Infrared Spectra of Organic Compounds: Alkanes, Alkenes, Alkynes, Aromatic Rings, Alcohols, Lecture notes of Chemistry

Download Infrared Spectra of Organic Compounds: Alkanes, Alkenes, Alkynes, Aromatic Rings, Alcohols and more Lecture notes Chemistry in PDF only on Docsity! 2015 Department of Chemistry, Illinois State University, Normal, IL 61790-4160 Page 1 CHE 231 AN INTRODUCTION TO LABORATORY PRACTICES IN ORGANIC CHEMISTRY Department of Chemistry, Illinois State University, Normal, IL 61790-4160, USA Infrared Spectroscopy Table of Contents Alkanes 2 Alkenes 4 Alkynes 6 Aromatic systems 9 Alcohols 12 Ethers 13 Phenols 14 Amines 15 Aromatic amines (anilines) 18 Ketones 19 Aromatic ketones 20 Aldehydes 21 Aromatic aldehydes 22 Carboxylic acids 24 Esters 25 Amides 26 Nitriles 27 Nitro compounds 28 2015 Department of Chemistry, Illinois State University, Normal, IL 61790-4160 Page 2 The Infrared Spectra of Alkanes C(sp3)-H stretching 2800 to 2960 cm-1 CH2 bending 1440 to 1500 cm-1 Infrared spectrum of n-heptane: 2961, 2926, 1468, 1379 cm-1. SDBSWeb: http://sdbs.db.aist.go.jp (National Institute of Advanced Industrial Science and Technology, January 8, 2015) Infrared spectrum of nujol (paraffin wax, long chain hydrocarbon, CnH2n+2): SDBSWeb: http://sdbs.db.aist.go.jp (National Institute of Advanced Industrial Science and Technology, January 8, 2015). 2015 Department of Chemistry, Illinois State University, Normal, IL 61790-4160 Page 5 The Infrared Spectra of Alkenes cis and trans-Alkenes may be distinguished based on two key features, the carbon-carbon double bond vibration and the out of plane bending of the vinylic protons. An infrared spectrum for cis-2-octene SDBSWeb: http://sdbs.db.aist.go.jp (National Institute of Advanced Industrial Science and Technology, January 8, 2015). An infrared spectrum for trans-2-octene SDBSWeb: http://sdbs.db.aist.go.jp (National Institute of Advanced Industrial Science and Technology, January 8, 2015). out of plane bending 964 cm-1 2015 Department of Chemistry, Illinois State University, Normal, IL 61790-4160 Page 6 The Infrared Spectra of Alkynes Dominant Observable Vibrations Stretching vibrations C(sp)-H stretching vibration ~3250-3350 cm-1 C≡C stretching vibration ~2100-2200 cm-1 Out of plane C-H bending vibrations C(sp)-H bending vibration ~610-700 cm-1 Infrared spectrum of 1-octyne (a terminal alkyne): 3323, 3024, 2926, 1636, 1466, 630 cm-1 SDBSWeb: http://sdbs.db.aist.go.jp (National Institute of Advanced Industrial Science and Technology, January 8, 2015). out of plane bending 630 cm-1 C C stretching 2015 Department of Chemistry, Illinois State University, Normal, IL 61790-4160 Page 7 The Infrared Spectra of Alkynes The infrared spectrum of 5-methyl-1-hexyne is illustrated below. The expected peaks are present. Notice the presence of the isopropyl group “doublet” at 1368 and 1388 cm-1. SDBSWeb: http://sdbs.db.aist.go.jp (National Institute of Advanced Industrial Science and Technology, January 8, 2015). Infrared spectrum of 2-octyne (an internal alkyne): the alkyne C≡C stretching vibration is very weak and not observable due to symmetry and lack of changing dipole moment. SDBSWeb: http://sdbs.db.aist.go.jp (National Institute of Advanced Industrial Science and Technology, January 8, 2015). 2015 Department of Chemistry, Illinois State University, Normal, IL 61790-4160 Page 10 The Infrared Spectra of Aromatic Rings Cumene (isopropylbenzene): There are a number of peaks that appear for aromatic systems. These peaks include aromatic C-H stretching, C=C from the aromatic ring, and out of plane bending. There are also out of plane CH bending overtones that appear between 2000 and 1667 cm-1. The intensity and shape of these peaks varies with the substitution of the aromatic ring. SDBSWeb: http://sdbs.db.aist.go.jp (National Institute of Advanced Industrial Science and Technology, January 8, 2015). 2015 Department of Chemistry, Illinois State University, Normal, IL 61790-4160 Page 11 Disubstituted aromatic rings Infrared spectrum of ortho-bromotoluene: 3061, 3016, 2924, 1470, 1031, 745 cm-1 Infrared spectrum of meta-bromotoluene: 3061, 3017, 2932, 1564, 834, 770, 681 cm-1 Infrared spectrum of para-bromotoluene: 3081, 3026, 2923, 1487, 1013, 801 cm-1 2015 Department of Chemistry, Illinois State University, Normal, IL 61790-4160 Page 12 The Infrared Spectra of The Alcohols (ROH) Dominant Observable Vibrations: O-H stretching ~3200-3600 cm-1 C-O stretching ~1000-1200 cm-1 Infrared spectrum of 1-octanol SDBSWeb: http://sdbs.db.aist.go.jp (National Institute of Advanced Industrial Science and Technology, January 8, 2015). Hydrogen bonding is an important phenomenon in the infrared spectra of alcohols. The frequency and shape of the peak is directly proportional to the level of hydrogen bonding. Image taken from https://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/Spectrpy/InfraRed/irspec1.htm 2015 Department of Chemistry, Illinois State University, Normal, IL 61790-4160 Page 15 The Infrared Spectra of The Amines Dominant Observable Vibrations: N-H stretching ~3200-3500 cm-1 N-H bend ~1550-1650 cm-1 Comments: Shows the –N-H stretch for NH2 as two signals between 3200-3500 cm-1 (s-m); symmetric and anti-symmetric modes NH2 group shows a deformation band from 1590-1650 cm-1. Additionally there is a “wag” band at 780-820 cm-1 that is not diagnostic
The N-H stretching vibration varies in intensity.
Primary amines have two N-H signals in the region between 3200 and 3500 cm-1.
Secondary amines have only one N-H signal in this region.
Tertiary amines do not have an N-H signal. Sometimes, amines are mistaken for alkenes because of the N-H bending vibration that appears near 1600 cm-1. 2015 Department of Chemistry, Illinois State University, Normal, IL 61790-4160 Page 16 The Infrared Spectra of Amines The infrared spectra of amines have only a few peaks that can be considered to be of diagnostic value. The symmetric and asymmetric N-H stretch is of greatest value. The NH2 scissoring peak will vary in intensity and may be obscured by other peaks. Primary amine: n-hexylamine SDBSWeb: http://sdbs.db.aist.go.jp (National Institute of Advanced Industrial Science and Technology, January 8, 2015). 2015 Department of Chemistry, Illinois State University, Normal, IL 61790-4160 Page 17 The Infrared Spectra of the Amines (primary, secondary, tertiary) Primary amine: Infrared spectrum of n-pentyl amine Secondary amine: Infrared spectrum of di-n-butyl amine Tertiary amine: Infrared spectrum of tri-n-butyl amine 2015 Department of Chemistry, Illinois State University, Normal, IL 61790-4160 Page 20 The Infrared Spectra of Aromatic ketones The frequency of the C=O stretching vibration for ketones will decrease to about 1690-1666 cm-1 if the C=O functional group is immediately next to an alkene or an aromatic ring. Infrared spectrum of propiophenone: SDBSWeb: http://sdbs.db.aist.go.jp (National Institute of Advanced Industrial Science and Technology, January 8, 2015). Infrared spectrum of benzophenone: SDBSWeb: http://sdbs.db.aist.go.jp (National Institute of Advanced Industrial Science and Technology, January 8, 2015). 2015 Department of Chemistry, Illinois State University, Normal, IL 61790-4160 Page 21 The Infrared Spectra of the Aldehydes The most important vibration associated with ketones is the C=O stretching vibration. This vibration occurs in the region of ~1725-1735 cm-1. The frequency of the carbonyl peak will vary based on the structure of the carbonyl. -C=O stretching ~1725-1735 cm-1 Aldehydic C-H ~2700-2800 cm-1 Infrared spectrum of n-hexanal: SDBSWeb: http://sdbs.db.aist.go.jp (National Institute of Advanced Industrial Science and Technology, January 8, 2015). 2015 Department of Chemistry, Illinois State University, Normal, IL 61790-4160 Page 22 The Infrared Spectra of the Aromatic Aldehydes Aromatic Aldehydes The frequency of the C=O stretching vibration for aldehydes will decrease to about 1685-1666 cm-1 if the C=O functional group is conjugated to an alkene or an aromatic ring. Infrared spectrum of 4-methoxybenzaldehyde (para-anisaldehdye) SDBSWeb: http://sdbs.db.aist.go.jp (National Institute of Advanced Industrial Science and Technology, January 8, 2015). 2015 Department of Chemistry, Illinois State University, Normal, IL 61790-4160 Page 25 The Infrared Spectra of Esters C=O stretch • saturated aliphatic esters: C=O: 1750–1735 cm–1 • formates, a,b-unsaturated, and benzoate esters: 1730-1715 cm–1 • phenyl or vinyl esters: 1770-1780 cm–1 C–O stretches (strong absorptions; asymmetrical coupled vibrations) • saturated aliphatic esters (except acetates): C–O: 1210–1163 cm–1 • acetates: 1240 cm–1 • α,β-unsaturated esters: 1300–1160 cm–1 • benzoate esters: 1310–1250 cm–1 Infrared spectrum of isopropyl acetate: SDBSWeb: http://sdbs.db.aist.go.jp (National Institute of Advanced Industrial Science and Technology, January 8, 2015). 2015 Department of Chemistry, Illinois State University, Normal, IL 61790-4160 Page 26 The Infrared Spectra of Amides Dominant Observable Vibrations: C=O stretching ~1660-1680 cm-1 N-H stretching ~3200-3500 cm-1 Infrared spectrum of octanamide: SDBSWeb: http://sdbs.db.aist.go.jp (National Institute of Advanced Industrial Science and Technology, January 8, 2015). Amide I band (C=O) Amide II band (NH bending) 2015 Department of Chemistry, Illinois State University, Normal, IL 61790-4160 Page 27 The Infrared Spectra of Nitriles -C≡N triple bond stretch ~2240-2280 cm-1 Infrared spectrum of heptanenitrile: SDBSWeb: http://sdbs.db.aist.go.jp (National Institute of Advanced Industrial Science and Technology, January 8, 2015). Comparison with terminal alkynes (R-C≡C-H) Both alkynes and nitriles possess triple bonded functional groups that appear in nearly the same region in the infrared spectrum. A key difference is that the alkynes (if they are terminal alkynes) will possess additional features that should readily allow for diagnostic confirmation for one functional group or the other. 1-octyne (a terminal alkyne): 3323, 3024, 2926, 1636, 1466, 964 cm-1.