Aldehydes, Ketones and Carboxylic Acids · Lesson 13 of 15
Reduction, Decarboxylation and Substitution of Acids
“Track carbon count through acid reduction, soda-lime treatment, electrolysis and substitution reactions.”
• Predict acid reduction products with appropriate reagent selectivity. • Compare carbon loss in soda-lime and Kolbe reactions. • Explain α-halogenation and its hydrogen requirement. • Predict ring substitution and connect acid structures with uses.
Reduction keeps the acid carbon but changes its oxidation level
LiAlH₄ reduces carboxylic acids to primary alcohols after appropriate aqueous work-up: RCOOH becomes RCH₂OH. The acid carbon is retained. NaBH₄ under ordinary conditions does not perform this acid-to-alcohol reduction; instead, acid can consume borohydride without giving the same useful transformation. LiAlH₄ is used under initially anhydrous conditions, with work-up only after the reduction stage.
Borane reagents, related to diborane in ethereal media, can preferentially reduce carboxylic acids under suitable conditions. This selectivity can preserve groups such as a nitro group that LiAlH₄ may also reduce. For example, a nitrobenzoic acid can give a nitrobenzyl alcohol with an appropriately controlled borane reduction. This is reagent- and condition-dependent selectivity, not a promise that every other functional group is inert to any amount of borane.
Problem
Prepare 3-nitrobenzyl alcohol from 3-nitrobenzoic acid.
- 1.The desired change is COOH → CH₂OH while ring NO₂ remains. A controlled borane reduction followed by work-up is a suitable selective strategy.
- 2.LiAlH₄ is not an interchangeable choice because it can reduce nitro functionality as well. The product is m-O₂NC₆H₄CH₂OH, with the original acid carbon now an alcohol carbon.
Soda lime removes one carbon
Heating a sodium carboxylate with soda lime, a mixture of NaOH and CaO commonly described in about a 3:1 ratio, gives a hydrocarbon with one fewer carbon than the acid. The carboxyl carbon is removed into carbonate. CaO supports the solid basic mixture and is not counted as a consumed carbon source in the net equation. Sodium ethanoate gives methane; sodium benzoate gives benzene.
Decarboxylation means loss of a carboxyl carbon as carbon dioxide or a related inorganic carbon product in the overall process. It is different from reducing COOH to CH₂OH, which keeps that carbon. When a target has one fewer carbon, decarboxylation can be useful; when the target keeps its carbon count, it is usually the wrong transformation.
Kolbe electrolysis couples two carbon fragments
At the anode in Kolbe electrolysis, a suitable carboxylate ion loses an electron and then CO₂, giving a carbon radical. Two such radicals couple to R–R. The simple idealised anode balance is 2RCOO⁻ → R–R + 2CO₂ + 2e⁻. Water reduction at the cathode gives H₂ and OH⁻. For a suitable saturated monocarboxylate with n carbons, the coupled hydrocarbon has 2(n−1) carbons. Competing reactions and substrate limitations can occur; this is the basic coupling model.
Problem
Compare sodium propanoate with soda lime and with Kolbe electrolysis.
- 1.Sodium propanoate is CH₃CH₂COONa. Soda lime gives CH₃CH₃, ethane: one ethyl group receives H and the acid carbon is removed.
- 2.In ideal Kolbe electrolysis, two CH₃CH₂ radicals couple to CH₃CH₂CH₂CH₃, butane. Starting acid carbon count n=3 gives 2(3−1)=4 product carbons.
- 3.The two processes both remove carboxyl carbon but differ in what happens to the remaining fragment. Calling both “decarboxylation” does not make their hydrocarbons identical.
Hell–Volhard–Zelinsky reaction targets the α-position
Carboxylic acids with an α-H can undergo α-chlorination or α-bromination using Cl₂ or Br₂ with a small amount of red phosphorus, followed by hydrolysis of the acyl-halide intermediate. The process is called the Hell–Volhard–Zelinsky, or HVZ, reaction. The phosphorus/halogen system forms an acyl halide that can enolise and react at α-carbon; subsequent hydrolysis restores COOH. A simple acid is less readily enolised directly than the activated acyl halide.
The bromine replaces an α-H rather than the acid OH. An acid such as 2,2-dimethylpropanoic acid has no α-H and does not give the ordinary α-bromination product. Benzene-ring bromination is a separate reaction with different activation and directing rules. A formula such as “acid + bromine” without conditions does not identify which site changes.
Aromatic acids are deactivated and meta-directing
A directly attached COOH withdraws electron density from an aromatic ring. Nitration of benzoic acid gives mainly 3-nitrobenzoic acid, and bromination with suitable Br₂/FeBr₃ conditions gives mainly 3-bromobenzoic acid. Ordinary Friedel–Crafts alkylation or acylation is ineffective on the strongly deactivated acid ring; the carboxyl oxygens can also bind the Lewis acid catalyst. The directing label describes ring electrophilic substitution, not the location of acid-base deprotonation.
Selected uses reflect different structures
Methanoic acid is used in rubber, textile, dyeing, leather and related processing. Ethanoic acid is a solvent and is present in vinegar as a dilute food-use solution. Hexanedioic acid is a building block for nylon-6,6, where its two acid groups permit chain formation with a diamine. Benzoate salts are used as preservatives under suitable conditions; benzoic acid esters have fragrance uses. Higher fatty acids supply long-chain carboxylates for soaps and related surfactants. A dilute food solution and a concentrated reagent are different formulations of a related chemical, and function depends on the complete material.
Problem
Convert benzoic acid to benzene, benzyl alcohol or 3-bromobenzoic acid.
- 1.For benzene, make the sodium salt and heat with soda lime; the carboxyl carbon is lost. For benzyl alcohol, reduce COOH using LiAlH₄ or suitable borane conditions and work up; the carbon is kept.
- 2.For 3-bromobenzoic acid, use aromatic bromination with appropriate activation such as Br₂/FeBr₃. COOH remains and directs substitution mainly meta. These are three different reactive-site choices, not variants of a single reduction.
Quick Quiz
Quiz
What does ordinary LiAlH₄ reduction of CH₃COOH give after work-up?
Explanation: The acid carbon remains and becomes CH₂OH.
Which carbon is removed in soda-lime decarboxylation?
Explanation: RCOONa gives RH while the acid carbon enters carbonate.
What ideal Kolbe hydrocarbon comes from sodium ethanoate?
Explanation: Two methyl radicals couple after two carboxyl carbons leave as CO₂.
What feature is required for ordinary HVZ α-bromination?
Explanation: The acyl-halide/enol pathway replaces an α-H by halogen.
Where does nitration of benzoic acid occur mainly?
Explanation: Directly attached COOH deactivates and directs electrophiles mainly meta.
Practice Problems
- Predict soda-lime products of sodium ethanoate and sodium benzoate.
- Compare the carbon counts for sodium butanoate by soda lime and ideal Kolbe electrolysis.
- Will benzoic acid or 2,2-dimethylpropanoic acid undergo ordinary HVZ α-bromination?
- Give the products and conditions for ethanoic acid to ethanol and propanoic acid to 2-bromopropanoic acid.
- Explain why hexanedioic acid is useful for a polyamide while ethanoic acid alone is not equivalent.
- Choose a reagent for nitrobenzoic acid to nitrobenzyl alcohol and justify selectivity.
CH₃COONa + NaOH → CH₄ + Na₂CO₃ on heating with CaO. C₆H₅COONa + NaOH → C₆H₆ + Na₂CO₃. Each hydrocarbon has one fewer carbon than its acid precursor; the R fragment receives hydrogen.
Key Takeaways
• Acid reduction retains the carboxyl carbon as CH₂OH. • NaBH₄ is not the ordinary reagent for acid-to-alcohol reduction. • Borane can provide useful acid-selective reduction under controlled conditions. • Soda lime gives RH with one carbon fewer than the acid. • Kolbe electrolysis couples two radicals after CO₂ loss. • HVZ halogenation replaces an α-H and needs the appropriate activation. • Directly attached COOH directs aromatic electrophilic substitution mainly meta.