Enterpretation of IR-spectra Tables and charts Assigning functional groups

Kort repetisjon/Short summery: IR er meget godt egnet til identifisering av funksjonelle grupper. IR is well suited for identification of functional groups Funksjoneller grupper har karakteristiske absorbsjoner Functional groups have characteristic absorptions Absorbsjoner fra funksjonelle grupper er vanligvis relativt konstant Absorption-frequencies from functional groups are relative constant Symmetriske molekyler er inaktive for de frekvenser som tilsvarer symmetrisk strekk Symmetrical molecules are inactive for symmeterical stretching frequencies

Alkanes C-H-stretch for CH2-groups in hydrocarbones occur in three areas: Two bands at 2966 og 2853 (s) Two band at 1375 og 1450 (m) One band at 720 (not always) Branching has no significant influence. Ring strain increases C-H stretching frequency. Cyclo- propanes : 3100 – 2990.

Alkenes Non conjugated system : Mono substituted : C=C stretch at 1640 (m to w) Di(trans)-, tri- og tetra-substituted: C=C stretch at 1670 (weak if symmetry) Disubstituted cis (also cyclic): C=C ved 1650 Conjugated system: Often two C=C bands at 1650 og 1600 (of unsymmetric) Conjugation with aromat gives C=C strekk ved 1625 C-H stretch generally over 3000.

Alkynes C-H stretch for mono substituted 3333 – 3267 (s) Broad absorption at 700 – 610 Triple bond stretch at 2260 – 2100 (m to w) No or low absorption if symmetric molecule

Aromatics Aromatic C-H stretch at 3000 – 3100 (s) C-H bend at 900 – 675 (often strong and informative) C-C stretch at 1600 – 1585 and 1500 – 1400 (m) Overtones and combination tones at 2000 – 1650 (w)

Alcoholes og Phenoles Characterized by O-H og O-C stretch ”Free” O-H stretch at 3650 – 3584 (sharp) H-bondet O-H stretch at 3550 – 3200 (s, broad) C-O stretch at 1260 – 1000 (s)

Ethers and epoxides Alifatic C-O stretch ved 1115 – 1085 Aryl alkyl ethers at 1275 – 1200 and 1075 - 1020

C=O stretch Ketones, aldehydes, carboxylic acids, esters, lactones, acid halides, anhydrides, amides og laktames : All have C=O stretch in the area 1870 – 1540 which are relatively constant and easily recognizable. The frequency depends on: Phase (solid, liquid or gas) Elektronic and mass effects by neighbor substituent Conjugation Hydrogen bonding Ring strain

Ketones Standard value for keton C=O stretch is 1715 (s) Conjugation gives delocalisation and reduction of C=O dobbel bond character. Weaker bond gives lower frequency. Conjugation with alken or aromat give C=O stretch at 1685 – 1666 Polar solvents reduce C=O stretch a little (5 – 15) Beta-diketones give keto-enol equil. and broad bands Ring strain increases C=O strekk (1775 for 4 ring) Electronegative alfa subs. increase frequency

Aldehydes Alifatic aldehyde C=O stretch at 1740 – 1720 (s) Electronegative substituents on alfa C increase frequency Conjugation give C=O stretch at 1710 – 1685 H-bonding reduce the C=O frequency C-H stretch at 2830 – 2695 A medium strong abs. at 2720 + C=O stretch give aldehyde

Carboxylic acids Dominenated by O-H stretch at 3300 – 2500 (H-bondet) Predominately dimers except at very low consentration C=O stretch for monomer vat 1760 (s) C=O stretch for dimer ved 1720 - 1706 Conjugation gives C=O stretch at 1710 - 1680 C=O stretch for carboxylate anion at 1650 – 1550 og 1400.

Esters and lactones Alifatic C=O stretch at 1750 – 1735 (s) Conjugation give C=O stretch ved 1730 – 1715 C-O stretch at 1300 – 1000 (s)

Carboxylic anhydrides Characterized by 2 C=O stretch at 1818 og 1750 (s) Conjugation give 2 C=O stretch ved 1775 og 1720 Ring strain increases the frequency C-O strekk ved 1047 (s)

Nitriles Alifatic C-N triple bond stretch at 2260 – 2240 (m) Conjugation give reduction to 2240 - 2222

Amines Primary amines give 2 bands at 3500 og 3400 (w) Secondary amines give 1 band at 3350 – 3310 H-bonding give lower frequency N-H bending at 1650 – 1580 (m)

Amides Amide C=O stretch at 1650 (s) N-H bending at 1655 – 1620 (m) N-H stretch ved 3520 – 3400 (2 bands for primary amide) Ring strain (lactames) increase C=O stretch frequency