1. Chapter 13 Structure Determination by Spectroscopy (I): IR
Organic Chemistry II
Chapter 13 Structure Determination by Spectroscopy (I): IR
“study of interaction of light with molecules”
electromagnetic radiation  light
E = hn = hc/l;n =frequency, l=wavelength
excitation in quantized orbitals,  502 Figure 13.1
spectrum: amount vs n of light,  504 Figure 13.2
spectrometer: a scheme,  503 middle
electromagnetic spectrum:  505 Figure 13.3
OrgChem-Chap13
Chapter 13

2. IR Spectroscopy: Functional Group
Organic Chemistry II
IR Spectroscopy: Functional Group
E of vibrational levels  E of IR radiation
vibrational modes: stretch & bend  506 top
functional group identification:  507 Figure 13.4
n  k / M; k=force constant, M=reduced mass
the stronger the bonding, the higher the wavenumber
higher stretching wavenumber than the bending
higher wavenumber for a light hydrogen atom:  508 top
the more polar the bonds, the stronger the absorption: %T
OrgChem-Chap13
Chapter 13

3. Hydrogen Region: 3600~2700 cm-1 bond strength: C-H < N-H < O-H
Organic Chemistry II
Hydrogen Region: 3600~2700 cm-1
bond strength: C-H < N-H < O-H
C-H: H-Csp3 (3000~2850 cm-1) < H-Csp2 (3100~ 3000) [H-C(O): 2830~2700] < H-Csp (ca. 3300);  509~511, Figure
O-H: hydrogen bonding; 3550 ~ 3200 cm-1 (br & s)  512, Figure 13.9
CO2H: very br & s (3000 cm-1);  513, Fig
N-H: w; 3400~3250,  513~4, Figure 13.11~12
practice:  514, Problem 13.5
OrgChem-Chap13
Chapter 13

4. Triple-Bond Region: 2300~2100 cm-1
Organic Chemistry II
Triple-Bond Region: 2300~2100 cm-1
CC: w, 2150~2100 cm-1,  510, Figure 13.7
CN: m, 2260~2220 cm-1,  515, Figure 13.13
CO: s, 2170 cm-1, CO2: s, 2400~2300 cm-1,  516~7, Focus On
practice:  515, Problem 13.6
OrgChem-Chap13
Chapter 13

5. Double-Bond Region: 1900~1500 cm-1
Organic Chemistry II
Double-Bond Region: 1900~1500 cm-1
C=O: typical; s, 1715 cm-1,  518 Table 13.1
conjugation effect: ↓ (20~40 cm-1),  518 top
ring-size effect: 1715 (6-ring) < 1751 (5) < 1775 (4)
C=N: m, 1689~1471 cm-1
C=C: w, aliphatic 1660~1640 cm-1
aromatic: 1600, 1580, 1500 & 1450 cm-1
NO2: s, 1550 & 1380 cm-1,  521 Fig
OrgChem-Chap13
Chapter 13

6. Fingerprint Region: < 1500 cm-1
Organic Chemistry II
Fingerprint Region: < 1500 cm-1
complex but identification of unknowns
single bond stretching & bending overlapped
C-O: s, 1300~1000 cm-1; ester & ether
C-N: m, 1250~1000 (Ar-N: s, 1340~1260)
Ar-H: s, 900~675 cm-1 (bending)
arenes: 1600~1450 & 3100~3000 cm-1
OrgChem-Chap13
Chapter 13

7. Interpretation of IR spectra
Organic Chemistry II
Interpretation of IR spectra
characteristic n:  522~3 Table 13.2~3
R-H: 3600~2700 cm-1  RR: 2300~2100 cm-1
R=R: 1800~1500 cm-1  C-O, C-N, Ar-H, etc.
practice:  523~9, Figure 13.15~13.22
Always double-check your assignment !!!
practice:  519, 530 Prob , Prob &  532 Problem 13.12
OrgChem-Chap13
Chapter 13