17 Carbonyl Compounds

how to increase [CN-] to increase the reaction rate

1. addition of a strong electrolyte containing CN- such as NaCN(aq) or KCN(aq) - NaCN(aq) --> Na+(aq) + CN-(aq) - the complete ionisation of the electrolyte provides the initial CN- ions for the nucleophilic attack on the carbonyl carbon 2. addition of a small amount of strong base such as NaOH(aq) and KOH(aq) - when base is added, H+ ions are consumed: OH- + H+ --> H2O - By LCP, the eqm position of HCN<-->H+ + CN- will shift to the right, resulting in an increase in [CN-]. hence this provides the initial CN- ions for nucleophilic attack on the carbonyl carbon

physical properties of aldehydes and ketones

1. as polar compounds, aldehydes and ketones have higher boiling poitns than alkanes of similar electron cloud size 2a. appreciable solubility of short-chain carbonyl compounds in water - the lone pair of electrons on the carbonyl oxygen allows carbonyl compounds to form hydrogen bonds with the H atom of water molecule 2b. as the length of the carbon chain increases, solubility in water decreases - molecule has a larger non0polar hydrophobic hydrocarbon portion, and id-id interactions become the predominant intermolecular forces

preparation of carbonyl compounds from alkenes

CR2=CR2 + 2[O] --> 2CR2(=O) R&C: KMnO4(aq), H2SO4(aq), heat

generation of nucleophile in formation of cyanohydrin

HCN(aq) <--> H+(aq) + CN-(aq)

oxidation of carbonyl cpds with CH3CO- to form tri-iodomethane

RC(CH3)(=O) + 3I2 + 4OH- --> RCO(=O) + CHI3 + 3I- + 3H2O R&C: I2(aq) with NaOH(aq)/KOH(aq), heat Observations: yellow ppt of CHI3

oxidation of aliphatic aldehydes to form carboxylate salts using Fehling's solution

RCH(=O) + 2Cu2+ + 5OH- --> RC(O-)(=O) + Cu2O + 3H2O R&C: Fehling's solution, heat or Benedict's solution, heat obsrevation: a brick-red ppt of Cu2O is formed - Fehling's soln: Alkaline soln of Cu2+ ion complexed with tartrate ion - Benedict's soln: Alkaline soln of copper(II) citrate (aromatic aldehydes and ketones do not react with Fehling's Benedict's soln)

oxidation of aldehydes to form carboxylate salts using Tollen's reagent

RCH(=O) + 2[Ag(NH3)2]+ + 3OH- --> RCO(=O) + 2Ag + 4NH3 + 2H2O R&C: Tollen's reagent [Ag(NH3)2], heat observation: A silver mirror/grey-black ppt of Ag is formed (used to distringuish aldehydes from ketones)

oxidation of aldehydes to form carboxylic acid

RCH(=O) + [O] -- RC(OH)(=O) R&C: K2Cr2O7(aq)/KMnO4(aq), H2SO4(aq), heat (happens for aldehydes because the hydrogen atom attached to the carbonyl carbon can be abstracted in oxidation)

reduction of cyanohydrines to form amines

RCH(OH)(CN) + 4[H]/2H2 --> RCH(OH)(CH2NH2) R&C: LiAlH4, dry ether OR H2, Ni, heat OR H2, pt

(Acidic) hydrolysis of cyanohydrins to form 2-hydroxycarboxylic acids

RCH(OH)(CN) + HCl + 2H2O --> RCH(OH)(COOH) + NH4Cl R&C: dilute HCl(aq)/H2SO4(aq), heat

preparation of aldehyde from primary alcohol

RCH2(OH) + [O] --> RCH(=O) + H2O R&C: K2Cr2O7(aq), H2SO4, heat with immediate distillation - oxidising agent is added slowly to alcohol so that the alcohol is always in ecvess to prevent any unreacted oxidising agent from further oxidising aldehyde to carboxylic acid - Immediate distillation of the aldehyde will prevent further oxidation - KMnO4 is too strong an oxidising agent to be used

preparation of ketone from secondary alcohol

RCHR(OH) + [O] --> RCR(=O) + H2O R&C: K2Cr2O&/KMnO4(aq), H2SO4(aq), heat

formation of cyanohydrins, with C(-CN)(-OH)

aldehyde + HCN --> RCH(OH)(CN) ketone + HCN --> RCR'(OH)(CN) R&C: HCN, a trace amount of KCN/KOH OR KCN, aq H2SO4

condensation reactions of carbonyl compounds

aldehydes and ketones react with the -NH2 group of 2-4-dinitrophenylhydrazine (2,4-DNPH) to form 2-4-dinitrophenylhydrazones with the elimination of water CH3-CHO + NH2-NH-phenyl(NO2)2 --> CH3-CH=N-NH-phenyl(NO2)2 + H2O - the 2,4-dinitrophenylhydrazones formed are orange crystalline solids, useful for deteciton of aldehydes andd ketones

reduction of carbonyl compounds to form alcohols

aldehydes are reduced to primary alcohols - RCH(=O) + 2[H] --> RCH2(OH) ketones are reduced to secondary alcohols - RCR'(=O) + 2[H] --> RCHR'(OH) R&C: NaBH4(specifically for carbonyl compounds OR LiAlH4 in dry ether OR H2, Ni, heat

preparation of carbonyl compounds from electrophilic substitution of benzene

benzene + RC(=O)Cl --> benzene-C(=O)R + HCl R&C: RCOCl, AlCl3, room temp (Anhydrous)

structure and bonding of carbonyl compounds

carbonyl carbon is sp2 hybridised - sigma bonds are 120deg apart (trigonal planar) - the reamining p-orbital of C overlaps with a p-orbital of O to form a pi bond.

stereochemistry of addition products

since the geometry attack the sp2 hybridised carbonyl C atom is trigonal planar, the nucleophile can attack it from either side of the plane with equal chance to form a racemic mixture containing equal proportions of the two enantiomers

mechanism of the nucleophilic addition reactions of HCN with aldehydes and ketones

step 1: Nucleophilic attack by CN- on the carbonyl carbon - a lone pair of electrons from CN- is used to form a bond with the carbonyl carbon - simultaneously, the electron pair of the C=O pi bond shifts to the oxygen atom, giving an anionic tetrahedral intermediate with a negative charge on O atom step 2: protonation of the anionic intermediate - the anionic intermediate is protonated to form the final cyanohydrin product - a proton can be abstracted from the undissociated HCN molecule (that acts as Bronsted acid)

aromatic carbonyl compounds are generally less reactive towards nucleophilic attack as compared to their aliphatic counterparts

the carbonyl C atom in an aromatic carbonyl compound is less electron-deficient due to the interaction of the pi electron cloud of the carbonyl group and those of the adjacent benzene ring

why do carbonyl compounds undergo nucleophilic addition?

- carbonyl compounds attract nucleophiles because the carbonyl carbon bears the partial positive charge as it is bonded to a more electronegative O atom. as a result, electron rich nucleophiles are attracted to this electron-deficient site - carbonyl compounds undergo addition reaction because there is a C=O bond that is unsaturated

why are aldehydes generall more reactive towards nucleophiles than ketones

- the carbonyl carbon in aldehydes is more electron deficient as it only has one electron-donating alkyl group while ketones have two such groups - there is less steric hindrance around the carbonyl carbon in aldehydes as it is only bonded to one alkyl group

why do carbonyl compounds undergo nucleophilic addition reactions but not alkenes

- the sp2 hybridised C atom of the -C=O group bears the partial positive charge as it is n=bonded to a more electronegative oxygen atom. as a result, electron rich nucleophiles are attracted to this electron deficient site - the sp2 hybridised C atoms of the C=C group in alkenes do not have a partial positive charge, and hence do not attract nucleophiles

susceptibility of the carbonyl carbon to nuclophilic attack is affected by

1. electronic factor - substituents on carbonyl carbon donate or withdraw electrons through: (a) inductive effect - donation or withdrawal of electrons through sigma bonds due to the electronegativity difference between atoms. - eg. alkyl groups are electron-donating by inductive effect, which reduces the magnitude of the partial positive charge on the carbonyl carbon, and hence decrease attraction for nucleophiles and the susceptibility of the carbonyl carbon to nucleophilic attack (b) resonance effect - donation or withdrawal of electrons through pi bonds due to the continuous side-on-p-orbital overlap of the substitutent and the carbonyl carbon - eg. Aryl groups are electron-donating by resonance 2. steric factor - bulkyl alkyl groups increase steric hindrance about the carbonyl carbon. hindering the approach of the attacking nucleophil and hence contribute to a reduction in reactivity

aldehyde

general formula RCHO have at least one H atom attached to the carbony carbon atom - aliphatic aldehyde: R is an alkyl group - aromatic aldehyde: R is an aryl group

ketone

general formula RCOR' have two alkyl or aryl groups attached to the carbonyl carbon atom