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Structure and Naming of Carboxylic Acids

Structure and Naming of Carboxylic Acids. Carboxylic acids contain a carbonyl attached to a hydroxyl group; this is called a carboxyl group Parent name ends in -oic acid Find longest chain containing the carboxyl group carbon Number C’s starting at carboxyl group carbon

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Structure and Naming of Carboxylic Acids

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  1. Structure and Naming of Carboxylic Acids • Carboxylic acids contain a carbonyl attached to a hydroxyl group; this is called a carboxyl group • Parent name ends in -oic acid • Find longest chain containing the carboxyl group carbon • Number C’s starting at carboxyl group carbon • Locate and number substituents and give full name • The smallest carboxylic acids are usually named by their common names • The carboxyl group takes precedence over all other groups

  2. Physical Properties of Carboxylic Acids • Carboxylic acids are very polar due to both the carbonyl group and the hydroxyl group • Carboxylic acids can H-bond with each other, and in fact exist primarily of dimers (two molecules held together by H-bonding) • Because of the above properties, carboxylic acids have high boiling points (higher than corresponding alcohols) • Those with less than 5 carbons are soluble in water - those with more than 5 C’s can be soluble when ionized

  3. Acidity and Salts of Carboxylic Acids • Carboxylic acids are weak acids (partially ionize in water) • They are stronger acids than alcohols, or even phenols, due to the high stability of their conjugate bases (resonance) • Carboxylic acids are neutralized by bases to form salts • Salts of carboxylic acids are useful because they are solids at room temperature, and most are soluble in water

  4. Preparation of Carboxylic Acids • Recall from chapters 14 and 15 that carboxylic acids can be prepared by oxidation of primary alcohols or aldehydes • Primary alcohols form acids when treated with Jones’ reagent (CrO3/H3O+), as well most other oxidizing agents (except PCC, which forms aldehydes) • Aldehydes can be oxidized to carboxylic acids with most oxidizing agents, such as Tollens’reagent (AgNO3/NH3) - alcohols do not react with Tollens

  5. Preparation of Esters from Carboxylic Acids • A carboxylic acid can be reacted with an alcohol to form an ester using an acid catalyst and heat (called Fischer esterification) - esters have an alkoxy group attached to the carbonyl • There are other types of esterification that we won’t study • Fischer esterification is a type of intermolecular dehyration • Because this is a reversible reaction, an excess of either the acid or the alcohol is normally used to shift the equilibrium towards products (sometimes H2O is removed as it forms)

  6. Mechanism of Fischer Esterification • First, the carbonyl oxygen is protonated • Next, the alcohol oxygen attacks the carbonyl carbon • A proton is then transferred from the alkoxy to a hydroxyl • Finally, the proton is removed from the carbonyl oxygen, and water is expelled

  7. Naming Esters • Parent name ends in -oate • First name the alkyl group attached to the oxygen • Follow by the carboxylic acid name converted to end in -oate • For IUPAC use the IUPAC name for both alkyl group and acid • Many small esters are also know by their common names

  8. Physical Properties of Esters • Because esters can’t H-bond with themselves, they have lower boiling points than carboxylic acids and alcohols • However, they are fairly polar, and have higher boiling points than ethers or hydrocarbons (similar to aldehydes and ketones) • Esters with less than 5 carbons are somewhat soluble in water - solubility is between that of ethers and aldehydes or ketones • Esters are not as flammable as ethers or hydrocarbons, but are similar to alcohols and ketones • Most esters have a pleasant smell and are often used as food additives to simulate or enhance natural flavors

  9. Hydrolysis of Esters • Esters can be hydrolyzed by heating with water and an acid catalyst (the reverse of Fischer esterification) - the hydrolysis is favored by adding an excess of water • Esters can also be hydrolyzed by heating with aqueous base (called saponification, this is how soap is made) - saponification produces a salt of the carboxylic acid

  10. Mechanism of Base Hydrolysis of an Ester • First, the hydroxyl group attacks the carbonyl carbon • Next, the alkoxide ion is eliminated as the carbonyl reforms • Finally, the alkoxide removes the proton from the acid and the resulting carboxylate ion forms a salt with the metal ion

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