Chem 353 IR Spectroscopy Lecture Notes PDF

Chem 353 – IR spectroscopy 2 September 17, 2024 CHEM CHEM 353...

Chem 353 – IR spectroscopy 2 September 17, 2024 CHEM CHEM 353 353 IR spectroscopy: chromophores & where to find them Ø Four main regions (–H, triple bond, double bond, fingerprint) Ø Peaks are affected by mass of atoms, and type of bond involved Ø There are characteristic frequencies and shapes Our goal is to be able to use data tables to predict IR peaks based on functional groups use IR peaks to deduce functional groups 1 CHEM CHEM 353 353 Double bond region (1900–1500 cm−1) more common than triple bonds carbonyl groups tend to give strong peaks (they are polar) alkene C=C are medium, but can be weak. 2 University of Saskatchewan 1 Chem 353 – IR spectroscopy 2 September 17, 2024 CHEM CHEM 353 353 C=O gives a characteristic shape: strong peak, in the middle of the spectrum C=C smaller, sharper 3 CHEM CHEM 353 353 Recall we discussed the importance of polarity on vibrational energy states: δ+ δ- hν δ+ δ- higher energy C H C H vibrational state The more polar the bond, the more intense the absorptions Ø A C=O bond is much more polar than a C=C bond C O C C 4 University of Saskatchewan 2 Chem 353 – IR spectroscopy 2 September 17, 2024 CHEM CHEM 353 353 Influence of symmetry on IR activity of the alkene stretch: H H H 3C CH3 H 3C CH3 C C C C C C H CH2CH3 H 3C CH3 H 3C CH2CH2CH2CH3 asymmetrical symmetrical small dipole moment: C=C bond: C=C bond: infrared active infrared active infrared inactive but very weak 5 CHEM CHEM 353 353 Carbonyl compounds O O O C aldehydes a C C H little higher n Aldehydes H H than ketones conjugation 1730 cm-1 1705 cm-1 1705 cm-1 lowers the n O O O C C C CH3 Ketones H 3C CH3 CH3 1715 cm-1 1685 cm-1 1685 cm-1 6 University of Saskatchewan 3 Chem 353 – IR spectroscopy 2 September 17, 2024 CHEM CHEM 353 353 Understanding the carbonyl frequencies C O C O structural features that structural features that increase the relative stability increase the relative stability of this canonical form of this canonical form increase the frequency of decrease the frequency of the C=O stretch the C=O stretch 7 CHEM CHEM 353 353 Conjugation effects O O- O C C more single-bond C character + 2-pentanone 2-cyclohexenone 1720 cm-1 1680 cm-1 Ø A C–O single bond frequency is near 1050 cm-1 8 University of Saskatchewan 4 Chem 353 – IR spectroscopy 2 September 17, 2024 CHEM CHEM 353 353 hexanal O=C-H 2722, 2836 O C=O 1728 cm-1 9 CHEM CHEM 353 353 2-hexanone O C=O 1710 cm-1 10 University of Saskatchewan 5 Chem 353 – IR spectroscopy 2 September 17, 2024 CHEM CHEM 353 353 Esters and amides Amides have a lone pair that can conjugate with the carbonyl group O O C polar form stabilized: C R N R R N R lower frequency H H Esters also have an adjacent lone pair, but the electronegativity of oxygen is the predominant effect O less resonance: C higher frequency R O R 11 CHEM CHEM 353 353 Ethyl butyrate O C=O C–O O 1739 cm-1 1188 cm-1 12 University of Saskatchewan 6 Chem 353 – IR spectroscopy 2 September 17, 2024 CHEM CHEM 353 353 Again, conjugation will affect the frequency O O O C CH3 H 2C C CH3 C CH3 H 3C O C O O H 1735 cm-1 1715 cm-1 1715 cm-1 13 CHEM CHEM 353 353 Be sure you think about the resonance structure O O C CH3 C CH3 O O 1715 cm-1 (shifted to lower n) methyl benzoate O O C C 1769 cm-1 H 3C O H 3C O (shifted to higher n) phenyl acetate 14 University of Saskatchewan 7 Chem 353 – IR spectroscopy 2 September 17, 2024 CHEM CHEM 353 353 hexanamide (a primary amide) O NH2 N-H 3363, C=O N-H bend 3193 cm-1 1663 cm-1 1633 cm-1 15 CHEM CHEM 353 353 N-ethyl propamide (a secondary amide) N-H 3299, 3097 cm-1 cis & trans C=O O forms 1648 cm-1 N-H bend present 1563 cm-1 N H University of Saskatchewan 8 Chem 353 – IR spectroscopy 2 September 17, 2024 CHEM CHEM 353 353 N,N-diethyl acetamide (a tertiary amide) O N C=O 1651 cm-1 17 CHEM CHEM 353 353 hexanoic acid O-H C-O 1247 cm-1 O C=O OH 1711 cm-1 18 University of Saskatchewan 9 Chem 353 – IR spectroscopy 2 September 17, 2024 CHEM CHEM 353 353 Anhydrides: Normally symmetrical, but the O O O O- electron-withdrawing effect means two types of bonds are C C C C+ always present R O R R O R Ø Result is two bands are observed, ~1740–1850 19 CHEM CHEM 353 353 propanoic anhydride C=O O O 1820, O 1762 cm-1 20 University of Saskatchewan 10 Chem 353 – IR spectroscopy 2 September 17, 2024 CHEM CHEM 353 353 hexanoyl chloride O C=O 1801 cm-1 Cl 21 CHEM CHEM 353 353 Cyclic carbonyls affected by strain 22 University of Saskatchewan 11 Chem 353 – IR spectroscopy 2 September 17, 2024 CHEM CHEM 353 353 Fingerprint region (