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CHAPTER 18: FUNCTIONAL DERIVATIVES OF CARBOXYLIC ACIDS
Thiols are the sulfur analogue of alcohols (sulfur replaces oxygen in the hydroxyl group of an alcohol). Thiols differ dramatically from alcohols in their oxidation chemistry. Following the aforementioned oxidation/reduction definitions and manipulations, mild oxidation (e.g. using O2) converts two thiols to a disulfide by taking one hydrogen atom from each thiol molecule (Scheme 2, left). These two removed hydrogen atoms may join with the oxidizing agent to form a water molecule. The reverse reaction occurs via reduction, in which two hydrogen atoms are added on each sulfur atom after cleavage of the sulfur-sulfur bond (Scheme 2, right). These two reactions are very important in the chemistry of proteins, as the thiol functional group is present in the amino acid cysteine. When the thiol groups of two cysteine residues are brought near each other in the course of protein folding, an oxidation reaction can generate a disulfide bond (-S-S-, called cystine). This disulfide bond can contribute to a protein’s tertiary structure if the cysteines are part of the same peptide chain. It can also contribute to the quaternary structure of multimeric proteins by forming fairly strong covalent bonds between different peptide chains.
Interestingly, when catalyzed by an acid, the esterification reaction establishes equilibrium where the reverse reaction (ester hydrolysis) also takes place. During hydrolysis, the ester linkage (represented by the carbonyl-oxygen single bond) is broken. The elements of water contribute to this reaction as the –OH is attached to the carbonyl carbon to form a carboxylic acid, and the –H is attached to the alcohol oxygen to form an alcohol (Scheme 4). In study groups and tutoring sessions, students often say, “During ester hydrolysis, the –OH from the water and the –OR’ from the ester swap places.”
Making esters from alcohols and acid anhydrides
N2 - 2-Pyrazolines 7-12 bearing a carboxylic acid ester or a carboxamide side-chain have been prepared by treatment of the appropriate chalcone derivatives 2-6 with hydrazine hydrate or phenylhydrazine in hot acetic acid. 1-(2-Carboxyphenyl)-2-pyrazolines 24-30 and 1-(4-carboxyphenyl)-2-pyrazolines 31-41 were synthesized by the reaction of chalcones with (2-carboxyphenyl) hydrazine and (4-carboxyphenyl)hydrazine in hot acetic acid. Structures of all new compounds have been elucidated by microanalyses, 1H-, 13C-NMR and IR spectroscopic measurements.
Amides are formed by the condensation reaction between a carboxylic acid and an amine (primary or secondary amines, or ammonia). Water is formed as a byproduct from the carboxylic acid –OH and the amine –H (Scheme 5). It could be very easy to understand amide formation if you recall the ester formation reaction (Scheme 3). However, unlike ester formation, the amide reaction equilibrium is highly unfavorable for the synthesis of products because a competing acid-base reaction occurs between a carboxylic acid and amine. Many methods are thus used to drive the reaction to the right. For example, either acid chloride or acid anhydride is used as a more reactive carboxylic acid derivative to prepare amides. In a similar way, the amide bond is formed between the carbonyl carbon and the amine nitrogen while the –Cl joins –H to form HCl (when using acid chloride) or the RCOO– joins –H to form a carboxylic acid (when using acid anhydride) (Scheme 5). In addition to learning these concepts in lecture, students engage in laboratory experiments such as the synthesis of nylon from adipoyl chloride and hexamethylemediamine that serve as good strategies to reinforce the concept of amide formation and solidify student understanding of this reaction.
Scheme 4. Ester hydrolysis under acid and basic conditions.
Alcohol oxidation is an important organic reaction because more than 90% of the ethyl alcohol that enters the body is metabolized by oxidation to acetic acid (Peters and Preedy, 1998). In terms of oxygen and hydrogen transfer, oxidation is a process of gaining oxygen or losing hydrogen atoms while reduction is a process of losing oxygen or gaining hydrogen atoms. We find this easy to remember using the acronym “OIL RIG:” Oxidation Is a Loss (of hydrogen atoms), Reduction Is a Gain (of hydrogen atoms). Although these are old definitions that are not used very much in current texts, we often will come across them in organic chemistry. As Scheme 1 shows of primary alcohols, removal of two hydrogen atoms (one from the hydroxyl group and another from the hydroxyl-bearing carbon atom) results in an aldehyde. In the presence of water, the aldehyde will transform into aldehyde hydrate, from which two hydrogen atoms are removed (one from the hydroxyl group and another from the hydroxyl-bearing carbon atom) to result in a carboxylic acid. The net effect for the second oxidation from the aldehyde to the carboxylic acid is the insertion of an oxygen atom. In the human body, this process happens in the liver and is mainly catalyzed by alcohol dehydrogenase (ADH) and acetaldehyde dehydrogenase (ALDH), respectively (Xiao, 1996).
Esters are derived from carboxylic acids. A carboxylic acid contains the -COOH group, and in an ester the hydrogen in this group is replaced by a hydrocarbon group of some kind. We shall just be looking at cases where it is replaced by an alkyl group, but it could equally well be an aryl group (one based on a benzene ring).
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Preparation of Carboxylic Acids
Esters are produced when carboxylic acids are heated with alcohols in the presence of an acid catalyst. The catalyst is usually concentrated sulphuric acid. Dry hydrogen chloride gas is used in some cases, but these tend to involve aromatic esters (ones containing a benzene ring). If you are a UK A level student you won't have to worry about these.
Synthesis of Aldehydes; Carboxylic Acids and Their Derivatives ..
The esterification reaction is both slow and reversible. The equation for the reaction between an acid RCOOH and an alcohol R'OH (where R and R' can be the same or different) is:
Author: Subject: Synthesis of Carboxylic Acid Esters
Carboxylic acids and alcohols are often warmed together in the presence of a few drops of concentrated sulphuric acid in order to observe the smell of the esters formed.
The ester can be separated from the carboxylic acid, alcohol, ..
Because the reactions are slow and reversible, you don't get a lot of ester produced in this time. The smell is often masked or distorted by the smell of the carboxylic acid. A simple way of detecting the smell of the ester is to pour the mixture into some water in a small beaker.
Making esters from alcohols and acid anhydrides.
Apart from the very small ones, esters are fairly insoluble in water and tend to form a thin layer on the surface. Excess acid and alcohol both dissolve and are tucked safely away under the ester layer.
5-Synthesis of Esters | Ester | Carboxylic Acid
To make a small ester like ethyl ethanoate, you can gently heat a mixture of ethanoic acid and ethanol in the presence of concentrated sulphuric acid, and distil off the ester as soon as it is formed.
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