Carboxylic Acids

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Description and Properties

• A carboxyl acid contains both a carbonyl group and a hydroxyl group, which are bonded to the same carbon. Always terminal groups
• Three bonds are to oxygen atoms, so this is the most oxidized functional group

Nomenclature

• When the carboxylic acid is the highest priority functional group: -oic acid replaces –e.
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• Cyclic carboxylic acids are named by listing the cycloalkane with the suffix carboxylic acid.
• Salts of carboxylic acids are named beginning with the cation, followed by the name of the acid with the ending –oate replacing –oic acid.
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• Dicarboxylic Acid: carboxylic acid group on each end of the molecule
• Smallest is oxalic acid, followed by malonic, succinic, glutaric, adipic, & pimelic acids.
• IUPAC names have the suffix –dioic acid.

Physical Properties

Hydrogen Bonding

• Carboxylic acids are polar because they contain a carbonyl group and can also form hydrogen bonds
• Display strong intermolecular attraction since both the hydroxyl oxygen and carbonyl oxygen can participate in hydrogen bonding.
• Dimers: pairs of molecules connected by two hydrogen bonds. Carboxylic acids tend to form dimers. This elevates the boiling and melting points.

Acidity

• Hydroxyl hydrogen is acidic, which results in a negative charge when the hydrogen is removed
• Resonance stabilization occurs between both of the electronegative oxygen atoms
  • Since the charge is delocalized, the carboxylate anion is very stable.
  • The more stable the conjugate base, the easier it is for proton to leave which indicates acid strength, Carboxylic acids have pK_a of around 4.8
• Substituents on carbon atom near the carbonyl group can influence the anion stability and thus its acidity
  • The presence of electron withdrawing compounds (such as NO₂ or halides) increases the acidity.
  • Electron donating such as NH₂ & OCH₃ destabilize the negative charge which decreases the acidity of the compound.
• Dicarboxylic acids are more acidic than monocarboxylic acids
  • If one proton is removed from the molecule, the carboxylate anion is formed which results in an immediate decrease in the acidity of the remaining carboxylic acid.
    • Thus the second proton is harder to remove (less acidic) as compared to an analogous monocarboxylic acid.
• b-dicarboxylic acid: dicarboxylic acids in which each carboxylic acid is position on the beta-carbon of the other – two carboxylic acids are separated by a single compound
  • alpha hydrogens have a high acidity (located on the carbon between two groups)
  • Loss of acidic hydrogen produces a carbanion which is stabilized by the electron withdrawing effects of both carboxyl groups.

Reactions of Carboxylic Acids

Synthesis of Carboxylic Acids

• Can be prepared via the oxidation of aldehydes and primary alcohols.
  • Oxidant is usually a dichromate salt, chromium trioxide, or potassium permanganate.
  • Secondary and tertiary alcohols cannot be oxidized to carboxylic acids since they already have at least two bonds to other carbons.

Nucleophilic Acyl Substitution

• One of the most common reaction for carboxylic acids and is similar to nucleophilic addition to an aldehyde or ketone
• Leaving group exists in carboxylic acid
• After the carbonyl is opened up via nucleophilic attack, the resulting tetrahedral intermediate can be reformed with a carbonyl (which kicks off the leaving group)
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• Acyl Derivatives: all molecules with a carboxylic acid-derived carbonyl (carboxylic acids, amides, esters, anhydrides, and others). Reaction is favored by good leaving groups.
• In the above reaction, the nucleophilic molecule replaces the leaving group of an acyl derivative.

Amides

• Carboxylic acid can be converted into amides if the incoming nucleophile is ammonia or an amine. Can be carried out in either an acidic or basic solution to drive the reaction forward.
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• Named by replacing the –oic acid with –amide.
  • Any alkyl groups on the nitrogen are placed at the beginning of the name with the same prefix N-.
• Exist in resonance states, where the electrons are delocalized between the N & O.
• Amides that are cyclic are called lactams
  • Named by replacing –oic acid with –lactam

Esters

• A hybrid between a carboxylic acid and an ether.
• Esterification: Can be made by reacting carboxylic acid with alcohols under acidic conditions. A condensation reaction with water as the side product.
• In acidic conditions, the carbonyl oxygen can be protonated which enhances the polarity of the bond, which subsequently places additional positive charge on the carbonyl carbon and increases its susceptibility to nucleophilic attack.
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• Names in the same manner as salts of carboxylic acids, end with an –ate.
• Cyclic esters are called lactones and are named by replacing –oic acid with –lactone.

Anhydrides

• Formed by the condensation of two carboxylic acids
• Named by replacing the acid with anhydride.
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Reduction

• Carboxylic acids can be reduced to primary alcohols by the use of lithium aluminum hydride (LiAlH₄)
• Reaction occurs by nucleophilic addition of hydride to the carbonyl group
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Decarboxylation

• The complete loss of the carboxyl group as carbon dioxide.
• When heat is added, carboxyl group is lost and replaced with hydrogen.
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Saponification

• Long chain carboxylic acids react with sodium or potassium hydroxide which results in the formation of a salt (soap)
• Soap can solvate nonpolar organic compounds in aqueous solution since they contain both a nonpolar tail and a polar carboxylate head.