CBSE Class 12 Chemistry Aldehydes Ketones And Carboxylic Acids Notes Set 06

These Class 12 CBSE Chemistry notes for Unit 8 start the study of carbonyl compounds. They cover the structure of the carbonyl group, IUPAC and common names, the preparation of aldehydes and ketones, their physical properties and the nucleophilic addition reactions that define them. Four solved examples show how to apply the reactivity order and the reagents.

Important Notes and Formulas

Carbonyl Group and Classification

1. Organic compounds that contain the carbonyl group C=O are called carbonyl compounds.
2. In an aldehyde the carbonyl carbon is attached to at least one hydrogen atom (the –CHO or formyl group), e.g. HCHO, CH₃CHO, C₆H₅CHO.
3. In a ketone the carbonyl carbon is attached to two carbon groups, e.g. CH₃COCH₃. Ketones are simple (both groups same, as in propanone) or mixed (different groups, as in butanone).
4. In a carboxylic acid the carbonyl carbon is attached to an –OH group (the –COOH group).
5. The general formula of saturated aliphatic aldehydes and ketones is CnH₂nO, so propanal and propanone are functional isomers.

Structure of the Carbonyl Group

1. The carbonyl carbon is sp² hybridised. It forms three σ bonds in one plane (bond angles about 120°) and one π bond with oxygen formed by sidewise overlap of p orbitals.
2. Oxygen is more electronegative than carbon, so the π electrons are pulled towards oxygen. The C=O bond is polar: C(δ+)=O(δ−), as shown by the resonance forms C=O ↔ C⁺–O⁻.
3. The carbon is electrophilic (attacked by nucleophiles) and the oxygen is nucleophilic (attacked by acids and electrophiles).
4. The C=O group has a larger dipole moment than C=C of alkenes, so aldehydes and ketones are polar molecules.

Nomenclature of Aldehydes and Ketones

1. Aldehydes: replace the final –e of the alkane name by –al. The CHO carbon is always C-1, so no locant is written for it. Aldehydes on a ring are named as carbaldehydes.
2. Ketones: replace the final –e by –one and number the chain so that the C=O carbon gets the lowest locant.
3. Choose the longest chain that contains the carbonyl carbon, then give substituents the lowest locants and list them alphabetically.

Isomerism

1. Chain isomerism: butanal and 2-methylpropanal.
2. Position isomerism (ketones): pentan-2-one and pentan-3-one.
3. Functional isomerism: an aldehyde and a ketone with the same formula, e.g. propanal and propanone (both C₃H₆O).
4. Metamerism (ketones): pentan-2-one and pentan-3-one also differ in the alkyl groups on either side of C=O.

Preparation of Aldehydes and Ketones

A. From alcohols (aldehydes and ketones)
1. Oxidation of a primary alcohol with pyridinium chlorochromate (PCC) or CrO₃ stops at the aldehyde: RCH₂OH —(PCC)→ RCHO. Stronger oxidants such as acidified K₂Cr₂O₇ or KMnO₄ go on to the carboxylic acid.
2. Oxidation of a secondary alcohol gives a ketone: R₂CHOH —([O])→ R₂C=O.
3. Dehydrogenation: the vapour of a 1° alcohol over Cu at 573 K gives an aldehyde and a 2° alcohol gives a ketone: CH₃CH₂OH —(Cu, 573 K)→ CH₃CHO + H₂. Tertiary alcohols lose water and give an alkene instead.

B. From hydrocarbons
1. Hydration of alkynes with dil. H₂SO₄ and HgSO₄ gives an enol that tautomerises to a carbonyl compound. Ethyne gives ethanal, and every other alkyne gives a ketone: CH₃C≡CH + H₂O —(Hg²⁺, H+)→ CH₃COCH₃.

C. Preparation of aldehydes only
1. Rosenmund reduction: acyl chloride + H₂ over palladium on barium sulphate gives an aldehyde: RCOCl + H₂ —(Pd/BaSO₄)→ RCHO + HCl. BaSO₄ partly poisons the catalyst so that the aldehyde is not reduced further to the alcohol. Methanal cannot be made this way because HCOCl is unstable.
2. Stephen reaction: a nitrile is reduced by SnCl₂ and HCl to an imine, which is hydrolysed to the aldehyde: RCN + 2[H] —(SnCl₂/HCl)→ RCH=NH —(H₃O⁺)→ RCHO.
3. Nitriles and esters are also reduced to aldehydes by diisobutylaluminium hydride (DIBAL-H) followed by hydrolysis: RCN --(i) AlH(i-Bu)₂, (ii) H₂O-→ RCHO.
4. Etard reaction: toluene is oxidised by chromyl chloride (CrO₂Cl₂) in CS₂ to a chromium complex, which is hydrolysed to benzaldehyde.
5. Toluene is also oxidised to benzaldehyde by CrO₃ in acetic anhydride, which forms benzylidene diacetate; hydrolysis with dilute acid gives C₆H₅CHO.
6. Gattermann–Koch reaction: benzene + CO + HCl in the presence of anhydrous AlCl₃ and CuCl gives benzaldehyde.

D. Preparation of ketones only
1. Acyl chloride + dialkylcadmium: 2RCOCl + R'2Cd → 2RCOR' + CdCl₂.
2. Nitrile + Grignard reagent, then hydrolysis: RCN + R'MgX → RR'C=NMgX —(H₃O⁺)→ RCOR'.
3. Friedel–Crafts acylation: benzene + RCOCl with anhydrous AlCl₃ gives an aryl ketone, e.g. benzene + CH₃COCl → acetophenone.

Physical Properties

1. Methanal is a gas, ethanal is a volatile liquid (boiling point 294 K) and the other lower aldehydes and ketones are liquids. Higher members are solids.
2. The boiling points of aldehydes and ketones are higher than those of hydrocarbons and ethers of comparable molar mass because of dipole–dipole attraction between polar C=O groups.
3. They are lower than those of alcohols of comparable molar mass because carbonyl compounds cannot form hydrogen bonds with each other.
4. Lower members (up to about four carbons) dissolve in water because the carbonyl oxygen forms hydrogen bonds with water. Solubility falls as the hydrocarbon part grows.
5. Aldehydes and ketones dissolve freely in organic solvents. Many of the lower aldehydes have sharp smells, while higher aldehydes and ketones have pleasant odours.

Order of boiling points for similar molar mass: alcohols > ketones > aldehydes > ethers > alkanes. Ketones boil slightly higher than aldehydes because the two alkyl groups push electrons towards C=O and make it more polar.

Uses

1. Methanal as a 40% aqueous solution (formalin) preserves biological specimens, and it is used to make bakelite (phenol–formaldehyde resin) and urea–formaldehyde glues.
2. Ethanal is a starting material for acetic acid, ethyl acetate, vinyl acetate and polymers.
3. Propanone and butanone are common industrial solvents.
4. Aldehydes and ketones such as vanillin, benzaldehyde, camphor and acetophenone are used in perfumes and flavourings.

Nucleophilic Addition Reactions

1. Mechanism: the nucleophile (Nu⁻) attacks the electrophilic carbonyl carbon from either face of the planar group. This slow step breaks the π bond and gives a tetrahedral alkoxide intermediate, which is quickly protonated by H⁺ to give the product.
2. Reactivity: aldehydes are more reactive than ketones. Electronically, a ketone has two electron-releasing alkyl groups (+I effect) that reduce the positive charge on carbon, while an aldehyde has only one. Sterically, a ketone has two bulky groups that hinder the attacking nucleophile in the crowded tetrahedral intermediate.
3. Order of reactivity: HCHO > CH₃CHO > CH₃COCH₃ > CH₃COC₂H₅ > C₆H₅COC₆H₅. Electron-withdrawing groups raise reactivity and electron-donating groups lower it.
4. Aromatic aldehydes such as benzaldehyde are less reactive than aliphatic aldehydes because the ring donates electrons to the carbonyl group through resonance.

Addition of HCN gives cyanohydrins. A small amount of base generates the stronger nucleophile CN⁻ and speeds the reaction.
RCHO + HCN —(NaOH)→ RCH(OH)CN (cyanohydrin)
CH₃COCH₃ + HCN → (CH₃)₂C(OH)CN (acetone cyanohydrin)
Cyanohydrins are useful because hydrolysis gives hydroxy acids: (CH₃)₂C(OH)CN —(H₃O⁺)→ (CH₃)₂C(OH)COOH (2-hydroxy-2-methylpropanoic acid).

Addition of sodium hydrogensulphite gives crystalline bisulphite adducts with aldehydes and aliphatic methyl ketones:
RCHO + NaHSO₃ ⇌ RCH(OH)SO₃Na
The adduct is water soluble and is converted back into the carbonyl compound by dilute acid or dilute alkali, so the reaction is used to separate and purify aldehydes and ketones. Bulky ketones such as benzophenone do not react.

Addition of alcohols: an aldehyde reacts with one equivalent of a monohydric alcohol in dry HCl to give a hemiacetal and then, with excess alcohol, an acetal (gem-dialkoxy compound).
CH₃CHO + 2C₂H₅OH —(dry HCl)→ CH₃CH(OC₂H₅)₂ + H₂O (ethanal diethyl acetal)
Ketones react with ethylene glycol in dry HCl to give cyclic ketals. Acetals and ketals are stable to bases and are used to protect the carbonyl group during a synthesis; dilute acid regenerates the carbonyl compound.
Addition of water: carbonyl compounds form unstable gem-diols (hydrates). Methanal is mostly hydrated in water, and chloral (CCl₃CHO) forms the stable chloral hydrate because of the three –I chlorine atoms.

Addition of Grignard reagents followed by hydrolysis gives alcohols: HCHO gives a primary alcohol, other aldehydes give secondary alcohols and ketones give tertiary alcohols. For example CH₃CHO + CH₃MgBr, then H₂O, gives propan-2-ol.

Reaction with ammonia derivatives (addition followed by loss of water, in weakly acidic medium): C=O + H₂N–Z —(H+)→ C=N–Z + H₂O

In semicarbazide only the NH₂ attached to NH reacts. The lone pair on the NH₂ next to C=O is delocalised into the carbonyl group, so that nitrogen is not nucleophilic. The coloured 2,4-DNP derivatives are used as a general test for the carbonyl group and for identification by melting point.

Solved Examples

Example 1. Arrange propanone, ethanal, methanal and benzophenone in the decreasing order of reactivity towards nucleophilic addition.
Answer:
Methanal has two H atoms and no alkyl groups, so its carbonyl carbon is the most positive and least hindered.
Ethanal has one methyl group, propanone has two methyl groups, and benzophenone has two bulky phenyl groups that hinder attack and donate electrons by resonance.
Order: HCHO > CH₃CHO > CH₃COCH₃ > C₆H₅COC₆H₅.

Example 2. Name the product when propanone reacts with HCN and the product is hydrolysed with dilute acid.
Answer:
CH₃COCH₃ + HCN → (CH₃)₂C(OH)CN (2-hydroxy-2-methylpropanenitrile).
Hydrolysis converts –CN into –COOH: (CH₃)₂C(OH)CN —(H₃O⁺)→ (CH₃)₂C(OH)COOH.
Product: 2-hydroxy-2-methylpropanoic acid.

Example 3. A mixture contains ethanal and ethyl benzene. How can the aldehyde be separated?
Answer:
Shake the mixture with saturated aqueous sodium hydrogensulphite. Ethanal forms the water-soluble adduct CH₃CH(OH)SO₃Na, while ethyl benzene does not react.
Separate the aqueous layer and treat it with dilute HCl or dilute NaOH to regenerate ethanal, then distil it.
The bisulphite addition and decomposition separates the carbonyl compound.

Example 4. Write the structure and name of the product formed when ethanal reacts with semicarbazide.
Answer:
CH₃CHO + H₂N–NH–CO–NH₂ —(H+)→ CH₃CH=N–NH–CO–NH₂ + H₂O.
The product is ethanal semicarbazone. Only the terminal –NH₂ on the NH group takes part.