Polarity, Resonance, and Electron Pushing: Crash Course Organic Chemistry #10

Summary

This Crash Course Organic Chemistry video explores electronegativity and its impact on molecular polarity. Deboki Chakravarti explains how electronegativity differences lead to polar covalent bonds and result in polar molecules like water, which have distinct charge regions. The video contrasts this with nonpolar molecules like carbon dioxide, highlighting the importance of molecular structure in chemical reactions. It also delves into resonance structures, showing how electrons are pushed in molecules to form different arrangements, affecting reactivity and stability.

Takeaways

• 🧲 In magnetism and organic chemistry, 'opposites attract' due to polarity and electronegativity.
• 🔬 Electronegativity is a measure of how much an atom attracts electrons in a bond, with fluorine being the most electronegative element.
• 🔗 A significant electronegativity difference between atoms results in ionic bonds, a small difference in nonpolar covalent bonds, and a medium difference in polar covalent bonds.
• 💧 The water molecule exemplifies a polar molecule with a bent shape and a dipole moment due to the unequal sharing of electrons between hydrogen and oxygen.
• 🌀 Carbon dioxide, despite having polar covalent bonds, is a nonpolar molecule due to its symmetrical linear shape which cancels out the dipole moments.
• 📊 Electronegativity differences create molecular hotspots for chemical reactions, as seen in 1-chloropropane which is slightly polar.
• 🔬 Resonance structures represent different possible arrangements of electrons in a molecule, and resonance hybrids are a blend of these structures.
• ➡️ Electron pushing, or arrow pushing, is a method used to track the movement of electrons in chemical reactions, avoiding the need to memorize each reaction.
• 📚 Resonance structures are prioritized based on neutrality, octet rule adherence, and the placement of charges on more electronegative elements.
• 🔮 Understanding electronegativity and resonance is crucial for predicting the behavior of molecules in organic chemistry.

Q & A

What is the relationship between 'opposites attract' in magnets and organic chemistry?

In magnets, 'opposites attract' is related to polarity and physics, while in organic chemistry, it refers to the attraction between opposite charges in polar molecules.

What is electronegativity and how does it relate to atomic bonding?

Electronegativity is an atomic property that measures the tendency of an atom to attract electrons in a bond. It helps determine the type of bond formed between atoms: a large difference indicates an ionic bond, a small difference indicates a nonpolar covalent bond, and a moderate difference indicates a polar covalent bond.

Who developed the relative electronegativity scale and what does it rank?

American chemist Linus Pauling developed the relative electronegativity scale, which ranks elements from most electronegative (fluorine) to least.

How does the electronegativity difference between hydrogen and oxygen in a water molecule affect its polarity?

The electronegativity difference of about 1.4 between hydrogen and oxygen in a water molecule places it in the 'polar covalent bond' zone, resulting in a polar molecule with a bent shape and regions of partial positive and negative charges.

What is a dipole and how is it represented?

A dipole is a molecule with a concentration of positive charge on one side and negative charge on the other, resulting in a lopsided charge distribution. It is represented with an arrow, where the arrowhead points to the negative side and the cross represents the positive side.

Why is carbon dioxide considered a nonpolar molecule despite having polar covalent bonds?

Carbon dioxide is nonpolar because its linear shape causes the polar covalent bonds to be symmetrically arranged, resulting in no net dipole moment.

How does the electronegativity difference between carbon and chlorine in 1-chloropropane affect its polarity?

The larger electronegativity difference between carbon and chlorine in 1-chloropropane results in a polar covalent bond, making the molecule slightly polar with a molecular hotspot for chemical reactions.

What is the significance of understanding electron pushing or arrow pushing in organic chemistry?

Understanding electron pushing helps avoid memorizing every reaction by learning guiding principles about how electrons move, allowing chemists to make educated guesses about reaction products.

What are resonance structures and how are they represented?

Resonance structures are different representations of a compound that differ in the placement of lone pairs and pi bonds. They are represented with a resonance arrow, indicating multiple valid structures.

How do you determine which resonance structures contribute more to the resonance hybrid?

Resonance structures contribute differently to the resonance hybrid based on factors like neutrality preference, maintaining an octet on oxygen and nitrogen, and placing charges on more electronegative elements.

What is a carbocation and how is it represented in resonance structures?

A carbocation is a carbon with a positive charge, represented by a p orbital with zero electrons. Its resonance structures are found by swapping electron positions to maintain the octet rule and formal charges.

Outlines

00:00

🔬 Electronegativity and Polar Molecules

In this segment, Deboki Chakravarti introduces the concept of electronegativity and its role in determining the type of chemical bonds formed between atoms. Electronegativity is defined as the ability of an atom to attract electrons in a bond, with fluorine being the most electronegative element. The difference in electronegativity between atoms dictates the bond type: a large difference results in ionic bonds, a small difference in nonpolar covalent bonds, and a moderate difference in polar covalent bonds. The example of water is used to illustrate how electronegativity leads to polar covalent bonds and the formation of a molecular dipole due to its bent shape. In contrast, carbon dioxide, despite having polar covalent bonds, is a nonpolar molecule because of its linear shape, which evenly distributes the charge. The importance of understanding electronegativity and molecular dipoles in organic chemistry is emphasized as it influences how molecules react with each other.

05:04

🧠 Electron Pushing and Resonance Structures

This section delves into the concept of resonance structures and how they are used in organic chemistry. Resonance structures are different ways of representing the distribution of electrons in a molecule, particularly useful for understanding the stability and reactivity of molecules. The process of 'electron pushing' or 'arrow pushing' is introduced as a method to track the movement of electrons during chemical reactions without needing to memorize each reaction. The example of the acetate ion is used to demonstrate how to draw resonance structures and how to use curved arrows to represent the movement of electron pairs. The segment also explains the concept of formal charges and how they relate to the distribution of electrons around an atom. The importance of understanding resonance structures is highlighted as it provides insights into the behavior of molecules in chemical reactions.

10:04

📚 Resonance Hybrids and Their Significance

The final paragraph focuses on resonance hybrids, which are average representations of a molecule that blend different resonance structures. It explains that not all resonance structures contribute equally to the resonance hybrid, and provides guidelines to determine which structures are more significant. These guidelines include preferring neutral forms, maintaining an octet on oxygen and nitrogen, and placing charges on more electronegative elements. The segment uses examples to illustrate these principles and emphasizes the importance of understanding resonance hybrids in predicting the properties of molecules. It concludes with a teaser for the next episode, which will explore how resonance structures relate to the strengths of organic acids and bases.

Keywords

💡Electronegativity

Electronegativity is a measure of the tendency of an atom to attract a bonding pair of electrons. It is a fundamental concept in understanding how atoms bond and influence the polarity of molecules. In the video, electronegativity is used to explain how atoms with different electronegativities can form ionic, polar covalent, or nonpolar covalent bonds. For instance, the difference in electronegativity between hydrogen and oxygen in a water molecule results in a polar covalent bond.

💡Polar Molecules

Polar molecules are those where the centers of positive and negative charge do not coincide, leading to an uneven distribution of charge. The video uses water as an example of a polar molecule, where the oxygen atom pulls electrons more strongly than the hydrogen atoms due to its higher electronegativity, creating a molecule with a bent shape and a dipole moment.

💡Dipole

A dipole refers to a separation of charge within a molecule, resulting in two poles where one end is slightly negative and the other is slightly positive. The video explains that water has a dipole due to its bent shape and the difference in electronegativity between oxygen and hydrogen, which creates regions of different charge.

💡Ionic Bond

An ionic bond is a type of chemical bond formed by the electrostatic attraction between oppositely charged ions. The video mentions that a large difference in electronegativity between two atoms leads to the formation of an ionic bond, as one atom donates its electron to the other.

💡Nonpolar Covalent Bond

A nonpolar covalent bond is a type of covalent bond where the electrons are shared equally between the two atoms because the electronegativity difference is very small. The video contrasts this with polar covalent bonds, stating that when electronegativity differences are small, nonpolar covalent bonds form.

💡Polar Covalent Bond

A polar covalent bond is a type of covalent bond where the electrons are shared unequally between the two atoms due to a difference in electronegativity. The video uses the water molecule as an example, where the oxygen-hydrogen bond is polar covalent because oxygen is more electronegative than hydrogen.

💡Resonance Structures

Resonance structures are different ways of drawing the same molecule that have the same arrangement of atoms but different arrangements of electrons. The video explains that resonance structures represent different possible distributions of electrons, especially in molecules like acetate, where electrons can be delocalized over multiple atoms.

💡Resonance Hybrid

A resonance hybrid is a concept in chemistry that describes the actual structure of a molecule as an average of different possible resonance structures. The video uses the example of the acetate ion to illustrate how the actual molecule is a blend of its resonance structures, with bond lengths that are intermediate between single and double bonds.

💡Electron Pushing

Electron pushing, also known as arrow pushing, is a method used in organic chemistry to track the movement of electrons in chemical reactions. The video emphasizes the importance of understanding how electrons move between atoms during reactions, which is crucial for predicting the products of organic reactions.

💡Formal Charge

Formal charge is a way of keeping track of the distribution of electrons in a molecule by assigning charges to individual atoms. The video explains how to calculate formal charges and uses them to understand the distribution of electrons in resonance structures, such as in the case of the acetate ion.

💡Carbocation

A carbocation is an ion with a positively charged carbon atom. The video discusses carbocations as an example of resonance structures, where the positive charge can be delocalized over different atoms, illustrating the concept of resonance and its impact on the stability of the molecule.

Highlights

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

The concept of 'opposites attract' is explored in both human relationships and magnetism, with a segue into organic chemistry.

In polar molecules, opposite charges attract each other, which is a fundamental concept in understanding molecular polarity.

Electronegativity is introduced as a key atomic property that influences how atoms attract electrons in a bond.

Linus Pauling's relative electronegativity scale is mentioned, which is crucial for understanding atomic bonding.

The difference in electronegativity between atoms can result in ionic, nonpolar covalent, or polar covalent bonds.

The water molecule is used as an example to explain how electronegativity leads to partial charges and molecular polarity.

The molecular shape of water is described as bent, contributing to its dipole moment and polar nature.

Carbon dioxide is contrasted with water to illustrate a molecule with polar covalent bonds but a nonpolar overall structure.

Electronegativity is highlighted as essential for understanding molecular reactivity in organic chemistry.

1-chloropropane is used to demonstrate how electronegativity differences create polar and nonpolar regions within a molecule.

The concept of electron pushing or arrow pushing is introduced as a method for tracking electrons in chemical reactions.

The acetate ion is used to explain the concept of resonance structures and how to draw them.

Formal charges are defined and calculated for atoms within a molecule.

Resonance hybrids are described as a blend of resonance structures, with a focus on stability and electron distribution.

Guidelines for determining the contribution of resonance structures to a hybrid are provided.

The importance of practicing electron pushing for understanding organic chemical reactions is emphasized.

A carbocation is used to demonstrate resonance structures and the process of electron pushing.

The episode concludes with a teaser for the next topic: combining electron pushing with acid-base chemistry.

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!

Whether we’re talking about romantic relationships or magnets, there's that cliche: “opposites attract.”

For people, well, it’s complicated.

Some psychologists say it’s because we seek what we don’t have, while others say it’s not really a thing.

So, who knows.

But for magnets, “opposites attract” has to do with polarity and physics.

And that goes for other science too, like organic chemistry.

In polar molecules, opposite charges do attract, and positive and negative regions of molecules are drawn together.

To understand polar molecules, we have to understand their reactive sites by thinking critically about those negatively-charged particles: electrons.

[Theme Music]

When atoms are bonded together, they don't necessarily share electrons equally.

One element could be a little greedier than another.

Electronegativity is the atomic property that helps us think about how much one atom will attract electrons in a bond, compared to other atoms.

Because electronegativity depends on two atoms in a relationship with each other, the electronegativity of a specific element can vary a little,

depending on what chemical compound it's a part of.

That gets a little complicated, but thankfully, scientists of the past have done a lot of work for us.

American chemist Linus Pauling developed a relative electronegativity scale, which ranks the elements from most electronegative (which is fluorine) to least.

We can use Pauling’s scale to determine a couple things about molecules.

First, electronegativity can tell us about atomic bonding, which is exactly what it sounds like: the bonds that hold atoms together.

A big difference in electronegativity between two atoms means an ionic bond, a small difference means a nonpolar covalent bond, and everything in between is a polar covalent bond.

For example, the electronegativity difference between a hydrogen atom and an oxygen atom in a water molecule is about 1.4, in the "polar covalent bond"-zone.

Second, electronegativity can tell us about regions of charge in molecules.

Sticking with our water molecule example, oxygen is quite electronegative and attracts electrons through its bonds, making it partially negative.

The hydrogens have lower electronegativity and with oxygen stealing electrons away, they're partially positive.

We can draw these partial charges with the lowercase Greek letter delta and a plus or minus sign.

Now, we also know that a water molecule has a bent molecular shape.

So one side of the molecule has a concentration of negative charge, while the other is positive.

We call this a dipole, two regions with a lopsided charge distribution.

To show a dipole, we use a little arrow.

The arrow head represents the negative side of the dipole, while the cross represents the positive side of the dipole.

It looks like a little plus sign, that’s how I remember!

So, water contains two polar covalent bonds.

And because of its bent shape it also has a dipole and is considered a polar molecule, one with two clear regions of different charge.

To really anchor these ideas about electronegativity, let’s look at another molecule: carbon dioxide, you know, the gas that plants use during photosynthesis.

If we draw the structure of CO2, we can see that its electron pair geometry and molecular shape are both linear.

If we look at Pauling’s electronegativity scale, we can see that carbon and oxygen have a difference in electronegativity of about 1.

Both bonds are polar covalent and the oxygen has a partial negative charge because of its greater electronegativity, while the carbons are partially positive.

So, is carbon dioxide polar?

Because the electrons are being pulled away from each other equally and symmetrically, this particular tug-of-war isn't lopsided, and carbon dioxide doesn’t have a dipole.

Therefore it isn’t polar!

It’s a nonpolar molecule.

To be clear, in a carbon dioxide molecule, the carbon-oxygen bonds are polar covalent, but the whole molecule is nonpolar.

Electronegativity is another key piece of our organic chemistry toolbox.

It reinforces the core idea we’ve been learning:

that the structure of molecules informs what they can do and how they react with other chemicals.

Organic chemists need to have a working knowledge of the types of bonds and electronegativity, even though we don't calculate it every single time.

So let’s get comfier with dipoles and continue building our chemical intuition by looking at 1-chloropropane.

From Pauling's electronegativity scale, we can see that the electronegativity difference between the carbons and hydrogens on most of the chain is really small,

so those are all nonpolar covalent bonds.

But there's a bigger electronegativity difference between carbon and chlorine, so that’s a polar covalent bond.

We have a partial positive charge on the carbon and a partial negative charge on the chlorine,

which means the electrons of the carbon-chlorine bond are being pulled unequally in that one spot.

And that means… we have a dipole, folks.

This molecule has a region that is polar and other regions that are non-polar.

But since it has a polar covalent bond, it's a slightly polar molecule.

It's definitely not as polar as water based on the difference in electronegativity and that three carbon chain, but it's more polar than carbon dioxide.

And the polar end creates a molecular hotspot, which is raring to go for chemical reactions.

In fact, whether we’re talking about reactions caused by cleaning products or carried out in plastic factories…

electronegativity and molecular dipoles are often involved.

Positive regions on a molecule can attract negative regions on another, and chemical bonds can be made and broken.

As we get into reactions, a big leap we have to take from general chemistry to organic chemistry is keeping track of electrons and moving them really precisely,

which we call electron pushing or arrow pushing.

If we can learn and remember some guiding principles about how electrons like to move, we can look at any chemical reaction and make reasonable guesses about its products.

Basically, practicing pushing electrons will help us avoid memorizing every single reaction in organic chemistry.

You might want to grab a pencil and a really good eraser, because mistakes will happen and electron pushing is a lot of trial and error at first.

All chemists have to start somewhere!

We'll start by looking at acetate, the anion of acetic acid.

I'll draw it out with all lone pairs of electrons, because that'll help us keep track of them this whole time.

One of the oxygens has one bond and three lone pairs of electrons, giving it a formal negative charge.

Remember, formal charges are the difference between a neutral atom’s valence electrons and the total electrons surrounding the atom in the molecule.

So in this case, a neutral oxygen atom has 6 valence electrons.

The single-bonded oxygen we're looking at has 6 electrons in lone pairs, which counts as 6 charges.

It also has 1 bond, which counts as 1 charge because the electrons are shared between two atoms.

6 - 7 is -1, which is the formal negative charge!

But we can also draw another valid Lewis structure for acetate, where the other oxygen atom has the single bond and formal charge.

These drawings are called resonance structures, which are representations of the compound that differ, in this case, in the placement of lone pairs and pi bonds.

We indicate multiple resonance structures with a resonance arrow, which is an arrow with arrowheads on both sides.

And we're not flipping the acetate ion around to get its two resonance structures — the atoms stay in place.

We’re actually changing where we show the electrons between the two oxygen atoms.

Drawing resonance structures can help us understand where the electrons could be, but in real life, things are a little mushier.

All molecules are a blend of their resonance forms, which we call a resonance hybrid.

The resonance hybrid is more stable than any individual resonance form, but it’s really hard to draw partial bonds, so we usually pick one form to draw at a time.

And we know resonance hybrids are a thing because we've measured bond lengths in these molecules.

For example, in an acetate ion, both carbon-oxygen bonds are shorter than average single bonds and longer than average double bonds.

The charge and pi bond are being equally shared across all three atoms.

In fact, a common theme with resonance structures is electrons shared across the p-orbitals of three atoms.

So even though identifying resonance forms can be really tricky, we can approach these puzzles three atoms at a time.

To start, we need to find a pi bond, and an adjacent p orbital with 0, 1, or 2 electrons in it.

These three atoms are involved in resonance, so let’s number and box them.

Now, whatever is at atom 1 moves to atom 3, and the stuff at 3 goes to 1.

In this case, our double bond moves to atom 3 and our extra lone pair and negative charge moves to atom 1.

Now that we have two different resonance structures as our beginning and end, we're ready to fill in the middle of the puzzle by pushing electrons.

In this episode, we’ll only be pushing pairs of electrons, which we represent using a curved arrow.

This arrow always starts on a pair of electrons, and points to where they end up.

To get started, let’s show how a lone pair of electrons on oxygen becomes a double bond.

We’ll start our arrow on that pair of electrons, and point it to where the double bond forms.

When we move a lone pair off an atom, it has to form a neighboring pi bond.

But we can’t stop here!

This would give a carbon with 5 bonds, which is not organically acceptable because it exceeds carbon’s maximum valence electrons.

We have to keep pushing electrons until we have a molecule that follows rules we've already learned about the maximum number of electrons and bonds.

We're doing science here, not science fiction.

So, next, let's draw an arrow to move a pair of electrons away from that double bond to a neighboring atom, which means our lucky recipient is the other oxygen atom.

The middle of the puzzle is filled in, and we've shown how our two resonance structures are connected!

An important thing to notice is that this whole time we only moved electrons in double bonds or lone pairs.

Electrons in single bonds don’t go anywhere.

We're going to have to understand resonance structures of many different molecules, so let’s try another example.

Here we have a carbon with a positive charge, called a carbocation.

We’ll start by finding our pi bond, and an adjacent p orbital with 0, 1, or 2 electrons in it.

The positive charge on carbon is a p orbital with zero electrons—let’s number our three atom unit.

Now, let’s swap what we have at atoms 1 and 3 to find our other resonance structure:

Nice. Let’s see if we can move between these structures with arrow pushing.

We always begin our arrows on electrons, so I’ll start my arrow on the double bond.

This moves the electrons away from that carbon and gives it a formal positive one charge.

And we’ve done it!

In both of these examples, the two resonance structures are very similar and contribute the same amount to the resonance hybrid.

But that's not always the case for molecules with resonance structures, like these two:

Resonance hybrids are usually more like a weighted average, sort of like grades.

Big tests might count more than weekly quizzes toward your final letter grade in a class, and some resonance structures matter more than others.

Here are a few key guidelines to figure out which resonance structures contribute more to the resonance hybrid.

Number 1: Neutral resonance forms are preferred (when possible)

So, for example, this:

is better than this:

Number 2: Keep an octet of electrons on oxygen and nitrogen (but carbon can be short)

And number 3: Negative charges are preferred on more electronegative elements, and positive charges are preferred on less electronegative elements.

For example, both of these resonance forms have an octet, but one has the negative charge on the more electronegative element.

So structure A is our winner, and contributes more to the resonance hybrid.

If pushing electrons is still sort of brain-bendy, don’t worry.

We’ll be doing plenty more of it for practice.

In this episode, we learned that:

Electronegativity describes how atoms attract electrons differently, which can lead to dipoles in some molecules

Resonance structures have different arrangements of electrons

And resonance hybrids are a mixture of resonance structures

Next time we’ll combine electron pushing with acid-base chemistry, and learn what resonance structures can tell us about the strengths of organic acids and bases.

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