Learning Guide for Chapter 3 - Infrared SpectroscopyI. Introduction to spectroscopy - p 1II. Molecular vibrations - p 3III. Identifying functional groups - p 6IV. Interpreting an IR spectrum - p 12I. Introduction to spectroscopyWhat does it mean to characterize a compound?figure out the identity and structure of a compoundWhat are some examples of when you would need to do this?confirming the identity of a compound you've madeisolating a useful compound from natureidentifying a pollutantWhat 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 hypothesisHow are chemical tests used to characterize organic compounds?test for whether a functional group is presentbromine test - add Br2 to a compound, if it has a C=C the brown color disappearsTollen's test - add Ag+ ions to a compound, if it has an aldehyde, Ag is formedWhat does spectroscopy use to characterize a compound?lightWhat 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 groupsNMR (nuclear magnetic resonance) - gives information about atoms, used to determine structureUV (ultraviolet) - gives information about pi bondsMS (mass spectroscopy) - gives MW, information about structure*** we will focus on theseCharacterizing compoundsLG Ch 3 p 2What kinds of light are present in the electromagnetic spectrum? How do they affect atoms and molecules?UVNMRIRradiomicrowaveinfraredvisiblex-raysgammalow energytransitions in energy levels of nucleusrotational energyvibrational energyelectrontransitionsionizing radiationUVatomic absorptionfrequencyhigh energywavelengthhighershorterlongerlowerWhat unit of frequency is used in IR spectroscopy? What is the range of frequencies used in IR spectroscopy?wavenumber = # of waves /cmhigh energylow energy4000 cm-1400 cm-1wavenumber lowerhigherWhat are the essential components of an IR spectrometer?lasersample cell detectorcomputercreates IR lightlight goes throughthe compound, some frequencies absorbedlight that is notabsorbed comesout, is detecteddata from detector used to create a spectrumWhat is FTIR?Fourier transform IR - uses all frequencies at once, sorts it out mathematicallyonly kind currently usedHow can you take the IR of a liquid compound?thin film, neat - a few drops on a salt plate or crystalHow can you take an IR of a solid compound?melt - heat the compound on a salt platenujol mull - make a paste with a hydrocarbon oilKBr pellet - grind compound with KBr, press it into a transparent pelletThe Electromagnetic SpectrumThe IR Spectrometer and and IR SpectrumLG Ch 3 p 3Which of the following represent the scissor, rocking, wagging, and twisting vibrations of a CH2 group?CH HCH HCHHHCHCHHCHCHCH HHHscissor rockingwaggingtwistingWhich vibrations in a molecule will we be concerned with?the ones that give us useful information (there will be many that won't!)II. Molecular vibrationsWhy do molecules absorb infrared light?frequency of the light matches the frequency of a bond vibration in the moleculeWhat are the two types of vibrations that molecules can undergo?stretching - bond length changesbending - bond angles changesWhat does a stretching vibration look like in an HCl molecule?H ClH ClWhat does a symmtrical and unsymmetrical vibration look like in a water molecule?HOHHOHHOHHOHsymmetricalunsymmetricalWhich of these is the molecule actually doing?both - at different frequenciesWhat does an IR spectrum look like?4000 cm-1400 cm-1frequency% transmittance100%Types of vibrations(how much light goes through)frequencies absorbed by the compound - called bandsLG Ch 3 p 4Predict which of the following bands will have the higher frequency, and explain why.C HC C3000 cm-11200 cm-1lighter atomhigher frequencyheavier atomlower frequency1) size of atoms2) bond orderC CC CC C1200 cm-11660 cm-12200 cm-1triple bondhigher frequencysingle bondlower frequency3) type of vibration2920 cm-11380 cm-1CHHCHHCHHCHHstretching vibrationhigher frequencybending vibrationlower frequencyPredict which of the following vibrations will absorb more light, and explain why.Ostrong dipole small dipole1) strength of dipoleno dipole2) number of bondsOO14 C-H bonds8 C-H bondsFactors which affect a bandfasterslowerslowerfasterbendingstretchingstretching(tight spring)(loose spring)lots of light absorbedsome lightabsorbedno light absorbedlots of light absorbedsome lightabsorbedLG Ch 3 p 5What are the two main regions of the frequency axis?functional group region - bands that are tell you the functional groupfingerprint region - bands that are unique to the compound4000 - 1300 cm-1functional group region1300 - 400 cm-1fingerprint regionThe spectra of two different alcohols are shown below. Which bands belong tot he functional group region, and which to the fingerprint region? If you take a spectum of an unknown compound, what can you deduce about it from the spectum?functional groupWhat would you need to be able to identify the compound?a spectrum of the real compound to match it up toRegions of the IR Spectrumfingerprint regionfingerprint regionO-HO-HC-HC-Hfunctional group bandsbands are the samefingerprint bandsbands are differentLG Ch 3 p 6III. Identifying functional groupsThe following bonds absorb IR light at known frequencies. They are useful for deducing the functional groups of compounds.C HC HC HO HN HO HonC OC CC CC NC OC CaromaticNH2 bendingCH2 bendingaromatic overtonesC HCOOH dimerC=C bendingC XC CC OononC ConC OFor 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)C CC Hon C=CC CC HonC CCH2 bending(most compounds have these)C=C bendingC HCH2 bendingC HCH2 bendingC Haromatic ring (5)alcohol (4)ether (3)C Caromaticaromatic overtonesC Hon C=CO HC OC OOHOC HCH2 bendingC HCH2 bendingC HCH2 bending(no C=C bending)LG Ch 3 p 7carboxylic acid (6)aldehyde (4)ketone (3)C OOHOOHOC OC Hon C=OC OO Hon C=OC OCOOH dimerC HCH2 bendingC HCH2 bendingC HCH2 bendingester (4)anhydride (4)amine (4)amide (5)nitrile (3)OOOOONH2NH2ONalkyl halide (3)ClC OC OC OC ON HNH2 bending (if 1o)C NN HNH2 bending (if 1o)C OC HCH2 bendingC HCH2 bendingC HCH2 bendingC HCH2 bendingC HCH2 bendingC HCH2 bendingC X(no C-N!)(no C-N!)Which bands would you expect to find in the following