Vibrational Spectroscopy Functional Groups

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1 hem 325 Vibrational Spectroscopy Functional roups Bonds to - single N- single - single egions of the I Spectrum The I spectrum normally spans the 4000 cm -1 to 400 cm -1 range (2500 nm to nm). This range is often divided into four ranges as described below. The low energy end of the spectrum is called the fingerprint region. Triple s N Double s = =N = Single Bonds - -N - Fingerprint egion 4000 cm cm cm cm cm-1 1. Alkanes Various - stretches and bends between cm -1 (m) - between methylene carbons ( 2 ) cm -1 (m) - between methylene carbons ( 2 ) and methyl ( 3 ) cm -1 (m) Show sp 3 - between cm -1 (s) 2. Alkenes = stretch occurs at cm -1, gets weaker as substitution increases. Vinyl - stretch occurs at cm -1 Note that the peaks from the alkane part are still present! The difference between alkane and alkene or alkynyl - is important! If slightly above 3000 it is vinyl sp 2 - or alkynyl sp -. If it is below it is alkyl sp 3 -. ctane 1-ctene 1

2 1-ctene vs ctane ctane egions of the I Spectrum The I spectrum normally spans the 4000 cm -1 to 400 cm -1 range (2500 nm to nm). This range is often divided into four ranges as described below. The low energy end of the spectrum is called the fingerprint region. Bonds to Triple s Double s Single Bonds 1-ctene - single N- single - single N = =N = - -N - Fingerprint egion 4000 cm cm cm cm cm-1 Band Intensities Missing Peak?? ule: For a vibrational mode to be I active the vibrational motion must cause a change in the dipole moment of the molecule. orollary: A vibrational mode which does not cause a change in the dipole moment of the molecule will be I inactive and will not produce an absorption band. 2

3 onjugated Dienes The = stretches of the two double s in dienes couple to give two different stretches an asymmetric and a symmetric stretch. Dienes For symmetric dienes, only the asymmetric stretch leads to a change in dipole moment, and is therefore the only I active mode. Symmetric Stretch Asymmetric Stretch Dienes For unsymmetrical dienes, both the symmetric and asymmetric stretches lead to changes in the dipole moment and are I active. The asymmetric peak is usually larger because it causes a bigger change. 3. Alkynes stretch occurs between cm -1 ; the strength of this band depends on asymmetry of, strongest for terminal alkynes, weakest for symmetrical internal alkynes (w-m) - for terminal alkynes occurs at cm -1 (s) emember internal alkynes ( - - ) would not have this band! 1-ctyne 3

4 Terminal vs Internal Alkynes 4. Aromatics Due to the delocalization of electrons in the ring, where the order between carbons is 1½, the stretching frequency for these s is slightly lower in energy than normal =. These s show up as a pair of sharp bands, 1500(s) & 1600 cm -1 (m), where the lower frequency band is stronger. - s off the ring show up similar to vinyl - at cm -1 (m). Ethyl benzene Substituted Aromatics If the region between cm -1 (w) is free of interference, a weak group of peaks is observed for aromatic systems. Analysis of this region, called the overtone of bending region, can lead to a determination of the substitution pattern on the aromatic ring. Monosubstituted 1,2 disubstituted (ortho or o-) Example: Ethylbenzene 1,2 disubstituted (meta or m-) Ethyl benzene 1,4 disubstituted (para or p-) 4

5 Example: o-cresol Example: m-xylene 3 3 Example: p-chloro-anisole Unsaturated Systems The substitution of aromatics and alkenes can also be discerned through the out-of-plane bending vibration region. owever, other peaks often appear in this region, so interference is common. 2 cm cm l

6 5. Ethers Show a strong band for the asymmetric -- stretch at cm -1 (s) therwise dominated by the hydrocarbon component of the rest of the molecule. eptane vs dipropyl ether Dipropyl ether 6. Alcohols Show a strong, broad band for the - stretch from cm-1 (s, br) this is one of the most recognizable I bands Alcohols Like ethers, a band for - stretch between cm -1 (s). Et 1-butanol N.B. an also be caused by wet sample! 6

7 Which is Which? 7. Primary Amines Shows the N- stretch for N 2 as a doublet between cm -1 (s-m); symmetric and anti-symmetric modes. 2-aminopentane 2N N 2 group shows a deformation band from cm -1 (w) Secondary Amines -N- band for 2 N- occurs at cm -1 (br, m) as a single peak weaker than - Tertiary Amines Tertiary amines have no N-, so nothing above 3000 cm -1. pyrrolidine N N 3 3 7

8 Example: Anilines Example: Trimethylamine N N 3 3 N 3 3 N 3 3 egions of the I Spectrum The I spectrum normally spans the 4000 cm -1 to 400 cm -1 range (2500 nm to nm). This range is often divided into four ranges as described below. The low energy end of the spectrum is called the fingerprint region. 8. Aldehydes Strong = (carbonyl) stretch from cm -1 (s) Band is sensitive to conjugation, as are all carbonyls (vide infra) Bonds to Triple s Double s Single Bonds - single N- single - single N = =N = - -N - Fingerprint egion yclohexyl carboxaldehyde 4000 cm cm cm cm cm-1 8

9 8. Aldehydes Example: Benzaldehyde Also displays a highly unique sp 2 - stretch as a doublet, 2720 & 2820 cm -1 (m - w) called a Fermi doublet yclohexyl carboxaldehyde 9. Ketones = stretch occurs at cm-1 (s) 10. Esters = stretch occurs at cm -1 (s) Also displays a strong band for - at a higher frequency than ethers or alcohols at cm -1 (s) 3-methyl-2-pentanone Ethyl pivalate 9

10 Which is Which? 11. arboxylic Acids = band occurs between cm-1 The highly dissociated - has a broad band from cm -1 (m, br) covering up to half the I spectrum in some cases 4-phenylbutanoic acid 12. Acid Anhydrides oupling of the anhydride though the ether oxygen splits the carbonyl band into two with a separation of 70 cm -1. Bands are at cm-1 and cm -1 (s) Mixed mode - stretch at cm -1 (s) 13. Amides Display features of amines and carbonyl compounds = stretch occurs from cm -1 (s) If the amide is primary (-N 2 ) the N- stretch occurs from cm -1 as a doublet (w-m) Propionic anhydride pivalamide N 2 10

11 13. Amides If the amide is secondary (-N) the N- stretch occurs at cm -1 (m) as a sharp singlet 14. Nitro Lewis structure gives a order of 1.5 from nitrogen to each oxygen Two bands are seen (symmetric and asymmetric) at cm -1 (m-s) and cm -1 (m-s) This group is a strong resonance withdrawing group and is itself vulnerable to resonance effects N 3 2-nitropropane N 15. Nitriles Alkyne vs Nitrile Principle group is the carbon nitrogen triple at cm -1 (s) Usually much more intense than that of the alkyne due to the electronegativity difference between carbon and nitrogen propanenitrile N N 11

12 Effects on I Bands onjugation by resonance, conjugation lowers the energy of a double or triple. The effect of this is readily observed in the I spectrum: 1684 cm cm -1 = = onjugation will lower the observed I band for a carbonyl by cm -1 provided conjugation gives a strong resonance contributor 3 Effects on I Bands X X = N 2 3 l N 2 2 N 3 Strong resonance contributor cm -1 vs. N 3 Poor resonance contributor (cannot resonate with =) Inductive effects are usually small, unless coupled with a resonance contributor (note 3 and l above) Effects on I Bands Steric effects usually not important in I spectroscopy, unless they reduce the strength of a (usually π) by interfering with proper orbital overlap: Strain effects changes in angle forced by the constraints of a ring will cause a slight change in hybridization, and therefore, strength Effects on I Bands =: 1686 cm -1 3 =: 1693 cm -1 ere the methyl group in the structure at the right causes the carbonyl group to be slightly out of plane, interfering with resonance 1815 cm cm cm cm cm -1 As angle decreases, carbon becomes more electronegative, as well as less sp 2 hybridized ( angle < 120 ) 12

13 Benzonitrile vs Phenylacetonitrile Set #1 Set #2 Br Br 13

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