IR spectra for hydrocarbons | Spectroscopy | Organic chemistry | Khan Academy

Summary

This video script delves into the analysis of carbon-hydrogen bonds in IR spectra, highlighting how bond wavenumbers vary with carbon hybridization states. It explains that sp-hybridized carbon-hydrogen bonds appear at 3300 wavenumbers, sp2 at 3100, and sp3 at 2900. The script guides viewers through interpreting IR spectra of hydrocarbons, including alkanes, alkynes, alkenes, and arenes, focusing on the diagnostic region and distinguishing features like bond strengths and vibration frequencies.

Takeaways

• 🔍 The wavenumber for a carbon-hydrogen bond stretch in an IR spectrum varies based on the hybridization state of the carbon atom involved.
• 🌉 For sp-hybridized carbon, the C-H bond stretch appears around 3300 wavenumbers, which is higher than for other hybridizations due to the bond's strength and shortness.
• 📏 The sp2-hybridized carbon's C-H bond stretch signal is found around 3100 wavenumbers, indicating a medium bond strength.
• 💠 With sp3-hybridized carbon, the C-H bond stretch is observed at approximately 2900 wavenumbers, reflecting the longest and weakest bond among the three hybridizations.
• 📊 Drawing a line at 3000 wavenumbers is a useful practice when analyzing IR spectra, as it helps to quickly identify the region where sp3-hybridized C-H bonds are expected to show their stretch.
• 🔬 The strength of a bond is directly related to the wavenumber at which its stretch appears in an IR spectrum; stronger bonds result in higher wavenumbers.
• 📚 In alkanes like decane, the IR spectrum is simple, with signals primarily below 3000 wavenumbers, indicating only sp3-hybridized C-H bonds.
• 🔎 In 1-octyne, the presence of an sp-hybridized C-H bond is indicated by a signal near 3300 wavenumbers, and a carbon-carbon triple bond stretch is seen at around 2100 wavenumbers.
• 📈 The diagnostic region of an IR spectrum (below 3000 wavenumbers) is particularly useful for identifying the type of carbon-hydrogen bonds present in a molecule.
• 🌐 Comparing IR spectra of different hydrocarbons, such as alkanes, alkenes, alkynes, and arenes, can reveal the presence of various hybridized carbons and functional groups like double and triple bonds.

Q & A

What is the approximate wavenumber for a carbon-hydrogen bond stretch in an IR spectrum?

The approximate wavenumber for a carbon-hydrogen bond stretch is a little bit over 3000 wavenumbers.

How does the hybridization state of carbon affect the wavenumber of a carbon-hydrogen bond stretch in an IR spectrum?

The wavenumber of a carbon-hydrogen bond stretch depends on the hybridization state of the carbon. For sp-hybridized carbon, the wavenumber is around 3300; for sp2-hybridized carbon, it's around 3100; and for sp3-hybridized carbon, it's around 2900.

What percentage of s character does an sp-hybridized orbital have?

An sp-hybridized orbital has 50% s character.

Why is the carbon-hydrogen bond where the carbon is sp-hybridized stronger than the one where the carbon is sp2- or sp3-hybridized?

The carbon-hydrogen bond where the carbon is sp-hybridized is stronger because the electron density is closest to the nucleus due to the higher percentage of s character in sp orbitals, making the bond shorter and stronger.

How does bond strength relate to the wavenumber in an IR spectrum?

A stronger bond has a higher force constant (k), which increases the frequency of bond vibrations, leading to a higher wavenumber in the IR spectrum.

What is the significance of drawing a line at 3000 wavenumbers when analyzing an IR spectrum of hydrocarbons?

Drawing a line at 3000 wavenumbers helps to distinguish between carbon-hydrogen bond stretches associated with sp3-hybridized carbons (below 3000) and those associated with sp2- or sp-hybridized carbons (above 3000).

What is the approximate wavenumber for the carbon-hydrogen bond stretch in decane, which is an alkane?

In decane, which only contains sp3-hybridized carbons, the carbon-hydrogen bond stretch appears under 3000 wavenumbers.

What is the approximate wavenumber for the carbon-hydrogen bond stretch where the carbon is sp-hybridized, as seen in 1-octyne?

The carbon-hydrogen bond stretch where the carbon is sp-hybridized, as in 1-octyne, appears around 3300 wavenumbers.

What is the characteristic wavenumber range for the carbon-carbon triple bond stretch in an alkyne?

The carbon-carbon triple bond stretch in an alkyne typically appears around 2100 wavenumbers.

How can the presence of an aromatic ring affect the wavenumber of a carbon-carbon double bond stretch in an IR spectrum?

In the presence of an aromatic ring, the carbon-carbon double bond stretch usually appears at a lower wavenumber than a typical aliphatic double bond, typically between 1600 to 1450 wavenumbers.

What is the difference in wavenumber between the carbon-hydrogen bond stretch of sp2-hybridized carbon and sp3-hybridized carbon in an IR spectrum?

The carbon-hydrogen bond stretch of sp2-hybridized carbon appears around 3100 wavenumbers, whereas for sp3-hybridized carbon, it appears around 2900 wavenumbers, showing a difference in wavenumber due to the different hybridization states.

Outlines

00:00

🔬 Carbon-Hydrogen Bond Wavenumbers and Hybridization

The paragraph discusses the relationship between the wavenumbers of carbon-hydrogen bond stretches in IR spectra and the hybridization state of the carbon atom involved. It explains that the wavenumber of a carbon-hydrogen bond stretch varies depending on whether the carbon is sp, sp2, or sp3 hybridized. An sp-hybridized carbon, associated with a triple bond, results in a wavenumber around 3300. An sp2-hybridized carbon, associated with a double bond, shows a wavenumber around 3100. An sp3-hybridized carbon, with single bonds, results in a wavenumber around 2900. The strength of the bond is directly related to the hybridization state, with more s character leading to a stronger bond, which in turn affects the force constant and the frequency of bond vibrations. This understanding helps in analyzing IR spectra of hydrocarbons, such as alkanes and alkynes.

05:01

📊 Analyzing IR Spectra of Hydrocarbons

This section delves into the practical application of understanding carbon-hydrogen bond wavenumbers to analyze IR spectra of hydrocarbons. It explains how drawing a line at 3000 wavenumbers can help differentiate between signals from sp3-hybridized carbon-hydrogen bonds and those from sp or sp2 hybridized bonds. The paragraph uses decane and 1-octyne as examples to illustrate the analysis process. It highlights the importance of recognizing the signal around 3300 wavenumbers as indicative of sp-hybridized carbon-hydrogen bonds and around 2100 wavenumbers for carbon-carbon triple bond stretches. The paragraph also introduces the concept of diagnostic and fingerprint regions in IR spectra and how they can be used to identify different types of bonds in molecules.

10:03

🌉 Comparing Alkenes and Arenes in IR Spectra

The final paragraph compares the IR spectra of alkenes and arenes, using 1-hexene and toluene as examples. It discusses how to identify sp2-hybridized carbon-hydrogen bonds by looking for signals around 3100 wavenumbers and carbon-carbon double bond stretches around 1650 wavenumbers. The paragraph also addresses the challenge of distinguishing between alkenes and arenes, noting that aromatic double bonds typically show up at lower wavenumbers (1600 to 1450) compared to alkenes. It suggests that the presence of multiple signals and the overall complexity of the spectrum can be indicative of an aromatic ring, such as in toluene. The summary emphasizes the importance of understanding these subtle differences for accurate analysis of IR spectra.

Keywords

💡Carbon-Hydrogen Bond

A carbon-hydrogen bond is a covalent bond between a carbon atom and a hydrogen atom. In the context of the video, this bond is crucial for understanding the infrared (IR) spectroscopy of hydrocarbons. The video discusses how the wavenumber at which a carbon-hydrogen bond stretch appears in an IR spectrum can vary depending on the hybridization state of the carbon atom involved. For instance, the script mentions that a carbon-hydrogen bond where the carbon is sp3-hybridized shows up around 2900 wavenumbers.

💡Wavenumber

Wavenumber is a measure of the frequency of a wave, often used in spectroscopy to describe the frequency of light absorbed or emitted. In IR spectroscopy, wavenumbers are used to identify different types of chemical bonds based on the energy of the absorbed light. The video script explains how different wavenumbers correspond to different types of carbon-hydrogen bonds, with higher wavenumbers indicating stronger bonds.

💡Hybridization State

Hybridization state refers to the arrangement of electron orbitals in an atom, particularly in carbon, which can form different types of bonds (single, double, or triple). The video explains how the hybridization state of carbon (sp, sp2, or sp3) affects the wavenumber at which the carbon-hydrogen bond stretch appears in an IR spectrum. For example, an sp-hybridized carbon-hydrogen bond shows up around 3300 wavenumbers, while an sp3-hybridized bond is around 2900 wavenumbers.

💡IR Spectrum

An IR spectrum is a graphical representation of the infrared absorption or emission of a molecule, which can be used to identify functional groups within the molecule. The video script uses IR spectra to illustrate how different types of carbon-hydrogen bonds in hydrocarbons produce signals at different wavenumbers, aiding in the identification and analysis of these compounds.

💡Bond Strength

Bond strength refers to the energy required to break a chemical bond. The video script connects bond strength to the wavenumber of the IR spectrum, explaining that stronger bonds, like those in sp-hybridized carbon-hydrogen bonds, result in higher wavenumbers due to a greater force constant. This relationship is key to interpreting IR spectra.

💡sp-hybridized Carbon

An sp-hybridized carbon is one that has two hybrid orbitals formed by the mixing of one s orbital and one p orbital. The video script mentions that sp-hybridized carbons are associated with triple bonds and result in carbon-hydrogen bonds that show up at around 3300 wavenumbers in an IR spectrum.

💡sp2-hybridized Carbon

An sp2-hybridized carbon has three hybrid orbitals formed by the mixing of one s orbital and two p orbitals. The video explains that carbon-hydrogen bonds where the carbon is sp2-hybridized, typically found in double bonds, show up around 3100 wavenumbers in an IR spectrum.

💡sp3-hybridized Carbon

An sp3-hybridized carbon has four hybrid orbitals formed by the mixing of one s orbital and three p orbitals. The video script notes that carbon-hydrogen bonds where the carbon is sp3-hybridized, commonly found in single bonds, show up around 2900 wavenumbers in an IR spectrum.

💡Alkanes

Alkanes are a class of hydrocarbons with single bonds only, consisting of hydrogen and carbon atoms. The video script uses decane as an example of an alkane, highlighting that its carbon-hydrogen bonds are all sp3-hybridized, resulting in IR spectrum signals below 3000 wavenumbers.

💡Alkynes

Alkynes are hydrocarbons that contain at least one carbon-carbon triple bond. The video script discusses 1-octyne as an example of an alkyne, explaining that it contains an sp-hybridized carbon-hydrogen bond that shows up at around 3300 wavenumbers in an IR spectrum, as well as a carbon-carbon triple bond stretch signal around 2100 wavenumbers.

Highlights

Carbon-hydrogen bond stretch wavenumber is approximately 3000.

Wavenumber varies with carbon hybridization state.

sp-hybridized carbon-hydrogen bond stretch appears around 3300 wavenumbers.

sp2-hybridized carbon-hydrogen bond stretch appears around 3100 wavenumbers.

sp3-hybridized carbon-hydrogen bond stretch appears around 2900 wavenumbers.

Hybridization affects electron density and bond strength.

sp-hybrid orbital has the most s character at 50%.

sp2-hybrid orbital has about 33% s character.

sp3-hybrid orbital has about 25% s character.

Bond strength correlates with wavenumber in IR spectra.

Stronger bonds have higher force constants, leading to higher wavenumbers.

IR spectra can be analyzed by drawing lines at key wavenumbers like 3000.

Decane, an alkane, shows a simple spectrum with signals under 3000.

1-Octyne, an alkyne, exhibits a signal near 3300 for sp-hybridized carbon-hydrogen bonds.

Carbon-carbon triple bond stretch in alkynes appears around 2100 wavenumbers.

1-Hexene, an alkene, shows a signal near 3100 for sp2-hybridized carbon-hydrogen bonds.

Aromatic compounds like toluene have unique IR spectra with signals around 3100 and below 1600 for double bonds.

Aromatic double bonds typically show up at lower wavenumbers compared to alkenes.

Transcripts

We've already looked at a carbon-hydrogen bond,

and in the last video,

we actually calculated an approximate wavenumber

for where we would expect the signal

for a carbon-hydrogen bond stretch to appear

on our IR spectrum.

And we got a value of a little bit over 3000 wavenumbers.

However, that wavenumber depends on the hybridization state

of this carbon.

So let's look at some examples here.

So, if we look at an example where the carbon is

sp-hybridized, so we know this is an sp-hybridized carbon

because we have a triple bond here,

so we're talking about a carbon-hydrogen bond,

where the carbon is sp-hybridized,

the signal for this carbon-hydrogen bond stretch

shows up about 3300 wavenumbers.

If we look at this next example here,

so now we have a carbon that has a double bond to it,

so it must be sp2-hybridized,

so we're talking about a carbon-hydrogen bond,

where the carbon is sp2-hybridized,

the signal for this carbon-hydrogen bond stretch

shows up about 3100 wavenumbers.

And then finally, if we look at a situation where we have

only single bonds to this carbon,

we're talking about an sp3-hybridized carbon here,

so we're talking about a carbon-hydrogen bond

where the carbon is sp3-hybridized.

The signal for this carbon-hydrogen bond stretch

shows up about 2900 wavenumbers.

And so, how do we explain these different wavenumbers,

because they're all carbon-hydrogen bonds?

Well, we need to think about the hybridizations.

So let's do that.

So if we look at the sp-hybridized carbon,

remember, that means that this carbon has two sp-hybrid orbitals.

And an sp-hybrid orbital has the most s character

out of all these orbitals we've discussed here.

So, actually 50% s character,

if you remember that from the videos on hybridizations.

So 50% s character for an sp-hybridized orbital.

For an sp2-hybridized orbital,

it's about 33% s character.

And finally for an sp3-hybridized orbital,

it's about 25% s character.

And so going back to the sp-hybridized carbon,

so the sp-hybrid orbital is 50% s character.

That means-- remember what that means.

The electron density is closest to the nucleus.

So, if that's the case,

then we're talking about this bond right here,

this bond being the shortest bond,

because the electron density is closest to the nucleus.

the more s character you have.

And if this is the shortest bond,

it must be the strongest bond out of these three

that we're talking about.

So this carbon-hydrogen bond,

where the carbon is sp-hybridized,

is stronger that the carbon-hydrogen bond

where the carbon is sp2-hybridized.

This bond, though, has more s character than this one,

so this bond is stronger than this one.

So this order of bond strength explains the wavenumbers,

because if you remember from the previous video,

the bond strength affects the force constant,

or the spring constant k,

so as you increase in bond strength,

you increase k, and we saw that increasing k

increases the frequency, or the wavenumber.

So this increases the frequency of bond vibrations,

or increases the wavenumber where you would find

the signal on your IR spectrum.

And so since this is the strongest bond,

this is the highest value for the wavenumber,

so we're going to find this signal more to the left

on our IR spectrum when we're looking at it.

Alright, now that we understand this idea,

so the hybridization,

we can look at some IR spectra for hydrocarbons,

and we can analyze those.

Let's do that.

First, let's compare alkanes and alkynes.

Let's go down here, and let's look at some spectra.

So let me just go down here and we can look at two IR spectra.

The first one is for this molecule, which is decane.

So hopefully I have the right number of carbons drawn there.

The second one is for 1-octyne,

so a triple bond in this molecule.

Let's compare these two, so you think about the differences.

Alright, one of the things that's sometimes helpful to do is

to draw a line around 3000.

So let me draw a line around 3000.

I'm going to try and draw it for both here too.

So I'm going to draw a line around 3000 for both,

so we can compare these two spectra.

Alright, we know that a carbon-hydrogen--

let me go and write this in here,

a carbon-hydrogen bond where the carbon is sp3-hybridized,

so that signal for that stretch shows up under 3000.

So that's why it's helpful to draw a line around 3000.

So that's what we're talking about when we're talking about

this complicated-looking thing in here.

So it's not really worth your time to analyze this in great detail,

and of course my drawing of it isn't perfect to being with,

but think about under 3000,

that's where you expect to find your signal

for your carbon-hydrogen bond,

we're talking about an sp3-hybridized carbon.

And those are the only types of carbons

that we have in decane.

So, if we think about the diagnostic region

versus the fingerprint region,

so if I draw a line here to separate those two regions,

in the diagnostic region, all we have is this.

So all we're thinking about is carbon-hydrogen

where the carbon is sp3-hybridized.

So very simple spectrum to analyze.

We move on to 1-octyne,

so now we're looking at this one down here.

So we see that same kind of thing,

because obviously we have carbon-hydrogen sp3-hybridized

also in this molecule.

And so this isn't going to really help you too much

when you're analyzing the spectra,

but it's useful to know what you're looking at,

drawing a line at 3000,

and thinking about that's what that represents.

So once you draw a line at 3000,

it allows you to see some differences.

So for example, this signal right here,

if we drop down, it's pretty close to 3300,

so this will be 3100, 3200, so 3300.

So approximately 3300 wavenumbers.

And we know what that signal represents.

We can go back up to here,

we can look at about 3300 is where we would expect

to see this carbon-hydrogen bond stretch

where that carbon is sp-hybridized.

So that's what we're looking at there.

Let's go down and look at the dot structure,

and see if we can figure out what it means on the dot structure.

So this signal must be a carbon-hydrogen bond,

where the carbon is sp-hybridized.

And that would be right here.

So we have a carbon here and we have a hydrogen right here.

We also have a carbon right here,

so that gives you your eight carbons.

And so this bond, let me go and highlight it here,

so this bond right here, this carbon-hydrogen bond,

where this carbon is sp-hybridized,

that's this signal on our spectrum.

So once again, it's useful for analyzing here.

Alright, we have something else

that shows up in the diagnostic region for this alkyne,

and it's this signal right here.

So if we drop down, what's the wavenumber

where this signal appears?

The wavenumber is about 2100,

so approximately 2100,

maybe a little bit higher than that.

And that's the carbon triple bond stretch,

so that's the carbon-carbon triple bond stretch

that we talked about in an earlier video.

So that's approximately the triple bond region

when you're looking at your spectrum.

And of course, obviously we have one.

This is an alkyne here.

And so hopefully, this just shows you the differences,

and once again, your fingerprint region over here is unique

for each of these molecules here.

So this shows you the differences and helps you to think

about how to analyze your IR spectrum.

Let's look at two more.

Actually, let's look at one more here,

and let's compare these two.

So now we have a spectrum for an alkene.

So here we have 1-hexene,

and let's see if we can analyze this one.

So we're going to do the same thing,

we're going to draw a line around 1500,

so draw a line around 1500,

we're going to draw a line around 3000,

and let's analyze this one.

So we know what this one is talking about,

we know it's talking about a carbon-hydrogen,

where the carbon is sp3-hybridized.

But what's this signal right here?

We drop down, and that's pretty close to 3100.

So that signal is approximately 3100.

And so, we know what that must represent.

That's a carbon-hydrogen bond,

where the carbon is sp2-hydribized,

so that stretch occurs at this frequency,

or this wavenumber, and so we know

an sp2-hybridized carbon must be present,

and obviously here with this double bond,

we know we have an sp2-hybridized carbon.

Notice the difference between this one

and the one we just talked about.

So let me go and highlight this here.

So this signal shows up around 3100.

So I draw a line up here,

and we saw this signal at 3300.

So it's useful to think about,

it helps you distinguish on your IR spectra,

if you draw a line in there,

and think about where the wavenumber is.

At what wavenumber does this signal appear?

We also have something else showing up in this one,

let me go ahead and draw this one in here.

What is this? What is this guy right here?

That looks like a pretty obvious signal.

So we can drop down here, and check out approximately

where does that show up?

What's that wavenumber?

Well this is 1500, this is 1600, this is 1700,

so that's a little bit-- that's pretty much in between,

it's approximately 1650.

And in an earlier video, we said that was in the double bond region.

So that's the carbon-carbon double bond stretch signal,

right in here.

And obviously, there's a double bond in our molecule.

Again, comparing these two, remember,

we talked about the fact that a triple bond vibrates faster

than a double bond.

And so, the signal is different.

The triple bond vibrates faster, so it has a higher wavenumber,

the double bond doesn't vibrate as fast,

so it has a lower wavenumber.

So these are important things to think about.

Finally, let's compare this alkene to an arene here.

So let's look at toluene right here.

Alright, so if we do the same thing,

if we draw a line around 3000,

so somewhere around 3000.

And we know this is below 3000,

so we know that this must be talking about carbon-hydrogen,

where the carbon is sp3-hybridized.

That's a carbon-hydrogen bond stretch

where the carbon is sp3-hybridized.

Well this carbon right here on toluene,

so this is toluene,

that carbon is sp3-hybridized,

so that makes sense.

We have this one little peak here,

this one little signal that's a little bit higher

than 3000, and so it's pretty close to 3100.

And we know that's approximately

where we would find a carbon-hydrogen bond stretch

where the carbon is sp2-hybridized.

So somewhere around 3100.

And so, this makes this-- at first you might think,

"Oh, well how do we tell this apart?"

So this is very similar to this situation.

So this looks very similar to this.

And so it can be tricky sometimes,

and just glancing at that part of your spectrum.

Let's think about the double bond region too.

So the double bond region right up here,

so this is where-- this is 1600,

that's for this one.

I'm going to draw a line down here,

and let's try to connect the 1600s right here like that.

And let's think about what happened.

So here's the signal for the carbon-carbon double bond here,

and when you're talking about an aromatic double bond stretch,

so carbon-carbon aromatic, that usually shows up lower than 1600.

So here it looks like we have two signals,

so it depends on what kind of compound you're dealing with.

But we can see that this usually shows up lower.

So this is somewhere usually around 1600 to 1450.

But we're talking about the

carbon-carbon double bond stretching here.

And there's some other subtle things that can clue you in,

like this right here.

So we don't really see that on this one, on this spectrum.

And then, these down here, we're missing those here too.

It would be way too much to get into in this video,

to talk about what these mean,

and that's a little bit more

than what we've talked about so far,

so that'll have to be a different video.

But there are subtle differences,

and the easiest one to think about

is to think about the fact that this aromatic

carbon-carbon double bond shows up

at a lower wavenumber than the one

that we talked about right here.

So just look at the sheer multitude of signals

and sometimes that clues you into the fact

that you're dealing with this benzene ring here for toluene.

So that sums up just a quick intro to looking

at IR spectra for hydrocarbons.