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19.16 a -Halogenation of Carboxylic Acids: The Hell-Volhard-Zelinsky Reaction

19.16 a -Halogenation of Carboxylic Acids: The Hell-Volhard-Zelinsky Reaction. O. O. R 2 CCOH. R 2 CCOH. H. X. a -Halogenation of Carboxylic Acids. analogous to a -halogenation of aldehydes and ketones

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19.16 a -Halogenation of Carboxylic Acids: The Hell-Volhard-Zelinsky Reaction

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  1. 19.16a-Halogenation of Carboxylic Acids:The Hell-Volhard-Zelinsky Reaction

  2. O O R2CCOH R2CCOH H X a -Halogenation of Carboxylic Acids • analogous to a-halogenation of aldehydes and ketones • key question: Is enol content of carboxylic acids high enough to permit reaction to occur at reasonable rate? (Answer is NO) + + X2 HX

  3. O O R2CCOH R2CCOH H X But... • reaction works well if a small amount ofphosphorus or a phosphorus trihalide is added tothe reaction mixture • this combination is called the Hell-Volhard-Zelinsky reaction P or PX3 + + X2 HX

  4. O CH2COH O CHCOH Br Example + Br2 PCl3 benzene80°C (60-62%)

  5. O O CH3CH2CH2COH CH3CH2CHCOH Br Value Br2 • a-Halogen can be replaced by nucleophilic substitution P (77%)

  6. O O CH3CH2CH2COH CH3CH2CHCOH Br O CH3CH2CHCOH OH Value Br2 P (77%) K2CO3H2Oheat (69%)

  7. O O (CH3)2CHCHCOH Br O (CH3)2CHCHCOH NH2 Synthesis of a -Amino Acids Br2 (CH3)2CHCH2COH PCl3 (88%) NH3H2O (48%)

  8. 19.17Decarboxylation of Malonic Acidand Related Compounds

  9. O RCOH Decarboxylation of Carboxylic Acids Simple carboxylic acids do not decarboxylatereadily. + RH CO2

  10. O RCOH O O O CH3COH HOCCH2COH Decarboxylation of Carboxylic Acids Simple carboxylic acids do not decarboxylatereadily. + RH CO2 But malonic acid does. 150°C + CO2

  11. O O O H O HO OH HO O H H H H Mechanism of Decarboxylation • One carboxyl group assists the loss of the other.

  12. O O O H O HO OH HO O H H H H O OH C H HO O H Mechanism of Decarboxylation One carboxyl group assists the loss of the other. • This compound is the enol form of acetic acid. +

  13. O O O H O HO OH HO O H H H H O O OH C HOCCH3 H HO O H Mechanism of Decarboxylation One carboxyl group assists the loss of the other. +

  14. O O O H O HO OH HO O H H H H O O OH C HOCCH3 H HO O H Mechanism of Decarboxylation One carboxyl group assists the loss of the other. These hydrogens play no role. +

  15. O O O H O HO OH HO O R R' R R' O O OH C HOCCHR' R' HO O R Mechanism of Decarboxylation One carboxyl group assists the loss of the other. Groups other than H may be present. + R

  16. CO2H 185°C CO2H 160°C CH(CO2H)2 CH2CO2H Decarboxylation is a general reaction for 1,3-dicarboxylic acids CO2H H (74%) (96-99%)

  17. O O O H O HO OH HO O R R' R R' O OH O C R' HOCCHR' HO O R R Mechanism of Decarboxylation One carboxyl group assists the loss of the other. This OH group plays no role. +

  18. O O O H O R" OH R" O R R' R R' O O OH + C R"CCHR' R' R" R O Mechanism of Decarboxylation One carboxyl group assists the loss of the other. Groups other than OH may be present. R

  19. O O R" OH R R' O R"CCHR' R Mechanism of Decarboxylation • Decarboxylation of a b-keto acid gives a ketone. This kind of compoundis called a b-keto acid. a b

  20. CH3 CH3 O O CO2H CH3C CH3C H C C CH3 CH3 Decarboxylation of a b -Keto Acid 25°C + CO2

  21. Section 19.18Spectroscopic Analysis ofCarboxylic Acids

  22. Infrared Spectroscopy • A carboxylic acid is characterized by peaks due toOH and C=O groups in its infrared spectrum. • C=O stretching gives an intense absorptionnear 1700 cm-1. • OH peak is broad and overlaps with C—H absorptions.

  23. 3500 3000 2500 2000 1500 1000 500 Figure 19.8 Infrared Spectrum of 4-Phenylbutanoic acid C6H5CH2CH2CH2CO2H O—H and C—H stretch C=O monosubstitutedbenzene Wave number, cm-1

  24. 1H NMR • proton of OH group of a carboxylic acid is normallythe least shielded of all of the protons in a 1HNMR spectrum: (d 10-12 ppm; broad).

  25. O CH2CH2CH2COH 12.0 11.0 10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0 Figure 19.9 Chemical shift (d, ppm)

  26. 13C NMR • Carbonyl carbon is at low field (d 160-185 ppm), but not as deshielded as the carbonyl carbon of an aldehyde or ketone (d 190-215 ppm).

  27. UV-VIS Carboxylic acids absorb near 210 nm, butUV-VIS spectroscopy has not proven to be very useful for structure determination of carboxylic acids.

  28. •• •+ O O •• •• + + ArC ArCOH ArCOH Ar O •• Mass Spectrometry Aliphatic carboxylic acids undergo a varietyof fragmentations. Aromatic carboxylic acids first form acylium ions,which then lose CO.

  29. End of Chapter 19

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