UV-Vis Spectroscopy of Conjugated Alkenes - Video Tutorials & Practice Problems

UV-Vis Spectroscopy of Conjugated Alkenes Concept 1

Hey, everyone. So we can say here that energy from UV or light energy can promote an electron from a lower energy to higher energy molecular orbital, or MO. Here, if we take a look, we have 13 butadiene. Here, it possesses 4 pi electrons. These pi electrons, we could fit them within their atomic p orbitals. Since there's 4 of them, we'd go up, up, up, up. And if we were to distribute it within this molecular orbital diagram here, we would be 1 up, 1 down, 1 up, 1 down. Here, HOMO represents the highest occupied molecular orbital. Basically, the orbital that's highest up with any electrons. LUMO would be our lowest unoccupied molecular orbital, the one that's right above it. Now here when we do UV radiation, we would excite one of these electrons, let's say this one, And it will be promoted up to a higher energy state, a higher molecular orbital. So we'd still have up, down, and then we'd have up, and then we'd have down here. As a consequence of this, this would no longer be our homo. This would no longer be our LUMO. This would be our new HOMO, the highest occupied molecular orbital, and then this would become our LUMO, the lowest unoccupied Molecular Orbital. One thing we need to see from here is that this promotion would have an impact on our lambda max, our wavelength in terms of this. Remember, the higher up we go in terms of molecular orbital, the smaller the distance gets between our individual molecular orbitals. Alright. So just remember, applying some UV or visible light can excite one of these electrons, making it promote itself up to the next highest energy molecular orbital.

UV-Vis Spectroscopy of Conjugated Alkenes Concept 2

So we can say here that the transition from our pi molecular orbital to our pi star molecular orbital corresponds to the lambda max, which depends on the energy gap between our HOMO and LUMO molecular orbitals. And we're going to say here that the energy gap itself, we're gonna say that it decreases with an increase in our conjugated double bonds. And this is gonna cause our lambda max to increase. So it's gonna giving it's gonna give us a higher lambda max. Now, remember from gen chem when we're talking about energy and wavelength, the equation that connects them together is that energy is equal to Planck's constant, which is h, times the speed of light, which is c, divided by my wavelength. Here you can see that energy acts as a numerator and wavelength acts as a denominator. This would mean that there's an inverse relationship between the 2 of them. As our energy goes down, our wavelength has to go up. So a decrease in energy would mean an increase in our lambda max. Now if we take a look here, we have 13 butadiene which has 4 pi electrons. We've applied UV radiation which caused one of our electrons to be promoted up to a higher level. Again, in this form, the LUMO would now be technically here, and this would technically be the HOMO, the one with the highest occupied molecular orbital with an electron. And we'd say here that the lambda max if we calculated the lambda max of 13 butadiene, it'd be 217 nanometers. Over here we have 135 hexatriene, so this has 6 pi electrons. So we would say here that we have initially up, down, up, down, up, down. Initially, it would be HOMO and this would be LUMO. But if we were to irradiate these electrons, this electron would be promoted up here. And, again, upon that promotion, this would technically become the HOMO, and this would become the LUMO here. Now we can say here that this is gonna cause, this is gonna give us a lambda max value now of 258 nanometers. So notice that UV radiation still caused the promotion of an electron from the HOMO up to the LUMO. But notice that your lambda max value is higher. That's because there's more conjugation within 13135 hexatriene than there is in 13 butadiene. Basically, the more conjugated we become as a compound, the higher our lambda max will be. And if we can get enough conjugation going on, we could transition from the UV spectrum to the visible light spectrum. We could actually interpret this increase in wavelength into colors. And a vast majority of plants in nature are due because of highly conjugated compounds residing within them. Right? So just remember, as we're increasing our conjugation, we're going to, we're gonna decrease our energy which is going to increase our lambda max. With enough of that, we can transition from UV to visible light.

UV-Vis Spectroscopy of Conjugated Alkenes Example 1

Hey, everyone. So in this question, it says, the following molecule is irradiated with UV light. Which pi orbitals correspond to HOMO and LUMO? If we take a look here, we have 8 pi electrons within this molecule. And because of that, we're gonna have 8 molecular orbitals. We fill this in up, down, up, down, up, down, up, down. Before we've done any radiation, this would be our HOMO, highest occupied molecular orbital. And this would be our LUMO. Now here, we would irradiate. And technically here, there should be a little bit more distance up here. Alright. So we're gonna say here we're irradiating this with UV light. So this would be up, down, up, down, up, down, up. But then this one here gets promoted up here, making this our new HOMO and our new LUMO. This is how we would depict the transition from non irradiated to irradiated. Again, we have 8 pi electrons, so we're gonna have 8 molecular orbitals. We would fill them up, filling from lowest to highest up, following the Aufbau principle. When we irradiate it, one of the electrons in the HOMO will be promoted up to the LUMO, giving us this new interpretation of the irradiated molecule. Right? So this is the approach you would take for a question like this.

Rank the following molecules in increasing order of wavelength of max absorption.

• A. ii < iv < iii < i
• B. ii < iv < i < iii
• C. iv < ii < i < iii
• D. iii < i < iv < ii