ketones/aldehydes – Flashcards
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Carbonyl Structure - 2 points
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Carbon is sp2 hybridized C=O is shorter, stronger and more polar than C=C
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Ketones and aldehydes are good solvents for ____ and ____
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water and alcohol. due to hydrogen bonding
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Draw Mclafferty Rearrangement
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The net result of this rearrangement is the breaking of the ?, ? bond, and the transfer of a proton from the gamma carbon to the oxygen. An alkene is formed as a product of this rearrangement through the tautomerization of the enol.
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Secondary alcohols are readily oxidized to ketones with ______ in ______or by _______
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sodium dichromate (Na2Cr2O7) sulfuric acid potassium permanganate (KMnO4).
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________ is selectively used to oxidize primary alcohols to aldehydes.
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Pyridinium chlorochromate (PCC)
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Friedel Crafts Reaction
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Reaction between an acyl halide and an aromatic ring will produce a ketone.
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Hydration of Alkynes
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The initial product of Markovnikov hydration is an enol, which quickly tautomerizes to its keto form. Internal alkynes can be hydrated, but mixtures of ketones often result.
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Hydroboration-Oxidation of Alkynes
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Hydroboration-oxidation of an alkyne gives anti-Markovnikov addition of water across the triple bond.
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Synthesis of Ketones and Aldehydes Using 1,3-Dithianes
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1,3-Dithiane can be deprotonated by strong bases such as n-butyllithium. The resulting carbanion is stabilized by the electron-withdrawing effects of two polarizable sulfur atoms.
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Alkylation of 1,3-Dithiane
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Alkylation of the dithiane anion by a primary alkyl halide or a tosylate gives a thioacetal that can be hydrolyzed into the aldehyde by using an acidic solution of mercuric chloride.
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Ketones from 1,3-Dithiane
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The thioacetal can be isolated and deprotonated. Alkylation and hydrolysis will produce a ketone.
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Synthesis of Ketones from Carboxylic Acids
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Organolithiums will attack the lithium salts of carboxylate anions to give dianions. Protonation of the dianion forms the hydrate of a ketone, which quickly loses water to give the ketone.
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Ketones from Nitriles
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A Grignard or organolithium reagent can attack the carbon of the nitrile. The imine is then hydrolyzed to form a ketone.
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Aldehydes from Acid Chlorides
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Lithium aluminum tri(t-butoxy)hydride is a milder reducing agent that reacts faster with acid chlorides than with aldehydes
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Lithium Dialkyl Cuprate Reagents
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A lithium dialkylcuprate (Gilman reagent) will transfer one of its alkyl groups to the acid chloride.
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Nucleophilic Addition
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A strong nucleophile attacks the carbonyl carbon, forming an alkoxide ion that is then protonated. Aldehydes are more reactive than ketones.
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The Wittig Reaction
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The Wittig reaction converts the carbonyl group into a new C?C double bond where no bond existed before. A phosphorus ylide is used as the nucleophile in the reaction.
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Preparation of Phosphorus Ylides
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Prepared from triphenylphosphine and an unhindered alkyl halide. Butyllithium then abstracts a hydrogen from the carbon attached to phosphorus.
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Mechanism for Wittig
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The oxaphosphetane will collapse, forming carbonyl (ketone or aldehyde) and a molecule of triphenyl phosphine oxide.
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Hydration of Ketones and Aldehydes
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In an aqueous solution, a ketone or an aldehyde is in equilibrium with its hydrate, a geminal diol. With ketones, the equilibrium favors the unhydrated keto form (carbonyl).
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Mechanism of Hydration of Ketones and Aldehydes
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Hydration occurs through the nucleophilic addition mechanism, with water (in acid) or hydroxide (in base) serving as the nucleophile.
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Cyanohydrin Formation
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The mechanism is a base-catalyzed nucleophilic addition: Attack by cyanide ion on the carbonyl group, followed by protonation of the intermediate. HCN is highly toxic.
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Formation of Imines
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Ammonia or a primary amine reacts with a ketone or an aldehyde to form an imine. Imines are nitrogen analogues of ketones and aldehydes with a C?N bond in place of the carbonyl group. Optimum pH is around 4.5
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Mechanism of Imine Formation
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Acid-catalyzed addition of the amine to the carbonyl compound group.
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Mechanism of Imine Formation
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Acid-catalyzed dehydration.
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Mechanism for Hemiacetal Formation
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Must be acid-catalyzed. Adding H+ to carbonyl makes it more reactive with weak nucleophile, ROH.
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Cyclic Acetals
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Addition of a diol produces a cyclic acetal. The reaction is reversible. This reaction is used in synthesis to protect carbonyls from reaction
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Acetals as Protecting Groups
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Hydrolyze easily in acid; stable in base. Aldehydes are more reactive than ketones.
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Reaction and Deprotection
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The acetal will not react with NaBH4, so only the ketone will get reduced. Hydrolysis conditions will protonate the alcohol and remove the acetal to restore the aldehyde.
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Oxidation of Aldehydes
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Aldehydes are easily oxidized to carboxylic acids.
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Reduction Reagents ... can reduce ketones to secondary alcohols and aldehydes to primary alcohols.
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Sodium borohydride, NaBH4
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Reduction Reagents...is a powerful reducing agent, so it can also reduce carboxylic acids and their derivatives.
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Lithium aluminum hydride, LiAlH4
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Reduction Reagents...can reduce the carbonyl, but it will also reduce any double or triple bonds present in the molecule.
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Hydrogenation with a catalyst
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Sodium borohydride, NaBH4
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NaBH4 can reduce ketones and aldehydes, but not esters, carboxylic acids, acyl chlorides, or amides.
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Lithium Aluminum Hydride
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LiAlH4 can reduce any carbonyl because it is a very strong reducing agent. Difficult to handle.
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Catalytic Hydrogenation
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Widely used in industry. Raney nickel is finely divided Ni powder saturated with hydrogen gas. It will attack the alkene first, then the carbonyl.
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Deoxygenation of Ketones and Aldehydes
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The Clemmensen reduction or the Wolff-Kishner reduction can be used to deoxygenate ketones and aldehydes.
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Wolff-Kishner Reduction
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Forms hydrazone, then heat with strong base like KOH or potassium tert-butoxide. Use a high-boiling solvent: ethylene glycol, diethylene glycol, or DMSO. A molecule of nitrogen is lost in the last steps of the reaction.