Aldehydes and Ketones

1. Aldehydes and Ketones Dr. Talat R. Al-Ramadhany

2. Introduction Aldehydes are compounds of the general formula RCHO; Ketones are compounds of the general formula RR´CO. The groups R and R´ may be aliphatic or aromatic. Both aldehydes and ketones contain the carbonyl group, C=O, and are often called carbonyl compounds.

3. An aldehyde is often written as RCHO. Remember that the H atom is bonded to the carbon atom, not the oxygen. • Likewise, a ketone is written as RCOR, or if both alkyl groups are the same, R2CO. Each structure must contain a C––O for every atom to have an octet. • The three bonds (carbon, oxygen, and the two other atoms attached to carbonyl carbon) lie in a plane; the three bond angels of carbon are very close to 120º.

4. Nomenclature Both IUPAC and common names are used for aldehydes and ketones. • NamingAldehydesintheIUPAC System To name an aldehyde using the IUPAC system: [1] If the CHOis bonded to a chain of carbons, find the longest chain containing the CHOgroup, and change the -eending of the parent alkane to the suffix -al. If the CHOgroup is bonded to a ring, name the ring and add the suffix -carbaldehyde. [2] Number the chain or ring to put the CHOgroup at C1

5. Example: Give the IUPAC name for the compound:

7. Give the IUPAC name for the compound:

9. Common Names for Aldehydes The common names of aldehydes are derived from the names of the corresponding carboxylic acids by replacing –icacidby –aldehyde. Greek letters are used to designate the location of substituents in common names. The carbon adjacent to the CHO group is the ` carbon, and so forth down the chain.

11. Naming Ketones in the IUPAC System • To name an acyclic ketone using IUPAC rules: [1] Find the longest chain containing the carbonyl group, and change the -eending of the parent alkane to the suffix -one. [2] Number the carbon chain to give the carbonyl carbon the lower number. Apply all of the other usual rules of nomenclature.

15. Common Names for Ketones Most common names for ketones are formed by naming both alkyl groupson the carbonyl carbon, arranging them alphabetically, and adding the word ketone. Using this method, the common name for 2-butanone becomes ethyl methyl ketone.

17. Physical properties: • Boiling point: • Aldehydes and ketones are polar compounds due to the polarity of carbonyl group and hence they have higher boiling points than non polar compounds of comparable molecular weight. • But they have lower boiling points than comparable alcohols or carboxylic acids due to the intermolecular hydrogen bonding.

18. Solubility: The lower aldehydes and ketones soluble in water, because of hydrogen bonding between carbonyl group and water, also they soluble in organic solvents.

19. Preparation of aldehydes & Ketones.

20. Preparation of aldehydes. Oxidation of primary alcohols: • Primary alcohols can be oxidized to give aldehydes by using of K2Cr2O7.

21. Oxidation of Methylbenzenes: In the case of methylbenzenes, oxidation of the side chain can be interrupted by trapping with acetic anhydride to form gem-diacetate, which on hydrolysis its return to aldehydes.

23. Partial reduction of acid chlorides Strong reducing agents (like LiAlH4) reduce acid chlorides all the way to primary alcohols. Lithium aluminum tri(t-butoxy)hydride is a milder reducing agent that reacts faster with acid chlorides than with aldehydes. Reduction of acid chlorides with lithium aluminum tri(t-butoxy)hydride gives good yields of aldehydes. (-78ºC)

24. Partial reduction of esters Stericallybulky reducing agents, e.g. Diisobutylaluminium hydride (DIBAH), can selectively reduce esters to aldehydes. The reaction is carried out at low temperature (-78ºC) in toluene. Diisobutylaluminium hydride (mild reducing agent) Ester Aldehyde

25. Reduction of Nitriles Reduction of nitrile with a less powerful reducing reagent, e.g. DIBAH, produces aldehyde. The reaction is carried out at low temperatures (-78ºC) in toluene.

27. Preparation of Ketones • Oxidation of Secondary alcohols: Secondary alcohols are oxidized to ketones by chromic acid (H2CrO4) in a form selected for the job at hand: aqueous K2Cr2O7, CrO3 in glacial acetic acid, CrO3 in pyridine, etc. Hot permanganate also oxidizes alcohols; it is seldom used for the synthesis of ketones.

28. Cleavage of Carbon–Carbon double bond by Ozone: Oxidative cleavage of an alkene breaks both the σand πbonds of the double bond to form two carbonyl groups. Depending on the number of R groups bonded to the double bond, oxidative cleavage yields either ketones or aldehydes.

29. Friedel-Crafts acylation. The Friedel-Crafts reaction involves the use of acid chlorides rather than alkyl halides. An acyl group (RCO–) becomes attached to the aromatic ring. Thus forming a ketone; the process is called acylation.

30. Synthesis of Ketones from Nitriles. A Grignard or organolithium reagent attacks a nitrile to give the magnesium salt of an imine. Acidic hydrolysis of the imine leads to the ketone.

31. Hydration of alkynes. Alkynes undergo acid-catalyzed addition of water across the triple bond in the presence of mercuric ion as a catalyst. A mixture of mercuric sulfate in aqueous sulfuric acid is commonly used as the reagent.

33. Reactions of aldehydes and Ketones Aldehydes and Ketones undergo many reactions to give a wide variety of useful derivatives. There are two general kinds of reactions that aldehydes and ketones undergo: [1] Reaction at the carbonyl carbon (Nucleophilic addition reactions).

34. [2] Reaction at the αcarbon. A second general reaction of aldehydes and ketones involves reaction at the α carbon. A C–H bond on the α carbon to a carbonyl group is more acidic than many other C–H bonds, because reaction with base forms a resonance-stabilized enolate anion.

35. [1] Nucleophilic addition reaction. Two general mechanisms are usually drawn for nucleophilic addition, depending on the nucleophile (negatively charged versus neutral) and the presence or absence of an acid catalyst. With negatively charged nucleophiles, nucleophilic addition follows the two-step process first (nucleophilic attack) followed by protonation.

36. Absence of an acid catalyzed nucleophilic addition Step [1]:The nucleophile attacks the carbonyl group,cleaving the π bond and moving an electron pair onto oxygen. This forms a sp3 hybridized intermediate with a new C–Nu bond. Step [2]: protonation of the negatively charged O atom by H2O forms the addition product.

37. Acid-catalyzed nucleophilic addition The general mechanism for this reaction consists of three steps (not two),but the same product results because H and Nu- add across the carbonyl π bond. In this mechanism protonation precedes nucleophilic attack. Step [1] Protonation of the carbonyl group

38. In Step [2], the nucleophile attacks, and then deprotonation forms the neutral addition product in Step [3]. Steps [2]–[3] Nucleophilic attack and deprotonation

39. a) Addition of Alcohols (Acetal Formation): Aldehydes and ketones react with twoequivalents of alcohol to form acetals. In an acetal, the carbonyl carbon from the aldehyde or ketone is now singly bonded to two OR" (alkoxy) groups.

40. b) Nucleophilic Addition of H- (Reduction reaction) Treatment of an aldehyde or ketone with either Sodium borohydride (NaBH4) or Lithium hydride (LiAlH4) followed by protonation forms a 1° or 2° alcohol.

41. Hydride reduction of aldehydes and ketones occurs via the two-step mechanism of nucleophilic addition, that is, nucleophilic attack of H:–followed by protonation.

42. Examples:

43. c) Reduction to alkane (Deoxygenation of Ketones and Aldehydes): • i) Clemmensen reduction. • ii) Wolff–Kishner reduction.

44. Clemmensen reduction: • The Clemmensen reduction is most commonly used to convert acylbenzenes (from Friedel-Crafts acylation) to alkylbenzenes, but it also works with other ketones or aldehydes that are not sensitive to acid. The carbonyl compound is heatedwith an excess of amalgamated zinc (zinc treated with mercury;Zn (Hg), and concentrated hydrochloric acid (HCl).The actual reduction occurs by a complex mechanism on the surface of the zinc. The Clemmensen reduction uses zinc and mercury in the presence of strong acid.

46. Wolff–Kishner reduction: Compounds that cannot survive treatment with hot acid can be deoxygenated using the Wolff–Kishner reduction. The ketone or aldehyde is converted to its hydrazone, which is heated with Hydrazine (NH2NH2), andstrong basesuch as KOH. Ethylene glycol, diethylene glycol, or another high-boiling solvent is used to facilitate the high temperature (140-200°C) needed in the second step.

48. Summary:

49. d) Nucleophilic Addition of CN– : Treatment of an aldehyde or ketone with NaCN and a strong acid such as HCl adds the elements of HCN across the carbon–oxygen π bond, forming a cyanohydrin.