SNS - Organic Chemistry - Aldehydes and Ketones Flashcards
Physical Properties
Governed by the presence of the carbonyl group
The dipole moments associated with the polar carbonyl groups align causing an elevation in BP relative to alkanes
The elevation is less than in alcohols as no hydrogen bonding is involved
Synthesis
- Friedel Crafts Acylation - produces ketones in the form R-CO-Ar
- Oxidation of alcohols - usually performed with PCC, Na or K dichromate or chromium trioxide (Jones reagent)
- Oxonolysis of alkenes - double bonds can be oxidatively cleaved to yield aldehydes and/or ketones typically with ozone
Alcohols and Ketones
Reactions
- Enolisation
- Addition - (a) Hydration, (b) Acetal and ketal formation, (c) Reaction with HCN, (d) Condensation with ammonia derivatives
- Aldol Condensation
- Wittig Reaction
- Oxidation and Reduction
Reactions
Enolisation
Protons alpha to carbonyl groups are relatively acidic (pKa ~20) due to resonance stabilisation of the conjugate base
A hydrogen atom that detaches itself from the alpha carbon has a finite probability of reattaching itself to the oxygen instead of the carbon
Therefore aldehydes and ketones exist in solution as a mixture of two isomers: familiar keto and the enol form (ene the double bond, ol the alcohol) with equilibrium lies far to the keto side. Are called tautomers - isomers differing only in the placement of a proton
Enols are the necessary intermediates in a number of reactions of aldehydes and ketones
Reactions
Enolisation
Michael Additions
The enolate carbanion, which is nucleophilic can be created with a strong base such as lithium diisopropyl amide (LDA) or potassium hydride (KH)
This nucleophilic carbanion reacts via SN2 with α,β-unsaturated carbonyl compounds in reactions called Michael additions
Reactions
Addition
Many of the reactions of aldehydes and ketones share the mechanism of nucleophilic addition to a carbonyl.
- Hydration
- Acetal and ketal formation
- Reaction with HCN
- Condensation with ammonia derivatives
Reactions
Addition
General Mechanism
- The C=O bond is polarised with a partial positive charge on C and a partial negative charge on O, making the carbon a good target for nucleophilic attack
- This results in the formation of a bond between the nucleophile and the C.
- The pi bond in the C=O then breaks generating a tetrahedral intermediate.
- If no good leaving group is present, the double bond can’t reform and the final product is almost identical to the intermediate except that usually the O- will accept a proton to form -OH
Reactions
Addition
Hydration
In the presence of water, aldehydes and ketones react to form gem diols (1,1-diols).
Water acts as a nucleophile, attacking at the carbonyl carbon
Proceeds slowly, although can be increased by the addition of a small amount of acid or base
Reactions
Addition
Acetal and Ketal Formation
A reaction similar to hydration occurs when are treated with alcohols - when one equivalent of alcohol added, produces a hemiacetal or hemiaketal. When two are added, an acetal or ketal is produced
Mechanism is as for hydration
Catalysed by anhydrous acid
Reactions
Addition
Reaction With HCN
Produces cyanohydrins
- HCN dissociates
- Nucleophilic cyanide anion attacks the carbonyl carbon
- Protonation of the oxygen atom produces a cyanohydrin. This is stable due to the newly formed C-C bond
Reactions
Addition
Condensation With Ammonia Derivatives
Ammonia and some of its derivatives are nucleophiles and can add to carbonyl compounds
In the simplest case, ammonia adds to the carbon atom. Water is lost (condensation reaction) and an imine produced (compound with a nitrogen atom double bonded to a carbon atom). The first part of this reaction follows the same mechanism as for nucleophilic addition, however after formation of the tetrahedral intermediate, the reaction proceeds further. The C=O bond reforms and a leaving group is eliminated in a mechanism called nucleophilic substitution
Common ammonia derivatives that react in this way are Hydroxylamine (H2NOH), hydraxine (H2NNH2) and semicarbazide (H2NNHCONH2). Form oximes, hydrazones and semicarbazones respectively
Reactions
Aldol Condensation
Basically follows the mechanism of nucleophilic addition to a carbonyl as for condensation with ammonia derivatives
An aldehyde acts as both nucleophile (enol form) and target (keto form)
When acetaldehyde (ethanal) is treated with base, an enolate ion is formed. Being nucleophilic, can react with the carbonyl group of another acetaldehyde molecule to produce 3-hydroxybutanal which contains both alcohol and aldehyde functionality (aldol)
With a stonger base and higher temp, condensation occurs and an α,β-unsaturated aldehyde is produced.
When heated this molecule can undergo elimination and lose H2O to form a double bond in an E1 reaction
Reactions
The Wittig Reaction
Method for forming carbon-carbon double bonds by converting aldehydes and ketones into alkenes
- Formation of phosphonium salt from SN2 reaction of an alkyl halide with the nucleophile triphenylphospine (C6H5)3P
- The phosphonium salt is then deprotonated (losing the proton alpha to he P) with a strong base to yield an electrically neutral compound called an ylide or phosphorane. An ylide is a type of carbanion and has nucleophilic properties.
- When combined with an aldehyde or ketone, an ylide attacks the carbonyl carbon to give an intermediate called a betaine which forms a 3-membered ring intermediate called oxaphosphetane.
- This decomposes to yield an alkene and triphenylphosphine oxide driven by the strength of the P-O bond that is formed
Reactions
Oxidation and Reduction
Oxidation
Aldehydes and ketones occupy the middle position of the oxidation-reduction continuum. They are more oxidised than alcohols but less than carboxylic acids
Can be oxidised with a number of different reagents such as KMnO4, CrO3, Ag2O or H2O2 to produce a carboxylic acid
Reactions
Oxidation and Reduction
Reduction
To Alcohols
A number of reagents will reduce aldehydes and ketones to alcohols. Most common is lithium aluminium hydride (LAH). Can also use NaBH4 when milder conditions needed
