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SURVEY OF SPECTRA

O-H 3600 N-H 3400 C-H 3000 C=N 2250 C=C 2150 C=O 1715 C=C 1650 C-O 1100. =. =. EFFECTS OF CONJUGATION AND RING SIZE ON C=O. ALKENES AND AROMATICS. ( C=C STRETCHING). SURVEY OF SPECTRA. CONJUGATION. Conjugation of C=O with C=C.

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SURVEY OF SPECTRA

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  1. O-H 3600 N-H 3400 C-H 3000 C=N 2250 C=C 2150 C=O 1715 C=C 1650 C-O 1100 = = EFFECTS OF CONJUGATION AND RING SIZE ON C=O ALKENES AND AROMATICS ( C=C STRETCHING) SURVEY OF SPECTRA

  2. CONJUGATION

  3. Conjugation of C=O with C=C • Conjugation of a carbonyl with a C=C bond shifts values to lower frequencies • For aldehydes, ketones and esters, subtract about 25-30 cm-1 for conjugation with C=O • Conjugated ketone = 1690 to 1680 cm-1 • Conjugated ester = 1710 to 1700 cm-1 • C=C becomes quite strong!!

  4. CONJUGATION LOWERS THE FREQUENCY OF C=O AND ALSO OF C=C (WHICH IS STRENGTHENED) resonance lengthens (weakens) C=O 1650 1625 cm-1 C=C is also lengthened (weakened) ….. and polarized ! 1715 1690 cm-1 lowered

  5. weak only one C=O : 1715 - 30 = 1685 KETONE conjugated C=C : 1650 - 25 = 1625 4-Methyl-3-penten-2-one doublet = gem dimethyl C=O C=C

  6. C=O : 1715 - 30 = 1685 AROMATIC KETONE conjugated Acetophenone benzene C-H stretch benzene C-H oops C=O benzene bonds 1400 - 1600

  7. RING SIZE / ANGLE STRAIN

  8. ANGLE STRAIN RAISES THE CARBONYL FREQUENCY 2 In response to more p character in the ring bonds, there is more s character in the bonds to C=O. 3 More s character leads to a shorter and stronger bond and a higher frequency. 120o angle is normal 1 A smaller angle requires more p character in the hybrids forming the ring.

  9. CONJUGATION AND RING SIZE EFFECTS 1690 1715 1705 1780 1745 1815 RING STRAIN CONJUGATION normal aliphatic ketones

  10. CYCLIC KETONE 5-ring Cyclopentanone expected = 1740 C-H aliphatic CH bend C=O

  11. ALKENES AROMATICS C=C STRETCHING

  12. 2.5 4 5 5.5 6.1 6.5 15.4 4000 2500 2000 1800 1650 1550 650 Typical Infrared AbsorptionRegions C=C WAVELENGTH (mm) C-Cl C=O C=N O-H C-H C N Very few bands C-O C=C N-H C C C-N X=C=Y C-C * N=O N=O (C,O,N,S) FREQUENCY (cm-1)

  13. The C=C stretching region • C=C double bond at 1650 cm-1 is often weak or not even seen. • C=C benzene ring shows peak(s) near 1600 and 1400 cm-1 , one or two at each value - CONJUGATION LOWERS THE VALUE. • When C=C is conjugated with C=O it is stronger and comes at a lower frequency.

  14. ALKENE 1-Hexene =C-H C=C C-H bend C-H aliphatic oops

  15. AROMATIC Toluene benzene C-H benzene C=C oops

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