Advices For Studying Organic Chemistry

9. Number of fundamental vibrations observed in IR will be influenced:
   (1) Overtones
   (2) Combination tones ⇒ increase the number of bands
   (3) Fall outside 2.5-15 μm region
   Too weak to be observed
   Two peaks that are too close
   Degenerate band
   Lack of dipole change

⇒ reduce the number of bands

10. Calculation of approximate stretching frequencies:

ν = 2 π^1 c Kμ ⇒ ν (cm–1) = 4.12 Kμ

ν = frequency in cm–1 c = velocity of light = 3 x 10^10 cm/sec

μ =
1 2

1 2

m m

mm

+ masses of atoms in grams or

( 1 2 )( 6. 02 x 1023 )

1 2

M M

M M

• masses of atoms
  in amu

μ =

1 2

1 2

M M

M M

+ where M^1 and M^2 are

atomic weights

K = force constant in dynes/cm

K = 5 x 10^5 dynes/cm (single)

= 10 x 10^5 dynes/cm (double)

= 15 x 10^5 dynes/cm (triple)

(1) C=C bond:

ν = 4.12 Kμ K = 10 x 10^5 dynes/cm μ =

C C

C C

M M

M M

+ = 12 12

( 12 )( 12 )

+ =^6

ν = 4.12^10 x 610

5

= 1682 cm–1 (calculated) ν = 1650 cm–1 (experimental)