CH 19: Aldehydes and Ketones Renee Y. Becker Valencia Community College CHM 2211
Some Generalizations About Carbonyl Compounds • The most important functional group in organic chemistry.
Some Generalizations About Carbonyl Compounds • carbonyl compounds are planar about the double bond with bond angles 120 due to the sp2 hybridized carbon. • Many types of carbonyl compounds have significant dipole moments. • The polarity of the C-O bond plays a significant role in the reactivity of carbonyl compounds.
Aldehydes and Ketones • Due to the polarity of the carbonyl C-O bond, aldehydes and ketones have higher BPs than alkanes with similar molecular weights. • The lack of H-bonding hydrogens, results in lower BPs than similar alcohols.
Naming Aldehydes • 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.
Example 2: Draw • 3-Methylbutanal • 3-Methyl-3-butenal • cis-3-tert-Butylcyclohexanecarbaldehyde
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
Naming Ketones • Ketones with Common Names
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
Ketones and Aldehydes as Substituents • 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
Example 3: Name 1. 3. 4. 2.
Example 4: Draw • 4-Chloro-2-pentanone • P-bromoacetophenone • 3-ethyl-4-methyl-2-hexanone
Preparation of Aldehydes • Oxidize primary alcohols using pyridinium chlorochromate
Preparation of Aldehydes • Oxidation of alkenes with a vinylic hydrogen
Preparation of Aldehydes • The partial reduction of certain carboxylic acid derivatives. (esters)
Example 5 How would you prepare pentanal from the following: 1. 1-Pentanol • 1-Hexene
Preparing Ketones • Oxidation of secondary alcohols
Preparing Ketones • Oxidation of alkenes if one unsaturated carbon is disubstituted
Preparing Ketones • Friedel-Crafts acylation of aromatic compounds with an acid chloride. Occurs only once!
Preparing Ketones • Hydrations of terminal alkynes • Methyl ketone synthesis • Hg2+ catalyst
Example 6 How would you carry out the following reactions? More than 1 step might be necessary. 1. 3-Hexyne 3-Hexanone 2. Benzene m-Bromoacetophenone 3. Bromobenzene Acetophenone
Reactions of Aldehydes and Ketones • Oxidation reactions • Nucleophilic addition reactions • Conjugate nucleophilic addition reactions
Oxidation of Aldehydes • Jones’ Reagent (preferred) • Preferred over other oxidation reagents due to Room temp. reaction with high yields • Run under acidic conditions (con) • Will react with C=C and any acid sensitive functionality
Oxidation of Aldehydes • Tollen’s reagent • For use with C=C double bonds
Oxidation of Ketones • Ketones are resistant toward oxidation due to the missing hydrogen on the carbonyl carbon • Treatment of ketones with hot KMnO4 will cleave the C-C bond adjacent to the carbonyl group:
Nucleophilic Addition Reactions of Aldehydes and Ketones • Nu- approaches 45° to the plane of C=O and adds to C • A tetrahedral alkoxide ion intermediate is produced
Nucleophiles • Nucleophiles can be negatively charged ( : Nu) or neutral ( : Nu) at the reaction site • The overall charge on the nucleophilic species is not considered
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)
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
Reactivity of Aromatic Aldehydes • Aromatic aldehydes are less reactive in nucleophilic addition than straight chain aldehydes • Due to electron-donating resonance effect of aromatic ring • Makes carbonyl group less electrophilic
Nucleophilic Addition of H2O: Hydration • Aldehydes and ketones react with water to yield 1,1-diols (geminal (gem) diols) • Hyrdation is reversible: a gem diol can eliminate water
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
Acid & Base-Catalyzed Addition of Water • Addition of water is catalyzed by both acid and base • The base-catalyzed hydration nucleophile is the hydroxide ion, which is a much stronger nucleophile than water • Acid-Catalyzed Addition of Water • Protonation of C=O makes it more electrophilic
Addition of H-Y to C=O • Reaction of C=O with H-Y, where Y is electronegative, gives an addition product (“adduct”) • Formation is readily reversible
Nucleophilic Addition of HCN: Cyanohydrin Formation • Aldehydes and unhindered ketones react with HCN to yield cyanohydrins, RCH(OH)CN
Mechanism of Formation of Cyanohydrins • Addition of HCN is reversible and base-catalyzed, generating nucleophilic cyanide ion, CN • Addition of CN to C=O yields a tetrahedral intermediate, which is then protonated • Equilibrium favors adduct
Uses of Cyanohydrins • Nitriles can be reduced with LiAlH4 to yield primary amines:
Uses of Cyanohydrins • Nitriles can be hydrolyzed with hot aqueous acid to yield carboxylic acids:
Nucleophilic Addition of Grignard Reagents and Hydride Reagents: Alcohol Formation • Treatment of aldehydes or ketones with Grignard reagents yields an alcohol • Nucleophilic addition of the equivalent of a carbon anion, or carbanion. A carbon–magnesium bond is strongly polarized, so a Grignard reagent reacts for all practical purposes as R :MgX +.
Mechanism of Addition of Grignard Reagents • Complexation of C=O by Mg2+, Nucleophilic addition of R :,protonation by dilute acid yields the neutral alcohol • Grignard additions are irreversible because a carbanion is not a leaving group