Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 328

Brief Organic Chemistry

Summer 2004

Lecture Notes: 1 July

© R. Paselk 2004
 
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Carboxylic Acids

Nomenclature, cont.

For substituted benzene ring compounds are named as benzoic acids with substituents numbered with the carboxyl group as number one.

When there are multiple substituents they are numbered and listed alphabetically:

Aromatic dicarboxylic acids are named as benzene dicarboxylic acids:

A number of common names are also widely used. In particular you should know: Formic acid (formyl grp), acetic acid (acetyl group), propionic acid, butyric acid, oxalic acid, and benzoic acid.

 

 

Structure and Properties of Carboxylic Acids

Like aldehydes and ketones carboxylic acids and their derivatives have a carbonyl carbon (C=O). Thus the carboxyl carbon is sp2 hybridized - planar with approximately 120° angles between substituents.

Hydrogen Bonding: The presence of the -OH group also means that carboxylic acids can H-bond in the pure state like alcohols. In fact most carboxylic exist as hydrogen-bonded dimers in the pure state:

As we would expect with this hydrogen bonding capability, boiling points are relatively high, even higher than similar MW alcohols (e.g. Acetic acid, bp = 118 °C for MW = 60.05 vs. n-Propyl alcohol, bp = 97.2 °C for MW = 60.09 vs. 2-Methylheptane, bp = 117.6 °C for MW = 114.23 {approx. 2 x acetic acid}; or Propionic acid, bp = 141.1 °C for MW = 74.09 vs n-butyl alcohol, bp = 117 °C for MW = 74.12).

The ability to participate in H-bonding will also make the low MW carboxylic acids soluble in water, like the alcohols, with the solubility decreasing with increasing MW as they become more alkane-like.

The solubility of long chain carboxylic acids in water also increases in the ionized state. In fact a common method for isolating carboxylic acids from other organic components is to extract them from an organic phase, such as an alkane, by vigorous shaking with an acidic aqueous phase. The carboxylic acids will become ionized and insoluble in the organic phase. The organic phase can then be removed, the aqueous phase can be neutralized and the acids can be extracted back to an organic phase.

Soaps and Detergents - just for fun: A problem occurs for carboxylic acid salts as the alkane part of carboxylic acids gets very long. The alkane part (the "tail") is very insoluble in polar solvents such as water, while the ionized salt part (the "head") is very insoluble in non-polar solvents, but is soluble in polar solvents like water. As a result these molecules form globules called micelles. In water they are arranged with the tails inside and the polar heads on the surface in contact with water. In oils they are reversed: polar heads inside, non-polar heads on the surface in contact with oil. Thus soaps can solubilize oils in water and water in oils by surrounding (coating) the immiscible species.

Acidity: As the name implies the carboxylic acids are significantly more acidic that other organic molecules such as alcohols or amines. So the question arises as to why the -OH group dissociates more readily in the acids than in alcohols. Put another way, why is the -O- charge more stable? As in previous examples of charge stabilization the explanation again lies in the ability of the carboxyl group to distribute the charge over a larger volume of space. In the case of the carboxyl group the charge distribution is particularly effective because once the H dissociates there is really no difference between the two oxygens - thus the charge is equally distributed over both of them:

Thus the charge is more effectively delocalized than, say, in phenol, and the pKa is lower (its more acidic). As with phenols, carboxylic acids become stronger as electron withdrawing groups are added, further delocalizing the charge.

Acid K pKa
Acetic 1.76 x 10-5  4.75
Iodoacetic 7 x 10-4 3.18
Bromoacetic 1.3 x 10-3 2.9
Chloroacetic 1.40 x 10-3  2.85
Fluoroacetic 2.6 x 10-3 2.59
Dichloroacetic 3.32 x 10-1 1.48
Trichloroacetic 2 x 10-1  0.7
Trifluoroacetic  strong strong

Carboxylic acids are also an excellent example to demonstrate the effect of bond distance on the induction effect:

Acid K pKa
Butanoic acid 1.5 x 10-5 4.82
4-Chlorobutanoic acid 3 x 10-5 4.52
3-Chlorobutanoic acid 1 x 10-4 3.98
2-Chlorobutanoic acid 1.5 x 10-3 2.83

As with other weak acids, the carboxylic acids react with strong bases until one or the other reactant is completely titrated. In general, whether water soluble or not, they form water soluble salts. They also react with sodium bicarbonate and sodium carbonate to give the sodium salts and carbon dioxide which is often lost as a gas, driving the reaction to completion.

 

 

CHEMISTRY OF THE CARBOXYLIC ACIDS

Synthesis of Carboxylic Acids

Like for aldehydes and ketones, we have seen a number of reactions resulting in the synthesis of carboxylic acids already.

Oxidation of substituted alkylbenzenes: In this reaction the benzilic carbon (that attached to the benzene ring) is subject to oxidation by permanganate or dichromate:

Oxidation of Primary Alcohols or Aldehydes: Primary alcohols can be oxidized to carboxylic acids using Chromium trioxide (CrO3) or sodium dichromate:

Aldehydes can by oxidized to carboxylic acids with mild oxidizing agents such as Ag+ (Tollen's reagent):

Hydrolysis of Nitriles: Nitriles can be hydrolyzed to in aqueous acid or base solution to give carboxylic acids.

Nitriles in turn may be synthesized from alkyl halides via an SN2 reaction. Substitution by cyanide ion followed by hydrolysis is a good method for transforming primary, or often secondary, alkyl halides into acids.

Carboxylation of Grignard Reagents: Grignard reagents can make a nucleophilic attack on carbon dioxide to give carboxylic acids:

 

Reactions of Carboxylic Acids

Reduction: Carboxylic acids and Esters can be reduced to give primary alcohols. Lithium aluminum hydride is often used to carry out this reaction:

Preparation of Esters by SN2 reaction of Carboxylic acids and Primary Alkyl Halides: Esters may be prepared via an SN2 attack of and acid salt on a primary alkyl halide, via a nucleophilic attack by the ionized acid:

Nucleophilic Acyl Substitution Reactions of Carboxylic Acids: The most important reactions of carboxylic acids are the substitutions of one nucleophile by another. The nucleophile attacks the carbonyl carbon to give a tetrahedral intermediate which relaxes to give a new carboxylic acid derivative:

Two reactions of carboxylic acids following this mechanism are:

Synthesis of Acid Chlorides from Acids: This reaction is usually done with thionyl chloride (SOCl2):

Note the use of thionyl chloride as seen earlier for synthesizing alkyl chlorides form alcohols (McMurry, pg 204). Recall that we said then that one of its advantages is that it makes -OH a better leaving group because the hydroxyl leaving group reacts with the sulfur. Of course making the -OH a better leaving group makes the reaction go better overall.

Synthesis of Esters from Acids: Esters can be made by nucleophilic acyl substitution in a reaction known as the Fischer esterification reaction. The reaction is acid catalyzed and fully reversible. Use of the alcohol as solvent drives it towards the ester, use of water as solvent drives it toward the acid:

The mechanism of the Fischer esterification reaction can serve as a model for nucleophilic acyl substitution reactions generally:


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Last modified 1 July 2004