Acidity: Crash Course Organic Chemistry #11

Summary

In this Crash Course Organic Chemistry episode, Deboki Chakravarti explores the concept of acids and bases through the Brønsted-Lowry definition, emphasizing the importance of understanding their strength in predicting chemical reactions. The video delves into the acid dissociation constant (Ka) and its inverse, pKa, to compare the strength of acids. Key factors influencing acidity, such as resonance stabilization, atom identity, inductive effects, and orbital hybridization, are discussed with illustrative examples. The episode concludes by highlighting the significance of acidity in organic chemistry.

Takeaways

• 🚀 The script introduces Crash Course Organic Chemistry with Deboki Chakravarti as the host.
• 🌌 A sci-fi reference to the movie 'Alien' is used to illustrate the concept of acidic strength.
• 🐜 Tawny ants and their formic acid are mentioned as an example of a weak acid in nature.
• 🔬 The Brønsted-Lowry definition of acids and bases is adopted for the course.
• ⚛️ Electron pushing between molecules is explained to demonstrate bond breaking and forming.
• 💧 Water's role as a base in dissolving acids like acetic and propanoic acid is highlighted.
• 📊 The importance of Ka (acid dissociation constant) and pKa in determining acid strength is discussed.
• 📉 A lower pKa indicates a stronger acid, while a higher pKa indicates a weaker acid.
• 🔍 Resonance stabilization is a key factor in understanding the acidity of compounds like acetic acid and phenol.
• 🌐 The periodic table's electronegativity and element size influence the stability of conjugate bases.
• 🔬 The inductive effect and the s character of hybrid orbitals are also factors that affect acidity.

Q & A

What is the significance of the acidic blood in the sci-fi movie 'Alien' mentioned in the script?

The acidic blood of the xenomorph in 'Alien' is used as an engaging example to illustrate the concept of acidity in chemistry. It is stronger than most acids produced by Earth creatures, providing a memorable and relatable context for discussing the strength of acids.

According to the script, what is the Brønsted-Lowry definition of an acid?

In the Brønsted-Lowry definition, an acid is defined as anything that can donate a proton (H+), while a base is anything that can accept a proton.

What is the role of water when carboxylic acids like acetic acid and propanoic acid are dissolved in it?

Water acts as a base in this context, accepting a proton from the carboxylic acid to form a hydronium ion and the corresponding carboxylate ion.

What does the Ka value represent in the context of acid-base chemistry?

The Ka value represents the acid dissociation constant, which describes the equilibrium between the products and reactants of an acid dissociation reaction.

How is pKa related to Ka, and what does it indicate about an acid?

pKa is the negative logarithm of Ka. It is used to compare the strength of acids; a lower pKa indicates a stronger acid because the equilibrium favors the products, while a higher pKa indicates a weaker acid as the equilibrium favors the reactants.

Why is the resonance stabilization of the acetate ion important for acetic acid's acidity?

The resonance stabilization of the acetate ion allows the negative charge to be distributed over two oxygen atoms, making it easier for the ion to carry the burden of the negative charge and thus facilitating the loss of a proton from acetic acid.

How does the size of an atom affect its ability to stabilize a negative charge?

Larger atoms have more electrons in more diffuse orbitals, which can spread out to stabilize negative charges better. This property is known as polarizability.

What is the inductive effect and how does it influence acidity?

The inductive effect refers to the influence of electronegativity throughout a molecule, where more electronegative atoms can pull negative charges towards them through covalent bonds, stabilizing the conjugate base and increasing the acidity of the molecule.

Why is trifluoroacetic acid a stronger acid than acetic acid?

Trifluoroacetic acid is stronger than acetic acid because the three fluorine atoms, being highly electronegative, pull on the negative charge in the conjugate base, stabilizing it and favoring the products side of the equilibrium.

What is the significance of the s character of hybrid orbitals in determining acidity?

The s character of hybrid orbitals affects acidity because orbitals with more s character (like sp hybridization) have electrons closer to the nucleus, which can stabilize a negative charge more effectively, making the corresponding acid more acidic.

What are the four key factors that help predict relative acidity in organic chemistry?

The four key factors are: 1) Atom identity - More electronegative and larger elements stabilize charge better; 2) Resonance stabilization - Multiple Lewis structures for conjugate bases indicate greater stability; 3) The inductive effect - Electronegative atoms can stabilize negative charges through covalent bonds; 4) The s character of the orbital - More s character stabilizes negative charge better.

Outlines

00:00

🔬 Introduction to Acid-Base Chemistry

The paragraph introduces the topic of organic chemistry with Crash Course, mentioning a fun fact about the movie 'Alien' to illustrate the concept of acidity. It explains the importance of understanding acids and bases in organic chemistry for predicting chemical reactions. The Brønsted-Lowry definition of acids and bases is introduced, where an acid is a proton donor and a base is a proton acceptor. The concept of conjugate acids and bases is discussed, using water and carboxylic acids as examples. The paragraph also introduces the acid dissociation constant (Ka) and its inverse, pKa, which are used to measure the strength of acids. The summaries of these concepts are given with examples such as formic acid, acetic acid, and propanoic acid, and the role of resonance structures in stabilizing the negative charge in conjugate bases is explained.

05:02

📚 Factors Affecting Acidity and pKa

This paragraph delves into the factors that affect the acidity of a compound and its pKa value. It discusses how resonance stabilization can influence acidity, using phenol and cyclohexanol as examples. The role of atom size and polarizability in stabilizing negative charges is explained, with a comparison between phenol and thiophenol. The paragraph also covers the inductive effect, where electronegativity can stabilize the negative charge in conjugate bases, exemplified by acetic acid and trifluoroacetic acid. Lastly, it touches on the s character of hybrid orbitals and how it affects acidity, comparing ethane, ethene, and ethyne, and explaining how the s character can stabilize a negative charge better in sp hybridized orbitals.

10:03

🧪 Predicting Relative Acidity

The final paragraph summarizes the key factors that can be used to predict the relative acidity of compounds. It lists four main factors: atom identity, resonance stabilization, the inductive effect, and the s character of orbitals. Each factor is explained in the context of how it helps stabilize negative charge in conjugate bases, thereby making the corresponding acid more acidic. The paragraph concludes by emphasizing the importance of understanding acidity in organic chemistry and hints at the upcoming episodes where these concepts will be applied to predict chemical reactions.

Keywords

💡Acid

In the context of the video, an acid is defined by the Brønsted-Lowry definition as anything that loses a proton (a hydrogen ion with a positive charge). This concept is central to understanding acid-base chemistry, which is a significant part of organic chemistry. For example, carboxylic acids like acetic acid and propanoic acid are Brønsted-Lowry acids that lose a proton in water to form a hydronium ion and a carboxylate ion.

💡Base

A base, as per the Brønsted-Lowry definition used in the video, is anything that accepts a proton. Bases are integral to acid-base chemistry and play a critical role in understanding how reactions occur. In the video, water is mentioned as a base that can accept a proton from carboxylic acids, forming a hydronium ion and a carboxylate ion.

💡pKa

pKa is the negative logarithm of the acid dissociation constant (Ka) and is used to express the strength of an acid. A lower pKa indicates a stronger acid, while a higher pKa indicates a weaker acid. The video uses pKa to compare the strength of different acids, such as hydrochloric acid, propanoic acid, acetic acid, and ethanol.

💡Resonance Stabilization

Resonance stabilization refers to the delocalization of electrons across multiple atoms, which can stabilize a molecule. In the video, the concept is used to explain why acetic acid is a stronger acid than ethanol. The acetate ion, formed when acetic acid loses a proton, can distribute the negative charge over two oxygen atoms through resonance, making it more stable.

💡Conjugate Acid

A conjugate acid is the species formed when a base accepts a proton. The video explains that when water acts as a base and accepts a proton from carboxylic acids, it forms a hydronium ion, which is the conjugate acid of water.

💡Conjugate Base

A conjugate base is the species formed when an acid loses a proton. The video uses the example of acetic acid, which forms the acetate ion as its conjugate base when it loses a proton.

💡Electronegativity

Electronegativity is the ability of an atom to attract electrons towards itself in a chemical bond. The video mentions that more electronegative elements can stabilize negative charges better, which is a key factor in determining the acidity of a molecule.

💡Inductive Effect

The inductive effect is the influence of electronegative atoms on the distribution of electron density within a molecule. The video explains how the presence of electronegative fluorine atoms in trifluoroacetic acid stabilizes its conjugate base more than acetic acid, making it a stronger acid.

💡Hybrid Orbitals

Hybrid orbitals are atomic orbitals that are mixed to form new orbitals suitable for bonding in molecules. The video discusses how the s character of hybrid orbitals affects acidity, with sp hybridized orbitals (found in alkynes) being better at stabilizing negative charges than sp2 or sp3 orbitals.

💡Phenol

Phenol is a compound with a hydroxyl group (-OH) attached to a benzene ring. The video uses phenol to illustrate how resonance stabilization in its conjugate base, phenoxide, allows the negative charge to be delocalized over the benzene ring, making phenol more acidic than cyclohexanol.

💡Ethanol

Ethanol is an alcohol with the formula C2H5OH. The video compares ethanol to acetic acid, noting that ethanol is a weaker acid because its conjugate base, ethoxide, lacks resonance stabilization and thus bears the full burden of the negative charge when a proton is removed.

Highlights

Crash Course Organic Chemistry app is available for Android and iOS.

Introduction to Crash Course Organic Chemistry by Deboki Chakravarti.

Sci-fi movie Alien's xenomorph has acidic blood, stronger than Earth creatures' acids.

Tawny ants use formic acid, which is relatively weak compared to xenomorph's blood.

Importance of predicting the strength of weak acids and bases in organic chemistry.

Introduction to Brønsted-Lowry definition of acids and bases.

Explanation of how to use arrows to push electrons between molecules.

Definition of a Brønsted-Lowry acid as a proton loser and a base as a proton acceptor.

Example of carboxylic acids dissolving in water to form hydronium and carboxylate ions.

Introduction to the acid dissociation constant (Ka) and its role in predicting acidity.

Explanation of how a large Ka indicates a strong acid and a small Ka indicates a weak acid.

Conversion of Ka to pKa for easier comparison of acid strengths.

Examples comparing hydrochloric acid, propanoic acid, and acetic acid's pKa values.

Explanation of how resonance stabilization affects acidity, using acetic acid and ethanol as examples.

Comparison of phenol and cyclohexanol's pKa values and their conjugate bases' stability.

Importance of atom identity in stabilizing negative charge and affecting pKa.

Role of atom polarizability in stabilizing negative charges and its effect on acidity.

Comparison of phenol and thiophenol to illustrate the effect of atom size on acidity.

Explanation of the inductive effect and its influence on acidity, using acetic acid and trifluoroacetic acid as examples.

Importance of the s character of hybrid orbitals in stabilizing negative charge and its effect on pKa.

Comparison of ethane, ethene, and ethyne to demonstrate the effect of hybrid orbitals on acidity.

Summary of four key factors that predict relative acidity in organic compounds.

Teaser for the next episode, which will focus on forming covalent bonds.

Transcripts

You can review content from Crash Course Organic Chemistry with the Crash Course app, available now for Android and iOS devices.

Hi! I’m Deboki Chakravarti and welcome to Crash Course Organic Chemistry!

A fun little detail in that sci-fi movie Alien -- you know, the one with Sigourney Weaver

-- is how the xenomorph has acidic blood.

When the humans cut one open, its blood eats right through the metal floor and leaves a hole.

It's creepy!

Xenomorph blood is way stronger than most acids produced by Earth creatures.

For example, Tawny ants produce formic acid to protect themselves from the basic venom of fire ants.

But formic acid is relatively weak, kind of like vinegar or lemon juice -- which definitely won’t eat through a spaceship.

As organic chemists, we need to know how to predict the strength of weak acids and bases.

Not just to dream up cool (and scientifically accurate) aliens, but because acids and bases are molecular hot spots where reactions can take place.

So let's add acid-base chemistry to our toolkit for predicting chemical reactions.

[Theme Music]

Part of what makes acid-base chemistry tricky is that there are different but overlapping definitions of acids and bases.

Arrhenius, Brønsted, Lowry, and Lewis all had their opinions on how an acid and base should be defined.

For this video, we’ll stick with the Brønsted-Lowry definition proposed in 1923.

And to help understand this definition, we’ll need to push some electrons around.

Last episode, we learned how to use arrows to push electrons within a molecule to understand resonance structures.

But we can also push electrons between molecules to show how bonds break and form using the same rules:

we start on electrons and point to where a new bond is made.

In the Brønsted-Lowry definition, an acid is anything that loses a proton (also known as a plus-one-charged hydrogen ion).

And a base is anything that accepts a proton.

Carboxylic acids, like acetic acid and propanoic acid, are Brønsted-Lowry acids.

When we dissolve these acids in water, the water acts as a base and removes a proton to form a hydronium ion and the corresponding carboxylate ion.

We say that hydronium is the conjugate acid of water, and the carboxylate is the conjugate base of the carboxylic acid.

In organic chemistry, we want to know how readily a molecule will gain or lose a proton.

This is a physical property, like boiling point or melting point, that we describe with Ka -- the acid dissociation constant.

Hank talks more about Ka in episode 30 of Crash Course Chemistry if you want to start with the basics.

But essentially, it describes the relationships between products and reactants when the rate of the forward reaction is equal to the reverse reaction -- in other words, when the reaction has reached equilibrium.

The size of the Ka tells us if we have more products or reactants.

If the Ka is large, the equilibrium favors the product side, and the molecule is a strong acid -- one that's very willing to get rid of a proton.

If the Ka is small, then the equilibrium favors the reactants, so the molecule is a weak acid -- one that's a little less willing to get rid of a proton.

So that logic is all fine and good, but it can be a little hard to tell whether the Ka is small or large from looking at a number like 1.8x10-5 .

We can get rid of the exponents and make these numbers more manageable by taking the negative log of a Ka to get pKa.

Just like the concentration of protons becomes pH with a little math, an acid’s equilibrium constant becomes pKa.

We can use pKa to compare two acids.

A lower pKa means the equilibrium prefers the product side and a molecule is a stronger acid.

And a higher pKa means the equilibrium prefers the reactant side and a molecule is a weaker acid.

Let's look at a couple examples.

We can see that hydrochloric acid has a negative pKa (which is REALLY low).

We consider it completely dissociated into products, so it's a strong acid.

Propanoic acid and acetic acid have similar pKa values, which makes sense because propanoic acid only has one extra CH2 group.

They're both weak acids.

But the pKa of ethanol is dramatically different from acetic acid, even though they're both losing a proton from an OH group.

That's the key though: they're similar, but not the same, when it comes to charge distribution.

When acetic acid loses a proton, it forms its conjugate base, acetate.

And remember last episode, we talked about the resonance structures of an acetate ion, and how the negative charge is spread out over its two oxygen atoms.

This is like going backpacking with a group of friends and splitting up gear.

Like, instead of carrying everything yourself, you take the tent, another friend takes the cooking gear, and another takes the food.

Distributing the weight means you all carry some of the burden.

The same is true here: distributing the negative charge over the two oxygen atoms makes it easier for the acetate ion to carry the burden of the negative charge.

Because of the resonance stabilization in the conjugate base, it’s not too tough for acetic acid to lose a proton.

On the other hand, when ethanol loses a proton, it forms the conjugate base ethoxide.

Ethoxide doesn't have any resonance structures, which means it doesn't have friends to share camping gear with.

So its oxygen atom is feeling the full burden of the negative charge.

This makes ethanol a weaker acid than acetic acid.

But, since it’s pretty tough to take the proton off of ethanol in the first place, if we do form the conjugate bases, ethoxide is a much stronger base than acetate.

Another great way to see how resonance stabilization can affect acidity is to consider two kinda similar ring compounds: phenol and cyclohexanol.

Phenol’s pKa is about 10 and cyclohexanol’s pKa is 16.

In cyclohexanol's conjugate base, that negative charge is stuck on oxygen, because there are no double bonds or other oxygens to share the burden.

So, like ethoxide, the conjugate base is less stable and really wants a proton back.

But in phenol's conjugate base, phenoxide, the negative charge can be pushed around the benzene ring to make four different resonance structures.

This stabilizes the conjugate base, so phenol is more acidic.

Resonance stabilization is one of four major factors that help us understand the role of pKa in our reactions.

Another key is the atom that loses the proton.

Within a row on the periodic table, more electronegative elements stabilize negative charge better, and within a group, larger elements form more stable conjugate bases.

Bigger atoms have more electrons, which end up in orbitals that are pretty diffuse.

So with bigger atoms, the electron cloud is easily smeared out and distorted, a property we call polarizability.

Imagine a cup of water as the electron density on a small atom.

If we pour the water into a frisbee, which is like a large atom, it has more space to move around and spread out.

Atom polarizability affects acidity and pKa, because this smeared-out-ness stabilizes negative charges.

It’s always useful to look at an example, so let's go back to phenol and compare it with its closely-related cousin thiophenol, which has a sulfur atom instead of an oxygen.

The only difference in structure is the size of the sulfur atom compared to the size of the oxygen atom.

To be precise, sulfur has 8 more electrons than oxygen.

So in thiophenol's conjugate base, electrons are more smeared out on the polarizable sulfur atom, stabilizing the conjugate base.

And the more stable the conjugate base, the more acidic the acid, making thiophenol more acidic than phenol.

The third key to pKa is hidden in covalent bonds.

The inductive effect has to do with electronegativity throughout a molecule, with more electronegative atoms pulling the negative charges toward them through bonds.

As an example, let’s compare two other very similar compounds: acetic acid and trifluoroacetic acid.

Both of their conjugate bases have resonance stabilization across the two oxygen atoms.

But the resonance stabilization is all that acetate has going for it, giving acetic acid its pKa of 4.76.

Now, trifluoroacetate also contains fluorine, an electronegative atom.

Those three fluorines pull on the negative charge and stabilize the conjugate base even more.

Sort of like having even more backpacking buddies who carry the poles and other pieces of the tent for you!

So the products side of the equilibrium is favored, and the pKa of trifluoroacetic acid is 0.23!

The last, and least powerful, key to understanding pKa also has to do with orbital shapes…..specifically the s character of hybrid orbitals.

Remember in episode 4, we talked about orbitals, places where we’re most likely to find electrons around atoms.

And we talked about orbitals combining when atoms bond to form hybrid orbitals.

The three most common hybrid orbitals for organic compounds are sp3, sp2, and sp, and this is approximately what they look like.

sp hybrid orbitals can be thought of as being 50% s-orbital and 50% p-orbital.

So we can say that they have more s character than sp2 and sp3 hybrid orbitals, which combine additional p orbitals into the mix.

So, sp orbitals are more similar to a plain old s orbital.

These shapes means their electrons are closer to the nucleus, and the atom can stabilize a negative charge better.

By contrast, sp2 and sp3 hybrid orbitals hold the electrons a little farther away from the nucleus and the atom doesn’t stabilize a negative charge as well.

To look at this in action, let's compare the pKa of ethane, ethene, and ethyne.

The carbons in ethyne have sp hybridized orbitals, which means the conjugate base has more s character and can stabilize the negative charge better.

So ethyne is the most acidic of this trio -- but, to be totally realistic, a pKa of 25 is not very acidic at all in the grand scheme of things and will definitely not be eating through spaceships or anything like that.

We can see the importance of the s character of hybrid orbitals by looking at the acidity of different protons within a molecule.

For example, this molecule has an alkene at one end (with the protons highlighted in blue) and an alkyne at the other (with a proton highlighted in red).

So which of these two protons is more acidic?

By pulling off a proton from each end of the molecule, we can create two different conjugate bases with a negative charge in different places.

Using what we know about hybrid orbitals, the alkene end has an sp2 hybridized carbon, and the alkyne end has an sp hybridized carbon.

That means the alkyne end has more s character and can stabilize the negative charge better, so that conjugate base is more stable, and that proton is more acidic.

So throughout this episode, we've learned four key things that we can use to predict relative acidity.

Each of these factors stabilizes negative charge in a conjugate base, which makes the corresponding acid more acidic.

Number 1: Atom identity -

More electronegative and larger elements stabilize charge better

Number 2: Resonance stabilization -

If we can draw multiple Lewis structures for conjugate bases, they are more stable

Number 3: The inductive effect -

Electronegative atoms can pull negative charge toward themselves through covalent bonds

And Number 4: The s character of the orbital

More s character stabilizes negative charge better

Acidity is definitely tricky, but it's an important part of organic compounds and will help us predict the products of chemical reactions.

In our next episode we’ll start to use all of the tools we’ve learned so far, and start forming covalent bonds at our molecular hotspots!

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