Larry Emme Chemeketa Community College

1. Aldehydes and Ketones Chapter 23 Larry Emme Chemeketa Community College

2. Structures of Aldehydes & Ketones

3. Both aldehydes and ketones contain a carbonyl ( C=O) group.

4. In a linear expression, the aldehyde group is often written as: • CHO

5. In the linear expression of a ketone, the carbonyl group is written as: • CO

6. Naming Aldehydes & Ketones

7. To establish the parent name, select the longest continuous chain of carbon atoms that contains the aldehyde group. • The carbons of the parent chain are numbered starting with the aldehyde group. Since the aldehyde group is at the beginning (or end) of a chain, it is understood to be number 1. IUPAC Rules for Naming Aldehydes

8. IUPAC Rules for Naming Aldehydes 3. Form the parent aldehyde name by dropping the –e from the corresponding alkane name and adding the suffix –al. 4. Other groups attached to the parent chain are named and numbered as we have done before.

9. Naming Aldehydes ethanal 4-methylhexanal

11. Common Names for Aldehydes

12. IUPAC Rules for Naming Ketones • To establish the parent name, select the longest continuous chain of carbon atoms that contain the ketone group. • Form the parent name by dropping the –e from the corresponding alkane name and add the suffix –one.

13. IUPAC Rules for Naming Ketones 3. If the chain is longer than four carbons, it is numbered so that the carbonyl group has the smallest number possible; this number is prefixed to the parent name of the ketone. 4. Other groups attached to the parent chain are named and numbered as we have done before.

14. 2-pentanone Naming Ketones

15. Common Names for Ketones

16. Bonding and Physical Properties

17. Bonding • The carbon atom of the carbonyl group is sp2-hybridized and is joined to three other atoms by sigma bonds. • The fourth bond is made by overlapping p electrons of carbon and oxygen to form a pi bond between the carbon and oxygen atoms.

18. Bonding • Because the oxygen atom is considerably more electronegative than carbon, the C=O group is polar. • Many of the chemical reactions of aldehydes and ketones are due to this polarity.

19. Properties • Unlike alcohols, aldehydes and ketones cannot hydrogen-bond to themselves, because no hydrogen atom is attached to the oxygen atom of the carbonyl group. • Aldehydes and ketones, therefore, have lower boiling points than alcohols of comparable molar mass.

20. Effect of Hydrogen Bonding on Physical Properties 20

21. Mole Weight Boiling point oC

22. Chemical Properties of Aldehydes & Ketones

23. Reactions of Aldehydes & Ketones • Oxidation • aldehydes only • Reduction • aldehydes and ketones • Addition • aldehydes and ketones

24. Oxidation of Aldehydes • Aldehydes are easily oxidized to carboxylic acids by a variety of oxidizing agents, including (under some conditions) oxygen of the air.

25. Tollens’ Silver Mirror Test Tollens’ reagent, which contains Ag+, oxidizes aldehydes, but not ketones. Ag+ is reduced to metallic Ag, which appears as a “mirror” in the test tube. Ag+ + e–→ Ag(s) 25

27. Fehling and Benedict Tests Benedict’s reagent, which contains Cu2+ ions in an alkaline medium,reacts with aldehydes that have an adjacent OH group. an aldehyde is oxidized to a carboxylic acid, while Cu2+ is reduced to give brick red Cu2O(s). 28

28. Increasing amounts of reducing sugar green orangeredbrown

29. Tollens, Fehling & Benedict Tests • Because most ketones do not give a positive with Tollens, Fehling, or Benedict solutions, these tests are used to distinguish between aldehydes and ketones.

30. Biochemical Oxidation of Aldehydes • When our cells ‘burn’ carbohydrates, they take advantage of the aldehyde reactivity. • The aldehyde is oxidized to a carboxylic acid and is eventually converted to carbon dioxide, which is then exhaled. • This stepwise oxidation provides some of the energy necessary to sustain life.

31. Reduction of Aldehydes & Ketones Aldehydes and ketones are easily reduced to alcohols using LiAlH4, NaBH4 , or H2/Ni . Aldehydes yield primary alcohols (1) while ketones yield secondary alcohols ( 2) . 32

32. Addition Reactions of Aldehydes & Ketones • Common addition reactions: • Addition of alcohols • hemiacetal, hemiketal, acetal, ketal • Grignard preparations of alcohols • 2,4-dinitrophenylhydrazine (2,4-DNP)

33. Addition of Alcohols Aldehydes react with alcohols and a trace of acid to yield hemiacetals as shown here. 34

34. Addition of Alcohols In the presence of excess alcohol and a strong acid such as dry HCl, aldehydes or hemiacetals react with a second molecule of the alcohol to yield an acetal. 35

35. Intramolecular Addition of Alcohols Cyclic hemiacetals or hemiketals can form when the alcohol and the carbonyl group exist within the same molecule . 36

36. Addition of Alcohols to Aldehydes and Ketones

37. Grignard preparations of alcohols • A Grignard reagent is an organic magnesium halide. It can be either an alkyl or an aryl compound (RMgX or ArMgX). Grignard (pronounced green yard) reagents were first prepared in France around 1900 by Victor Grignard (1871-1935).

38. Grignard reagents are usually made by reacting an organic halide and magnesium metal in an ether solvent:

39. In the Grignard reagent, the bonding electrons between carbon and magnesium are shifted away from the electropositive Mg to form a strongly polar covalent bond. As a result the charge distribution in the Grignard reagent is such that the organic group (R) is partially negative and the –MgX group is partially positive. This charge distribution directs the manner in which Grignard reacts with other compounds.

40. The Grignard reagent is one of the most versatile and widely used reagents in organic chemistry. We will consider only its reactions with aldehydes and ketones at this time. Grignards react with aldehydes and ketones to give intermediate products that form alcohols when hydrolyzed. With formaldehyde, primary alcohols are formed; with other aldehydes, secondary alcohols are formed; with ketones, tertiary alcohols are formed.

41. Examples Formaldehyde

42. Examples Benzaldehyde

43. Examples Acetone

44. Explanation • The Grignard reaction with acetone may be explained in this way. In the first step of the addition of ethyl magnesium bromide, the partially positive –MgBr of the Grignard bonds to the oxygen atom, and the partially negative CH3CH2– bonds to the carbon atom of the carbonyl group of acetone.

46. Explanation • In the hydrolysis step, a proton [H+] from water bonds to the oxygen atom, leaving the hydroxyl group [–OH] to combine with the +MgBr. So, the alcohol is formed.

48. 2,4-dinitrophenylhydrazine (2,4-DNP)

49. 2,4-dinitrophenylhydrazine (2,4-DNP) • The carbonyl carbon in both aldehydes and ketones reacts with 2,4-DNP to form heavy yellow to orange crystalline solids. • These solids were used extensively for identification purposes before the use of spectrometers. • The solid is purified by crystallization and its melting point compared to those of known structure.