Introduction to carboxylic acids 1. Learning Objectives Identify the general structure of carboxylic acids Give the IUPAC and the common names Explain.

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Introduction to carboxylic acids

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Learning Objectives

• Identify the general structure of carboxylic acids

• Give the IUPAC and the common names• Explain the physical properties of the

carboxylic acids ; e.g solubility , boiling point, etc

• Give examples and uses of carboxylic acids

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Functional Groups

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R = Alkyl Groups

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Carboxylic Acids

A class of organic compounds containing at least one carboxyl group

Introduction

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R C

O

OH

R = alkyl group Aliphatic acid

R = aryl group aromatic acid

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(1) The IUPAC names of carboxylic acids

Replace the ‘e’ with ‘oic acid’ at the end of the name of the hydrocarbon

Naming Carboxylic Acids

H H

H C C H

H Hethane

H O

H C C O H

Hethan oic acid

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• Number substituents from the carboxyl carbon 1.

CH3 O | ║

CH3—CH—CH2—C—OH 4 3 2 1

3-methylbutanoic acid

IUPAC Names

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The common names of simple carboxylic acids HCOOH formic acid, CH3—COOH acetic acid

• Locate substituents using greek letters , , γ for the carbon atoms adjacent to the carboxyl carbon.

CH3

γ α | CH3—CH—CH2—COOH

α-methylbutryic acid ( IUPAC ; 2-methylbutanoic acid )

(2) Common Names

CH3CH2CH2CHCH2COOH

Ph

-phenylcaproic acid ε δ γ α

Common and IUPAC Names of Some Carboxylic Acids

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(3) Naming Cyclic Acids

Cycloalkanes bonded to -COOH are named as cycloalkanecarboxylic acids

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Br

O

HO

3-bromo-cyclohexane carboxylic acid

O

OH

1-cyclopentene carboxylic acid

COOH

Cyclohexanecarboxylic acid

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(4) Naming Aromatic Acids

OHC

O

3-chlorobenzoic acid 4-aminobenzoic acid m-chlorobenzoic acid p-aminobenzoic acid

Locates substituent by assigning 1 to the carbon attached to the carboxyl group. Prefixes ortho, meta, and para for 2 substituents:

ortho 1, 2 locationmeta 1, 3 location para 1, 4 location

Benzoic acid

o-hydroxybenzoic acid2-hydroxybenzoic acid

(salicylic acid)

OHC

O

OHC

O

ClNH2

COOH

OH

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Dicarboxylic Acids• Aliphatic diacids are usually called by their

common names.• For IUPAC name, number the chain from the

end closest to a substituent.

3-bromohexanedioic acid-bromoadipic acid

HOOCCH2CHCH2CH2COOH

Br

Some examples of dicarboxylic acid

Common name IUPAC name(Historical name) alkanedioic acid

HOOC-COOH

HOOCCH2COOH

HOOC(CH2)2COOH

oxalic acid

malonic acid

succinic acid

ethanedioic acid

propanedioic acid

butanedioic acid

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COOH

COOH

phthalic acidO-phthalic acid

COOHHOOC 1,3-benzenedicarboxylic acid

HOOC

COOH

1,4-benzenedicarboxylic acid

Common name IUPAC name

1,2-benzenedicarboxylic acid

isophthalic acidm-phthalic acid

terephthalic acidp-phthalic acid

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PHYSICAL PROPERTIES

carboxylic acids are soluble in organic solvents ; e.g alcohols they are also soluble in water due to hydrogen bonding

small ones dissolve readily in cold water ( up to 4 carbons) as mass increases, the solubility decreases

benzoic acid is fairly insoluble in cold, but soluble in hot water

Solubility

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The boiling points of carboxylic acids• Are higher than alcohols, ketones, and aldehydes of similar

mass.

• Are high because they form dimers in which hydrogen bonds form between the polar groups in the two carboxyl groups.

O H—O || |

CH3—C C—CH3

| ||

O—H O

A dimer of acetic acid

Boiling Points

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Melting Points• Aliphatic acids with more than 8 carbons are solids at

room temperature.• Double bonds (especially cis) lower the melting

point. The following acids all have 18 carbons:– Stearic acid (saturated): 72C

– Oleic acid (one cis double bond): 16C

– Linoleic acid (two cis double bonds): -5C

CO2H

CO2H

CO2H

Polarity of Carboxylic Acids

Polarity isCarboxylic acids are strongly polar because they have

two polar groups hydroxyl (OH) and carbonyl (C=O)

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Acidity of Carboxylic Acids

• A carboxylic acid may dissociate in water to give a proton and a carboxylate ion (known as conjugate base).

• The equilibrium constant Ka for this reaction is called the acid-dissociation constant, The acidity constant, Ka,, is about 10-5 for a typical carboxylic acid (pKa ~ 5)

• The acid will be mostly dissociated if the pH of the solution is higher than the pKa of the acid.

• are weak acids compared to inorganic acids

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Acidity of Carboxylic Acids Carboxylic acids are significantly more acidic than water or alcohols.

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4 -electrons delocalizedover three p-prbitals

C-O bond length of a carboxylates are the same

This greater acidity of carboxylic acids is attributed to greater stabilization of carboxylate ion by:

a. Inductive effect of the C=O group

b. Resonance stabilization of the carboxylate ion

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Electron delocalization stabilizes carbonyl group

Carbon is sp2 hybridized.

Bond angles are close to 120.

RC

OH

O••

•• ••

•• RC

OH

O••

•• ••

••+••

– RC

OH

O••

••

••

+

••

–

Electron delocalization; resonance

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Electronegative substituents increase acidity

X CH2C-OH

O X Ka pKa

HF

Cl

1.8 x 10-5 4.72.5 x 10-3 2.6

1.4 x 10-3 2.9

Substituent Effects on Acidity

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Substituent Effects on Acidity

• Multiple electronegative substituents have synergistic effects on acidity

• Fluoroacetic, chloroacetic, bromoacetic, and iodoacetic acids are stronger acids than acetic acid

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The magnitude of a substituent effect depends on its distance from the carboxyl group.

Substituent Effects on Acidity

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Aromatic Carboxylic Acids

Electron-withdrawing groups (e.g NO2) enhance the acid strength while electron-donating groups ( e.g alkyl ) decrease the acid strength.

Effects are strongest for substituents in the ortho and para positions.

Substituent Effects

• An electronegative group will drive the ionization equilibrium toward dissociation, increasing acidity

• An electron-withdrawing group (-NO2) increases acidity by stabilizing the carboxylate anion, and an electron-donating (activating) group (OCH3) decreases acidity by destabilizing the carboxylate anion

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Substituent Effects in Substituted Benzoic Acids

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So:• Carboxylic acids are weak acids, average

pH = 5• Are Polar because of ( the high

electronegative O)• Soluble in water• Have high boiling and melting points

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Learning Check

Write the balanced equation for the ionization of butanoic acid in water and identify the carboxylate ion.

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Solution

Write the balanced equation for the ionization of butanoic acid in water and identify the carboxylate ion.

O O

CH3CH2COH + H2O CH3CH2CO– + H3O+

carboxylic acid carboxylate ion

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Naming Carboxylic Acid Salts

• First name the cation.• Then name the anion by replacing the

-ic acid with -ate.

potassium 3-chloropentanoate

C H3 C H2 C HC H2 C O O- K

+

C l

Neutralization of Carboxylic Acids

Carboxylic acid salts • are a product of the neutralization of a carboxylic acid with a

strong base

CH3—COOH + NaOH CH3—COO– Na+ + H2Oacetic acid sodium acetate

(carboxylic acid salt)• are used as :• preservatives and flavor enhancers

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Properties of Acid Salts

• Usually solids with no odor.

• Carboxylate salts of Na+, K+, Li+, and NH4+

are soluble in water.• Soap is the soluble sodium salt of a long

chain fatty acid.• Salts can be formed by the reaction of an

acid with NaHCO3, releasing CO2.

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Uses of Organic Acids

Alpha hydroxy acids Acetic acid

skin care products. in vinegar as preservative and flavourings

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Uses of Organic Acids

Making of drugs, dyes, paints, insecticides, plastics.

Making of esters.

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Limes, lemons

Citric acid

Formic acid

Insects

Other Important Organic Acids

Malic acid

Apples and pears

Lactic acid

Sour milkTartaric acid

Grape juice

Oxalic acid

Spinch

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VITAMIN C (Ascorbic

acid)