemical anchor for creating stable, intense color in these big organic molecules.
It's the key.
Okay, so nitrogen is this crucial connector.
To understand it, we have to start with the simplest group, the amines.
Exactly, they're just derivatives of ammonia where you've swapped out one or more hydrogens for a carbon chain, an alkyl or an aryl group.
And we classify them as primary, secondary, or tertiary.
Right, it's just counting how many carbon chains are attached directly to that nitrogen.
One for primary, two for secondary, three for tertiary.
It's that simple.
But the defining feature, the thing that makes an amine, an amamine,
is that lone pair of electrons on the nitrogen.
Absolutely.
And that lone pair dictates their most important chemical property.
Which is their basicity.
Their basicity.
They are proton acceptors, H plus acceptors, because that lone pair is available to form a new bond.
But they're not all equally good at it.
Right, and this is the first really critical comparison.
If you line up, say, ethylamine, ammonia, and phenylamine.
You see a huge difference in strength.
Ethylamine is the strongest by far.
Phenylamine, on the other hand, is surprisingly weak.
So why is ethylamine my winner here?
Well, it comes down to something called the inductive effect.
That alkyl group, the ethyl group, is electron donating.
It pushes electron density toward the nitrogen.
Exactly.
It makes that nitrogen atom even more electron rich.
So the lone pair is, you know, more available, more eager to grab a proton.
That makes it a stronger base.
And ammonia is sort of the baseline with no push.
Right.
And then you have phenylamine, which is the complete opposite.
Because of the benzene ring it's attached to.
Precisely.
The lone pair on the nitrogen gets completely disrupted.
Its p orbital overlaps with the whole pi bonding system of the benzene ring.
So the lone pair isn't just sitting on the nitrogen anymore.
No, it's pulled into the ring.
It's delocalized.
And the key chemical insight here is, if that lone pair isn't localized on the nitrogen, it can't easily and bond with the proton.
It's just not available.
That makes perfect sense.
So let's talk about actually making these things.
The methods are pretty different for aryl amines versus alkylamines, right?
They are.
For something like phenylamine, the standard route is a reduction reaction.
You start with mitrobenzene.
You do.
You heat it under reflux with tin and concentrated hydrochloric acid.
That reduces the nitro group.
Then you have an intermediate ion and you just add some sodium hydroxide to neutralize it and release the final phenylamine.
Okay.
And for the alkylamines?
You've got more options.
The classic one is nucleophilic substitution.
You take a halogen alkyne, something like bromothane, and react it with ammonia.
But there's a catch here, isn't there?
A big one.
You have to use excess ammonia.
A huge excess and it has to be hot and ethanolic.
Why so much?
Because if you don't, the primary amine you just made will immediately act as a nucleophile itself and attack another molecule of the halogen alkyne.
You'll end up with a messy mixture of secondary, tertiary, and even quaternary products.
So excess ammonia basically ensures the halogen alkyne is more likely to bump into an ammonia molecule than one of your product molecules.
Exactly.
It's a numbers game.
And there are other methods, ones that can actually extend the carbon chain.
Yes.
And this is very powerful, the reduction of nitriles.
This route always adds one extra carbon atom to your chain.
How does that work?
You'd start with your halogen alkyne, react it to form the nitrile, which has that C triple bond N group, and then you reduce that.
You can use hydrogen with a nickel catalyst or a really strong reducing agent.
Like LiOH4?
Yes.
Lithium tetrahydroaluminate in dry ether.
And that stuff is incredibly versatile.
It can also reduce the carbonyl group in an imide straight down to an amelamine.
Okay.
Let's circle back to the practical chemistry, back to the dyes.
Phenolamine has this super reactive benzene ring.
It does.
That NH2 group is just pouring electron density into it, activating it.
Which is why if you add aqueous bromine, the reaction is instant.
You get a white precipitate.
Right.
Because it brominates at positions two, four, and six all at once, doesn't wait around.
And this high reactivity is what we exploit to make those vivid azo dyes.
It's a brilliant two -step process.
Step one is called diazotization.
You react phenolamine with nitric three acid.
Which is unstable.
So you have to make it right there in the beaker in situ from sodium nitrite and HCl.
And here's the crucial condition you always see in the textbook.
Keep it cold.
The mixture absolutely must stay below 10 degrees Celsius.
So the ice bath, why?
Because the product you make, this benzene diazonium ion, is incredibly unstable.
Above 10 degrees, it just decomposes violently sometimes.
But you need that unstable intermediate for the second step.
It do.
That's the coupling reaction.
The positively charged diazonium ion is now a great electrophile and it attacks an alkaline solution of phenol or another similar compound.
And that creates the azo dye.
That's right.
You get this N double bond N bridge, the azo group, which connects the two ring systems.
The real insight is that this bridge creates a massive delocalized pi system across the whole molecule.
And that huge delocalized system is what absorbs visible light and gives us those stable bright colors.
That's the secret to the color.
Okay.
So from industrial color to the building blocks of life, let's pivot now to amino acids.
It's a perfect transition because they combine the two functional groups we've been talking about.
The basic amino group, NH2, and the acidic carboxylic acid group, COOH.
So they are inherently amphoteric.
They can act as both an acid and a base all in one molecule.
Their general structure is always the same apart from that R group.
And that R group is everything.
It's the only thing that changes, but it dictates the entire identity and function of that specific amino acid.
Now in solution, these two groups, the acid and the base, they don't just ignore each other, do they?
No, they react with each other.
It's an internal acid base reaction.
The COOH group donates its proton to the NH2 group.
And that forms what's called a zwitterion.
From the German, for two ions, you get a negatively charged COO minus group and a positively charged NH3 plus group.
So it's doubly charged.
The overall charge is zero.
Exactly.
It's electrically neutral.
And because it's essentially an internal salt, it has strong intermolecular forces.
That's why amino acids are crystalline solids and they're pretty soluble in water.
But that charge isn't static.
It depends entirely on the pH of the solution you put it in.
Right.
pH is the master switch.
If you put the zwitterion in a very acidic solution, a low pH.
There are lots of H plus ions around, so the negative COO minus group will pick one up.
And the whole molecule becomes positively charged.
Conversely, in an alkaline solution, high pH, the NH3 plus group will donate its proton.
Making the whole molecule negatively charged.
And that one specific pH, where the amino acid exists mainly as the neutral zwitterion, with no net charge, that's called its isoelectric point, or PI.
This ability to have different charges is what allows them to link up.
Yes.
Through a condensation reaction, the carboxylic acid end of one reacts with the amino end of another.
A molecule of water is eliminated and you form a dipeptide.
And the bond you form is called an amide bond, or in a biological context, a peptide bond.
The CONH link.
And since the new molecule still has a free amino end and a free carboxyl end, the chain can just keep growing.
Which brings us to the general class of amides.
What's so interesting is that when you form that amide group, you effectively kill the basicity we saw in the amides.
So amides are neutral.
They are.
And the reason goes right back to electron availability.
That nitrogen in the amide is right next to a carbonyl group.
The C double bond O.
And that oxygen is strongly electron withdrawing.
It pulls electron density away from the nitrogen, making that lone pair completely unavailable to accept a proton.
It's just not basic anymore.
And to make them, you typically use something highly reactive like an acyl chloride.
Right.
React an acyl chloride with concentrated ammonia or a primary amine.
It's a fast reaction, happens right at room temperature.
And that peptide bond is strong, but we can break it chemically.
We can through hydrolysis.
You just reflux the amamide with either an acid or an alkali.
And you get different products depending on which you choose.
You do.
With acid hydrolysis, you'll break the bond to form a carboxylic acid.
And the ammonium salt of the amim.
And with alkaline hydrolysis.
You'll form the salt of the carboxylic acid, the sodium salt, for example, and the free neutral primary amine.
So you choose your conditions based on which product you want to end up in which form.
This brings us to our final topic, an analytical tool that uses all of this charge chemistry,
electrophoresis.
Right.
This is the technique that really proves the whole principle of PI and pH control.
You just apply an electric field to a mixture of amino acids in a buffer solution.
And the ions move.
Positive ions head to the negative electrode.
And negative ions go to the positive one.
Simple as that.
But how fast they move depends on a couple of things.
Their size bigger ions are slower and they're charged.
A higher charge means a stronger pull, so it moves faster.
The separated molecules form these visible bands.
And the key to the whole process is the pH of that buffer solution.
It is the absolute core principle.
The buffer's pH sets the charge on every single amino acid in your sample.
So it determines if they're positive, negative, or neutral.
Which in turn determines if they move, which direction they move, and how fast they move.
Electrophoresis is physical separation based entirely on this amphoteric chemical behavior.
That feels like a perfect place to stop and summarize.
Let's just quickly run through the biggest insights from today.
First, amine basicity is all about electron availability.
It's a competition between the electron donating inductive effect in alkylamines, which makes them strong, and… And the electron stealing delocalization into the benzene ring, which makes arylamines weak.
Second, those amazing synthetic dyes are made by a process that balances on a knife edge.
You need the highly temperature -sensitive diazotization to make an unstable intermediate.
Which then couples to form that incredibly stable, highly delocalized, N -double bond and azo group that gives us the color.
Third, amino acids are dynamic.
They form these internal salts called sweterians, but their actual net charge is completely at the mercy of the surrounding pH, relative to their unique isoelectric point.
And finally, the amide bond, the peptide link, is neutral because that adjacent oxygen atom wins the tug of war for electrons.
This makes it chemically distinct from a basic amine, but we can still precisely break it with acid or alkaline hydrolysis.
So that leaves us with a final thought for you to consider.
Knowing all this, how could you specifically design an electrophoretic separation to isolate only the amino acids that have basic R groups from a complex mixture?
Think about the buffer pH you would need to choose to make sure those specific amino acids carry a unique charge, allowing them to be separated from all the acidic and neutral ones.
It's a great puzzle.
And on that note, thanks for joining us for this deep dive into organic nitrogen compounds.
We'll see you next time.