Aldehydes and Ketones and Amines (Chapter 5) PDF

Preparation of aldehydes and ketones  Alcohols can be oxidized to form aldehydes and ketones.  Primary (1o) alcohols are oxidized to aldehydes aldehyde (and subsequently to 1o alcohol...

Preparation of aldehydes and ketones  Alcohols can be oxidized to form aldehydes and ketones.  Primary (1o) alcohols are oxidized to aldehydes aldehyde (and subsequently to 1o alcohol [O] carboxylic acids)  Secondary (2o) alcohols are oxidized to ketones 2o alcohol ketone [O] [O] = KMnO4 or K2Cr2O7 Reactions of Aldehydes & ketones 1) Oxidation 2) Haloform 3) Reduction 4) Nucleophilic addition to carbonyl i)- Addition of cyanide ii)- Addition of derivatives of ammonia iii)- Addition of alcohols 5) Cannizzaro reaction 6) Aldol condensation 6) Addition of Grignard reagents Page 111  1- Oxidation Aldehydes can be oxidized easily to carboxylic acids Ketones are resistant to oxidation There are several tests that have been developed to determine the presence of aldehydes, based on their oxidation to carboxylic acids: Tollen’s NH3, H2O heat + Ag Ag aldehyde carboxylic acid silver metal Benedict’s test Cu2+ Cu2O aldehyde carboxylic acid reddish solid Page 112  2- Haloform reaction: Reaction of methyl ketones and aldehydes containing acetyl group with halogen in the presence of base produces haloform after C-C cleavage (i.e the aldehyde or ketone must contain CH3CO- group). 3- Reduction Carbonyl groups (C=O) in both aldehydes and ketones converted to methylene (CH2) groups by the Clemmensen reduction (Zn / Hg/Conc. HCl) or the Wolff–Kishner reduction (heat with hydrazine “NH2NH2” and KOH in a high boiling alcohol). O Zn / Hg / Conc. HCl (Clemmensen reduction) R R1 R R1 O NH2NH2. H2O R R1 (Wolff–Kishner reduction) R R1 KOH O Zn / Hg / Conc. HCl Ex1 H3C CH3 H 3C H or NH2NH2. H2O, KOH O Zn / Hg / Conc. HCl Ex2 H3C CH2 CH3 H 3C CH3 or NH2NH2. H2O, KOH Page 113 Aldehydes are reduced to 1ry alcohols, and ketones are reduced to 2ry alcohols by a variety of reducing agents. e.g. Catalytic hydrogenation over a metal catalyst (Pd/H2); reduction with sodium borohydride (NaBH4) or lithium aluminum hydride (LiAlH4). 1- Reduction of Aldehydes gives 1ry alcohols 2- Reduction of Ketones gives 2ry alcohols O OH O LiAlH4 LiAlH4 OH R R R R Ex: H3C CH3 or NaBH4 or NaBH4 H3C CH3 or H2 / Pt or H2 / Pt 4- Nucleophilic addition to carbonyl  O OY C + Y Z C  Z Mechanism: nucleophilic addition to carbonyl Page 114  4i) Addition of cyanide O H+ HO O R1 R1 R1 + CN CN R CN R R cyanohydrine O OH i)- NaCN a)- H3C CH3 + H 3C CH3 ii)- H CN Acetone cyanohydrin O OH i)- NaCN b)- H3C H + H3C H ii)- H CN Acetaldehyde cyanohydrin i)- NaCN OH c)- O ii)- H+ CN cyclopentanone cyanohydrin 4ii) Addition of derivatives of ammonia O (H+) + H2N G + H2O N G Page 115 O OH C + H+ C OH OH + H2N G C C NH2 G OH C + H2O + H+ C N NH2 G G Page 116  4iii- Reactions of aldehydes and ketones with alcohols (Hemiacetals, Acetals, Hemiketals and Ketals)  When aldehydes and ketones react with alcohols in the presence of an acid or base, the resulting product is called a hemiacetal (in case of aldehydes) or hemiketal (in case of ketones).  Hemiacetals or hemiketals can further react with alcohols in an acid to form acetals or ketals respectively: Formation of Hemiacetals or Hemiketals, catalyzed by base catalyst B + H O R B H O R O O R1 fast and reversible R1 R 2 + O R R2 OR R1 = H, aldehyde R2 = alkyl, ketone O OH R2 R1 + H O R R2 R1 + OR OR OR R1 = H (hemiacetal) R2 = alkyl (hemiketa)l Page 117  Formation of Hemiacetals or Hemiketals, catalyzed by acid catalyst fast and H O reversible O Protonation of + H A + A carbonyl (making the R2 R1 R2 R1 oxygen more electronegative) R1 = H (aldehyde) R2 = alkyl (ketone) O H H Attack of the (poor) R2 R1 O + H O R nucleophile on (good) R1 O H electrophile. 2 R R O H OH R 2 R1 R2 R1 Deprotonation O OR H R R1 = H (hemiacetal) A R2 = alkyl (hemiketa)l Hemiacetal to Acetal and Hemiketals to Ketals in Acid catalyst Only Protonate the hemiacetal, setting up leaving group. Departure of leaving group. Attack of nucleophile Deprotonation Page 118  Example 5- Cannizzaro reaction (self oxidation/reduction) a reaction of aldehydes without α-hydrogens The Cannizzaro reaction is a redox reaction in which two molecules of an aldehyde are reacted to produce a primary alcohol and a carboxylic acid using a hydroxide base Page 119  Crossed Cannizzaro Formaldehyde is the most easily oxidized aldehyde. When mixed with another aldehyde that doesn’t have any alpha-hydrogens and conc. NaOH, all of the formaldehyde is oxidized and all of the other aldehyde is reduced. 6- Aldol Condensation Dimerization of aldehydes containing α-hydrogen atom in alkaline media to produce β- hydroxyaldehydes: Page 120  Cross Aldol Condensation It occurs when reacting two aldehydes one of them containing α-hydrogen and the other does not have H R CH CHCHO CH2CHO heat - H2O OH O O OH H2O R C CH2CHO R CH CH2CHO R CH CH2CHO H carbanion 7) Addition of Grignard reagents. O O MgBr C + RMgX C R O MgBr OH C + H2O C + Mg(OH)Br R R larger alcohol Page 121  Amines An amine with 1C attached to N is a primary amine, an amine with two C is a secondary amine, and an amine with three C is a tertiary amine. Physical Properties Amines, which are more polar than alkanes but less polar than alcohols. Alkylamines have boiling points higher than those of alkanes but lower than those of alcohols. Preparation of Amines by Alkylation of Ammonia Preparation of Amines by Reduction Acid amides reduced to the corresponding amines Page 122  The R and R’ may be alkyl groups or H: Inductive Effects and Basicity The presence of electron donating group (+I) increase the basicity while, the presence of electron withdrawing group (-I) decrease it. Page 123