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Organic Chemistry II CHEM 202 2nd Term 2011-2012 Instructor: Assoc. Prof. Khaled S. Abdel Halim [email protected] Chapter IV : Aldehydes & Ketones In this chapter, we shall study the following topics: • Nomenclature of aldehydes & ketones. • Preparation of aldehydes & ketones. • Main chemical reactions. • Uses of aldehydes & ketones. 2 Overview * The general formula of aldehyde is RCHO while that of ketone is RCOR. * Aldehydes and ketones are common in living systems such as ribose sugar (aldehyde) and progesterone (ketone, female hormone). *Many aldehydes and ketones have distinctive odors . Aldehydes are generally pungent- smelling and ketones are sweet- smelling. •The carbonyl group of aldehyde and ketones are characterized by: •SP2 hybridization • flatness •Polar •Two pairs of unshared valence electrons exist on oxygen atom. 3 Nomenclature 4 Preparation of aldehydes & Ketones The most common way to prepare aldehyde or ketone in laboratory is by the oxidation of alcohol. Aryl ketones can be prepared by Friedel - Crafts acylation. 5 Reactions of aldehydes & Ketones: 1. 2. 3. 4. 5. Addition reactions: (H2O, ROH, HCN, RMgX) Addition – elimination reactions: ( RNH2, NH2NH2, phosphonium ylide) Reduction Oxidation Halogenation Addition reactions Carbonyl group is polar, so the compound may be attacked by a nucleophile or electrophile Carbonyl group is stabilized by adjacent alkyl groups (e-donor), so ketone is more stable than aldehyde. Steric effect also play a role in the relative reactivities of aldehydes and ketones. 6 Reaction with water Carbonyl compounds react with water to form 1,1 diol (gem-diol or hydrate) Both formaldehyde and chloral are more reactive than most other aldehydes or ketones because carbonyl carbon in each has a large amount of positive charge. Reaction with alcohol 7 Reaction with hydrogen cyanide HCN can be obtained from reaction of KCN or NaCN with strong acid. HCN is toxic . It forms cyanohydrin when reacts with carbonyl group. The reaction requires alkaline condition such as buffer solution of NaCN -HCN Cyanohydrins are usuful synthetic intermediate, CN group can be hydrolyized to carboxylic or ester group. Hydroxyl group also can be replaced by amonia to form amino acid. 8 Reaction with Grignard reagent 9 10 Addition – Elimination reactions of carbonyl group Reaction with ammonia and primary amine to form imine Reaction with secondary amine to form enamine (vinyl amine) CH3CHO + (CH3)2NH 11 Reaction with nitrogen compounds to form hydrazone 12 Reaction with phosphonium ylide (Witting reaction) to form alkenes + (C6H5)3 P= O Triphenylphosphoine oxide 13 Reduction of aldehydes & ketones Aldehydes and ketones can be reduced to alcohols (using H2 or metal hydrides), or to hydrocarbons or amines (using ammino compounds). Hydrogenation reactions If C=C exist in the carbonyl compounds, the reduction take place first at C=C and after that C=O can be reduced. 14 Reduction with metal hydride Aldehydes and ketones can be reduced to corresponding alcohols using strong reducing agents such as LiAlH4 or mild reducing agents such as NaBH4 Metal hydride reduced only carbonyl group and can not reduce C=C or ester group if exist in carbonyl compound 15 Reduction to hydrocarbons C=O group in aryl ketones can be reduced to CH2 group using Wolff-Kishner reduction (hydrazine reagent in alkaline medium) or Clemmensen reduction (Zn amalgam in acidic medium) Reduction to amine compounds The carbonyl group can be converted to amino group using ammonia or primary amine in the presence of H2 and catalyst 16 Oxidation of aldehydes & ketones Aldehydes can be easily oxidized to corresponding carboxylic acids while ketones does not. Oxidizing agents such as Pot. Permanganate, Pot. dichromate or Tollens reagent (Silver mirror). Tautomerism and alpha hydrogen The presence of hydrogen atom between two carbonyl groups make it acidic due to resonance structures. Such kind of carbonyl group form tautomerism (special type of structural isomers), keto form and enol form. More stable by H-bond 17 A ketone that form tautomerism can be oxidized by strong reducing agent such as conc. HNO3 2 CH3CH2CO2H CH3CH2CH2CO2H + CH3CO2H Propanoic acid butanoic acid acetic acid Alpha halogenation & haloform reaction Ketones can be halogenated in acidic or basic medium at alpha carbon. The reaction is the basis of haloform test (iodoform, chloroform, bromoform). The reaction can be used to get carboxylic acids. 18 Use of aldehydes & ketones Aldehydes and ketones can be used to synthesis of many organic compounds. In all these reactions, carbonyl group can be retained (halogenation), or extended to more carbon skeleton (Grignard and Witting reaction), or converted to another functional group (reduction) 19 Homework (Ch.13) • • • • • • • • • • 13.10 13.11 13.15 13.18 13.19 13.23 13.27 13.31 13.32 13.35 20 21 Organic Chemistry II CHEM 202 2nd Term 2011-2012 Instructor: Assoc. Prof. Khaled S. Abdel Halim [email protected] Chapter V : Carboxylic acids In this chapter, we shall study the following topics: • Nomenclature of carboxylic acid. • Preparation of carboxylic acid. • Acid strength. • Reactions of carboxylic acids. 2 Overview Carboxylic group contains hydroxyl group and carbonyl group, its planar, polar and has unshared electrons. Carboxylic compounds are important biologically and commercially. Aspirin is a common carboxylic acid. They are weak acids (pKa around 5) compared with mineral acids (pKa around -1), but they are more acidic than alcohols or phenols (because of resonance structure). 3 Nomenclature of carboxylic acids IUPAC Common names 4 Preparation of carboxylic acids (i) Hydrolysis of carboxylic acid derivatives (ii) Oxidation of alcohols (iii) Grignard reaction Hydrolysis reactions Oxidation reactions 5 Grignard reaction The reaction between Grignard reagent and CO2 in the presence of aqueous acid gives carboxylic compounds 6 Acid strength It is the extent of ionization of acid in water. The greater the amount of ionization, the stronger is the acid. The acidity of carboxylic compounds depends on its structure. There are many factors affecting the stability of A- and consequently the strength of HA such as: Electronegativity of atom in A- , Size of A- , hybridization of atom in A- , inductive effect of atoms attached to A- , resonance stabilization of A- , solvation of A- Effect of EN Effect of Size 7 Effect of hybridization Inductive effect Effect of Substituted groups Effect of resonance 8 Reactions of carboxylic acids (i) Neutralization (ii) Esterification (iii) Reduction (i) Reaction with base (neutralization): Acetate salt such as (ii) Reaction with alcohols (Esterification) Esterification is acid catalyzed and reversible . The rate depends on steric hindrance in the alcohol and acid. Water is from by loss of H from alcohol and OH of acid. 9 (iii) Reduction of carboxylic acid Polyfunctional carboxylic acids Dicarboxylic acids and carboxylic acid containing another functional group such as keto group show unique chemical properties 10 Homework (Ch.14) • • • • • • • • • 14.4 14.6 14.7 14.10 14.14 14.20 14.22 14.26 14.29 Write a report on : “ Comparative study between acidity of mineral acids and that of carboxylic acid 11 12