Aldehydes, Ketones and Carboxylic Acids • Topic 1 of 3

Carbonyl Compounds: Structure & Preparation

Aldehydes and ketones both contain the carbonyl group (>C=O), so they are together called carbonyl compounds. In an aldehyde the carbonyl carbon carries at least one hydrogen (general formula R–CHO), while in a ketone the carbonyl carbon is bonded to two carbon atoms (R–CO–R'). The simplest aldehyde is methanal (HCHO, formaldehyde) and the simplest ketone is propanone (CH3COCH3, acetone).

Nomenclature

In IUPAC names the aldehyde group is denoted by the suffix -al and the longest chain is numbered so the –CHO carbon is C-1 (e.g. CH3CH2CHO is propanal; CH3CH(CH3)CHO is 2-methylpropanal). Ketones take the suffix -one with a locant for the carbonyl carbon (CH3COCH2CH3 is butan-2-one). Common names of aldehydes derive from the acid (formaldehyde, acetaldehyde, benzaldehyde); ketones are named as alkyl/aryl + ketone (ethyl methyl ketone). Benzaldehyde (C6H5CHO) and acetophenone (C6H5COCH3) are common aromatic members.

Structure & nature of the carbonyl group

The carbonyl carbon is sp2 hybridised; the three σ-bonds lie in a plane at about 120° and the unhybridised p-orbital overlaps sideways with a p-orbital of oxygen to give the π-bond. Because oxygen is far more electronegative than carbon, the π-electrons are pulled towards oxygen, making the bond strongly polar: the carbon is electron-poor (δ+) and the oxygen electron-rich (δ−). This polarity, written C=O ↔ +C–O−, controls all of carbonyl chemistry — the δ+ carbon is attacked by nucleophiles and the δ− oxygen by electrophiles/acids. The polar C=O also gives these compounds appreciable dipole moments and higher boiling points than non-polar hydrocarbons of similar mass.

Methods of preparation

(a) By oxidation of alcohols. Primary alcohols give aldehydes (e.g. with PCC, pyridinium chlorochromate, which stops at the aldehyde stage); secondary alcohols give ketones (with acidified KMnO4 or K2Cr2O7). (b) By dehydrogenation of alcohols over hot Cu at 573 K: 1° alcohols → aldehydes, 2° alcohols → ketones. (c) From acyl chlorides (Rosenmund reduction): RCOCl + H2 → RCHO over Pd poisoned with BaSO4 to prevent over-reduction. (d) From nitriles and esters: nitriles on partial reduction (Stephen reaction, SnCl2/HCl, then H2O) or with DIBAL-H give aldehydes; esters with DIBAL-H also give aldehydes. (e) From hydrocarbons: ozonolysis of alkenes yields aldehydes/ketones depending on substitution; Gattermann–Koch reaction converts benzene to benzaldehyde with CO + HCl/anhydrous AlCl3–CuCl. (f) Friedel–Crafts acylation: an aromatic ring + RCOCl/anhydrous AlCl3 gives an aryl ketone (e.g. benzene + CH3COCl → acetophenone).

Physical properties

Lower members are colourless liquids (methanal is a gas) with characteristic odours. They cannot form intermolecular H-bonds among themselves (no O–H), so their boiling points are lower than the corresponding alcohols/acids but higher than comparable hydrocarbons/ethers because of dipole–dipole attraction. Lower aldehydes and ketones are fairly soluble in water (H-bonding with water through the carbonyl O), the solubility falling as the hydrocarbon part grows.

Structure and polarity of the carbonyl group (sp2 carbon)COπ + σ (polar)RR'/Hδ+δ−sp² carbon, ~120° bond angles; nucleophile attacks δ+ C
Solved Examples
1
Worked Example
Give the IUPAC name of CH3CH2CH2CHO and of CH3COCH2CH3.
Solution
  1. The first compound has a four-carbon chain ending in –CHO; number from the CHO carbon (C-1).
  2. Chain of 4 carbons with the -al suffix → butanal.
  3. The second has a four-carbon chain with C=O on C-2: butan-2-one.

Answer: Butanal and butan-2-one.

2
Worked Example
Which reagent converts a primary alcohol to an aldehyde without over-oxidising it to a carboxylic acid, and why?
Solution
  1. Acidified KMnO4/K2Cr2O7 oxidise the aldehyde further to the acid.
  2. PCC (pyridinium chlorochromate) in CH2Cl2 is a mild, anhydrous oxidant.
  3. It stops at the aldehyde because no water is present to form the hydrate that gets over-oxidised.

Answer: PCC; it is a mild anhydrous oxidant that halts oxidation at the aldehyde stage.

3
Worked Example
How is benzaldehyde prepared directly from benzene? Name the reaction and reagents.
Solution
  1. Benzene cannot be formylated by an acyl halide of formic acid (it is unstable).
  2. Use a mixture of CO and HCl with anhydrous AlCl3 (and CuCl).
  3. This in-situ generates the formyl cation; it attacks the ring.

Answer: Gattermann–Koch reaction: C6H6 + CO + HCl ⟶(anhyd. AlCl3/CuCl) C6H5CHO.

4
Worked Example
Write the product when acetyl chloride (CH3COCl) is treated with H2 over Pd/BaSO4. Name the reaction.
Solution
  1. An acyl chloride reduced by H2 on a poisoned Pd catalyst gives an aldehyde.
  2. BaSO4 (plus S/quinoline) poisons Pd so reduction stops at –CHO.
  3. CH3COCl + H2 → CH3CHO + HCl.

Answer: Acetaldehyde (CH3CHO) by Rosenmund reduction.

5
Worked Example
What carbonyl products form on ozonolysis of 2-methylbut-2-ene, (CH3)2C=CHCH3?
Solution
  1. Ozonolysis cleaves the C=C double bond.
  2. The (CH3)2C= fragment has no H on the doubly bonded carbon → ketone (propanone).
  3. The =CHCH3 fragment has one H → aldehyde (ethanal).

Answer: Propanone (acetone) and ethanal (acetaldehyde).

6
Worked Example
Arrange propanal, propan-1-ol and butane in increasing order of boiling point and justify.
Solution
  1. Butane is non-polar (only weak van der Waals forces) — lowest b.p.
  2. Propanal is polar (dipole–dipole) but has no O–H, so no intermolecular H-bonding — intermediate.
  3. Propan-1-ol forms strong intermolecular hydrogen bonds — highest b.p.

Answer: butane < propanal < propan-1-ol.

Key Points

  • Carbonyl group >C=O has an sp2 carbon (~120°) and is strongly polar (C is δ+, O is δ−).
  • Aldehydes (R–CHO, suffix -al) have H on the carbonyl carbon; ketones (R–CO–R', suffix -one) have two C.
  • Oxidation/dehydrogenation of 1° alcohols → aldehydes, 2° alcohols → ketones.
  • Rosenmund (RCOCl + H2/Pd-BaSO4) and Stephen/DIBAL-H (from nitriles/esters) make aldehydes; Gattermann–Koch makes benzaldehyde; Friedel–Crafts acylation makes aryl ketones.
  • No self H-bonding, so b.p. lower than alcohols/acids but higher than hydrocarbons; lower members are water-soluble.
Quick Check — 4 MCQs
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Q1.The carbonyl carbon in an aldehyde is:
Explanation: It forms three sigma bonds in a plane (~120°) and one pi bond, so it is sp2.
Q2.Rosenmund reduction of an acyl chloride uses:
Explanation: The Pd catalyst is poisoned by BaSO4 so reduction stops at the aldehyde.
Q3.Which reaction converts benzene into benzaldehyde using CO and HCl with anhydrous AlCl3/CuCl?
Explanation: Gattermann–Koch formylates benzene directly with CO + HCl.
Q4.Oxidation of a secondary alcohol gives a:
Explanation: A 2° alcohol has the OH on a carbon bearing two C groups, so oxidation yields a ketone.
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