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Carbonyl Compounds and Nucleophilic Addition. Carbonyl compounds. Contain at least one carbonyl group. R = R’ or R R’. propanal. butanal. pentanal. Aldehydes terminal carbonyl groups. No need to specify the position of the carbonyl group. pentan-2-one. pentan-3-one. cyclohexanone.
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Carbonyl compounds Contain at least one carbonyl group. R = R’ or R R’
propanal butanal pentanal Aldehydes terminal carbonyl groups No need to specify the position of the carbonyl group
pentan-2-one pentan-3-one
cyclohexanone cyclohexanecarbaldehyde cyclohexylmethanal cyclohexylethanal
benzaldehyde 1-phenylethanone diphenylmethanone benzophenone
4-oxopentanoic acid 5-oxopentanoic acid 4-oxopentanal
Physical Properties 1. Most simple aliphatic ketones and aldehydes are liquids at room temperature except methanal (b.p. = 21C) and ethanal (b.p. = 20.8C) Aliphatic aldehydes have an unpleasant and pungent smell Ketones and aromatic aldehydes have a pleasant and sweet odour
Boiling point : - (similar molecular masses) carboxylic acid > alcohol > aldehyde, ketone > CxHy Presence of polar group Absence of –OH group
Solubility Small aldehydes and ketones show appreciable solubilities in water due to the formation of intermolecular hydrogen bonds with water
Solubility Ethanal and propanone are miscible with water in all proportions. Propanone(acetone) is volatile and miscible with water Once used to clean quick-fit apparatus potentially carcinogenic
Solubility Methanal gas dissolves readily in water Aqueous solutions of methanal (Formalin) are used to preserve biological specimens Methanal(formaldehyde) is highly toxic
Industrial preparation By dehydrogenation (oxidation) of alcohols Further oxidation is prohibited Out-dated
Laboratory preparation • Oxidation of alcohols 1 alcohol aldehyde carboxylic acid 2 alcohol ketone Further oxidation of aldehyde to carboxylic acid is prohibited by (i) using a milder O.A., e.g. H+/ Cr2O72
Laboratory preparation • Oxidation of alcohols 1 alcohol aldehyde carboxylic acid 2 alcohol ketone Further oxidation of aldehyde to carboxylic acid is prohibited by (i) using a milder O.A., e.g. H+/ Cr2O72 (ii) distilling off the product as it is formed
Heating under reflux Ethanol ethanoic acid
carboxylic acid High T 2 alcohol ketone Further oxidation of ketone to carboxylic acid has not synthetic application since 1. it requires more drastic reaction conditions 2. it results in a mixture of organic products
2. Reduction of acid chlorides The catalyst Pd or BaSO4 is poisoned with S to prevent further reduction to alcohol
oxidation reduction Carboxylic acid or acyl chloride Aldehyde Alcohol Aldehydes Intermediate oxidation state Preparation must be well controlled.
Friedel-Crafts acylation (Preparation of aromatic ketones)
4. Decarboxylation of calcium salts Symmetrical ketones can be obtained by heating a single calcium carboxylate
4. Decarboxylation of calcium salts Aldehydes can be obtained by heating a mixture of two calcium carboxylates Cross decarboxylation is preferred
NaOH(s) from soda lime CH3COONa(s) CH4 + Na2CO3 fusion NaOH(s) from soda lime + Na2CO3 fusion Decarboxylation of sodium salts gives methane or benzene.(p.30 and p.49)
ketone 5. Catalytic hydration of alkynes enol Keto-enol tautomerism
5. Catalytic hydration of alkynes enol Keto-enol tautomerism aldehyde
1. O3 2. Zn dust / H2O Unsymmetrical alkenes give a mixture of two carbonyl compounds making subsequent purification more difficult.
Reactions of Aldehydes and Ketones • Nucleophilic Addition Reactions (AdN) • Condensation Reactions (Addition-Elimination) • Iodoform Reactions (Oxidation) • Oxidation Reactions • Reduction Reactions
Bonding in the Carbonyl Group The carbonyl carbon atom is sp2-hybridized sp2 – 2p head-on overlap bond 2p – 2p side-way overlap bond The and bonds in the C = O bond
The carbonyl group is planar(sp2-hybridized) and highly polarized due to (i) Polarization of bond (inductive effect) (ii) Polarization of bond (mesomeric effect) +
50% + 50% Susceptible to nucleophilic attack
Bond Enthalpy (kJ/mol) : C=C (611) < 2 C–C (346) ( bond < bond) C=O (749) > 2 C–O (358) ( bond > bond) Due to polarization of the C-O bond
Nucleophilic Addition Reactions (AdN) Acid-catalyzed More susceptible to nucleophilic attack
Nucleophilic Addition Reactions (AdN) Base-catalyzed Stronger nucleophile
(Non-polar) + AdN vs AdE
Reaction mechanism Slow (r.d.s.) H+ and Nu added across the C=O bond Fast
50% 50% 50% 50% H+ Q.52 A racemic mixture
Reactivity depends on two factors : - (i) Electronic effect (ii) Steric effect
(i) Electronic effect Electron-deficiency of carbonyl C • Ease of nucleophilic attack • Reactivity
Decreasing reactivity > > Decreasing positive charge on carbonyl carbon
Carbonyl C is less positive due to delocalization of positive charge to the benzene ring. Or, The C-O bond has less character less mesomeric effect
> Reactivity : -
(ii) Steric effect No. /bulkiness of R groups at carbonyl C • Steric hindrance • Reactivity
Decreasing reactivity > > Increasing steric hindrance
Reactivity of AdN : - • Aldehydes > ketones • Aliphatic > aromatic