Compare the experimental values for C-H vs. C-D frequencies in the benzene serie

Question

Compare the experimental values for C-H vs. C-D frequencies in the benzene series. WIIY is the C-D absorption different from the C-H absorption? WHY is the calculated absorption value different from the experimental value? Compare the calculated values for the C.O vs. C = O frequencies in the propanol and acetone. Compare the experimental values for the C-O vs. C = O frequencies in the propanol and acetone. WHY is the C-O absorption different from the C = O absorption? WHY is the calculated absorption value different from the experimental value.

Explanation / Answer

2. Experimental value for C-H frequency in benzene series: 3000-3100 cm-1

Experimental value for C-D frequency in benzene series: 2206 cm-1

So, C-D < C-H

3. From Hooke's Law we get, (in cm-1) = (1/2c)(K/)     = 4.12 * (K/)

K( force constant)= 5*105 dynes/cm for single bond

= 10*105 dynes/cm for double bond

= 5*105 dynes/cm for triple bond

and

= Reduced Mass = m1m2/(m1 + m2)

If reduced mass is higher will be smaller. For C-H bond value of = (12*1/12+1) = 0.923 and for C-D bond = (12*2/12+2) = 1.71

So C-D < C-H .

4. Calculated absorption value for C-D = 4.12 (K/)

= 4.12 (5*105 /1.71) = 2228 cm-1 .

Calculated absorption value for C-H = 4.12 (K/)

= 4.12 (5*105 /0.923) = 3032 cm-1 .

In the equation = 4.12 * (K/), only major factors that influence the stretching frequency of a compound have been considered. These calculated values are different from the experimental values due to reasons that has not been considered into the equation, e.g., effect of dipole moment, resonance, hybridization effect etc. In aromatic compounds like benzene resonance and coupling with other C-H bonds play a significant role which vary the experimental absorption frequencies from the calculated one.

5. Calculated absorption value for C-O in propanol = 4.12 (K/)

= 4.12 (5*105 /6.86) = 1112 cm-1

Calculated absorption value for C-O in acetone = 4.12 (K/)

= 4.12 (10*105 /6.86) =1573 cm-1

So, C-O in acetone > C-O ­in propanol

6. Experimental absorption value for C-O in propanol 1100 cm-1

Experimental absorption value for C-O in acetone 1715 cm-1

So, experimental value of C-O in acetone > C-O ­in propanol

7. C-O absorption is different from the C=O absorption because of the difference in force constants. s character in a double bond is higher than the s character in single bond, so double bond is stronger than the single bond. Thus energy required to vibrate the C=O bond is greater than that required for C-O bond. So, C-O in acetone > C-O ­in propanol

8. Calculated absorption value is different from the experimental one because effects arising from the environment of that bond within the molecule have not been considered in the equation.

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