How to Interpret an Infrared (IR) Spectrum

Transcription

1 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.( cm -1 ) The shape and location define which bond type is present. -O-H cm -1 Alcohol; Broad, rounded and distinct -NH cm -1 Primary Amine; Sharp double point -N-H cm -1 Secondary Amine; Sharp single point -COO-H cm -1 Carboxylic Acid; Very broad. Extends outside designated region. CH Bonds The exact location defines the type of C-H bond present. ( 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 cm -1 sp 3 Carbon-Hydrogen; Multi-peaked, strong. Almost always present sp 2 -C-H 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. ( cm -1 ) C N cm -1 Nitrile; C C 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. ( cm -1 ) C=O cm -1 Carbonyl; Strong distinct absorbance. C=C 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 cm -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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3 Bonds Position (cm -1 ) strength and Shape O-H/ NH (Hydrogen bonded to Nitrogen & Oxygen) -O-H rounded -N-H, -NH single or double pointed -COOH very broad C-H , 3300 (Hydrogen bonded to Carbon) sp C-H 3300 narrow and very pointed sp 2 =C-H shoulder to left of sp 3 C-H sp 3 C-H multi-peaked (Triple Bond; Carbon to carbon and carbon to nitrogen) C N sharp C C-H sharp = (Double Bond; Carbon to ) C=O strong slightly rounded C=C medium, sharp Carbon to oxygen single bond limited value -C-O use only when necessary Note: All spectra from SDBSWeb: : (National Institute of Advanced Industrial Science and Technology, August 31, 2016) Page 3

4 cm -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 cm -1. SECONDARY AMINES- Shows one short pointed absorbance between cm -1. TERTIARY AMINES- There are no N-H bonds, so no N-H absorbance. Typical shapes in cm-1 Region. Alcohol Terminal alkyne Secondary Amine Carboxylic Acid Primary Amine Page 4

5 Page 5 Pattern differences between Carboxylic Acids, Alcohols, Terminal Alkynes and Amines

6 cm cm -1. Sp 2 and sp 3 carbonhydrogen bonds. Almost every organic molecule will have an absorbance in the cm -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 Sp 2 C-H sp 3 C-H sp 3 C-H Alkane, Pentane Alkene, 1-pentene cm -1. Triple bonds. -C N or C C. ALKYNES and NITRILES have an absorbance in a usually barren area of cm -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

7 cm -1 & cm -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 between 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) cm -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

8 Alkane. Only sp 3 C-H at cm -1. Alkene. Sp 2 C-H at 3100 cm -1, sp 3 C-H at cm -1, C=C at 1620cm-`. [ ] cm -1 (O--H,N-H) [ ] cm -1 (O--H,N-H) [X ] cm -1 (=C-H) [X ] cm -1 (C=C) Primary Amine,NH 2 at 3400cm -1, sp 3 C-H at cm -1, [X ] cm -1 (O--H,N-H) Secondary Amine,NH at 3400cm -1, sp 3 C-H at cm -1, X ] cm -1 (O--H,N-H) X ] cm -1 (O--H,N-H) [X ] cm -1 (C=O) [ ] cm -1 (C=C) [X] cm -1 (C-O) Page 8

9 Carboxylic Acid. VERY BROAD -O-H which spills over into normal sp 3 C-H at cm - 1 region, C=O at 1710cm -1, [X] cm -1 (O--H,N-H) [X ] cm -1 (C=O) [x ] cm -1 (C-O) Alcohol. -O-H broad at 3300cm -1, sp 3 C-H at cm -1, (-C-O at 1100cm-1), [X] cm -1 (O--H,N-H) [X ] cm -1 (C-O) Internal alkyne or nitrile sp 3 C-H at cm -1, C C or C N at 2250cm-1,. [ ] cm -1 (O--H,N-H) [X ] cm -1 ( ) terminal alkyne Sp C-H at 3300cm -1, sp 3 C-H at cm -1, C C or C N at 2250cm-1, [X] cm -1 (O--H,N-H) [X ] cm -1 ( ) Page 9

10 Ketone or Aldehyde, sp 3 C-H at cm -1, C=O at 1720 cm-1, [ ] cm -1 (O--H,N-H) [X ] cm -1 (C=O) Ester sp 3 C-H at cm -1, C=O at 1750 cm-1, (-C-O at 1250cm -1 ) [ ] cm -1 (O--H,N-H) [X ] cm -1 (C=O) [X ] cm -1 (C-O) Amide -NH2 at 3300-cm-1, sp 3 C-H at cm -1, C=O at 1750 cm-1, [X] cm -1 (O--H,N-H) [X ] cm -1 (C=O) Ethersp 3 C-H at cm -1 (-C-O at 1150 cm-1), [ ] cm -1 (O--H,N-H) [X ] cm -1 (C-O) Page 10

11 IR bond absorbance regions by functional group C n H (2n+2) Alkane Alkene Terminal alkyne [ ] cm -1 (O-H,N-H) or [ ] cm -1 (O-H,N-H) [X] cm -1 (=C-H) [ ] cm -1 [X ] cm -1 (C=C) [X] 3300cm -1 ( C-H) [X ] cm -1 (C-H) [X] cm -1 (C C) Nitrile or internal Alkyne Alcohol Ether [ ] 3300cm -1 ( C-H) [X] cm -1 (C N, C C) [X] cm -1 (-O-H) [ ] cm -1 (C N) [X] cm -1 (C-O) [ ] cm -1 (O-H,N-H) [ ] cm -1 (C N) [X] cm -1 (C-O) and Ketone and aldehydes Ester Carboxylic Acid [ ] cm -1 (O-H) [X] cm -1 (C-H) [ ] cm -1 (C N, C C) [X ] cm -1 (C=O) [ ] cm -1 (O-H) [X] cm -1 (C-H) [X] cm -1 (C=O) [X] cm -1 (C-O) [X] cm -1 (COO-H) [X] cm -1 (C-H) [X] cm -1 (C=O) [X] cm -1 (C-O) Amine Amide [X] cm -1 (N-H) [X] cm -1 (N-H) [X] cm -1 (C-H) [X] cm -1 (C-H) [X] cm -1 (C=O) Revised November 13, 2017 s.l. weaver Page 11