Aldehydes, Ketones and Carboxylic Acids · Lesson 3 of 15
Ketone Preparation and Carbonyl Physical Properties
“Prepare ketones and explain volatility, solubility and applications through molecular interactions.”
• Predict ketones from alcohols, acyl chlorides, nitriles and arenes. • Track carbon groups through ketone-forming reactions. • Explain boiling-point and water-solubility trends. • Connect carbonyl properties with selected industrial uses.
Keep two carbon groups attached to the new C=O
Oxidising a secondary alcohol removes the O–H hydrogen and the hydrogen on the alcohol-bearing carbon: RCH(OH)R′ + [O] → RCOR′ + H₂O. Cyclohexanol therefore gives cyclohexanone with a suitable oxidant such as CrO₃. Dehydrogenation over heated copper also gives the ketone and H₂. A tertiary alcohol lacks the necessary carbinol C–H bond and does not undergo this simple oxidation while keeping its carbon skeleton intact.
Hydration of propyne with HgSO₄ in aqueous H₂SO₄ gives an enol that tautomerises to propanone. Reductive ozonolysis provides ketones from alkene carbons bearing two carbon substituents. These routes are complementary: hydration retains the carbon framework, whereas ozonolysis cleaves a C=C bond. Always draw the starting structure rather than recalling only an alkene or alkyne name.
Choose a carbon nucleophile that stops at a ketone
Acyl chlorides react with dialkylcadmium reagents to give ketones. The organocadmium reagent can be represented as prepared from 2RMgX + CdCl₂ → R₂Cd + 2MgXCl. Then 2R′COCl + R₂Cd → 2R′COR + CdCl₂. The relatively moderated carbon nucleophile substitutes at the acyl centre without the usual extensive second addition of a Grignard reagent. This is a classical preparation principle, not a claim that all organometallic reagents share its selectivity.
A Grignard reagent adds once to a nitrile to give a magnesium imine species. Acidic hydrolysis then replaces C=N by C=O, yielding a ketone: RCN + R′MgX → bound imine → RCOR′. The ketone appears during aqueous work-up, after the organomagnesium reagent has been destroyed, which prevents it from simply undergoing a second Grignard addition in the original dry mixture. Dry ether or another appropriate anhydrous ethereal medium is essential before work-up.
Problem
Prepare propiophenone from propanenitrile.
- 1.Propanenitrile is CH₃CH₂CN. Its ethyl group will remain on one side of the eventual carbonyl; the CN carbon becomes the carbonyl carbon.
- 2.Add phenylmagnesium bromide in dry ether, then perform acidic hydrolysis. Product: CH₃CH₂COC₆H₅, 1-phenylpropan-1-one, commonly propiophenone. Total carbons: 3 + 6 = 9.
Friedel–Crafts acylation creates an aryl ketone
Benzene reacts with an acyl chloride in the presence of anhydrous AlCl₃, followed by aqueous work-up, to give an aryl ketone: C₆H₆ + RCOCl → C₆H₅COR + HCl. The Lewis acid helps generate an acyl electrophile. The ring forms a new C–C bond, then loses H⁺ to restore aromaticity. The acyl fragment retains its C=O group. Acetyl chloride gives acetophenone; benzoyl chloride gives benzophenone. Suitable acid anhydrides can also supply an acyl group.
AlCl₃ also coordinates to product oxygen, so work-up is part of obtaining free ketone. Strongly deactivated rings, such as nitrobenzene, do not undergo ordinary Friedel–Crafts acylation readily. When a carbonyl group is directly attached to an aromatic ring, it withdraws electron density and directs later electrophilic substitution mainly meta. Therefore benzene → acetophenone → m-nitroacetophenone is sensible, whereas attempting ordinary acylation of nitrobenzene is not.
Dipoles attract, but a carbonyl lacks an O–H donor
Methanal is gaseous near room temperature and ethanal is very volatile; higher members are liquids or solids according to size and shape. Carbonyl molecules attract through dispersion and dipole–dipole forces. Compared with nonpolar hydrocarbons of similar size, stronger association generally raises their boiling points. However, ordinary aldehydes and ketones lack O–H or N–H bonds and cannot form a self-sustaining hydrogen-bond network as donors. Comparable alcohols therefore often boil higher. These comparisons require similar molecular mass and shape; they are not an absolute ranking for arbitrary molecules.
| Compound | Approximate normal boiling point / K | Dominant comparison |
|---|---|---|
| n-Butane | 273 | Mostly dispersion |
| Methoxyethane | 281 | Weaker dipolar association |
| Propanal | 322 | Carbonyl dipole |
| Propanone | 329 | Carbonyl dipole; shape also matters |
| Propan-1-ol | 370 | Intermolecular hydrogen bonding |
Carbonyl oxygen accepts hydrogen bonds from water: water O–H points toward an oxygen lone pair. Small carbonyl compounds consequently dissolve well; ethanal and propanone mix readily with water, while methanal dissolves extensively and reacts to hydrated forms. As the hydrocarbon part grows, its nonpolar surface becomes less compatible with water and solubility falls. The ability to accept hydrogen bonds has not disappeared; its contribution is increasingly outweighed by the larger hydrophobic portion.
Problem
Compare butanal, butan-1-ol, ethoxyethane and pentane.
- 1.Their molecular masses are close, so intermolecular interactions are useful comparators. Pentane is nonpolar, ethoxyethane has a modest dipole, butanal has a strong carbonyl dipole, and butan-1-ol can donate and accept hydrogen bonds.
- 2.The expected increasing boiling-point order is pentane < ethoxyethane < butanal < butan-1-ol. If the hydrocarbon were butane instead, its lower mass reinforces its low boiling point but it is no longer part of a closely matched mass set.
Properties explain uses without defining identity
Propanone and butan-2-one are common solvents because they dissolve many organic substances and evaporate readily. Methanal solutions, often described as formalin at roughly 37–40% formaldehyde, are used for specimen preservation and in phenol-formaldehyde and urea-formaldehyde resins. Ethanal is an intermediate for other chemicals. Benzaldehyde is used in fragrance and dye chemistry. Vanillin, camphor and several other carbonyl compounds contribute characteristic aromas. “Carbonyl compound” does not imply a common smell or a common biological effect; the entire structure controls the property.
Quick Quiz
Quiz
Which alcohol directly oxidises to cyclohexanone?
Explanation: The secondary ring alcohol gives a ketone at the same carbon.
What does CH₃CN give with CH₃MgBr then hydrolysis?
Explanation: The nitrile carbon becomes C=O between the original and added methyl groups.
What reagent class classically converts acyl chlorides selectively to ketones?
Explanation: The moderated organocadmium nucleophile provides the carbon substituent and limits further addition.
Why does propanone dissolve well in water?
Explanation: A hydrogen-bond acceptor need not be a donor.
Which sequence is suitable for m-nitroacetophenone?
Explanation: The acetyl group directs nitration meta; nitrobenzene resists ordinary acylation.
Practice Problems
- Predict the product of benzoyl chloride with dimethylcadmium.
- Prepare benzophenone by two different carbon–carbon bond-forming methods.
- Explain why a large ketone can be poorly water-soluble despite having oxygen.
- Arrange propane, dimethyl ether, ethanal and ethanol by increasing boiling point.
- Why does propanenitrile plus methylmagnesium bromide give butan-2-one rather than propanone?
2C₆H₅COCl + (CH₃)₂Cd → 2C₆H₅COCH₃ + CdCl₂. The phenyl group comes from the acyl chloride and methyl comes from cadmium; their attachment to C=O gives acetophenone.
Key Takeaways
• Secondary alcohols give ketones without skeleton cleavage. • Nitrile carbon becomes the ketone carbonyl carbon after Grignard addition and hydrolysis. • Classical dialkylcadmium and Grignard reactions have different selectivities. • Friedel–Crafts acylation requires a suitable aromatic substrate. • Carbonyl molecules have dipoles but lack an intrinsic O–H donor. • Small carbonyl compounds accept hydrogen bonds from water. • Boiling-point comparisons must control for size and shape.