Carboxylic Acid and Esters Topic 10 (PDF)

CARBOXYLIC ACID AND ESTERS  Carboxylic Acids The functional group of a carboxylic acid is a carboxyl group, which can be represented in any one of three ways  Carboxylic Acids Derivatives of carboxylic acids – Anhydrides, Esters, and Amides – Made by reacting a carbox...

CARBOXYLIC ACID AND ESTERS  Carboxylic Acids The functional group of a carboxylic acid is a carboxyl group, which can be represented in any one of three ways  Carboxylic Acids Derivatives of carboxylic acids – Anhydrides, Esters, and Amides – Made by reacting a carboxyl acid group with another molecule. H2O is formed in each reaction  Naming Carboxylic Acids IUPAC names –Take longest carbon chain that contains the carboxyl group as the parent alkane –Change the final -e from the name of the parent alkane to -oic acid  Naming Carboxylic Acids IUPAC names –Number the chain so that the carboxyl group carbon is number 1 –Carboxyl carbon is understood to be carbon 1, so we don’t need to include the number in the name  Naming Carboxylic Acids – Examples: (common name shown in parentheses) – an -OH substituent is indicated by the prefix hydroxy- – an -NH2 substituent by the prefix amino-  Dicarboxylic Acids – Add the suffix -dioic acid to the name of the parent alkane that contains both carboxyl groups – Carboxylic acid groups must be at ends of chain, so we do not need to number them  Nomenclature – Common names use the Greek letters alpha (α), beta (β), gamma (γ), etc. to locate substituents  Nomenclature When a carboxyl group is added to a ring the suffix - carboxylic acid is added to the name of the cyclic compound. The ring carbon attached to the carboxyl group is given the #1 location number. Seatwork Seatwork  Physical Properties The carboxyl group contains three polar covalent bonds; C=O, C-O, and O-H – Polarity of carboxyl group determines the major physical properties of carboxylic acids  Physical Properties – Highly polar group – Two hydrogen bonds can form between groups – Two carboxyl groups create a dimer that behaves as a higher-molecular-weight compound – Much higher boiling points than other types of organic compounds of comparable molecular weight  Physical Properties – More soluble in water than comparable alcohols, ethers, aldehydes, and ketones  Fatty Acids (carboxylic acids) Fatty acids: long chain carboxylic acids – derived from animal fats, vegetable oils, or phospholipids of biological membranes. – Over 500 have been isolated from various cells and tissues. Generally 12 and 20 carbons in an unbranched chain—with even number of carbons May be unsaturated: – cis isomer predominates; trans isomers are rare. – Unsaturated fatty acids have lower melting points than saturated fatty acids. Fatty Acids (carboxylic acids)  Fatty Acids (carboxylic acids) Unsaturated fatty acids generally have lower melting points than their saturated counterparts.  Fatty Acids (carboxylic acids) Saturated fatty acids are solids at room temperature – Hydrocarbon chains can to pack together in such a way as to maximize interactions (by London dispersion forces) between their chains.  Fatty Acids (carboxylic acids) Unsaturated fatty acids are liquids at room temperature because the cis double bonds interrupt the regular packing of their hydrocarbon chains.  Fatty Acids (carboxylic acids) Fatty acid: an unbranched-chain carboxylic acid derived from hydrolysis of animal fats, vegetable oils, or membrane phospholipids – Usually unbranched chain with 10-20 carbons – EVEN number of carbons – May be saturated or unsaturated (C=C)  Fatty Acids (carboxylic acids) – Unsaturated generally have cis double bonds – Unsaturated fatty acids have lower melting points than their saturated fatty acids – Most abundant are palmitic acid (16:0), stearic acid (18:0), and oleic acid (18:1) Fatty Acids  Reactions of Carboxylic Acids I: Carboxylic Acids Acid-Base Properties – Ionization and pH – Reaction with base Esterification (Reaction with Alcohol) Reduction (NaBH4 or LiAlH4 ) Decarboxylation II. Derivatives of Carboxylic Acids Reaction with Acids: Anhydride formation Reaction with Amines: Amide formation  Acidity of Carboxylic Acids Carboxylic acids are weak acids – Ka generally in range of 10-4 to 10-5 for most unsubstituted aliphatic and aromatic carboxylic acids – pKa is pH at which half of acid has lost its H – pKa range is 4 - 5 Example: Acetic Acid  Acidity of Carboxylic Acids – Highly electronegative substituents, such as -OH, -Cl, and -NH3+, near the carboxyl group increase the acidity of carboxylic acids – Pull electron density away from carboxyl group – Both dichloroacetic acid and trichloroacetic acid are stronger acids than H3PO4 (pKa 2.1)  Reaction With Bases All carboxylic acids, whether soluble or insoluble in water, react with strong bases (NaOH, KOH) to form water-soluble salts – Also form water-soluble salts with ammonia and amines (weak bases)  Reaction With Bases – Carboxylic acids react with sodium bicarbonate and sodium carbonate to form water-soluble sodium salts and carbonic acid, H2CO3 – Carbonic acid then decomposes to give water and carbon dioxide gas CO2  Reduction The most common reagent for the reduction of a carboxylic acid to a 1° alcohol is the very powerful reducing agent lithium aluminum hydride, LiAlH4. Reduction of a carboxyl group with this reagent is commonly carried out in diethyl ether.  Fischer Esterification Fischer esterification is commonly used to make esters – Carboxylic acid is reacted with an alcohol in the presence of an acid catalyst, such as concentrated sulfuric acid – Fischer esterification is reversible – Can drive reaction in either direction by altering experimental conditions (Le Chatelier’s principle)  Fischer Esterification – Alcohol adds to the carbonyl group of the carboxylic acid to form a tetrahedral carbonyl addition intermediate – Intermediate then loses H2O to form an ester  Soaps Natural soaps are prepared by boiling lard or other animal fat with NaOH, in a reaction called saponification (Latin, sapo, soap)  Soaps In water, soap molecules spontaneously cluster into micelles, a spherical arrangement of molecules such that their hydrophobic parts are shielded from the aqueous environment, and their hydrophilic parts are in contact with the aqueous environment.  Soaps Soaps clean by acting as emulsifying agents – Long hydrophobic hydrocarbon chains cluster so as to minimize their contact with water – Polar hydrophilic carboxylate groups remain in contact with the surrounding water molecules – These two forces cause soap molecules to form micelles  Soaps When soap is mixed with dirt (grease, oil, and fat stains), soap micelles “dissolve” these nonpolar, water-insoluble molecules.  Soaps Natural soaps form water-insoluble salts in hard water. Hard water contains Ca(II), Mg(II) and Fe(III) ions.  Detergents Can overcome problem of precipitates in by using a molecule containing a -SO3- group (sulfonic acid group) in the place of a -CO2- group. – Calcium, magnesium and iron salts of sulfonic acids, RSO3H, are more soluble in water than salts of fatty acids. – Synthetic detergents can be synthesized from SDS, a linear alkylbenzene sulfonate (LAS), an anionic detergent. Seatwork Seatwork