Justify the carbonyl stretching frequencies for a series of m...

Question

Justify the carbonyl stretching frequencies for a series of methoxybenzaldehydes. Specifically, why are the 2- and 4-methoxy derivatives similar to each other but different from the 3-methoxy derivative?

IR spectra of methoxybenzaldehydes: 4- and 2-methoxy at 1688-1689 cm⁻¹, 3-methoxy at 1702 cm⁻¹.

Accepted Answer

All right. Hi, everyone. So this question says that when properly positioned substituents can transmit electronic data through the benzene ring. This enables us to discover this idea using Ir spectroscopy and apply it to an example for the given series of substituted aceto explain the carbona stretching frequencies. Why are the carbon stretching frequencies of two and four metox derivatives distinct from the three metox derivative but similar to each other. And here on the screen is four methoxsalen, two methoxsalen and three methoxsalen. No recall that when it comes to infrared or Ir spectroscopy, each functional group has their own characteristic bands that help with its identification, right? Because each band is found in a different region of the spectrum because different functional groups, different bonds can absorb radiation at different frequencies, right. So the IR band itself has two different characteristics. It's the intensity of the band itself. In other words, the length of the bond or the stretch. And we also have the wave number which is related to the frequency, right. So there are a few things that affect the wave number or the frequency because we can discuss either the strength of the bond itself. And we can also discuss, discuss the reduced mass of the two atoms bound together. Now, generally speaking, when the strength of a given bond decreases, so does is stretching frequency. And I want to bring this up because notice how both four methoxsalen and two methoxsalen have lower wave numbers than three methio doin. And this has to do with their reduced strength of the car bono bond itself. Now, generally speaking, recall the the car Bono itself is very polar, right? Because there is a strong dipole moment between the oxygen and carbon, which means that the carbonel stretch is one of the most intense in a given ir spectrum. However, sometimes we can see variations in its wave number depending on a few different factors. One is conjugation because conjugation can impact the strength of the carbonel stretch itself and also substituents on the benzene ring can have different effects either via resonance or via inductive effects. Now, it's worth noting that for the three substituted aceto phenoms, there are the carbel is in conjugation with the pi electrons of the ring for all of them. So therefore, right, what that does is if I scroll up here, it actually increases the single bond character of of the car bono itself. So what that means right is that when a molecule has a conjugated system, the car bottle behaves more like a single bond instead of a double bond. And what that does is weaken the carbon stretch overall which results in a decrease in stretching frequency. Now, the reason why two or four and two metox aopen have lower wave numbers is because the car Bonal bonds are weaker due to resonance. And to showcase this, I'm going to start off by drawing out form metox acito feno why? Because in four metox aceto pone, the oxygen of the further extends the conjugated system. Meaning that if I consider the resonance structure a form methoxsalen, then I get a resonance structure like. So right. So in my resonance structure, I do have a resonance structure in which there is a single bond in between carbon and oxygen. And furthermore, because of the extended conjugation, there is less positive charge density on the carbon carbon atom because notice how in my resonance structure, the positive charge extends all the way towards the metox oxygen. So the charge density on carbon specifically is therefore reduced, which increases the single bond character. And we can actually observe something similar in two metox aid doina, right. So here is two methoxsalen and in this case, the meth oxy oxygen can contribute to the resonance and therefore the conjugation in a similar way. So that creates another resonance structure in which there's a single bond between carbon, the carbon carbon and oxygen. And I also end up with a positive charge on the meth oxy oxygen. So once again, positive charge density on carbon is reduced, which therefore increases the single bond character of the carbo itself. And because single bonds are weaker than double bonds, the car bono itself becomes weaker, which means that it vibrates at a lower frequency. However, 33 metox ofen does not have this problem, right? Because its metox group only affects pi electrons through the inductive effect. Now, the inductive effect means that the positive charge density on carbon actually increases, which makes a carbon bond overall stronger due to the reduction or the single bond character being reduced and there you have it. So here is your final answer, right? The carbon stretching frequencies of two and four methoxylated are lower due to the increase in single bond character. Because of extended conjugation, the increase in single bond character makes the car bal overall weaker, which means that the frequency is lower. And with that being said, thank you so very much for watching. And I hope you found this helpful.

Justify the carbonyl stretching frequencies for a series of methoxybenzaldehydes. Specifically, why are the 2- and 4-methoxy derivatives similar to each other but different from the 3-methoxy derivative?

Key Concepts

Electronic Effects of Substituents

Infrared (IR) Spectroscopy

Resonance and Inductive Effects