79.02015-04,e2.d1/25/9027wag30⊕ Infrared Spectroscop 0-H 3400cm-l C=C 2150cm N-H 715 C-H 2250 1100 HOWATOLANCOACH THE ANALYSIS OF A SPECTRUM (OR WHAT YOU CAN When analying the spectrum of an unknown.concentrate your first cffon determining the e)of a few major functional groups The C=0. -0. they are present.Do not try to make a detailed analysis of the near 3000cm tures can'tmisifcm 2.If C-is present,check the following types(if it is absent,go to step 3): ACIDS IsO-Halso present? absorption near 3400-2400 cm(usually overlaps AMIDES Is N-H also present Medium absorption near 3400 cm sometimes a double peak with equivalent halves. ESTERS IsC-Oalso present? Strong-intensity absorptions near 1300-1000 cm- ANHYDRIDES Two C=O absorptions near 1810 and 1760 cm-. ALDEHYDES Is aldehyde C-H present? nd 2750 cm on righ KETONES 3.If C=0 is absent ALCOHOLS.PHENOLS Check for O-H. Broad absorption near 3400-3300 cm- Confirm this by finding C-O near 1300-1000 cm-. AMINES Check for N-H. .Medium absorption(s)near3400 cm-. ETHERS
TABLE 2.4 BASE VALUES FOR ABSORPTIONS OF BONDS OIH 3400 cm−1 CKC 2150 cm−1 NIH 3400 CJO 1715 CIH 3000 CJC 1650 CKN 2250 CIO 1100 2.9 HOW TO APPROACH THE ANALYSIS OF A SPECTRUM (OR WHAT YOU CAN TELL AT A GLANCE) When analyzing the spectrum of an unknown, concentrate your first efforts on determining the presence (or absence) of a few major functional groups. The CJO, OIH, NIH, CIO, CJC, CKC, CKN, and NO2 peaks are the most conspicuous and give immediate structural information if they are present. Do not try to make a detailed analysis of the CIH absorptions near 3000 cm−1 ; almost all compounds have these absorptions. Do not worry about subtleties of the exact environment in which the functional group is found. Following is a major checklist of the important gross features. 1. Is a carbonyl group present? The CJO group gives rise to a strong absorption in the region 1820–1660 cm−1 . The peak is often the strongest in the spectrum and of medium width. You can’t miss it. 2. If CJO is present, check the following types (if it is absent, go to step 3): ACIDS Is OIH also present? • Broad absorption near 3400–2400 cm−1 (usually overlaps CIH). AMIDES Is NIH also present? • Medium absorption near 3400 cm−1 ; sometimes a double peak with equivalent halves. ESTERS Is CIO also present? • Strong-intensity absorptions near 1300–1000 cm−1 . ANHYDRIDES Two CJO absorptions near 1810 and 1760 cm−1 . ALDEHYDES Is aldehyde CIH present? • Two weak absorptions near 2850 and 2750 cm−1 on right side of the aliphatic CIH absorptions. KETONES The preceding five choices have been eliminated. 3. If CJO is absent: ALCOHOLS, PHENOLS Check for OIH. • Broad absorption near 3400–3300 cm−1 . • Confirm this by finding CIO near 1300–1000 cm−1 . AMINES Check for NIH. • Medium absorption(s) near 3400 cm−1 . ETHERS Check for CIO near 1300–1000 cm−1 (and absence of OIH near 3400 cm−1 ). 30 Infrared Spectroscopy 14782_02_Ch2_p015-104.pp2.qxd 1/25/08 10:27 AM Page 30
14782_02_ch2_p015-104.Pp2.gxd1/25/0810:27age31⊕ 2.10 Hydrocarbons:Alkanes,Alkenes,and Alkynes 31 4.Double bonds and/or aromatic rings .C-Cis a weak absorption near 1650 cm 3000 cm-(aliphatic C-H occurs to the right of this value) 5.Triple bonds C=N is a medium.sharp absorption near 2250 cm .C=C is a weak,sharp absorption near 2150 cm-. .Check also for acetylenic C-H near 3300 cm- 6.Nitro groups 7.Hydrocarbons None of the preceding is found Major absorptions are in C-H region near 3000-. to nosi h orabsence.This is best done by carefilly shdying thelativera A SURVEY OF THE IMPORTANT FUNCTIONAL GROUPS.WITH EXAMPLES Ttoiggaestskiberiosofiampotnfnctioagoupsowadinfarodadion 1.The basic information about the functional group or type of vibration is abstracted and placed in a Spectral Analysis Box,where it may be consulted easily. groups an 2.10 HYDROCARBONS:ALKANES,ALKENES,AND ALKYNES A.Alkanes Alkanes show very few absorption bands in the infrared spectrum.They yield four or more C-H stretching peaks near 3000 cm plus CH,and CH:bending peaks in the range 1475-1365 cm
4. Double bonds and/or aromatic rings • CJC is a weak absorption near 1650 cm. • Medium-to-strong absorptions in the region 1600–1450 cm−1 ; these often imply an aromatic ring. • Confirm the double bond or aromatic ring by consulting the CIH region; aromatic and vinyl CIH occur to the left of 3000 cm−1 (aliphatic CIH occurs to the right of this value). 5. Triple bonds • CKN is a medium, sharp absorption near 2250 cm−1 . • CKC is a weak, sharp absorption near 2150 cm−1 . • Check also for acetylenic CIH near 3300 cm−1 . 6. Nitro groups • Two strong absorptions at 1600–1530 cm−1 and 1390–1300 cm−1 . 7. Hydrocarbons • None of the preceding is found. • Major absorptions are in CIH region near 3000−1 . • Very simple spectrum; the only other absorptions appear near 1460 and 1375 cm−1 . The beginning student should resist the idea of trying to assign or interpret every peak in the spectrum. You simply will not be able to do it. Concentrate first on learning these major peaks and recognizing their presence or absence. This is best done by carefully studying the illustrative spectra in the sections that follow. A SURVEY OF THE IMPORTANT FUNCTIONAL GROUPS, WITH EXAMPLES The following sections describe the behaviors of important functional groups toward infrared radiation. These sections are organized as follows: 1. The basic information about the functional group or type of vibration is abstracted and placed in a Spectral Analysis Box, where it may be consulted easily. 2. Examples of spectra follow the basic section. The major absorptions of diagnostic value are indicated on each spectrum. 3. Following the spectral examples, a discussion section provides details about the functional groups and other information that may be of use in identifying organic compounds. 2.10 HYDROCARBONS: ALKANES, ALKENES, AND ALKYNES Alkanes show very few absorption bands in the infrared spectrum. They yield four or more CIH stretching peaks near 3000 cm−1 plus CH2 and CH3 bending peaks in the range 1475–1365 cm−1 . A. Alkanes 2.10 Hydrocarbons: Alkanes, Alkenes, and Alkynes 31 14782_02_Ch2_p015-104.pp2.qxd 1/25/08 10:27 AM Page 31
Infrared Spectroscop SPECTRAL ANALYSIS BOX ALKANES The spectrum is usually simple,with few peaks C-H Stretch occurs around 3000 cm-1. strained ds).spC-H absorption always occurs a e C- (see Sections 2.10B and 2.10C). CH. Methylene groups have a characteristic bending absorption of approximately 1465 cm CH Methyl groups have a characteristic bending absorption of approximately 1375 cm CHz C-c Stretch not interpretatively useful:many weak peaks. Examples:decane (Fig.2.7).mineral oil (Fig.2.8).and cyclohexane (Fig.2.9). 45烟 FIGURE 2.7 The infran d sp rum of d ne (neat liquid.KBr plates). Nujol- FIGURE 2.8 The infrared: of mineral oil(ea liquid.KBr plates)
32 Infrared Spectroscopy SPECTRAL ANALYSIS BOX ALKANES The spectrum is usually simple, with few peaks. CIH Stretch occurs around 3000 cm−1 . In alkanes (except strained ring compounds), sp3 CIH absorption always occurs at frequencies less than 3000 cm−1 (3000–2840 cm−1 ). If a compound has vinylic, aromatic, acetylenic, or cyclopropyl hydrogens, the CIH absorption is greater than 3000 cm−1 . These compounds have sp2 and sp hybridizations (see Sections 2.10B and 2.10C). CH2 Methylene groups have a characteristic bending absorption of approximately 1465 cm−1 . CH3 Methyl groups have a characteristic bending absorption of approximately 1375 cm−1 . CH2 The bending (rocking) motion associated with four or more CH2 groups in an open chain occurs at about 720 cm−1 (called a long-chain band). CIC Stretch not interpretatively useful; many weak peaks. Examples: decane (Fig. 2.7), mineral oil (Fig. 2.8), and cyclohexane (Fig. 2.9). 4000 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000 800 600 400 WAVENUMBERS (CM–1) 2.5 3 4 5 6 7 8 9 10 11 12 13 14 15 16 19 25 100 90 80 70 60 50 40 30 20 10 0 MICRONS % TRANSMITTANCE CH3 bend CH2 bend long-chain band sp3 C–H stretch Nujol (mineral oil) FIGURE 2.8 The infrared spectrum of mineral oil (neat liquid, KBr plates). 4000 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000 800 600 400 WAVENUMBERS (CM–1) 2.5 3 4 5 6 7 8 9 10 11 12 13 14 15 16 19 25 100 90 80 70 60 50 40 30 20 10 0 MICRONS % TRANSMITTANCE CH3(CH2) 8 CH3 CH3 bend CH2 bend long-chain band sp3 C–H stretch FIGURE 2.7 The infrared spectrum of decane (neat liquid, KBr plates). 14782_02_Ch2_p015-104.pp2.qxd 1/25/08 10:27 AM Page 32
14782_02_ch2_p015-104.Pp2.gxd1/25/0810:27age3⊕ 2.10 Hydrocarbons:Alkanes,Alkenes,and Alkynes 33 215份 FIGURE 2. The infrared spe n of cycloh e(neat liquid.KBr plates) B. Alkenes Alkenes show many more peaks than alkanes.The principal peaks of diagnostic value are the C-H stretching peaks for the spcarbon at values greater than 3000cmalong with C-H peaks for the compounds,you sh ⊕ SPECTRAL ANALYSIS BOX ALKENES -C-H Stretch for spC-H occurs at values greater than 3000 cm(3095-3010 cm). -C-H Out-of-plane(oop)bending occurs in the range 100-650cm These bands can be used to determine the degree of substitution on the double bond (see discussion). C-c Stretch occu at1660-1600cm-1. conjugation moves C-C stretch to lower fre quencies and increases the intensity. Symmetrically substituted bonds (e.2.3-dimethyl-2-butene)do not absorb in the infrared (no dipole change). Symmetrically disubstituted (trans)double bonds are often vanishingly weak in absorption:cis are stronger. Example (Fig)-n ( 210
2.10 Hydrocarbons: Alkanes, Alkenes, and Alkynes 33 B. Alkenes Alkenes show many more peaks than alkanes. The principal peaks of diagnostic value are the CIH stretching peaks for the sp2 carbon at values greater than 3000 cm−1 , along with CIH peaks for the sp3 carbon atoms appearing below that value. Also prominent are the out-of-plane bending peaks that appear in the range 1000–650 cm−1 . For unsymmetrical compounds, you should expect to see the CJC stretching peak near 1650 cm−1 . 4000 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000 800 600 400 WAVENUMBERS (CM–1) 2.5 3 4 5 6 7 8 9 10 11 12 13 14 15 16 19 25 100 90 80 70 60 50 40 30 20 10 0 MICRONS % TRANSMITTANCE CH2 bend sp3 C–H stretch no long-chain band FIGURE 2.9 The infrared spectrum of cyclohexane (neat liquid, KBr plates). SPECTRAL ANALYSIS BOX ALKENES JCIH Stretch for sp2 CIH occurs at values greater than 3000 cm−1 (3095–3010 cm−1 ). JCIH Out-of-plane (oop) bending occurs in the range 1000–650 cm−1 . These bands can be used to determine the degree of substitution on the double bond (see discussion). CJC Stretch occurs at 1660–1600 cm−1 ; conjugation moves CJC stretch to lower frequencies and increases the intensity. Symmetrically substituted bonds (e.g., 2,3-dimethyl-2-butene) do not absorb in the infrared (no dipole change). Symmetrically disubstituted (trans) double bonds are often vanishingly weak in absorption; cis are stronger. Examples: 1-hexene (Fig. 2.10), cyclohexene (Fig. 2.11), cis-2-pentene (Fig. 2.12), and trans- 2-pentene (Fig. 2.13). 14782_02_Ch2_p015-104.pp2.qxd 1/25/08 10:27 AM Page 33
CH F IG URE 2.10 The infrared spectrum of 1-hexene(neat liquid.KBr plates). FIGURE 2.11 The infr n of cyclohexene (neat liquid.KBr plates) FIGURE tliquid.KBr plates)
4000 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000 800 600 400 WAVENUMBERS (CM–1) 2.5 3 4 5 6 7 8 9 10 11 12 13 14 15 16 19 25 100 90 80 70 60 50 40 30 20 10 0 MICRONS % TRANSMITTANCE sp2 C–H stretch vinyl oop overtone of 910 cm–1 C C–– (CH2) 3 CH3 – – – – H H H sp3 C–H stretch C C stretch –– FIGURE 2.10 The infrared spectrum of 1-hexene (neat liquid, KBr plates). 4000 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000 800 600 400 WAVENUMBERS (CM–1) 2.5 3 4 5 6 7 8 9 10 11 12 13 14 15 16 19 25 100 90 80 70 60 50 40 30 20 10 0 MICRONS % TRANSMITTANCE CH2 bend sp2 C–H stretch sp3 C–H stretch cis oop cis C C stretch –– FIGURE 2.11 The infrared spectrum of cyclohexene (neat liquid, KBr plates). 4000 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000 800 600 400 WAVENUMBERS (CM–1) 2.5 3 4 5 6 7 8 9 10 11 12 13 14 15 16 19 25 100 90 80 70 60 50 40 30 20 10 0 MICRONS % TRANSMITTANCE cis oop cis C C stretch –– C C–– – – – – CH3 H CH2CH3 H sp2 C–H stretch sp3 C–H stretch FIGURE 2.12 The infrared spectrum of cis-2-pentene (neat liquid, KBr plates). 14782_02_Ch2_p015-104.pp2.qxd 1/25/08 10:27 AM Page 34