2a. IR-2 2018 HO - Chemistry | A&S | SU

1 Infrared Spectroscopy Carbonyl Compounds O O O O O O O O O O. R O R R Cl R O R R OR R H R R R OH R NR2. 1810 cm-1 1800 cm-1 1760 cm-1 1735 cm-1 1725 cm-1 1715 cm-1 1710 cm-1 1690 cm-1. (band 1) (band 2). both present Inductive Effects Resonance Effects stronger O. bond O O. R W R W. R W. stronger bond weaker weaker O bond O O bond R D R D R D. Infrared Spectroscopy Carbonyl Compounds O O O O O O O O O O. R O R R Cl R O R R OR R H R R R OH R NR2. 1810 cm-1 1800 cm-1 1760 cm-1 1735 cm-1 1725 cm-1 1715 cm-1 1710 cm-1 1690 cm-1. (band 1) (band 2). both present Inductive Effects Hydrogen Bonding H O H O. H C R R. O. H O H O. H weaker weaker bond bond O O. Infrared Spectroscopy Carbonyl Compounds Other Factors that Influence Position of C=O Absorbance 1.

2 Conjugation conjugation shifts carbonyl absorbance right 25-45 cm-1; additional unsaturation 15 cm-1. O O O. O O O. H H OH. OH. 1727 cm-1 1703 cm-1 1716 cm-1 1690 cm-1 1712 cm-1 1687 cm-1. may see two bands O R. R O. s-cis s-trans Infrared Spectroscopy Carbonyl Compounds 1-penten-2-one O. C=C. C=O. Infrared Spectroscopy Carbonyl Compounds Other Factors that Influence Position of C=O Absorbance 2. Ring Size (Angle Strain). decreasing ring size shifts carbonyl absorbance to higher frequencies O O O O. O O. 1717 cm-1 1747 cm-1 1735 cm-1 1770 cm-1. O O. NH NH. 1666 cm-1 1700 cm-1. Infrared Spectroscopy Carbonyl Compounds Other Factors that Influence Position of C=O Absorbance 3. Alpha Substituion the presence of an electronegative group at the a position shifts the C=O.

3 Stretch to the right (10-45 cm-1). O O. Cl 1715 cm-1 1726 cm-1. influence by both inductive and field effects O O. - X X. + - +. X withdraws electron density from carbonyl repulsion of nonbonding electrons on O. (inductive effect) (field effect). strengthens C=O bond strengthens C=O bond Infrared Spectroscopy Carbonyl Compounds Other Factors that Influence Position of C=O Absorbance 3. Alpha Substituion magnitude of shift is influenced by conformation O Cl OH O R'. H R' R' Cl Cl H. R R R. A B C. syn-eclipsed (cis) anti-staggered (gauche). O O O. H Cl H. H H Cl 1713 cm-1 1750 cm-1 1725 cm-1. may see multiple absorbances Infrared Spectroscopy Carbonyl Compounds methyl chloroacetate O. Cl OCH3. C=O. Infrared Spectroscopy Carbonyl Compounds Other Factors that Influence Position of C=O Absorbance 4.

4 Internal Hydrogen-Bonding effects can be additive O O. OMe OMe OMe OH. 1729 cm-1 1680 cm-1. Infrared Spectroscopy Aldehydes C=O stretch between 1740-1720 cm-1. - typically about 1725 cm-1 for aliphatic aldehydes - absorbance is lowered by conjugation conjugated aldehydes: 1700-1660 cm-1. C-H stretch of CHO shows two weak bands (2860-2800 and 2760-2700 cm-1). - first band may be obscured by sp3 C-H absorbances Infrared Spectroscopy Aldehydes octanal Infrared Spectroscopy Aldehydes crotonaldehyde Infrared Spectroscopy Ketones C=O stretch between 1720-1708 cm-1. - typically about 1715 cm-1 for aliphatic ketones - absorbance is lowered by conjugation conjugated ketones: 1700-1680 cm-1; or lower if doubly conjugated - frequency increases with decreasing ring size bending vibration appears as medium intensity peak between 1300-1100 cm-1.

5 1,3-diketones give more complicated spectra Infrared Spectroscopy Ketones cyclohexanone overtone O. C=O. Infrared Spectroscopy Ketones mesityl oxide H. O O O O. Infrared Spectroscopy Ketones keto form enol form 1723 and 1706 cm-1 1622 and 3200-2400 cm-1. 2,4-pentanedione O-H. O O OH O. C=O. (ketone) C=O. (enol). Infrared Spectroscopy Carboxylic Acids O-H stretch between 3400-2400 cm-1. - typically very broad - absorbance often centered about sp3 C-H region C=O stretch appears between 1730-1700 cm-1. - strong, usually more intense than than of aldehydes and ketones - conjugation moves absorption to lower frequency C-O stretch between 1320-1210 cm-1. Infrared Spectroscopy Carboxylic Acids isobutyric acid Infrared Spectroscopy Carboxylic Acids benzoic acid (nujol).

6 Infrared Spectroscopy Carboxylic Acids benzoic acid O. OH. 1-octanol OH. Infrared Spectroscopy O O. Carboxylic Acids R O R O. sodium benzoate O. O- +Na symmetric asymmetric stretch stretch Infrared Spectroscopy Esters C=O stretch appears between 1750-1735 cm-1. - conjugation with C=O moves absorption to lower frequencies (10-25 cm-1). O. OR (band moves to right). - conjugation with ester oxygen moves band to higher frequencies (15-25 cm-1). O (band moves to left). R O. - frequency increases with decreasing ring size C-O stretch gives two bands between 1300-1100 cm-1. - one often stronger than the other Infrared Spectroscopy Esters ethyl acetate O. OCH2CH3. C=O. C-O. 1743 cm-1. Infrared Spectroscopy Esters methyl benzoate 1724 cm-1.

7 Infrared Spectroscopy O O. Esters R O R O. phenylacetate O. O. C=O C-O. 1765 cm-1. Infrared Spectroscopy Esters Effect of Ring Size & Conjugation on C=O Absorbance effect of effect of ring size effects , -conjugation conjugation with O. O O O. O O O. 1735 cm-1 1725 cm-1 1760 cm-1. O O O. O O O. 1770 cm-1 1750 cm-1 1800 cm-1. O. O. 1820 cm-1. Infrared Spectroscopy Amides C=O stretch between 1680-1630 cm-1. N-H stretch occurs between 3500-3300 cm-1. - signal varies with amide structure 1 amines show two bands (~3350 and 3180 cm-1). 2 amines have one band (~3300 cm-1). 3 amines have no absorbance in this region N-H bend vibrations between 1640-1550 cm-1. O O O. H H R. R N R N R N. H R R. 1 2 3 . Infrared Spectroscopy Amides isobutylamide O.

8 NH2. NH2 C=O. Infrared Spectroscopy Amides N-methylacetamide Infrared Spectroscopy Amides N,N-dimethylacetamide O. CH3. N. CH3. Infrared Spectroscopy Amides Effect of Ring Size on C=O Absorbance O. O. O. NH. NH. NH. 1660 cm-1 1705 cm-1 1745 cm-1. Infrared Spectroscopy Acid Chlorides C=O stretch occurs between 1810-1775 cm-1. - conjugation shifts frequency to right (1780-1760 cm-1). C-Cl stretch between 730-550 cm-1. Infrared Spectroscopy Acid Chlorides acetyl chloride benzoyl chloride Infrared Spectroscopy Acid Anhydrides C=O stretch always has two bands - absorption at 1830-1800 and 1775-1740 cm-1. may vary in size - conjugation shifts frequency to right (lower frequency). - ring strain moves absorption to left (higher frequency).

9 C-O stretch - multiple bands between 1300-900 cm-1. Infrared Spectroscopy Anhydrides propionic anhydride Infrared Spectroscopy Imines, Oximes, and Hydrazones R' OH NR'2. N N N. R R R R R R. imine oxime hydrazone C=N 1690-1640 cm-1 1650-1610 cm-1. OH 3650-2600 cm-1. Infrared Spectroscopy Imines, Oximes, Hydrazones 2-butanone oxime OH. N. C=N. N-OH (1665 cm-1). Infrared Spectroscopy Imines, Oximes, Hydrazones acetone dimethylhydrazone NMe2. N. C=N. benzaldehyde phenylhydrazone H. N. N Ph N-H. C=N. (1602, 1593 cm-1). Infrared Spectroscopy Nitriles, Isocyanates, and Related Compounds + - R C N R N C O R N N N R N C N R R N C S. nitrile isocyanate azide carbodiimide isothiocyanate 2250 cm-1 2270 cm-1 2140 cm-1 2130 cm-1 2125 cm-1.

10 (broad). R R'. C C C O C O. R R'. allene carbon dioxide 1950 cm-1 2349 cm-1. (medium). Infrared Spectroscopy Nitriles cyclohexanecarbonitrile C N. o-tolunitrile C N. Infrared Spectroscopy Isocyanate and Isothiocyanate butyl isocyanate N=C=O. cyclohexane isothiocyanate N=C=S. Infrared Spectroscopy Azides and Carbodiimides ethyl 2-azidoisovalerate N3. N,N'-dicyclohexylcarbodiimide N=C=N. Infrared Spectroscopy Allenes 1,2-pentadiene H H. C C C. CH3CH2 H. sp2 C-H. C=N. N-OH C=C=C. Infrared Spectroscopy Other Groups R F R Cl R Br R I. fluorides chloride bromide iodide 1400-1000 cm-1 785-540 cm-1 650-510 cm-1 600-485 cm-1. R NO2 Ar NO2 R SH. nitro group nitro group thiol 1600-1530 cm-1 1550-1490 cm-1 2550 cm-1. 1390-1300 cm-1 1355-1315 cm-1.