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Organic Chemistry

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Carboxylic acid

Prepared by Layne A. MorschThe University of Illinois - Springfield

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Carboxylic Acid Structure

• Carboxylic acids are compounds containing a carboxy group (COOH).

• The structure of carboxylic acids is often abbreviated as RCOOH or RCO2H, but keep in mind that the central carbon atom of the functional group is doubly bonded to one oxygen atom and singly bonded to another.

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• The C–O single bond of a carboxylic acid is shorter than the C–O bond of an alcohol.

• This can be explained by looking at the hybridization of the respective carbon atoms.

• Because oxygen is more electronegative than either carbon or hydrogen, the C–O and O–H bonds are polar.

Bonds in Carboxylic Acids

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• In the IUPAC system, carboxylic acids are identified by a suffix added to the parent name of the longest chain with different endings being used depending on whether the carboxyl group is bonded to a chain or a ring.

• If the COOH is bonded to a chain, find the longest chain containing the COOH, and change the “e” ending of the parent alkane to the suffix “oic acid”.

• If the COOH is bonded to a ring, name the ring and add the words “carboxylic acid”.

• Number the carbon chain or ring to put the COOH group at C1, but omit this number from the name.

• Apply all the other usual rules of nomenclature.

Naming Carboxylic Acids

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• Greek letters are used to designate the location of substituents in common names.

• The carbon adjacent to the COOH is called the carbon, followed by the carbon, followed by the carbon, the carbon and so forth down the chain.

• The last carbon in the chain is sometimes called the carbon.

• The carbon in the common system is numbered C2 in the IUPAC system.

Common Naming for Carboxylic Acids

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• Compounds containing two carboxy groups are called diacids.

• Diacids are named using the suffix-dioic acid.

• Metal salts of carboxylate anions are formed from carboxylic acids in many reactions.

• To name the metal salt of a carboxylate anion, put three parts together:

Naming Diacids and Salts

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Naming Metal Salts of Carboxylate Anions

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• Carboxylic acids exhibit dipole–dipole interactions because they have polar C–O and O–H bonds.

• They also exhibit intermolecular hydrogen bonding.• Carboxylic acids often exist as dimers held together by

two intermolecular hydrogen bonds.Two molecules of acetic acid(CH3COOH) held together by

two hydrogen bonds

Hydrogen Bonds Cause Dimers

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• Acetic acid is the sour component of vinegar.

• The air oxidation of ethanol to acetic acid is the process that makes “bad” wine taste sour.

• Acetic acid is an industrial starting material for polymers used in paints and adhesives.

Acetic Acid

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• Butanoic acid is an oxidation product that contributes to the disagreeable smell of body odor.

Butanoic Acid

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•Oxalic acid and lactic acid are simple carboxylic acids that are quite prevalent in nature.

Carboxylic Acids in Nature

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• Salts of carboxylic acids are commonly used as food preservatives.

• Sodium benzoate is a fungal growth inhibitor and is also used as a preservative in soft drinks.

• Potassium sorbate is an additive that prolongs the shelf-life of baked goods and other foods.

Interesting Salts of Carboxylic Acids

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• Aspirin (acetylsalicylic acid) is a synthetic carboxylic acid, similar in structure to salicin, a naturally occurring compound isolated from willow bark, and salicylic acid, found in meadowsweet.

Aspirin and Related Compounds

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• Aspirin lessens pain and decreases inflammation because it prevents the synthesis of prostaglandins, the compounds responsible for both of these physiological responses.

• Aspirin inactivates cyclooxygenase, an enzyme that converts arachidonic acid to PGG2, an unstable precursor of PGF2 and other prostaglandins.

Arachidonic Acid and Prostaglandins

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[1] Oxidation of 1° alcohols

[2] Oxidation of alkyl benzenes

Preparation of Carboxylic Acids

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[3] Oxidative cleavage of alkynes

Preparation of Carboxylic Acids

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• The most important reactive feature of a carboxylic acid is its polar O–H bond, which is readily cleaved with base.

Acid–Base Reactions of Carboxylic Acids

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• The nonbonded electron pairs on oxygen create electron-rich sites that can be protonated by strong acids (H–A).

• Protonation occurs at the carbonyl oxygen because the resulting conjugate acid is resonance stabilized (Possibility [1]).

• The product of protonation at the OH group (Possibility [2]) cannot be resonance stabilized.

Protonation of Carboxylic Acids

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• The polar C–O bonds make the carboxy carbon electrophilic.

• Thus, carboxylic acids react with nucleophiles.

• Nucleophilic attack occurs at an sp2 hybridized carbon atom, so it results in the cleavage of the bond as well.

Reaction with Nucleophiles and Bases

• Carboxylic acids are strong organic acids, and as such, readily react with BrØnsted–Lowry bases to form carboxylate anions.

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• An acid can be deprotonated by a base that has a conjugate acid with a higher pKa.

• Because the pKa values of many carboxylic acids are ~5, bases that have conjugate acids with pKa values higher than 5 are strong enough to deprotonate them.

Deprotonation of Carboxylic Acids

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• Amino acids contain two functional groups—an amine group (NH2) and a carboxy group (COOH).

• Amino acids are the building blocks of proteins.

• The simplest amino acid, glycine, has R=H. When R is any other group, the carbon is a stereogenic center.

Amino Acids

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• An amino acid is both an acid and a base.

• Amino acids are never uncharged neutral molecules, they exist as salts, so they have very high melting points and are very water soluble.

Zwitterion Character of Amino Acids

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• An amino acid can exist in three different forms depending on pH.

pH and Amino Acid Structure

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• Alanine exists in three different forms depending on the pH of the solution in which it is dissolved.

• As the pH of a solution is gradually increased from 2 to 10, the following process occurs:

Summary of Acid–Base Reactions

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• Because a protonated amino acid has at least two different protons that can be removed, a pKa value is reported for each of these protons.

• The pH at which the amino acid exists primarily in its neutral form is called its isoelectric point, abbreviate as pI.

• Generally, the isoelectric point is the average of both pKa values of an amino acid:

Isoelectric Point

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What is the name of the following carboxylic acid?

A) 3-propyllbutanoic acidB) 3-methyl-3-propylpropanoic acidC) 3-methylpentanoic acidD) 3-methylhexanoic acid

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Ans:  D

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What is the IUPAC name of the following compound?

A) 4-hydroxy-3-methylbutanoic acidB) 3-hydroxy-2-methylbutanoic acidC) 1-hydroxy-2-methylbutanoic acidD) 3-(hydroxymethyl)butanoic acid

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Ans:  A

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The cleansing action of soaps is due to the formation of  A) water insoluble micelles with lipophilic interiors. B) water soluble micelles with lipophilic interiors. C) water insoluble micelles with hydrophilic interiors. D) water soluble micelles with hydrophilic interiors. 

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Ans:  B

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What is the name of the compound below?

 A) trans-3-bromocyclohexanecarboxylic acidB) cis-3-bromocyclohexanecarboxylic acidC) trans-5-bromocyclohexanecarboxylic acidD) cis-5-bromocyclohexanecarboxylic acid 

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Ans:  B

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Chloroacetic acid, ClCH2CO2H, is a stronger acid than acetic acid. Which one of the following best explains this?  ClCH2CO2¯ is more stable than CH3CO2¯ because of the electron-withdrawing Cl. ClCH2CO2¯ is more soluble in water than CH3CO2¯ because of the Cl. ClCH2CO2¯ is more stable than CH3CO2¯ because of an additional resonance form. ClCH2CO2¯ is more stable than CH3CO2¯ because of hydrogen bonding. 

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Ans:  A

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Which of the following has only a single peak in its proton NMR spectrum?

A) only I B) only II C) I and II D) I, II, and III

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Ans:  C

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A compound is either cyclononane or cyclodecane. Which of the following is the most useful technique in identifying the compound?

A)IR spectroscopy B) mass spectrometry C) proton NMR D) C-13 NMR

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Ans:  B

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How many different sets of equivalent protons are there in the following compound?

A) three B) four C) five D) six

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Ans:  A

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Which compound below fits the following proton NMR data?  triplet delta 1.22 (3H) singlet delta 1.98 (3H) quartet delta 4.07 (2H)

A) A B) B C) C D) D

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Ans:  B

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What region of the electromagnetic spectrum is used in nuclear magnetic resonance spectroscopy?

A) radio wave B) X-ray C) ultraviolet D) microwave

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Ans:  A

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