Learning Guide for Chapter 3 - Infrared Spectroscopy

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1 Learning Guide for hapter 3 - Infrared Spectroscopy I. Introduction to spectroscopy - p 1 II. Molecular vibrations - p 3 III. Identifying functional groups - p 6 IV. Interpreting an IR spectrum - p 12 haracterizing compounds I. Introduction to spectroscopy What does it mean to characterize a compound? figure out the identity and structure of a compound What are some examples of when you would need to do this? confirming the identity of a compound you've made isolating a useful compound from nature identifying a pollutant What are some physical properties of organic compounds, and how can they be used in characterization? mp, bp, solubility, color, odor, density -only useful to confirm a hypothesis ow are chemical tests used to characterize organic compounds? test for whether a functional group is present bromine test - add Br 2 to a compound, if it has a = the brown color disappears Tollen's test - add Ag + ions to a compound, if it has an aldehyde, Ag is formed What does spectroscopy use to characterize a compound? light What are the most common kinds of spectroscopy used in organic chemistry, and what kinds of information do they give? * * IR (infrared) - gives information about bonds, used to determine functional groups MR (nuclear magnetic resonance) - gives information about atoms, used to determine structure UV (ultraviolet) - gives information about pi bonds MS (mass spectroscopy) - gives MW, information about structure * we will focus on these

2 The Electromagnetic Spectrum LG h 3 p 2 What kinds of light are present in the electromagnetic spectrum? ow do they affect atoms and molecules? high energy higher shorter frequency wavelength lower longer low energy gamma x-rays ionizing radiation UV visible electron transitions UV atomic absorption infrared vibrational energy IR microwave rotational energy radio transitions in energy levels of nucleus MR What unit of frequency is used in IR spectroscopy? What is the range of frequencies used in IR spectroscopy? wavenumber = # of waves /cm high energy low energy higher wavenumber lower 4000 cm cm -1 The IR Spectrometer and and IR Spectrum What are the essential components of an IR spectrometer? laser sample cell detector computer creates IR light What is FTIR? light goes through the compound, some frequencies absorbed light that is not absorbed comes out, is detected data from detector used to create a spectrum Fourier transform IR - uses all frequencies at once, sorts it out mathematically only kind currently used ow can you take the IR of a liquid compound? thin film, neat - a few drops on a salt plate or crystal ow can you take an IR of a solid compound? melt - heat the compound on a salt plate nujol mull - make a paste with a hydrocarbon oil KBr pellet - grind compound with KBr, press it into a transparent pellet

3 What does an IR spectrum look like? 100% 4000 cm -1 frequency 400 cm -1 LG h 3 p 3 % transmittance (how much light goes through) Types of vibrations frequencies absorbed by the compound - called bands II. Molecular vibrations Why do molecules absorb infrared light? frequency of the light matches the frequency of a bond vibration in the molecule What are the two types of vibrations that molecules can undergo? stretching - bond length changes - bond angles changes What does a stretching vibration look like in an l molecule? Which of the following represent the scissor, rocking, wagging, and twisting vibrations of a 2 group? scissor rocking l l What does a symmtrical and unsymmetrical vibration look like in a water molecule? symmetrical unsymmetrical Which of these is the molecule actually doing? both - at different frequencies wagging twisting Which vibrations in a molecule will we be concerned with? the ones that give us useful information (there will be many that won't!)

4 Factors which affect a band LG h 3 p 4 Predict which of the following vibrations will absorb more light, and explain why. 1) strength of dipole strong dipole lots of light absorbed small dipole some light absorbed no dipole no light absorbed 2) number of bonds 14 - bonds lots of light 8 - bonds absorbed some light absorbed Predict which of the following bands will have the higher frequency, and explain why. 1) size of atoms 3000 cm cm -1 lighter atom higher frequency faster heavier atom lower frequency slower 2) bond order 2200 cm cm cm -1 triple bond higher frequency single bond lower frequency 3) type of vibration faster (tight spring) 2920 cm cm -1 stretching vibration higher frequency (loose spring) vibration lower frequency slower stretching stretching

5 Regions of the IR Spectrum LG h 3 p 5 What are the two main regions of the frequency axis? functional group region - bands that are tell you the functional group fingerprint region - bands that are unique to the compound cm -1 functional group region cm -1 fingerprint region The spectra of two different alcohols are shown below. Which bands belong tot he functional group region, and which to the fingerprint region? fingerprint region - - fingerprint region - - functional group bands bands are the same fingerprint bands bands are different If you take a spectum of an unknown compound, what can you deduce about it from the spectum? functional group What would you need to be able to identify the compound? a spectrum of the real compound to match it up to

6 III. Identifying functional groups LG h 3 p 6 The following bonds absorb IR light at known frequencies. They are useful for deducing the functional groups of compounds. 2 on 2 on = on dimer on aromatic overtones aromatic X For each of the functional groups below, write down which of these bands it should have. The number of bands you should look for is given in ( ). alkane (2) alkene (5) alkyne (4) 2 (most compounds have these) 2 on = = 2 on aromatic ring (5) alcohol (4) ether (3) 2 aromatic 2 2 on = aromatic overtones (no = )

7 LG h 3 p 7 ketone (3) 2 aldehyde (4) 2 carboxylic acid (6) 2 (no - on =!) on = on = dimer ester (4) anhydride (4) amine (4) (x 2) amide (5) nitrile (3) (no -!) 2 (if 1 o ) 2 (if 1 o ) (no -!) alkyl halide (3) l 2 X Which bands would you expect to find in the following ranges? cm cm cm cm cm -1 on on = on = on = dimer 2 aromatic overtones (fingerprint) X =

8 LG h 3 p 8 Using the charts on the previous pages, identify the important bands for each functional group. Alkane hexane cm , cm-1 useful reference points most compounds contain them Alcohol 1-heptanol cm-1 (must be large!) confirmation band cm-1 (broad, round) main band to look for Alkyne small dipole cyclohexylacetylene - on --- on --- ~ 3300 cm-1 no dipole! cm-1 *internal alkynes only no on no (no dipole) What would it look like? alkane

9 LG h 3 p 9 Ketone 3-pentanone = cm-1 Anhydride propanoic anhydride = (two) cm-1 (two prominent bands) cm-1 only one, very large Amine 2 butyl amine 1o amine cm-1 (broad) 2o amine 2 2 ~1600 cm-1 (often a triangular) *only if 1o only one - band no 2 3o amine no - bands no 2 (like an alkane!)

10 LG h 3 p 10 Alkene 1-octene - on = = = 2 on cm cm cm -1 (confirmation only) itrile butanenitrile cm -1 2 arboxylic acid butanoic acid - on = = on = dimer cm - -1 very broad overlaps - stretching dimer 2700 cm -1 (sometimes two) cm cm -1 (not always visible)

11 LG h 3 p 11 Alkyl halide Br bromomethyl cyclohexane -X 2 X -Br cm-1 -l cm-1 often hard to distinguish (must be large!) Amide 2 -methylpropanamide - = 2 2 cm (broad) 1o bands 2o band 3o - - no - bands cm-1 2 ~1600 cm-1 (broad) *only if 1o Aldehyde 3-methylpentanal = - on = - may show up equilibrium with hydrate 2 - on = ~2720 cm-1 (2 bands) cm-1

12 LG h 3 p 12 aromatic overtones cm-1 Aromatic 2 sec-butyl benzene - on = = aromatic overtones - - on = cm-1 = in aromatic ring cm-1 Ester ethyl butyrate - = - 2 = cm cm-1 (must be large!) Ether 2 dibutyl ether cm-1 (must be large!)

13 IV. Interpreting an IR spectrum LG h 3 p 13 If you think you know the identity of a compound that you have a spectrum of, what should you do? figure out all of the bands it should have, see if they are there alcohol -, - ester =, - you need to know which bands each functional group has from memory! flash cards alkene =, - on =, = If you don't know the identity of the compound you have a spectrum of, what should you do? 1) find - and 2 to use as reference points 2) look in each area for bands that stick out -, -, - on =, ---, ---, = 3) then look for bands that confirm, or narrow it down if you see an -, look for: if you see an -, look for: - (alcohol) 2 (amine/amide) = (amide) if you see a or, look for: - on --- (alkyne will have it, nitrile won't) if you see a =, look for: - (ester) if there are no obvious bands, look for: - on =, dimer (carboxylic acid) - on = (aldehyde) -, 2 (amide) two of them, - (anhydride) none of these (ketone) =, - on = (alkene) - (ether) -X (alkyl halides)

14 ow would you tell spectra of the following compounds apart? LG h 3 p 14 aldehyde vs. ketone? aldehyde has - next to =, ketone doesn't alkane vs. ether? ketone vs. ester? amide vs. amine? nitrile vs. alkyne? ether has -, alkane doesn't ester has -, ketone doesn't amide has =, amine doesn't alkyne has - next to ---, nitrile doesn't, --- larger alcohol vs. amine? - and - are in the same region, but - is broader if 2, two bands + 2, alcohol has - alcohol vs. carboxylic acid? alcohol smaller, doesn't cover has =, dimer band What would you look for in the IR spectum of the following reactions? alcohol - - r 3 ketone = - should disappear - should disappear = should appear 2 a 3 2 Br 1 o amide = 2 o amide - (2) 2 = - (1) = should still be there, unchanged - should go from 2 bands to 1 band 2 should disappear aromatic carboxylic acid 2 S 4 ethanol = aromatic - ester dimer = aromatic overtones - on = large - should disappear = should still be there, may shift - should appear = aromatic, - on =, overtones still there = - = aromatic overtones - on =

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