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Presentacion de Interpretacion de espectros infrarrojos
- 1. Hydroxyl and AminoGroups 1. Alcohols and Phenols
- 2. 3580-3650 3200-3550 970-1250 var str str O-H (free), usuallysharp O-H (H-bonded), usually broad C-O 1330-1430 650-770 med var-wk O-H bending (in-plane) O-H bend (out-of-plane) Alargamiento Flexión
- 3. Alcohols also displayC-O stretching absorption at 970 to 1250 cm-1
- 4. 2. Carboxylic Acids 2500-3300(acids) overlap C-H 1705-1720 (acids) 1210-1320 (acids) str str med-str O-H (very broad) C=O (H-bonded) O-C (sometimes 2-peaks) 1395-1440 med C-O-H bending Alargamiento Flexión
- 6. 3. Amines Primary (1°) TheN-H stretching absorption is less sensitive to hydrogen bonding than are O-H absorptions. In the gas phase and in dilute CCl4 solution free N-H absorption is observed in the 3400 to 3500 cm-1 region. A smaller absorption near 3200 cm-1 (shaded orange in the spectra) is considered to be the result of interaction between an overtone of the 1600 cm-1 band with the symmetric N-H stretching band. C-N stretching absorptions are found at 1200 to 1350 cm-1 for aromatic amines, and at 1000 to 1250 cm-1 for aliphatic amines. Strong in-plane NH2 scissoring absorptions at 1550 to 1650 cm-1 , and out-of-plane wagging at 650 to 900 cm-1 (usually broad) are characteristic of 1°-amines Alargamiento Flexión
- 8. Secondary (2°) Only one absorptionnear 3420 cm-1 . Hydrogen bonding in concentrated liquids shifts these absorptions to lower frequencies by about 100 cm-1 . Again, this absorption appears at slightly higher frequency when the nitrogen atom is bonded to an aromatic ring. The C-N absorptions are found in the same range, 1200 to 1350 cm- 1 (aromatic) and 1000 to 1250 cm-1 (aliphatic) as for 1°-amines. A weak N-H bending absorption is sometimes visible at 1500 to 1600 cm-1 . A broad wagging absorption at 650 to 900 cm-1 may be discerned in liquid film samples
- 9. Tertiary (3°) No N-Habsorptions. The C-N absorptions are found in the same range, 1200 to 1350 cm-1 (aromatic) and 1000 to 1250 cm-1 (aliphatic) as for 1°-amines. Aside from the C-N stretch noted on the left, these compounds have spectra characteristic of their alkyl and aryl substituents
- 10. Carbonyl Compounds 1. Aldehydesand Ketones 2690-2840(2 bands) 1720-1740 1710-1720 1690 1675 1745 1780 med str str str str str str C-H (aldehyde C-H) C=O (saturated aldehyde) C=O (saturated ketone) aryl ketone α, β-unsaturation cyclopentanone cyclobutanone 1350-1360 1400-1450 1100 str str med α-CH3 bending α-CH2 bending C-C-C bending Alargamiento Flexión Since alkyl substituents stabilize the carbocation, ketone carbonyls have slightly lower stretching frequencies, 1715 ± 7 cm-1, compared with aldehydes, 1730 ± 7 cm-1.
- 12. Three factors areknown to perturb the carbonyl stretching frequency: 1. Conjugation with a double bond or benzene ring lowers the stretching frequency. 2. Incorporation of the carbonyl group in a small ring (5, 4 or 3-membered), raises the stretching frequency. This shift also occurs in the presence of the previous conjugative lowering of the stretching absorption.
- 13. 3. Changing analkyl substituent of a ketone for an electron releasing or withdrawing group. This effect, which may shift the carbonyl stretching frequency up or down, is particularly important when an alkyl substituent is replaced by a hetero atom such as N, O or X (halogen). Strongly electron withdrawing group (-CCl3) and a Electron donating group (-OCH3
- 14. 2. Carboxylic AcidDerivatives 2500-3300 (acids) overlap C-H 1705-1720 (acids) 1210-1320 (acids) 1785-1815 ( acyl halides) 1750 & 1820 (anhydrides) 1040-1100 1735-1750 (esters) 1000-1300 1630-1695(amides) str str med-str str str str str str str O-H (very broad) C=O (H-bonded) O-C (sometimes 2-peaks) C=O C=O (2-bands) O-C C=O O-C (2-bands) C=O (amide I band) 1395-1440 1590-1650 1500-1560 med med med C-O-H bending N-H (1¡-amide) II band N-H (2¡-amide) II band
- 15. The inductive effectof a halogen substituent on the carbonyl stretching frequency is demonstrated. An overtone near 3600 is often observed in concentrated samples. A second overtone related absorption is often seen on the low frequency side of the strong carbonyl absorption Carbonyl Derivative Carbonyl Absorption Acyl Halides (RCOX) X = F X = Cl X = Br C=O stretch 1860 ± 20 cm-1 1800 ± 15 1800 ± 15
- 16. The inductive effectof the acyloxy substituent on the carbonyl stretching frequency is demonstrated. Two carbonyl absorptions due to asymmetric and symmetric coupling are observed, the former having the higher frequency. The asymmetric absorption is stronger in the case of acyclic anhydrides. Two C-O stretching bands are usually evident Acid Anhydride, (RCO)2 O acyclic 6-membered ring 5-membered ring C=O stretch (2 bands) 1750 & 1820 cm-1 1750 &1820 1785 & 1865
- 17. Esters & Lactones(RCOOR') esters 6-membered lactone 5-membered lactone 4-membered lactone C=O stretch 1740 cm ± 10 cm-1 1740 cm ± 10 1765 cm± 5 1840 cm ± 5
- 18. Primary amides showtwo strong N-H absorptions Strong C=O absorption occurs at lower frequencies than in ketones, due to the extensive p-pi conjugation in amides.There are usually two strong bands in this region, the higher frequency absorption (amide I band) is essentially C=O stretching, the lower frequency (amide II band) is chiefly N-H bending. All these absorptions may display H-bonding association shifts. Amides & Lactams (RCONR2 ) 1° & 2°-amides 3°-amides C=O bands 1510 to 1700 cm-1 (2 bands) 1650± 15 (one band)
- 21. To understand thefunctioning of these factors consider the following analyses: 1. Conjugation extends the dipolar character of the carbonyl group to the double bond (or aromatic ring) so that the beta-carbon atom shares the positive character of the carbonyl carbon. As illustrated by the following resonance equation, this not only explains conjugate addition reactions of nucleophiles, but also suggests that the carbonyl double bond has slightly more single bond character than does an unconjugated function. The bond energy (and force constant) of the conjugated C=O group is correspondingly reduced, and this results in a lower stretching frequency. 2. Under ideal conditions the carbon atom of a carbonyl group is essentially sp2 hybridized, which implies that the bond angles will be 120° and the C-O sigma bond has 33% s-character. If this group is incorporated in a small ring, the C-CO-C bond angle is reduced to 108° (5-membered ring), 90° (4-membered ring) or 60° (3-membered ring). When this happens, the C-C bonds of the ring assume greater p-character and the C-O sigma bond has correspondingly greater s-character. The double bond of the carbonyl group is therefore shorter and stronger, and exhibits a larger stretching frequency. 3. Electron donating substituents on the carbonyl group stabilize the ionic resonance contributor, and increase the single bond character of the C=O bond. The stretching frequency is therefore decreased, as noted in the right hand example below. Electron withdrawing groups have an opposite influence, and increase the stretching frequency of the carbonyl group. Trichloroacetaldehyde (left below) provides a good example.