Download Carboxylic Acids: Properties, Naming, and Synthesis and more Study Guides, Projects, Research Organic Chemistry in PDF only on Docsity!
Carboxylic Acids
When a carbonyl carbon also bears a hydroxyl group, then these compounds are appreciably acidic, and are called carboxylic acids. Carboxylic acids are classified according to the substituent that is bonded to the carboxyl carbon: Aliphatic acids have an alkyl group bound to the carboxyl group. An aromatic acid has an aryl group bound to the carboxyl group. The simplest acid is formic acid. A carboxylic acid donates protons by the heterolytic cleavage of the O-H bond, generating a carboxylate ion. R C O O-H RCO 2 H RCOOH
Nomenclature IUPAC formulation The root name is based on the longest continuous chain of carbon atoms bearing the carboxyl group. The - e is replaced by - oic acid. The chain is numbered starting with the carboxyl carbon atom. The carboxyl group takes priority over any other functional groups previously discussed. E.g.
Typically aromatic acids of the form Ar-CO 2 H are named as derivatives of benzoic acids, with ortho , meta and para indicating the location relative to the carboxyl group. ( Recall that this is non-IUPAC ).
Dicarboxylic Acids Aliphatic dicarboxylic acids are named by simply adding the suffix - dioic acid to the root name. The root name comes from the longest carbon chain containing both carboxyl groups. Numbering starts at the end closest to a substituent. E.g. Structure of the Carboxyl Group The most stable conformation of formic acid is an almost planar arrangement of the molecule. The carbon is sp^2 hybridized, and the O-H bond lies in the plane described by the sp^2 carbon, eclipsing the C=O double bond. This unexpected geometric arrangement can be explained by resonance (or conjugation).
Values of pKa for common alkyl carboxylic acids are around 5 (Ka ~ 10-^5 ).
E.g. ethanoic acid has pKa = 4.74, (alcohols have pKa ~ 1 6 , so carboxylic acids are about 10^11 times more acidic than alcohols). The reason why carboxylic acids are much more acidic than alcohols is because the carboxylate anion is much more stable than the alkoxide anion. Both alcohols and carboxylic acids are acidic since their respective O-H bonds can be broken heterolytically, giving a proton and an oxygen anion. The difference lies in the fact that the carboxylate anion has the negative charge spread out over two oxygen atoms, whereas the alkoxide has the negative charge localized on a single oxygen atom.
Substituent Effects on Acidity Any substituent that stabilizes a negative charge is going to enhance the dissociation process, and therefore result in a stronger acid. Thus electronegative elements can enhance the acid strength, through inductive effects. E.g. The closer the substituent to the anion, the more profound the effect.
Salts of carboxylic Acids Strong bases can completely deprotonate carboxylic acids, thus salts of carboxylic acids are formed. E.g. The acid can be regenerated by protonation (acidification) of the salt. E.g.
Cleavage of Alkynes and Alkenes Alkenes react with concentrated KmnO 4 to produce intermediate glycols which react further to produce either carboxylic acids or ketones (depending on the original alkene substituents). E.g. Alkynes also react with conc. KMnO 4 to give carboxylic acids, and the same transformation can be achieved by the use of ozonolysis. E.g.
Alkylbenzenes Benzoic acid derivatives can be made by the oxidation of alkylbenzenes with either hot KMnO 4 or hot chromic acid. The vigorous conditions means this can only be used when there are no oxidizable groups present in the molecule. E.g.
Formation and Hydrolysis of Nitriles Nitriles can be hydrolyzed by dilute acid to generate carboxylic acids. E.g. Nitriles are easily made by the action of cyanide ion as a nucleophile on alkyl halides (or tosylates). Again the overall transformation is from alkyl halide to a carboxylic acid with an extra carbon atom.
Reactions of Carboxylic Acids (and Derivatives) Ketones and aldehydes have a carbonyl group and undergo nucleophilic addition , whereas carboxylic acids (and their derivatives) undergo nucleophilic acyl substitution - this is where one nucleophile replaces a leaving group on the acyl carbon. E.g. Carboxylic acid derivatives differ in the nature of the group bound to the acyl group.
- OH is an acid
- Cl is the acid chloride
- OCOR' is the anhydride
- OR' is the ester
- NR 2 is the amide Nucleophilic acyl substitution can interconvert all of these different acid derivatives.
The Fischer Esterification Recall that acid and alcohol ester and water The overall transformation is that the - OH of an acid is replaced by the - OR' of an alcohol. E.g. The Fischer esterification is an example of acid catalyzed nucleophilic acyl substitution. The carbonyl group of a carboxylic acid is not sufficiently electrophilic to be attacked by the alcohol. The acid catalyst protonates the carbonyl oxygen, and activates it toward nucleophilic attack. The alcohol attacks, and after deprotonation of the alcohol oxygen, the hydrate of an ester is formed.
The ester is produced via acid catalyzed dehydration of the ester hydrate. As usual, the hydroxyl oxygen is protonated, thus creating a good leaving group. This leaves with assistance from the neighboring hydroxyl oxygen. The cation remaining is resonance stabilized, and deprotonation yields the desired ester. The overall mechanism is quite long, but both steps have been seen before (acid catalyzed addition to a carbonyl; acid catalyzed dehydration), and so should not be viewed as difficult, or as new work to learn.
