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Transcript
Chapter 13. Aldehydes and Ketones Based on McMurry’s Organic Chemistry, 6th edition 1 Aldehydes and Ketones Aldehydes and ketones are characterized by the carbonyl functional group (C=O) The compounds occur widely in nature as intermediates in metabolism and biosynthesis They are also common as chemicals, as solvents, monomers, adhesives, agrichemicals and pharmaceuticals 2 The Carbonyl Group 3 The Carbonyl Group 4 Naming Aldehydes and Ketones Aldehydes are named by replacing the terminal -e of the corresponding alkane name with –al The parent chain must contain the CHO group The CHO carbon is numbered as C1 If the CHO group is attached to a ring, use the suffix -carbaldehyde 5 Naming Ketones Replace the terminal -e of the alkane name with –one Parent chain is the longest one that contains the ketone group Numbering begins at the end nearer the carbonyl carbon 6 Common Names IUPAC retains well-used but unsystematic names for a few ketones: 7 Ketones and Aldehydes as Substituents The R–C=O as a substituent is an acyl group is used with the suffix -yl from the root of the carboxylic acid CH3CO: acetyl; CHO: formyl; C6H5CO: benzoyl; ArCO: aroyl The prefix oxo- is used if other functional groups are present and the doubly bonded oxygen is labeled as a substituent on a parent chain 8 Preparation of Aldehydes and Ketones Preparing Aldehydes Oxidize primary alcohols using pyridinium chlorochromate (PCC) Preparing Ketones Oxidize secondary alcohols using (PCC), or CrO3 or Na2Cr2O7 9 Preparing Ketones: Other Methods Friedel–Crafts acylation of an aromatic ring with an acyl chloride in the presence of AlCl3 catalyst Hydration of terminal alkynes in the presence of Hg2+ catalyst 10 Oxidation of Aldehydes CrO3 in aqueous acid oxidizes aldehydes to carboxylic acids efficiently Silver oxide, Ag2O, in aqueous ammonia (Tollens’ reagent) oxidizes aldehydes (no acid) 11 Nucleophilic Addition Reactions of Aldehydes and Ketones Nu- approaches 45° to the plane of C=O and adds to C, pushing p electrons onto oxygen A tetrahedral alkoxide ion intermediate is produced 12 Nucleophiles Nucleophiles can be negatively charged ( : Nu) or neutral ( : Nu) at the reaction site The overall charge on the nucleophilic species is not considered 13 14 Relative Reactivity of Aldehydes and Ketones Aldehydes are generally more reactive than ketones in nucleophilic addition reactions The transition state for addition is less crowded and lower in energy for an aldehyde (a) than for a ketone (b) Aldehydes have one large substituent bonded to the C=O: ketones have two 15 Electrophilicity of Aldehydes and Ketones Aldehyde C=O is more polarized than ketone C=O As in carbocations, more alkyl groups stabilize + character Ketone has more alkyl groups, stabilizing the C=O carbon inductively 16 Reactivity of Aromatic Aldehydes Less reactive in nucleophilic addition reactions than aliphatic aldehydes Electron-donating resonance effect of aromatic ring makes C=O less reactive electrophilic than the carbonyl group of an aliphatic aldehyde 17 Nucleophilic Addition of H2O: Hydration Aldehydes and ketones react with water to yield 1,1-diols (geminal (gem) diols) Hydration is reversible: a gem diol can eliminate water 18 Relative Energies Equilibrium generally favors the carbonyl compound over hydrate for steric reasons Acetone in water is 99.9% ketone form Exception: simple aldehydes In water, formaldehyde consists is 99.9% hydrate 19 Base-Catalyzed Addition of Water Addition of water is catalyzed by both acid and base In base-catalyzed hydration nucleophile is the hydroxide ion, which is a much stronger nucleophile than water 20 Acid-Catalyzed Addition of Water Protonation of C=O makes it more electrophilic 21 Nucleophilic Addition of Alcohols: Acetal Formation Two equivalents of ROH in the presence of an acid catalyst add to C=O to yield acetals, R2C(OR)2 These can be called ketals if derived from a ketone 22 Formation of Acetals Alcohols are weak nucleophiles but acid promotes addition forming the conjugate acid of C=O Addition yields a hydroxy ether, called a hemiacetal (reversible); further reaction can occur Protonation of the OH and loss of water leads to an oxonium ion, R2C=OR+ (an oxygen bearing a positive charge) to which a second alcohol adds to form the acetal 23 Uses of Acetals Acetals can serve as protecting groups for aldehydes and ketones It is convenient to use a diol, to form a cyclic acetal (the reaction goes even more readily) 24 Cyclic Structures of Monosaccharides: Hemiacetal Formation glucose 25 Hydride Addition: reduction Convert C=O to CH-OH LiAlH4 and NaBH4 react as donors of hydride ion Protonation after addition yields the alcohol 26 Reduction Reagent: Sodium Borohydride NaBH4 is not sensitive to moisture and it does not reduce other common functional groups Lithium aluminum hydride (LiAlH4) is more powerful, less specific, and very reactive with water Both add the equivalent of “H-” 27 End of chapter 13 28