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Infrared Spectroscopy. Infrared Absorbance IR- Empirical Comparisons Identifying functional groups in organic molecules.

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Presentation on theme: "Infrared Spectroscopy. Infrared Absorbance IR- Empirical Comparisons Identifying functional groups in organic molecules."— Presentation transcript:

1 Infrared Spectroscopy

2 Infrared Absorbance

3 IR- Empirical Comparisons Identifying functional groups in organic molecules

4 Region of infrared that is most useful lies between 2.5-16  m (4000-625 cm -1 ) depends on transitions between vibrational energy states Stretching: higher energy / higher wave number (cm -1 ) Bending: lower energy / lower wave number (cm -1 ) Infrared Spectroscopy

5 A bond must have a dipole or an induced dipole in order to have an absorbance in the IR spectrum. When the bond stretches, the increasing distance between the atoms increases the dipole moment. Therefore, the greater the dipole, the more intense the absorption. (i.e., The greater the molar extinction coefficient (  ) in Beer’s law, A =  bc.

6 The energy (IR frequency/ wave number) and the intensity of the absorption band also depends on the concentration of solution from Beer’s law, A =  bc. It is easier to stretch an O–H bond if it is hydrogen bonded

7 The presence & absence of absorption bands must be considered in identifying a possible structure in IR spectroscopy. Empiricism is critical to successful identification. NOTE: Bonds which lack dipole moments are not detected. Analyzing Structure: Functions & Infrared Spectra The molecular formula is a critical piece of information, which limits the functional possibilities.

8 Structural/Functional Components

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10 The peaks are quantized absorption bands corresponding to molecular stretching and bending vibrations An Infrared Spectrum

11 The fingerprint region The functional group stretching region

12 Structural unitFrequency, cm -1 Stretching vibrations (single bonds) O—H (alcohols)3200-3600 O—H (carboxylic acids) 3000-3100 N—H3350-3500 Infrared Absorption Frequencies First examine the absorption bands in the vicinity of 4000-3000 cm –1

13

14 IR Spectrum of a Primary Amine(1 o ) The N–H bending vibration occurs at ~1600 cm –1

15 Structural unitFrequency, cm -1 Stretching vibrations (single bonds) sp C—H3310-3320 sp 2 C—H3000-3100 sp 3 C—H2850-2950 sp 2 C—O1200 sp 3 C—O1025-1200 Infrared Absorption Frequencies

16 Structural unitFrequency, cm -1 Stretching vibrations (single bonds) sp C—H3310-3320 sp 2 C—H3000-3100 sp 3 C—H2850-2950 sp 2 C—O1200 sp 3 C—O1025-1200 Infrared Absorption Frequencies

17 Structural unitFrequency, cm -1 Stretching vibrations (multiple bonds) Infrared Absorption Frequencies C C 1620-1680—CN —CC— 2100-2200 2240-2280

18 Some hydrocarbon absorption bands

19 Structural Components & Functional Differences: Therefore the amide has a longer (weaker) C=O bond (1680-1700 cm -1 ) and the ester (1730-1750 cm -1 ) is shorter (stronger). The nitrogen of an amide is less electronegative than the oxygen of an ester.

20 Structural unitFrequency, cm -1 Stretching vibrations (carbonyl groups) Aldehydes and ketones1710-1750 Carboxylic acids1700-1725 Acid anhydrides1800-1850 and 1740-1790 Esters1730-1750 Amides1680-1700 Infrared Absorption Frequencies C O

21 Cyclic aliphatic ketone

22 Mono substituted aromatic methyl ketone

23 Mono substituted aromatic ketone

24 Aliphatic ester I

25 Aliphatic ester II

26 Aliphatic ester III

27 Mono substituted aromatic ester

28 Mono substituted aromatic conjugated ester

29 Structural unitFrequency, cm -1 Stretching vibrations (single bonds) sp 2 C—O1200 sp 3 C—O1025-1200 Infrared Absorption Frequencies

30 Dihexyl Ether ~1100 cm -1 1025-1200 cm -1

31 ~1200 cm -1

32 Structural unitFrequency, cm -1 Bending vibrations of alkenes Infrared Absorption Frequencies CH 2 RCH R2CR2CR2CR2C CHR' cis-RCH CHR' trans-RCH CHR' R2CR2CR2CR2C910-990 890 665-730 960-980 790-840

33 wavenumber (cm –1 ) assignment 3075 2950 1650 and 890 ??????

34 cis- 665-730 trans- 960-980

35 Summary: C–H bond absorption and hybridization of the carbon atom

36 Distinctive Stretch of C–H Bond in an Aldehyde (the “waggle” vibration)

37 Aliphatic aldehyde

38 Mono-substituted aromatic aldehyde

39 Mono-substituted aromatic conjugated aldehyde

40 Mono substituted aromatic ester

41 Para di-substituted aromatic ether & aldehyde

42 Infrared Spectroscopy Common Functional Groups Handout Handout http://chemconnections.org/general/chem121/Spectroscopy/IR-handout-11.htm

43 Structural unitFrequency, cm -1 Bending vibrations of derivatives of benzene Monosubstituted730-770 and 690-710 Ortho-disubstituted735-770 Meta-disubstituted750-810 and 680-730 Para-disubstituted790-840 Aromatic Absorption Frequencies

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45 200035003000250010001500500 Wave number, cm -1 Infrared Spectrum of tert-butylbenzene H—C Ar—H Monsubstituted benzene C 6 H 5 C(CH 3 ) 3

46 Chemical Communication: Smell / Pheromones Pheromone Synthesis [20:40-23:51] http://chemconnections.org/COT/COT-chemcomm-eg.html http://www.learner.org/resources/series61.html

47 Infrared Spectroscopy Common Functional Groups QUIZ

48 Question Is the following IR of cis or trans 2- pentene? A)cisB) trans

49 A)benzyl alcohol B)1,4,6-heptatrien-3-one C)2,4,6-heptatrienaldehyde D)benzaldehyde C7H6OC7H6OC7H6OC7H6O Question Identify the compound from the IR above.

50 Question C 10 H 12 O Identify the compound from the IR above. A)4-phenylbutanaldehyde B)phenylpropyl ketone C)meta-isopropylbenzaldehyde D)1-phenyl-2-butanone

51 Question C3H4OC3H4OC3H4OC3H4O A)cyclopropanone B)propynol C)2-cyclopropenol D)1,2-propadienol Identify the compound from the IR above.

52 Question C 3 H 7 NO Identify the compound from the IR above. A)N-methylacetamide B)N,N-dimethylformamide C)3-aminopropanal D)N-methylamino-ethanal

53 Match the ortho, meta and para isomers of xylene. I III III II II orthometapara A)IIIIII B)IIIIII C)IIIIII D)IIIIIIQuestion

54 Question A)methylbenzoate B)phenylacetate C)p-anisaldehyde D)o-anisaldehyde Identify the compound from the IR above. C8H8O2C8H8O2C8H8O2C8H8O2


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