Tell how you would distinguish between the two compounds of each pair of the fol

Question

Tell how you would distinguish between the two compounds of each pair of the following list. Give the type of vibration you would look for, and roughly the position of the band in cm^-1. Immediately below is an example of how you should answer. a. CH_3CO_2H and CH_3CO_2CH_3 The first compound will have an OH stretch about 3300 cm^-1. The second will not have this band b. CH_3CHO and CH_3CH(OCH_3)_2 c. CH_3CH_2OH and CH_3CO_2H d. (CH_3)_2C=C(CH_3)_2 and CH_3CH=CHCH_3 e. C_6H_5C CH and C_6H_5CH CH_2 CH_3CH=CHCH_2OH and CH_2=CHCH_2OCH_3 (CH_3)_3N and (CH_3)CHNH_2

Explanation / Answer

For the questions related to position of band, it completely depends on how stiff or tough is the bond between the atoms. The stiffness can increase or decrease based on the number of bonds between atom (double, single or triple), dipole moment, or resonance, or Hydrogen bonding between them like N-H, O-H or F-H bonds.

triple >double>single bond
higher dipole results in stronger stretch.
more resonance weakens the stretch
more Hydrogen-bond = higher stretch

a) CH3-CO-OH has resonance between O and O-H, so it loosens the double bond between C and O.

Similarly fo CH3-CO-O-CH3, we can say that there is resonance between O and O-CH3 which loosens the C=O double bond, but here the CH3 is a good electron donor and thus it facilitates more electrons to O, which further can give those electrons to O which is attached in C=O through resonance. So, this will decrease the bond strength of C=O and thus the stretch than CH3-CO-OH.


b) CH3-CO-H , here there is resonace between C=O and alpha hydrogen present in CH3.
For CH3-CH-(OCH3)2 , we see that there is no double bond and also no Hydrogen bond. So, we can atleast expect that there will be lesser stretch between the atoms as compared to CH3-CO-H aldehyde which has C=O double bond.


c) In CH3-CH2-OH we see that there is hydrogen bond O-H which increases the stretch energy.
In CH3-CO-OH there is resonance which decreases the stretch energy.
Here we can argue that C=O should be given more preference. But actually the H-bond has greater influence than C=O. which makes the alcohol have greater stretch.


d) For (CH3)2-C=C-(CH3)2 , we can apply similar logic that there is huge resonance with double bond and alpha-H i.e. with 12 alpha-H. So it shoould lessen the stretch. Also see that there are all CH3 present at double bond. So all of them have dipole moment 0.
For, CH3-CH=CH-CH3 we see that if the CH3 are present at trans position then the net dipole will be 0 and will have same stretch as 1st compond.
If they are in cis position then the net dipole will have some value and shall increase sretch.

e) In C6H5-C=-CH, the triple bond more stretch than C6H5-C=CH2 which has double bond only.


f) For 1st compound we have CH3 attached and also O-H hydrogen bond.
In 2nd compound we have O-CH3 which has no H-bond.
So we can say that 1st compound will have higher band.


g) SImilarly 2nd compiund will have higher stretch due to N-H hydrogen bond.


Please respond back if you have any doubts. I haven't given the accurate answers as per the infrared spectroscopy results. I have given answers based on the concepts that can applied like bondng and structure of compunds.

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