Chapter 17 & 18: Carboxylic Acids, Carboxylic Anhydrides, Esters and Amides

Carboxylic acids 

organic compound containing carbonyl group

Carbonyl group

• functional group of a carboxylic acid

• can be represented in 3 ways

anhydrides, esters, and amides

• 3 classes of compounds derived from carboxylic acids 

• each is related to a carboxyl group by loss of H2O

Anhydrides 

• functional group is 2 carbonyl groups bonded to the same oxygen

• symmetrical or mixed

• To name it, drop the word “acid” and add the word “anhydride”

Esters 

• functional group is a carbonyl group bonded to an -OR group 

  • R may be alkyl or aryl

• IUPAC and common names are derived from the names of the parent carboxylic acids

• “-ic” becomes “-ate”

Lactone 

cyclic ester 

Amide

• functional group is a carbonyl group

• To name it, drop the ‘oic” acids

• also bonded to an alkyl or aryl group, name the group and show its location on nitrogen by N-; two alkyl or aryl groups by N,N-di-

Lactam

cyclic amide

Penicillin

referred to as β-lactam antibiotics

Cephalosporins

are also β-lactam antibiotics

C=O, C-O, O-H

• 3 polar covalent bonds in carboxyl group

• polarity of these bonds determines the major physical properties of carboxylic acids

Carboxylic acids

• have significantly higher boiling points than other types of organic compounds of comparable molecular weight

• more soluble in water than are alcohols, aldehydes, and ketones of comparable molecular weight

Polarity and hydrogen bonding 

• causes the higher boiling points of carboxylic acids

• creation of dimer

Dimer

behaves as a higher-molecular-weight compound

Fatty acids

• long chain carboxylic acids derived from animal fats, vegetable oils, or phospholipids of biological membranes

• more than 500 have been isolated from various cells and tissues

• most have between 12 and 20 carbons in an unbranched chain 

Unsaturated fatty acids

• cis isomer predominates, trans isomers are rare

• generally have lover melting points than their saturated counterparts

Saturated fatty acids 

• lauric acid

• myristic acid

• palmitic acid

• stearic acid 

• arachidic acid 

Unsaturated fatty acids

• palmitoleic acid

• oleic acid

• linoleic acid

• linolenic acid

• arachidonic acid

Saturated fatty acids

• solids at room temperature 

• the regular nature of their hydrocarbon chains allows them to pack together in such a way as to maximize interaction (by london dispersion forces) between their chains

Unsaturated fatty acids

are liquids in room temperature because the cis double bonds interrupt the regular packing of their hydrocarbon chains

Natural soaps

• sodium or potassium salts of fatty acids

• prepared from a blend of tallow and palm oils (triglycerides)

Tallow

insoluble triglyceride layer that forms on the top when solid fats from cattle are melted with steam 

Triglycerides

triesters of glycerol

Preparation of soaps

begins by boiling the triglycerides with NaOH

Saponification

the reaction that takes place in preparation of soaps by boiling triglycerides with NaOH

Micelles

• in water, soap molecules spontaneously clusters into this

• 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

Nonpolar hydrocarbon inner parts

when soaps and dirt (grease, oil, and fats stains) are mixed in water, this part of the soap micelles “dissolve” the nonpolar substances

Natural soaps

from water-insoluble salts in hard water

Hard water

contains Ca2+, Mg2+, and Fe3+ ions

Sodium soap

soluble in water as micelles

Calcium salt of a fatty acid

insoluble in water 

Sulfonate group (—SO3-)

the problem of formation of precipitates in hard water was overcome by using a molecule containing this group in the place of a carboxylate (—CO2-) group 

Calcium, magnesium and iron salts of sulfonic acids (RSO3H)

more soluble in water than are their salts of fatty acids

SDS (sodium dodecyl sulfate)

• synthetic detergent

• a liner alkylbenzenesulfonate (LAS), an anionic detergent

Foam stabilizers, bleaches, and optical brighteners 

most common additives to detergents 

Carboxylic acids

• weak acids

• Values of Ka for most unsubstituted aliphatic and aromatic carboxylic acids fall within the range 10^–4 to 10^–5 (pKa 4.0 – 5.0)

10^–4 to 10^–5 (pKa 4.0 – 5.0)

Values of Ka for most unsubstituted aliphatic and aromatic carboxylic acids fall within the range

Substituents of high electronegativity (-OH, -Cl, -NH3+)

increase the acidity of carboxylic acids

Dichloroacetic acids and trichloroacetic acid

stronger acids than H3PO4 (pKa 2.1)

Increasing acid strength

• acetic acid

• chloroacetic acid

• dichloroacetic acid 

• trichloroacetic acid 

pH of the solution in which it is dissolved

when a carboxylic acid is dissolved in aqueous solution, the form of the carboxylic acid present depends on the,,,

NaOH, KOH, and other string bases

all carboxylic acids, whether soluble or insoluble in water, react with these to form water soluble salts

Ammonia and amines

carboxylic acids also form water-soluble salts with them

Sodium bicarbonate and sodium carbonate

like inorganic acids, carboxylic acids react with these to form water soluble sodium salts and carbonic acids

Carbonic acid

decomposes to give water and carbon dioxide, which evolves as a gas

Fischer esterification 

• one of the most commonly used methods for the preparation of esters

• a carboxylic acid is treated with an alcohol in the presence of an acid catalyst, most commonly concentrated sulfuric acid

• reversible

• possible to drive it in either direction by the choice of experimental conditions (Le Chatelier’s principle)

Tetrahedral carbonyl addition intermediate 

in fischer esterification, the alcohol adds to the carbonyl group of the carboxylic acid to form a,,,

Ester

the intermediate loses H2O to give an,,,

Decarboxylation

loss of CO2 from a carboxyl group

Thermal decarboxylation

almost all carboxylic acids, when heated to a very high temperature will undergo this

Most carboxylic acids

• resistant to moderate heat and melt and even boil without undergoing decarboxylation

• an exception is any carboxylic acid that has a carbonyl group on the carbon β to the COOH group

Decarboxylation of a β-ketoacid

Mechanism of thermal decarboxylation 

involves (1) redistribution of electrons in a cyclic transition state followed by (2) keto-enol tautomerism 

Oxidation of foodstuff in the tricarboxylic acid (TCA) cycle

an important example of decarboxylation of a β-ketoacid in biochemistry

Oxalosuccinic acid

• one of the intermediates in this cycle

• has a carbonyl group (in this case a ketone) β to one of its three carboxyl groups

Amides

• can be formed with an amine and removing -OH from the acid and an -H from the amine

• In practice, what occurs if the two are mixed is an acid-base reaction to form an ammonium salt

• if this slat is heated to a high enough temperature, water is eliminated and an amide forms

Ammonium salt

occurs if the carboxylic acid and amine are mixed in an acid base reaction

Amide

if ammonium salt is heated to a high enough temperature, water is eliminated and it is formed,,,

Treating an anhydride with an amine

• much more common way of preparation of amides