Structure and ionization of the carboxyl group
The carboxyl group combines a carbonyl oxygen ($=O$) and a hydroxyl group ($-OH$). Their interaction causes a strong mesomeric effect. The carbonyl carbon pulls the $\pi$-bond electrons toward itself, while the hydroxyl oxygen donates its lone electron pair. As a result, the $\sigma$-bond electron density shifts from hydrogen to oxygen ($O \leftarrow H$), which facilitates the release of a proton ($H^+$).
The carboxylate ion formed after dissociation is highly stable. This is achieved through the delocalization of the $\pi$-bond and negative charge between the two oxygen atoms, which sharply reduces the likelihood of reverse proton addition.
Effect of substituents on acid strength
Introducing electron-withdrawing groups with a negative inductive effect ($-I$) into the radical pulls electron density away and further stabilizes the anion. A classic example is halogen-substituted acids. Their acidity increases as the number of halogen atoms grows: Acetic < Monochloroacetic < Dichloroacetic < Trichloroacetic.
Acid strength also increases in the presence of conjugated double bonds, such as in the dicarboxylic oxalic acid.
Classification and homologous series
Organic acids are classified according to the nature of the hydrocarbon radical and the number of carboxyl groups:
- Saturated monocarboxylic: from formic ($HCOOH$) and acetic ($CH_3COOH$) to higher acids such as palmitic ($C_{15}H_{31}COOH$) and stearic ($C_{17}H_{35}COOH$).
- Dicarboxylic: contain two $-COOH$ groups. Examples: oxalic, malonic, succinic, glutaric.
- Unsaturated: contain double bonds in the radical (acrylic, methacrylic, oleic, linoleic).
- Aromatic: contain a benzene ring (benzoic, phthalic acid isomers).
Physical properties and toxicology
Lower members of the series are infinitely miscible with water due to hydrogen bonding. Acid molecules also form strong intermolecular hydrogen bonds with each other, forming cyclic dimers.
As the hydrocarbon chain lengthens, solubility drops sharply, and the compounds acquire surfactant (surface-active agent) properties. Anhydrous ("glacial") acetic acid poses a serious hazard: it rapidly penetrates deep skin layers, causing severe, slow-healing chemical ulcers, surpassing sulfuric acid in its local damaging effect.
Specific chemical properties
Due to their structural features, nucleophilic addition at the $C=O$ bond is not typical for acids; nucleophilic substitution reactions occur instead (e.g., esterification with alcohols to form esters).
- Radical reactions: In the presence of chlorine or bromine and red phosphorus, hydrogen substitution occurs at the $\alpha$-carbon atom (adjacent to the carboxyl).
- Reducing properties: Formic acid participates in the silver mirror test, while oxalic acid decolorizes potassium permanganate solution.
- Complexation: Polybasic acid ions bind heavy metal ions. Potassium oxalate reacts with iron(III) chloride to form photosensitive green crystals of potassium tris(oxalato)ferrate(III).
Briefly: answers to topical questions
What are the main mechanisms of intermolecular hydrogen bond and dimer formation in carboxylic acids?
The mechanism of intermolecular hydrogen bond formation in carboxylic acids involves the interaction of a positively charged hydrogen atom with electronegative oxygen atoms. The carboxyl group forms two types of hydrogen bonds:
- With water molecules — ensures solubility.
- With another carboxyl group — leads to dimer formation.
Dimers have a cyclic structure formed by two hydrogen bonds between molecules. The dimer structure is as follows: $R-C(=O)\dots H-O-C(-R)=O\dots H-O$.
Salt names often originate from Latin. Key correspondences to remember: Formic — Formates (formica), Acetic — Acetates (acetum), Butyric — Butyrates (butyrum), Oxalic — Oxalates, Succinic — Succinates.
Class and exam questions
Why are nucleophilic addition reactions atypical for carboxylic acids?
The presence of the hydroxyl group blocks this pathway. Following nucleophilic addition, the elimination of a water molecule would be required, which is energetically highly unfavorable. Therefore, substitution reactions proceed instead.
How is a soap micelle structured in an aqueous solution?
Soaps (salts of higher fatty acids) are amphiphilic. In solution, they form bilayers where polar salt-like hydrophilic groups face outward (toward the water), while nonpolar hydrophobic hydrocarbon radicals are sequestered inside.
What happens during the thermal decomposition of formic acid?
In the presence of a dehydrating agent (concentrated sulfuric acid), it decomposes into water and carbon monoxide ($CO$). This method is frequently used to produce carbon monoxide in the laboratory.
What else to study on this topic
- Mechanism of the esterification reaction with alcohols and the role of the acid catalyst.
- Redox reactions of formic and oxalic acids.
- Effect of $\alpha$-halogenation on the properties and further transformations of acids.
- Synthetic methods: oxo synthesis and the use of Grignard organomagnesium reagents.
- Formation of coordination compounds with metal ions using potassium tris(oxalato)ferrate(III) as an example.
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