SpectroscopyInfrared Spectra

Infrared spectra in this presentation are taken by permission from the SDBS web site:

SDBSWeb: http://www.aist.go.jp/RIODB/SDBS/

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Spectroscopyseeing the unseeableUsing electromagnetic radiation as a probe to obtain information about atoms and molecules that are too small to see.Electromagnetic radiation is propagated at the speed of light through a vacuum as an oscillating wave.

electromagnetic relationships: = c 1/E = hE E = hc/E 1/

= wave length = frequencyc = speed of lightE = kinetic energyh = Plancks constant

c

Two oscillators will strongly interact when their energies are equal.E1 = E21 = 21 = 2If the energies are different, they will not strongly interact!We can use electromagnetic radiation to probe atoms and molecules to find what energies they contain.

some electromagnetic radiation rangesApprox. freq. rangeApprox. wavelengths Hz (cycle/sec) meters

Radio waves 104 - 1012 3x104 - 3x10-4Infrared (heat) 1011 - 3.8x1014 3x10-3 - 8x10-7Visible light 3.8x1014 - 7.5x1014 8x10-7 - 4x10-7Ultraviolet 7.5x1014 - 3x1017 4x10-7 - 10-9X rays 3x1017 - 3x1019 10-9 - 10-11Gamma rays > 3x1019 < 10-11

Infrared radiation = 2.5 to 17 m = 4000 to 600 cm-1These frequencies match the frequencies of covalent bond stretching and bending vibrations. Infrared spectroscopy can be used to find out about covalent bonds in molecules.

IR is used to tell:1. what type of bonds are present2. some structural information

IR source sample prism detector graph of % transmission vs. frequency=> IR spectrum40003000200015001000 500v (cm-1)100

%T

0

toluene

Some characteristic infrared absorption frequenciesBONDCOMPOUND TYPEFREQUENCY RANGE, cm-1C-Halkanes2850-2960 and 1350-1470alkenes3020-3080 (m) andRCH=CH2910-920 and 990-1000R2C=CH2880-900cis-RCH=CHR675-730 (v)trans-RCH=CHR965-975aromatic rings3000-3100 (m) andmonosubst.690-710 and 730-770ortho-disubst.735-770meta-disubst. 690-710 and 750-810 (m)para-disubst.810-840 (m)alkynes3300O-Halcohols or phenols3200-3640 (b)C=Calkenes1640-1680 (v)aromatic rings1500 and 1600 (v)CCalkynes2100-2260 (v)C-Oprimary alcohols1050 (b)secondary alcohols1100 (b)tertiary alcohols1150 (b)phenols1230 (b)alkyl ethers1060-1150aryl ethers1200-1275(b) and 1020-1075 (m) all abs. strong unless marked: m, moderate; v, variable; b, broad

IR spectra of ALKANESCH bond saturated(sp3) 2850-2960 cm-1 + 1350-1470 cm-1

-CH2- + 1430-1470-CH3 + and 1375 -CH(CH3)2 + and 1370, 1385-C(CH3)3 + and 1370(s), 1395 (m)

n-pentaneCH3CH2CH2CH2CH3

3000 cm-11470 &1375 cm-12850-2960 cm-1satd C-H

CH3CH2CH2CH2CH2CH3n-hexane

2-methylbutane (isopentane)

2,3-dimethylbutane

cyclohexaneno 1375 cm-1no CH3

IR of ALKENES=CH bond, unsaturated vinyl(sp2)3020-3080 cm-1 + 675-1000RCH=CH2 +910-920 & 990-1000R2C=CH2 +880-900cis-RCH=CHR +675-730 (v)trans-RCH=CHR +965-975

C=C bond 1640-1680 cm-1 (v)

1-decene910-920 & 990-1000 RCH=CH2C=C 1640-1680unsatdC-H

3020-3080 cm-1

4-methyl-1-pentene910-920 & 990-1000 RCH=CH2

2-methyl-1-butene880-900R2C=CH2

2,3-dimethyl-1-butene880-900R2C=CH2

IR spectra BENZENEs=CH bond, unsaturated aryl(sp2) 3000-3100 cm-1 + 690-840mono-substituted +690-710, 730-770ortho-disubstituted + 735-770meta-disubstituted + 690-710, 750-810(m)para-disubstituted + 810-840(m)

C=C bond1500, 1600 cm-1

ethylbenzene690-710, 730-770 mono-1500 & 1600Benzene ring3000-3100 cm-1Unsatd C-H

o-xylene735-770ortho

p-xylene810-840(m)para

m-xylenemeta 690-710, 750-810(m)

styreneno satd C-H910-920 & 990-1000RCH=CH2mono1640C=C

2-phenylpropenemono880-900R2C=CH2Satd C-H

p-methylstyrenepara

IR spectra ALCOHOLS & ETHERS

CO bond1050-1275 (b) cm-11o ROH10502o ROH11003o ROH1150ethers1060-1150

OH bond3200-3640 (b)

1-butanolCH3CH2CH2CH2-OHC-O 1o3200-3640 (b) O-H

2-butanolC-O 2oO-H

tert-butyl alcohol C-O 3oO-H

methyl n-propyl etherno O--HC-O ether

2-butanone C=O ~1700 (s)

C9H12C-H unsatd & satd1500 & 1600benzenemonoC9H12 C6H5 = -C3H7isopropylbenzenen-propylbenzene?

n-propylbenzene

isopropyl split 1370 + 1385isopropylbenzene

C8H6C-H unsatd1500, 1600benzenemonoC8H6 C6H5 = C2Hphenylacetylene3300C-H

C4H81640-1680C=C880-900R2C=CH2isobutylene CH3 CH3C=CH2Unstd

Which compound is this?2-pentanone1-pentanol1-bromopentane2-methylpentane1-pentanol

What is the compound?1-bromopentane1-pentanol2-pentanone2-methylpentane2-pentanone

In a matching problem, do not try to fully analyze each spectrum. Look for differences in the possible compounds that will show up in an infrared spectrum.

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