How to Interpret an Infrared (IR) Spectrum Infrared (IR) Spectroscopy allows the identification of particular bonds present within molecules. In this class we have simplified IR analysis by only focusing on 5 regions of the spectrum. Do not try to examine and identify each and every peak within the entire spectrum. Instead, sequentially look at each of the designated 5 regions. Yes, you are expected memorize the exact location for each of these regions and to know the distinctive peak shape for the N-H/O-H region. First determine if there is a substantial absorbance present in each of the designated regions. Then determine which bonds are present. From the bonds present, determine which functional group is responsible. That is the functional group of the molecule. You need to know your functional groups to be able to do this successfully. Also review your lecture notes and read Klein text book. N-H/OH Bonds. This is toward the far left of the spectra.(3200-3600 cm -1 ) The shape and location define which bond type is present. -O-H 3200-3600 cm -1 Alcohol; Broad, rounded and distinct -NH 2 3200-3600 cm -1 Primary Amine; Sharp double point -N-H 3200-3600 cm -1 Secondary Amine; Sharp single point -COO-H 2500-3600 cm -1 Carboxylic Acid; Very broad. Extends outside designated region. CH Bonds The exact location defines the type of C-H bond present. (2800-3100 cm -1 ) Draw a line at 3000 cm-1, peaks just to right are sp 3 to left are sp 2. sp 3 -C-H 2800-3000 cm -1 sp 3 Carbon-Hydrogen; Multi-peaked, strong. Almost always present sp 2 -C-H 3000-3100 cm -1 sp 2 Carbon-Hydrogen; Usually a shoulder on left of sp 3 C-H absorbance. sp-c-h 3300 cm -1 Terminal Alkyne: Icicle-like within typical NH/OH region Triple Bonds Two types of bonds overlap. (2100-2300 cm -1 ) C N 2100-2300 cm -1 Nitrile; C C 2100-2300 cm -1 Terminal alkyne; weak (often not seen at all for internal alkynes.) Double Bonds Two types of bonds overlap. The shape and exact location defines which bond is present. (1600-1800 cm -1 ) C=O 1650-1800 cm -1 Carbonyl; Strong distinct absorbance. C=C 1600-1680 cm -1 Alkene; Sharp medium size Fingerprint Region This area is used to confirm identity but is not very useful for assigning identity. C-O 1000-1300cm -1. This area is difficult to distinguish. Use only when necessary. This area will be necessary to distinguish between an alkane and an ether, and between a ketone and an ester. Page 1
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Bonds Position (cm -1 ) strength and Shape O-H/ NH 3200-3600 (Hydrogen bonded to Nitrogen & Oxygen) -O-H 3200-3600 rounded -N-H, -NH 2 3200-3600 single or double pointed -COOH 2500-3600 very broad C-H 2800-3100, 3300 (Hydrogen bonded to Carbon) sp C-H 3300 narrow and very pointed sp 2 =C-H 3000-3100 shoulder to left of sp 3 C-H sp 3 C-H 2800-3000 multi-peaked 2100-2300 (Triple Bond; Carbon to carbon and carbon to nitrogen) C N 2100-2300 sharp C C-H 2100-2300 sharp = 1600-1800 (Double Bond; Carbon to ) C=O 1650-1800 strong slightly rounded C=C 1600-1680 medium, sharp Carbon to oxygen single bond 1000-1300 limited value -C-O- 1000-1300 use only when necessary Note: All spectra from SDBSWeb: : http://sdbs.db.aist.go.jp (National Institute of Advanced Industrial Science and Technology, August 31, 2016) Page 3
3200-3600cm -1 Oxygen-hydrogen, nitrogen-hydrogen (and sp hybridized carbon-hydrogen bonds). Shape dependent. Include Alcohols, Carboxylic Acids, Amines, Terminal Alkynes. ALCOHOLS have very distinctive broad rounded absorbance between 3200 and 3600 cm -1. There is a distinct break between the OH absorbance and the sp 3 CH absorbance. CARBOXYLIC ACID- The OH of a carboxylic acid maximum absorbance shifts over toward 3000cm -1. This is a very broad absorbance extending from 2500 to 3500cm-1 which spills over the sp 3 CH absorbance. TERMINAL ALYKNES- The terminal C-H bond of C-H has a very sharp icicle like absorbance around 3300cm -1. PRIMARY AMINES- Show TWO short pointed absorbances between 3200-3500 cm -1. SECONDARY AMINES- Shows one short pointed absorbance between 3200-3500 cm -1. TERTIARY AMINES- There are no N-H bonds, so no N-H absorbance. Typical shapes in 3200-3600 cm-1 Region. Alcohol Terminal alkyne Secondary Amine Carboxylic Acid Primary Amine Page 4
Page 5 Pattern differences between Carboxylic Acids, Alcohols, Terminal Alkynes and Amines
3100-3000cm -1 3000-2800cm -1. Sp 2 and sp 3 carbonhydrogen bonds. Almost every organic molecule will have an absorbance in the 3000-2800cm -1 region because most organic molecules contain sp 3 C-H bonds. Different alkanes have little discernible difference in this area. It is often useful to draw a vertical line at 3000cm -1, any peak to the left of this line will be sp 2 C-H bonds and those to the right will be sp 3 C-H bonds. If alkene =C-H is present it will often look like shoulder to left of regular C-H absorbance around 3100. Sp 2 C-H sp 3 C-H sp 3 C-H Alkane, Pentane Alkene, 1-pentene 2100-2300cm -1. Triple bonds. -C N or C C. ALKYNES and NITRILES have an absorbance in a usually barren area of 2100-2300cm -1. The nitrile peak is usually stronger than alkyne. If a terminal alkyne is present the sp C-H ( C-H) bond will show an additional absorbance around 3300cm -1. R-C N nitrile -C CH alkyne Page 6
1600-1680 cm -1 &1650-1800cm -1 Double Bond region, Carbonyl (C=O) and Alkene (C=C) Carbon-Oxygen double bond (C=O or carbonyl) has a very distinctive and strong absorbance between1650-1800 cm -1 A normal ketone typically exists close to 1720 cm -1. Carbon-Carbon double bonds, (C=C or alkene), absorb between 1620 and 1680cm -1. Alkenes tend to be sharper and less intense (weaker). Carbonyl(~1720cm-1) Alkene (~1650cm-1) 1000-1300cm -1. C-O Single bond. This is a difficult to correctly interpret region. Use this region only when necessary, such as when trying to distinguish an ether from an alkane or between an ester from a ketone. Sizable strong absorbance little or weak absorbance Page 7
Alkane. Only sp 3 C-H at 2800-3000cm -1. Alkene. Sp 2 C-H at 3100 cm -1, sp 3 C-H at 2800-3000cm -1, C=C at 1620cm-`. [ ] 3200-3600cm -1 (O--H,N-H) [ ] 3200-3600cm -1 (O--H,N-H) [X ] 3000-3100 cm -1 (=C-H) [X ]1600-1680 cm -1 (C=C) Primary Amine,NH 2 at 3400cm -1, sp 3 C-H at 2800-3000cm -1, [X ] 3200-3600cm -1 (O--H,N-H) Secondary Amine,NH at 3400cm -1, sp 3 C-H at 2800-3000cm -1, X ] 3200-3500cm -1 (O--H,N-H) X ] 3200-3500cm -1 (O--H,N-H) [X ] 3200-3600 cm -1 (C=O) [ ]3000-3100 cm -1 (C=C) [X] 2800-3000 cm -1 (C-O) Page 8
Carboxylic Acid. VERY BROAD -O-H which spills over into normal sp 3 C-H at 2800-3000cm - 1 region, C=O at 1710cm -1, [X] 3200-3600cm -1 (O--H,N-H) [X ] 1650-1800 cm -1 (C=O) [x ] 1000-1300 cm -1 (C-O) Alcohol. -O-H broad at 3300cm -1, sp 3 C-H at 2800-3000cm -1, (-C-O at 1100cm-1), [X] 3200-3600cm -1 (O--H,N-H) [X ] 1000-1300 cm -1 (C-O) Internal alkyne or nitrile sp 3 C-H at 2800-3000cm -1, C C or C N at 2250cm-1,. [ ] 3200-3000cm -1 (O--H,N-H) [X ] 2100-2300 cm -1 ( ) terminal alkyne Sp C-H at 3300cm -1, sp 3 C-H at 2800-3000cm -1, C C or C N at 2250cm-1, [X] 3200-3600cm -1 (O--H,N-H) [X ] 2100-2300 cm -1 ( ) Page 9
Ketone or Aldehyde, sp 3 C-H at 2800-3000cm -1, C=O at 1720 cm-1, [ ] 3200-3600cm -1 (O--H,N-H) [X ] 1650-1800 cm -1 (C=O) Ester sp 3 C-H at 2800-3000cm -1, C=O at 1750 cm-1, (-C-O at 1250cm -1 ) [ ] 3200-3600cm -1 (O--H,N-H) [X ] 1650-1800 cm -1 (C=O) [X ] 1000-1300 cm -1 (C-O) Amide -NH2 at 3300-cm-1, sp 3 C-H at 2800-3000cm -1, C=O at 1750 cm-1, [X] 3200-3600cm -1 (O--H,N-H) [X ] 1650-1800 cm -1 (C=O) Ethersp 3 C-H at 2800-3000cm -1 (-C-O at 1150 cm-1), [ ] 3200-3600cm -1 (O--H,N-H) [X ] 1000-1300 cm -1 (C-O) Page 10
IR bond absorbance regions by functional group C n H (2n+2) Alkane Alkene Terminal alkyne [ ] 3200-3600cm -1 (O-H,N-H) or [ ] 3200-3600cm -1 (O-H,N-H) [X] 3000-3100 cm -1 (=C-H) [ ] 2100-2300 cm -1 [X ]1600-1680 cm -1 (C=C) [X] 3300cm -1 ( C-H) [X ] 2800-3000 cm -1 (C-H) [X] 2100-2300 cm -1 (C C) Nitrile or internal Alkyne Alcohol Ether [ ] 3300cm -1 ( C-H) [X] 2100-2300 cm -1 (C N, C C) [X] 3100-3600cm -1 (-O-H) [ ] 2100-2300 cm -1 (C N) [X] 1000-1300 cm -1 (C-O) [ ] 3200-3600cm -1 (O-H,N-H) [ ] 2100-2300 cm -1 (C N) [X] 1000-1300 cm -1 (C-O) and Ketone and aldehydes Ester Carboxylic Acid [ ] 3200-3600cm -1 (O-H) [X] 2800-3100 cm -1 (C-H) [ ] 2100-2300 cm -1 (C N, C C) [X ] 1650-1800 cm -1 (C=O) [ ] 3200-3600cm -1 (O-H) [X] 2800-3100 cm -1 (C-H) [X] 1650-1800 cm -1 (C=O) [X] 1000-1300 cm -1 (C-O) [X]3200-3600cm -1 (COO-H) [X] 2800-3100 cm -1 (C-H) [X] 1650-1800 cm -1 (C=O) [X] 1000-1300 cm -1 (C-O) Amine Amide [X] 3200-3600cm -1 (N-H) [X] 3200-3600cm -1 (N-H) [X] 2800-3100 cm -1 (C-H) [X] 2800-3100 cm -1 (C-H) [X] 1650-1800 cm -1 (C=O) Revised November 13, 2017 s.l. weaver Page 11