Carboxylic acidsCarboxylic acids

Dr AKM Shafiqul IslamDr AKM Shafiqul Islam

School of Bioprocess EngineeringSchool of Bioprocess Engineering

06.10.0806.10.08

Carboxylic AcidsCarboxylic Acids

The functional group of a carboxylic acid is a The functional group of a carboxylic acid is a carboxyl groupcarboxyl group, , CarbonylCarbonyl with with hydroxyhydroxy

represented in one of three waysrepresented in one of three ways

CO2HCOOHC-OHO

Fill in the table below.

formic acid methanoic acidacetic acid ethanoic acidpropionic

acidpropanoic

acidbutyric acid butanoic acidvaleric acid pentanoic acidcaproic acid hexanoic acidbenzoic acid benzoic acid

2. Carboxylic acids that have branches are often given IUPAC names, where the numbering of the longest carbon chain begins at the Carbonyl carbon. Name the following compounds.

2-bromopropanoic acid

3-chlorobutanoic acid

4-amino-3-methylhexanoic acid

Carboxylic Aromatic AcidsCarboxylic Aromatic AcidsC

OHO

benzoic acid

COHO

CO

OH

o-phthalic acid(benzene-1,2-dicarboxylic)

OH

COHO

salicylic acid(2-hydroxybenzenecarboxylic)

OH

OH

HO

COHO

gallic acid(3,4,5-trihydroxy

-benzenecarboxylic)

CH3O

OH

COHO

vanillic acid(4-hydroxy-3-methoxy-benzenecarboxylic)

CCH

CHO

OH

cinnamic acid(3-phenylprop

-2-enoic)

Carboxylic acids and their derivatives:Carboxylic acids and their derivatives:

Tart taste of citrus fruits, vinegar, and rhubarbTart taste of citrus fruits, vinegar, and rhubarb

Sharp sting of red antsSharp sting of red ants

Unsavory smell and taste of rancid butterUnsavory smell and taste of rancid butter

Vitamin C is a carboxylic acidVitamin C is a carboxylic acid

Pleasant taste and odor of fruits are due to carboxylic acid Pleasant taste and odor of fruits are due to carboxylic acid

derivatives: estersderivatives: esters

Where are they to be found?Where are they to be found?

Physical PropertiesPhysical Properties The carboxyl group contains three polar The carboxyl group contains three polar

covalent bonds; C=O, C-O, and O-Hcovalent bonds; C=O, C-O, and O-H the polarity of these bonds determines the major the polarity of these bonds determines the major

physical properties of carboxylic acidsphysical properties of carboxylic acids

δδ--O – HO – Hδδ++ δδ++C=OC=Oδδ--

RR

RRCC

OOHH

OO••••

•••• ••••

•••• RRCC

OOHH

OO••••

•••• ••••

••••++••••

––

Electron DelocalizationElectron Delocalization

stabilizes carbonyl groupstabilizes carbonyl group

RRCC

OOHH

OO••••

•••• ••••

•••• RRCC

OOHH

OO••••

•••• ••••

••••++••••

–– RRCC

OOHH

OO••••

••••

••••

++

••••

––

Electron DelocalizationElectron Delocalization

Physical PropertiesPhysical Properties The carbonyl group has a large dipoleThe carbonyl group has a large dipole The hydroxy group is capable of hydrogen The hydroxy group is capable of hydrogen

bonding.bonding. The molecules can H-bond to each otherThe molecules can H-bond to each other How does this affect boiling point?How does this affect boiling point?

Higher than aldehydes and ketone – no H-bondsHigher than aldehydes and ketone – no H-bonds Higher than alcohols – H-bonds, not strong Higher than alcohols – H-bonds, not strong

dipoledipole

H3C C

O

O

H

CH3C

O

O

H- +

+ -

hydrogen bondingbetween two molecules

HH33CCCC

OO HH OO

OOHHOO

HH

HH

HH

Solubility in WaterSolubility in WaterCarboxylic acids are similar to alcohols in Carboxylic acids are similar to alcohols in respect to their solubility in waterrespect to their solubility in waterForm hydrogen bonds to waterForm hydrogen bonds to water

Physical PropertiesPhysical Properties carboxylic acids are more soluble in water than are carboxylic acids are more soluble in water than are

alcohols, ethers, aldehydes, and ketones of comparable alcohols, ethers, aldehydes, and ketones of comparable molecular weightmolecular weight

CH3COOHCH3CH2CH2OHCH3CH2CHO

CH3(CH2)2COOHCH3(CH2)3CH2OHCH3(CH2)3CHO

acetic acid

1-propanolpropanal

60.5

60.158.1

1189748

16388.1butanoic acid1-pentanol 88.1 137

103pentanal 86.1

Structure NameMolecularWeight

Boiling Point (°C)

Solubility(g/100 mL H2O)

infinite

infinite

16infinite

2.3slight

Alkoxide anions don’t have resonance-stabilization.

Alcohols are weaker acids than carboxylic acids.

R OH H2O H3O+

+ +

acid base alkoxide anion

R O

Physical PropertiesPhysical Properties

Sharp and or sour odor/tasteSharp and or sour odor/taste Vinegar, rancid butter, sweat, sauerkrautVinegar, rancid butter, sweat, sauerkraut

When the carbonyl group is substituted by atoms other than carbon or hydrogen, as in

R C

Z

O

where Z is oxygen, nitrogen, or halogen, the compound becomes a carboxylic acid or a carboxylic acid derivative.

Carboxylic acids derivativesCarboxylic acids derivatives

Ester Acid anhydride AmideEster Acid anhydride Amide Acid Acid

halidehalide

Carboxylic acid and derivativesCarboxylic acid and derivatives

O

R

X

O

R

O

O

RO

R

R'O

O

R

H2N

Carboxylic Acid Derivatives Carboxylic Acid Derivatives Hydroxy AcidsHydroxy Acids

Beta-hydroxy acidBeta-hydroxy acid

OH

O

OH

Salicylic Acid

exfoliant

oil soluble – penetrates oil containing pores to remove dead skin cells

less irritating than alpha acids

Reactions carboxylic acidsReactions carboxylic acids

1.1. as acidsas acids2.2. conversion into functional derivativesconversion into functional derivatives

a)a) acid chloridesacid chloridesb)b) estersestersc)c) amidesamides

3.3. reductionreduction4.4. alpha-halogenationalpha-halogenation5.5. EASEAS

Reactions : Acid/BaseReactions : Acid/Base Carboxylic acids are weak acidsCarboxylic acids are weak acids

Give up the H bonded to O to water or baseGive up the H bonded to O to water or base Tendency for acid to give up proton (HTendency for acid to give up proton (H++) is indicated by ) is indicated by

pKa; lower pKa indicates stronger acidpKa; lower pKa indicates stronger acid Carboxylic acids: pKCarboxylic acids: pKaa 4.0 - 5.0 4.0 - 5.0

In comparison:In comparison: Hydrochloric acid Hydrochloric acid pKa = -7pKa = -7 Sulfuric acid Sulfuric acid pKa = -3pKa = -3 Alcohol Alcohol pKa = 15-16pKa = 15-16

CH3COH H2O CH3CO H3O+

O O

Comparison Of pKa ValuesComparison Of pKa Values

Hydrochloric acid Hydrochloric acid pKa = -7pKa = -7 Sulfuric acid Sulfuric acid pKa = -3pKa = -3 Alcohol Alcohol pKa = 15-16pKa = 15-16 Carboxylic acidCarboxylic acid pKa = 4 pKa = 4

1. Both acetic acid and ethanol have an acidic hydrogen on the OH group. Why is the pKa value for acetic acid (pKa = 4.74) lower than the pKa for ethanol (pKa = 15.9)? In other words, why are carboxylic acids more acidic than alcohols. Consider resonance-stabilized anions.

R C

O

OH H2O R C

O

O H3O+

+ +

acid base carboxylate anion

Acidity of Carboxylic AcidsAcidity of Carboxylic Acids

=R CO

OR C

O

O

R CO

O

½-

½-

The greater the stability of the carboxylate anion, the farther the shift to the right.

resonance-stabilized carboxylate anion

R C

O

OH H2O R C

O

O H3O+

+ +

acid base carboxylate anion

The farther the shift to the right, the greater the acidity.

As acids:As acids:

a) with active metalsa) with active metals

RCORCO22H + Na H + Na RCO RCO22-Na+ + H-Na+ + H22(g)(g)

b) b) with baseswith bases

RCORCO22H + NaOH H + NaOH RCO RCO22-Na+ + H-Na+ + H22OO

c) c) relative acid strength?relative acid strength?

CHCH44 < NH < NH33 < HC < HCCH < ROH < HOH < HCH < ROH < HOH < H22COCO33 < RCO< RCO22H < H < HFHF

d)d) quantitativequantitative

HA + HHA + H22O O H H33OO++ + A + A-- onization in water onization in water

Ka = [HKa = [H33OO++] [A-] / [HA]] [A-] / [HA]

Dehydration of Carboxylic AcidDehydration of Carboxylic Acid

* water must be removed (to avoid hydrolysis)* water must be removed (to avoid hydrolysis)

* limited use* limited use

H3C O CH3

O O+ H2O(g)H3C COOH2 800o

COOH

COOH200o

O

O

O

+ H2O(g)

Reaction With BasesReaction With Bases All carboxylic All carboxylic acidsacids react with strong react with strong basesbases to to

form water-soluble form water-soluble saltssalts

COOH NaOHH2O

COO- Na

+H2O+ +

Benzoic acid(slightly soluble in water)

Sodium benzoate(60 g/100 mL water)

COOH NH3H2O

COO- NH4

++

Ammonium benzoate(20 g/100 mL water)

Benzoic acid(slightly soluble in water)

Reactions with basesReactions with bases

Salts of carboxylic acidsSalts of carboxylic acids Drop the –ic acidDrop the –ic acid Change to –ateChange to –ate

Sodium benzoate & monosodium glutamate Sodium benzoate & monosodium glutamate Sodium benzoate – inhibit moldSodium benzoate – inhibit mold MSG – flavor enhancerMSG – flavor enhancer

OHC

CH

O

NH2

CH2CH2

CO

-

O

Na+

O-

O

Na+

sodium benzoate MSG

R COH

O SOCl2

or PCl3orPCl5

R CCl

O

CO2H + SOCl2 COCl

CH3CH2CH2 CO

OH

PCl3CH3CH2CH2 C

O

Cl

a) acid chlorides

Conversion into functional derivativesConversion into functional derivatives

“direct” esterification: H+

RCOOH + R´OH RCO2R´ + H2O

-reversible and often does not favor the ester-use an excess of the alcohol or acid to shift equilibrium-or remove the products to shift equilibrium to completion

“indirect” esterification:RCOOH + PCl3 RCOCl + R´OH RCO2R´

-convert the acid into the acid chloride first; not reversible

)b esters

C CO

O

CH3

+ H2O

SOCl2

CCH3OH

O

OH+ CH3OH

O

Cl

H+

)c amides

“indirect” only!

RCOOH + SOCl2 RCOCl + NH3 RCONH2

amide

Directly reacting ammonia with a carboxylic acid results in an ammonium salt:

RCOOH + NH3 RCOO-NH4+

acid base

OH

O

3-Methylbutanoic acid

PCl3

Cl

O NH3

NH2

O

CO

OH

PCl3C

O

ClC

O

NH2

NH3

NH3

amide

CO

O NH4

ammonium salt

3. Reduction:

RCO2H + LiAlH4; then H+ RCH2OH

1o alcohol

Carboxylic acids resist catalytic reduction under normal conditions.

RCOOH + H2, Ni NR

CH3CH2CH2CH2CH2CH2CH2COOH

Octanoic acid(Caprylic acid)

LiAlH4 H+

CH3CH2CH2CH2CH2CH2CH2CH2OH

1-Octanol

CH2 CO

OH

H2, PtNR

LiAlH4

H+

CH2CH2OH

4. Alpha-halogenation: (Hell-Volhard-Zelinsky reaction)

RCH2COOH + X2, P RCHCOOH + HX X α-haloacid X2 = Cl2, Br2

COOH

Br2,PNR (no alpha H)

CH3CH2CH2CH2COOH + Br2,P CH3CH2CH2CHCOOH

Brpentanoic acid2-bromopentanoic acid

RCH2COOH + Br2,P RCHCOOH + HBr

Br

NH2OH

RCHCOOH RCHCOOH

RCH=CHCOOHRCH2CHCOOH

Br

aminoacid

NaOH;then H+

NH3

KOH(alc)

then H+

5. EAS: (-COOH is deactivating and meta- directing)

CO2H

CO2H

NO2

CO2H

SO3H

CO2H

Br

NR

HNO3,H2SO4

H2SO4,SO3

Br2,Fe

CH3Cl,AlCl3

benzoic acid

N

NH2

N

H2N

N

H2N

melamine

N

NH

N

HN N NCH2NH

N

NH

N

N NH

CH2

N

CH2

NCH2NH

N

NH

N

N

N

HN

N

HN N

N

NH2

N

H2N N NH2

H2C=O-H2O

Melamine

formaldehyde

Melmac