Different fundamental vibrations

1. 1
Stretching and Bending
Symmetric stretching Antisymmetric stretching
Scissoring Rocking Wagging Twisting
Bending Vibrations
Stretching Vibrations

2. 2
Q1) What is the correct order of stretching frequency of C=O bond for the following
compounds?
Ans. (i) > (iii) > (ii)
In (ii) lone pair of electrons from oxygen as well as alkene double bond electrons
can be delocalized to carbonyl double bond and will decrease its bond order
significantly. In (iii), oxygen only oxygen lone pairs can be delocalized to carbonyl
double bond. So in this case C=O bond order will be higher than (ii) case. Finally,
in (i), oxygen lone pairs can be delocalized into alkene double bond as well as
carbonyl double bond, therefore, C=O in (i) will have highest bond order and thus
stretching frequency.

3. 3
Q2) What is the correct order of stretching frequency of C=O bond for the following
compounds?
Ans. (i) > (iii) > (ii)
Explanation similar to question no. 1

4. 4
Q3) What is the correct order of stretching frequency of C=O bond for the following
compounds?
Ans. (iii) > (i) > (ii)
Because of the resonance, C=O bond will get partial
double bond character in all the three cases with a
positive charge on carbon at para position. OMe
(electron donating) will stabilize the positive charge,
whereas, nitro group will destabilize the positive
charge. Therefore, bond strength of C=O will be highest
in (iii) and least in case of (ii).

5. 5
Q4) What is the correct order of stretching frequency of C=O bond for the following
compounds?
Ans. (iv) > (iii) > (ii) > (i)
External C=O double bond give an increase in absorption frequency with decreasing ring
size. Smaller rings require the use of more p character to make the ring C-C bonds form the
requisite small angles (recall the trend: sp = 180˚, sp2 = 120˚, sp3 = 109˚, sp>3 = < 109˚). This
removes the p character from the sigma bond of the external double bond (C=O) but gives
it more s character, thus strengthening and stiffening the double bond. The force constant
K is then increased, and the absorption frequency increases.

6. 6
C5H10O

7. 7
C6H12O

8. 8

9. 9

10. 10
For CnHn+r
For CnHn+r o
For CnHn+r Nx or CnHn+r NxO
Hydrogen deficiency: [n-(r-x)]+2
2
For CnHn+r Xx or CnHn+r XxO (X: F, Cl, Br, I)
Hydrogen deficiency: [n-(r+x)]+2
2
Hydrogen deficiency Index