Carbonyl Compounds Cl H H N 2 1810 cm -1 (band 1) 1800 cm -1 1760 cm -1 both present (band 2) 1735 cm -1 1725 cm -1 1715 cm -1 1710 cm -1 1690 cm -1 Inductive Effects esonance Effects stronger bond W W W stronger bond weaker bond weaker bond D D D
Carbonyl Compounds Cl H H N 2 1810 cm -1 (band 1) 1800 cm -1 1760 cm -1 both present (band 2) 1735 cm -1 1725 cm -1 1715 cm -1 1710 cm -1 1690 cm -1 Inductive Effects Hydrogen Bonding H H H H C weaker bond H H weaker bond
Carbonyl Compounds ther Factors that Influence Position of C= Absorbance 1. Conjugation conjugation shifts carbonyl absorbance right 25-45 cm -1 ; additional unsaturation 15 cm -1 H H H H 1727 cm -1 1703 cm -1 1716 cm -1 1690 cm -1 1712 cm -1 1687 cm -1 may see two bands s-cis s-trans
Carbonyl Compounds 1-penten-2-one C=C C=
Carbonyl Compounds ther Factors that Influence Position of C= Absorbance 2. ing Size (Angle Strain) decreasing ring size shifts carbonyl absorbance to higher frequencies 1717 cm -1 1747 cm -1 1735 cm -1 1770 cm -1 NH NH 1666 cm -1 1700 cm -1
Carbonyl Compounds ther Factors that Influence Position of C= Absorbance 3. Alpha Substituion the presence of an electronegative group at the a position shifts the C= stretch to the right (10-45 cm -1 ) Cl 1715 cm -1 1726 cm -1 influence by both inductive and field effects δ - X δ + X δ - δ + X withdraws electron density from carbonyl (inductive effect) strengthens C= bond repulsion of nonbonding electrons on (field effect) strengthens C= bond
Carbonyl Compounds ther Factors that Influence Position of C= Absorbance 3. Alpha Substituion magnitude of shift is influenced by conformation Cl H ' H ' ' Cl Cl A B C syn-eclipsed (cis) anti-staggered (gauche) H H 1713 cm -1 H Cl H Cl 1750 cm -1 1725 cm -1 H may see multiple absorbances
Carbonyl Compounds methyl chloroacetate Cl CH 3 C=
Carbonyl Compounds ther Factors that Influence Position of C= Absorbance 4. Internal Hydrogen-Bonding effects can be additive Me Me 1729 cm -1 H 1680 cm -1 Me
Aldehydes C= 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 CH shows two weak bands (2860-2800 and 2760-2700 cm -1 ) - first band may be obscured by sp 3 C-H absorbances
Aldehydes octanal
Aldehydes crotonaldehyde
Ketones C= 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 1,3-diketones give more complicated spectra
Ketones cyclohexanone overtone C=
Ketones mesityl oxide
Ketones 2,4-pentanedione H keto form enol form 1723 and 1706 cm -1 1622 and 3200-2400 cm -1 -H H C= (ketone) C= (enol)
Carboxylic Acids -H stretch between 3400-2400 cm -1 - typically very broad - absorbance often centered about sp 3 C-H region C= stretch appears between 1730-1700 cm -1 - strong, usually more intense than than of aldehydes and ketones - conjugation moves absorption to lower frequency C- stretch between 1320-1210 cm -1
Carboxylic Acids isobutyric acid
Carboxylic Acids benzoic acid (nujol)
Carboxylic Acids benzoic acid H 1-octanol H
Carboxylic Acids sodium benzoate - + Na symmetric stretch asymmetric stretch
Esters C= stretch appears between 1750-1735 cm -1 - conjugation with C= moves absorption to lower frequencies (10-25 cm -1 ) (band moves to right) - conjugation with ester oxygen moves band to higher frequencies (15-25 cm -1 ) (band moves to left) - frequency increases with decreasing ring size C- stretch gives two bands between 1300-1100 cm -1 - one often stronger than the other
Esters ethyl acetate CH 2 CH 3 C= C- 1743 cm -1
Esters methyl benzoate 1724 cm -1
Esters phenylacetate C= C- 1765 cm -1
Esters Effect of ing Size & Conjugation on C= Absorbance ring size effects effect of α,β-conjugation effect of conjugation with 1735 cm -1 1725 cm -1 1760 cm -1 1770 cm -1 1750 cm -1 1800 cm -1 1820 cm -1
Amides C= 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 N H N H N H 1 2 3
Amides isobutylamide NH 2 NH 2 C=
Amides N-methylacetamide
Amides N,N-dimethylacetamide N CH 3 CH 3
Amides Effect of ing Size on C= Absorbance NH NH NH 1660 cm -1 1705 cm -1 1745 cm -1
Acid Chlorides C= stretch occurs between 1810-1775 cm -1 - conjugation shifts frequency to right (1780-1760 cm -1 ) C-Cl stretch between 730-550 cm -1
Acid Chlorides acetyl chloride benzoyl chloride
Acid Anhydrides C= 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) C- stretch - multiple bands between 1300-900 cm -1
Anhydrides propionic anhydride
Imines, ximes, and Hydrazones N ' N H N N' 2 imine oxime hydrazone C=N H 1690-1640 cm -1 3650-2600 cm -1 1650-1610 cm -1 http://www.ochemonline.com/infrared_spectroscopy_absorption_table
Imines, ximes, Hydrazones 2-butanone oxime N H N-H C=N (1665 cm -1 )
Imines, ximes, Hydrazones acetone dimethylhydrazone N NMe 2 C=N benzaldehyde phenylhydrazone H N N Ph N-H C=N (1602, 1593 cm -1 )
Nitriles, Isocyanates, and elated Compounds C N N C + - N N N N C N N C S nitrile isocyanate azide carbodiimide isothiocyanate 2250 cm -1 2270 cm -1 (broad) 2140 cm -1 2130 cm -1 2125 cm -1 ' C C C ' C allene carbon dioxide 1950 cm -1 (medium) 2349 cm -1
Nitriles cyclohexanecarbonitrile C N o-tolunitrile C N
Isocyanate and Isothiocyanate butyl isocyanate N=C= cyclohexane isothiocyanate N=C=S
Azides and Carbodiimides ethyl 2-azidoisovalerate N 3 N,N -dicyclohexylcarbodiimide N=C=N
Allenes 1,2-pentadiene sp 2 C-H H H C C C CH 3 CH 2 H N-H C=C=C C=N
ther Groups F Cl Br I fluorides chloride bromide iodide 1400-1000 cm -1 785-540 cm -1 650-510 cm -1 600-485 cm -1 N 2 nitro group 1600-1530 cm -1 1390-1300 cm -1 Ar N 2 nitro group 1550-1490 cm -1 1355-1315 cm -1 SH thiol 2550 cm -1
Halides carbontetrachloride iodoform I H C I I C-I stretch
Halides chloroform chloroform-d C-D Cl D C Cl Cl
Nitro Compounds 2-nitropropane N N 2
Thiols butanethiol S-H
What Can the I Tell You? Evaluation of Unknown Samples by I Preliminary Evaluation: Identification of Major Functional Groups - Determine what you might expect to see (peak position, # of peaks) for each of the possible major organic functional groups. - Determine which functional groups are potentially present. Is there a peak between 3600-3000 cm -1? Consider and alcohol or amine how can you differentiate the two? - You may also be able to eliminate possibilities based on peaks that are not present (e.g. no C= stretch? esters, carboxylic acids, aldehydes and ketones are eliminated from consideration). - Consider also any special circumstances that could affect what you see (e.g. hydrogen bonding, amine substitution, etc.) Subsequent Evaluation: Indications of Minor Functional Groups - Consider the presence of minor functional groups including double and triple bonds, halides, nitro groups, ethers, etc. - Be very careful here. I spectra are usually very complex. Don't try to read more into the spectrum than is actually there.
Practice Problem 1 What does this I tell you about structure? C 8 H 12
Practice Problem 1 what does this I tell you about structure? C 8 H 12 (same compound, now I taken as solution in CCl 4 )
Practice Problem 2 3429 70 2817 29 1602 30 1248 74 913 52 3085 53 2766 64 1584 60 1184 72 892 49 3065 42 2719 37 1493 12 1156 74 865 35 3030 31 1953 77 1453 13 1120 60 760 12 2979 20 1807 72 1391 38 1067 41 701 5 2956 33 1734 6 1373 55 1022 17 616 77 2876 41 1724 4 1301 66 1000 58 526 30 C 9 H 10 What does this I tell you about structure?