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  Reactions of Carboxylic Acids Homework Help - K-12 Grade Level, College Level Chemistry

Introduction to Reactions of Carboxylic Acids

1. Salt Formation

Due to their improved acidity, carboxylic acids react with bases to form ionic salts, as displayed in the following equations. In the example of alkali metal hydroxides and simple amines (or ammonia) the resulting salts have marked ionic character and are generally soluble in water. Heavy metals like silver, mercury and lead form salts having more covalent character (3rd instance), and the water solubility is reduced, particularly for acids composed of four or more carbon atoms.

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arboxylic acids and salts having alkyl chains longer than six carbons show strange behavior in water because of the existence of both hydrophilic (CO2) and hydrophobic (alkyl) regions in similar molecule. Such type of molecules are termed as amphiphilic (Gk. amphi = both) or amphipathic. Depending on the behaviour of the hydrophilic portion these compounds may form monolayers on the sphere-like clusters or water surface, called micelles, in solution.

2. Substitution of the Hydroxyl Hydrogen

This reaction class could be termed as electrophilic substitution at oxygen and is described as follows (E is an electrophile). Some instances of this substitution are presented in equations (1) through (4).

RCO2-H   +   E(+)RCO2-E  +  H(+)

As in the first equation, If E is a strong electrophile it will attack the nucleophilic oxygen of the carboxylic acid directly giving positively charged intermediate that then loses a proton. If E is a weak electrophile, like an alkyl halide, it is essential to convert the carboxylic acid to the more nucleophilic carboxylate anion to make easy the substitution. This is the process employed in reactions 2 and 3. Equation 4 depicts the use of the reagent diazomethane (CH2N2) for the preparation of methyl esters. This explosive and toxic gas is all the time used as an ether solution (bright yellow in color). The reaction is simply followed by the evolution of nitrogen gas and the disappearance of the reagent's color. This reaction is believed to carry on by the quick bonding of a strong electrophile to a carboxylate anion.
The nature of SN2 reactions, like in equations no.2 & 3, has been explained later.

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3. Substitution of the Hydroxyl Group

Reactions where the hydroxyl group of a carboxylic acid is substituted by other nucleophilic group are significant for preparing functional derivatives of carboxylic acids. The alcohols present a helpful reference chemistry against which this class of transformations might be evaluated. Generally, the hydroxyl group proved to be a poor leaving group and almost all alcohol reactions where it was lost involved a prior conversion of -OH to a better leaving group. This has confirmed to be right for the carboxylic acids as well. 
Four instances of these hydroxyl substitution reactions are existed by the following equations. In each instance, the new bond to the carbonyl group is colored magenta and the nucleophilic atom that has substituted the hydroxyl oxygen is colored green. The hydroxyl moiety is frequently lost as water, but in reaction no.1 the hydrogen is lost as HCl and the oxygen as SO2. This reaction parallels a identical transformation of alcohols to alkyl chlorides, even though its technique is distinct. Other reagents that make a identical conversion to acyl halides are PCl5 and SOBr2.
The amide and anhydride formations displayed in equations no.2 & 3 require strong heating, and milder procedures that achieve these transformations will be explained later.

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Reaction no.4 is called esterification, because it is generally employed to convert carboxylic acids to their ester derivatives. Esters might be prepared in several different ways; certainly, equations no.1 and no.4 in the preceding diagram depict the formation of tert-butyl and methyl esters correspondingly. The acid-catalyzed formation of ethyl acetate from acetic acid and ethanol displayed here is reversible, with the equilibrium constant near 2. The reaction can forced to completion by removing the water as it is produced. This sort of esterification is often considered as Fischer esterification. As supposed, the opposite reaction, acid-catalyzed ester hydrolysis, can be take out by adding excess water. 
The following equations where the hydroxyl oxygen atom of the carboxylic acid is colored red (in the diagram) and that of the alcohol is colored blue (in the below diagram), demonstrate this difference (note: the starting compounds are in the center).

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Reductions & Oxidations of Carboxylic Acids

1. Reduction

The carbon atom of a carboxyl group is in a comparatively high oxidation state. Reduction to a 1º-alcohol occurs quickly on treatment with the powerful metal hydride reagent, lithium aluminum hydride, as displayed by the following equation. 1/3 of the hydride is lost as hydrogen gas and the initial result contains metal salts which must be hydrolyzed to generate the alcohol. These reductions occur by the addition of hydride to the carbonyl carbon, in identical way noted earlier for aldehydes and ketones. The resultant salt of a carbonyl hydrate then breaks down to an aldehyde that goes through further reduction.

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2. Oxidation

Since it is already in a high oxidation state, additional oxidation removes the carboxyl carbon like carbon dioxide. Depending on the reaction circumstances, the oxidation state of the remaining organic structure might be unchanged, higher, or lower. The following reactions are all instances of decarboxylation (loss of CO2). In first, bromine substitutes the carboxyl group, so both of the carboxyl carbon atom and the remaining organic moiety are oxidized. Silver salts have also been employed to start this transformation, which is termed as the Hunsdiecker reaction. The 2nd reaction is an interesting bis-decarboxylation, where the atoms of the organic remains keep their original oxidation states. Lead tetraacetate will also the oxidize mono-carboxylic acids in a way identical to reaction no.1. At last, the third instance depicts the general decarboxylation of β-keto acids, that leaves the organic residue in a reduced state (note: the CO2 carbon has increased its oxidation state.). 

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