ORGO Question

[jz123]

I do not understand why triple bonds have high frequencies than double and single bonds. Doesn't a high frequency mean that it is not stable? However, it is known that triple bonds are the stronger than double and single bonds? So shouldn't a triple bond have the lowest frequency?Please clarify, thanks.

 

[dentalstudent2021]

the shorter the bond, the more energy it has thus it vibrates at more frequency. this means it takes more energy (so higher frequency) to get a triple bond to vibrate than a single bond.
hope that makes sense 🙂

 

[DAT Destroyer]

I do not understand why triple bonds have high frequencies than double and single bonds. Doesn't a high frequency mean that it is not stable? However, it is known that triple bonds are the stronger than double and single bonds? So shouldn't a triple bond have the lowest frequency?Please clarify, thanks.

The IR values are given in units of a wavenumber. The larger the wave number, the stronger the bond. When a molecule is irradiated with IR radiation, there is insufficient energy to break the bond, BUT enough energy to allow it to VIBRATE. We can measure the vibrational frequencies and use that information to obtain important structural information. Our scale could be EITHER in wavenumber or frequency units. It is more convenient to use the wave number. It takes more energy to make a triple bond vibrate thus a stronger bond absorbs more IR light at a higher frequency. The Double bond is weaker, hence we get a value of about 1650 cm-1 vs. a 2250 cm-1 for a triple bond.

I hope this helps.

Dr. Romano

 

[jz123]

It requires more energy to stretch a bond than to bend it. Since triple bonds are stronger than double or single bonds, you therefore need more energy to stretch it.
More energy means higher frequency.

Hope this helps.

Okay. So what about a conjugated alkene? Based on the reasoning, a conjugated alkene is very stable, so it would need to absorb a high energy (high frequency as well). Then why does conjugation decreases the frequency ?

 

[DAT Destroyer]

Okay. So what about a conjugated alkene? Based on the reasoning, a conjugated alkene is very stable, so it would need to absorb a high energy (high frequency as well). Then why does conjugation decreases the frequency ?

Consider a ketone.....we see a value of about 1720 cm-1. Now consider a ketone with a conjugated double bond. Is the energy the same that stretches the C=O bond ? No......a ketone has essentially little contribution from its resonance hybrid..thus we can imagine it as a double bond. The conjugated ketone has SIGNIFICANT resonance contribution, and has LESS double bond character. In other words, the carbonyl group has less of a double bond than that of the non-conjugated molecule. This is reflected in the fact that it will be easier to stretch this bond,,,,,it is not as strong. We would see a value of about 1690 cm-1. Remember.....conjugated molecule have LESS double bond character and will give lower values for both carbonyl and C=C bonds. The alkene bond would be about 1600 cm-1. Hope this helps on a VERY important DAT question.

 

[jz123]

Consider a ketone.....we see a value of about 1720 cm-1. Now consider a ketone with a conjugated double bond. Is the energy the same that stretches the C=O bond ? No......a ketone has essentially little contribution from its resonance hybrid..thus we can imagine it as a double bond. The conjugated ketone has SIGNIFICANT resonance contribution, and has LESS double bond character. In other words, the carbonyl group has less of a double bond than that of the non-conjugated molecule. This is reflected in the fact that it will be easier to stretch this bond,,,,,it is not as strong. We would see a value of about 1690 cm-1. Remember.....conjugated molecule have LESS double bond character and will give lower values for both carbonyl and C=C bonds. The alkene bond would be about 1600 cm-1. Hope this helps on a VERY important DAT question.

Thanks for the explanation!

 

[jz123]

Consider a ketone.....we see a value of about 1720 cm-1. Now consider a ketone with a conjugated double bond. Is the energy the same that stretches the C=O bond ? No......a ketone has essentially little contribution from its resonance hybrid..thus we can imagine it as a double bond. The conjugated ketone has SIGNIFICANT resonance contribution, and has LESS double bond character. In other words, the carbonyl group has less of a double bond than that of the non-conjugated molecule. This is reflected in the fact that it will be easier to stretch this bond,,,,,it is not as strong. We would see a value of about 1690 cm-1. Remember.....conjugated molecule have LESS double bond character and will give lower values for both carbonyl and C=C bonds. The alkene bond would be about 1600 cm-1. Hope this helps on a VERY important DAT question.

one last question, orgo odyssey chapter 6 #33, does the question asking which absorbs the lowest frequency (thus energy)?
Dr. Romano. Could you please explain how I should approach questions like this? I am vey confused about UV radiation (wavelength,,,frequency) stuff... Please help. Thank you.

 

[deleted738762]

It requires more energy to stretch a bond than to bend it. Since triple bonds are stronger than double or single bonds, you therefore need more energy to stretch it.
More energy means higher frequency.

Hope this helps.

You can actually test this out using short & long distensible objects and it becomes apparent which one requires more energy.

 

[DAT Destroyer]

one last question, orgo odyssey chapter 6 #33, does the question asking which absorbs the lowest frequency (thus energy)?
Dr. Romano. Could you please explain how I should approach questions like this? I am vey confused about UV radiation (wavelength,,,frequency) stuff... Please help. Thank you.

 

[DAT Destroyer]

You need to remember 3 vital concepts. 1. Energy is directly related to frequency. For example, Cosmic Ray > Gamma Rays > X Rays > Ultra Violet > Visible > IR > Microwave>TV> Radio waves. #2. Wavelength is INDIRECTLY related to frequency. A large frequency wave has a short wavelength. Thus a Cosmic Ray has a very small wavelength while radiowaves have long wavelengths !!! You can hear radio waves for many many miles while listening to your favorite tunes in your car ..... ( My favorites are Abba and Foreigner !!! ) Finally #3. Wavelength varies indirectly with Energy.

I hope this helps.

Dr. Romano

 

[rak173]

You need to remember 3 vital concepts. 1. Energy is directly related to frequency. For example, Cosmic Ray > Gamma Rays > X Rays > Ultra Violet > Visible > IR > Microwave>TV> Radio waves. #2. Wavelength is INDIRECTLY related to frequency. A large frequency wave has a short wavelength. Thus a Cosmic Ray has a very small wavelength while radiowaves have long wavelengths !!! You can hear radio waves for many many miles while listening to your favorite tunes in your car ..... ( My favorites are Abba and Foreigner !!! ) Finally #3. Wavelength varies indirectly with Energy.

I hope this helps.

Dr. Romano

One of the mnemonics that I came across was: Raging Martians Invaded ROYGBIV Using X-ray Guns. Didn't know about the TV or Cosmic rays.