450
9 UV/Vis
9.4 Aromatic Hydrocarbons
9.4.1 Monosubstituted Benzenes
Typical Ranges for Monosubstituted Benzenes (λ max in nm)
Transition
λ max
ε
π   π* (allowed)
180–230
2000–10000
π   π* (forbidden)
250–290
100–2000
π   π* (substituent delocalized by aryl; K band)
220–250
10000–30000
n  π* (substituent with lone pair; R band)
275–350
10–100
Specific Examples of Monosubstituted Benzenes (λ max in nm)
π   π*
(allowed)
π   π*
(forbidden)
π   π*
(K band)
n  π*
(R band)
Substituent R (solvent) λ max
ε
λ max
ε
λ max
ε
λ max
ε
–H (cyclohexane)
198 8000 255
230
–CH 3 (hexane)
208 7900 262
230
–CH=CH 2 (ethanol)
282
450 244 12000
–C– – – CH (hexane)
278
650 236 12500
–Cl (ethanol)
210 7500 257
170
–OH (water)
211 6200 270 1450
–O – (water)
235 9400 287 2600
–NH 2 (water)
230 8600 280 1430
–NH 3
+ (water)
203 7500 254
160
–NO 2 (Hexan)
208
213
9800
8100
270
800 251 9000 322 150
–C– – – N (water)
271 1000 224 13000
–CHO (hexane)
280 1400 242 14000 ≈330 ≈60
–COCH 3 (ethanol)
278 1100 243 13000 319
50
–COOH (water)
202 8000 270
800 230 10000