10F. Carboxylic Acid

Question 1

Carboxylic Acid

Answer 1

• an organic molecule containing a carboxyl group -COOH (carbonyl + hydroxyl)
• also represented by -CO₂H
• one of the most widely occurrung functional groups in chemistry + biochemistry
• not only are carboxylic acids important, but the carboxyl group is the parent group of a large family of compounds

Question 2

Naming

Answer 2

• similar to other functional groups where the parent chain contains the carboxyl group + it is numbered from the carboxyl group end
• the last -e of the parent alkane is replaced by -oic acid
• because the carboxyl group can only be at the end of the parent chain, its location is not included
• when carboxylic acids contain an -OH group or an NH₂ group, hydroxy- + amino- are used respectively
• for dicarboxylic acids, the parent chain includes both carboxyl groups, one at either end, and -dioc acid is used
• common names for aliphatic carboxylix acids are often derived from the name of the natural source from which it could be isolated. Alpha, beta, gamma, etc are often added as a prefix to locate substituents
• unbranched carboxylic acids with 12 to 20 carbon atoms are known as fatty acids. Those with 16, 18 + 20 carbon atoms are particularly abundant in animal fats, vegetable oils + the phospholipid components of biological membranes

Question 3

Physical Properties

Answer 3

• the carboxyl group contains three polar covalent bonds

C=O, C-O + O-H

• the polarity of these bonds determines the major physical properties of carboxylic acids
• have significantly higher boiling points to other organic compounds of comparable molecular weight as a result of hydrogen bonding between two carboxyl groups creating a dimer, which behaves as a higher molecular weight compound
• more soluble in water than alcohols, ethers, aldehydes + ketones of comparable molecular weight due to hydrogen bonding through both the carbonyl + hydroxyl groups
• as the size of the hydrocarbon chain increases relative to the carboxyl group, water solubility decreases
• liquid carboxylic acids from propanoic acid to decanoic acid have sharp, often disagreeable odours
• carboxylic acids have a characteristic sour taste

Question 4

Reactions

Answer 4

• carboxylic acids are weak acids, however, substituents of high electronegativity, especially -OH, -Cl+ -NH₃⁺, near the carboxylic group increase the acidity, often by several orders of magnitude
• electronegative atoms on the carbon adjacent to a carboxyl group increase acidity because they pull electron density away from the O-H bond, thereby facilitating ionizationof the carboxyl group + making it a stronger acid

Question 5

Solubility reactions

Answer 5

• when a carboxylic acid is dissolved in an aqueous solution, the form of the carboxylic acid present depends on the pH of the solution in which it is dissolved
• all carboxylic acids, whether soluble or insoluble in water, react with NaOH, KOH + other strong bases, as well as ammonia + amines, to form water-soluble salts
• carboxylic acids react with sodium bicarbonate + sodium carbonate to form water-soluble sodium salts + carbonic acid. The carbonic acid then decomposes to produce water + carbon dioxide gas
• salts of carboxylic acids are named in the same manner as the salts of inorganic acids: the cation is named first, followed by th anion which is derived from the name of the carboxylic acid by replacing the suffix -ic acid with -ate
• a consequence of the water solubility of carboxylic acid salts is that water insoluble carboxylic acids can be converted to water soluble ammonium or alkali metal salts + extracted into aqueous solution. The salt can be transformed back to the free carboxylic acid by treatments with HCl, H2SO4 or another strong acid
• these reactions allow an easy separation of water insoluble carboxylic acids from water insoluble non acidic compounds

Question 6

Reduction reactions

Answer 6

• the carboxylic group is one of the organic functional groups most resistant to reduction. It is not affected by catalytic reduction under conditions that readily reduce alkenes to alkanes, or by sodiumborohydride which readily reduces aldehydes + ketones to alcohols
• to reduce a carboxylic acid to a primary alcohol, the powerful reducing agent lithium aluminium hydride, LiAlH4, is used. Commonly carried out in diethyl ether, the initial product is aluminium alkoxide, which is then treated with water to produce the primary alcohol + lithium hydroxide + aluminium hydroxide. The two hydroxides are insoluble in diethyl ether + are removed by filtration. Evaporation of the other solvent leaves the primary alcohol

Question 7

Fischer Esterification

Answer 7

• the process of forming an ester by refluxing a carboxylic acid + an alcohol in the presence of an acid catalyst, commonly sulfuric acid
• in this process the alcohol adds to the carbonyl group of the carboxylic acid to form a tetrahedral carbonyl addition intermediate, which then collapses by the loss of a water molecule to produce the ester
• by controlling the conditions, fischer esterification can be used to prepare esters in high yields
• an excess of alcohol will drive the equilibrium to achieve a high conversion of carboxylic acid to ester
• alternatively, removal of the water will also drive the reaction to produce more ester

Question 8

Decarboxylation

Answer 8

• the loss of CO2 from a carboxyl group
• almost any carboxylic acid, when heated to a very high temperature , undergoes thermal decarboxylation

however, carboxylic acids are quite resistant to moderate heat + melt or even boil without decarboxylation

exceptions are carboxylic acids that have a carbonal group ß to the carboxylic group

• note that the CO2 is formed from the carboxyl group
• thermal decarboxylation is unique to ß-ketoacids + does not occur with alpha-ketoacids, which undergo oxidative decarboxylation