ACIDITY, EFFECT OF SUBSTITUENTS ON ACIDITY AND IMPORTANT REACTIONS OF BENZOIC ACID.

BENZOIC ACID

Conversion into functional derivatives:

1. 🡪 acid chlorides

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2. 🡪 esters

“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

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“indirect” esterification:

RCOOH + PCl₃ 🡪 RCOCl + R´OH 🡪 RCO₂R´

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

3. 🡪 amides

“indirect” only!

RCOOH + SOCl₂ 🡪 RCOCl + NH₃ 🡪 RCONH₂

amide

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

RCOOH + NH₃ 🡪 RCOO^-NH₄⁺

acid base

Reduction:

RCO₂H + LiAlH₄; then H⁺ 🡪 RCH₂OH

1^o alcohol

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Carboxylic acids resist catalytic reduction under normal conditions.

RCOOH + H₂, Ni 🡪 NR

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

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RCH₂COOH + X₂, P 🡪 RCHCOOH + HX

X

α-haloacid

X₂ = Cl₂, Br₂

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

ACIDITY OF SUBSTITUTED BENZOIC ACIDS�

• ELECTRON-WITHDRAWING GROUPS
• THE CONJUGATE BASE OF BENZOIC ACID IS DESTABILIZED BY ELECTRON-DONATING GROUPS.  THIS MAKES THE ACID LESS ACIDIC

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• ELECTRON-WITHDRAWING GROUPS DEACTIVATE THE BENZENE RING TO ELECTROPHILIC ATTACK AND MAKE BENZOIC ACIDS MORE ACIDIC.

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ELECTRON-DONATING GROUPS

• THE CONJUGATE BASE OF BENZOIC ACID IS STABILIZED BY ELECTRON-WITHDRAWING GROUPS.  THIS MAKES THE ACID MORE ACIDICX

• ELECTRON-WITHDRAWING GROUPS ACTIVATE THE BENZENE RING TO ELECTROPHILIC ATTACK AND MAKE BENZOIC ACIDS LESS ACIDIC.

SUBSTITUENT EFFECTS ON ACIDITY

• WATER IS LESS ACIDIC THAN HYDROGEN PEROXIDE BECAUSE HYDROGEN IS LESS ELECTRONEGATIVE THAN OXYGEN, AND THE COVALENT BOND JOINING THESE ATOMS IS POLARIZED IN THE MANNER SHOWN. ALCOHOLS ARE SLIGHTLY LESS ACIDIC THAN WATER, DUE TO THE POOR ELECTRONEGATIVITY OF CARBON, BUT CHLORAL HYDRATE, CL₃CCH(OH)₂, AND 2,2,2,-TRIFLUOROETHANOL ARE SIGNIFICANTLY MORE ACIDIC THAN WATER, DUE TO INDUCTIVE ELECTRON WITHDRAWAL BY THE ELECTRONEGATIVE HALOGENS (AND THE SECOND OXYGEN IN CHLORAL HYDRATE). IN THE CASE OF CARBOXYLIC ACIDS, IF THE ELECTROPHILIC CHARACTER OF THE CARBONYL CARBON IS DECREASED THE ACIDITY OF THE CARBOXYLIC ACID WILL ALSO DECREASE.

• SIMILARLY, AN INCREASE IN ITS ELECTROPHILICITY WILL INCREASE THE ACIDITY OF THE ACID. ACETIC ACID IS TEN TIMES WEAKER AN ACID THAN FORMIC ACID (FIRST TWO ENTRIES IN THE SECOND ROW), CONFIRMING THE ELECTRON DONATING CHARACTER OF AN ALKYL GROUP RELATIVE TO HYDROGEN, AS NOTED EARLIER IN A DISCUSSION OF CARBOCATION STABILITY. ELECTRONEGATIVE SUBSTITUENTS INCREASE ACIDITY BY INDUCTIVE ELECTRON WITHDRAWAL. AS EXPECTED, THE HIGHER THE ELECTRONEGATIVITY OF THE SUBSTITUENT THE GREATER THE INCREASE IN ACIDITY (F > CL > BR > I), AND THE CLOSER THE SUBSTITUENT IS TO THE CARBOXYL GROUP THE GREATER IS ITS EFFECT (ISOMERS IN THE 3RD ROW). SUBSTITUENTS ALSO INFLUENCE THE ACIDITY OF BENZOIC ACID DERIVATIVES, BUT RESONANCE EFFECTS COMPETE WITH INDUCTIVE EFFECTS. THE METHOXY GROUP IS ELECTRON DONATING AND THE NITRO GROUP IS ELECTRON WITHDRAWING (LAST THREE ENTRIES IN THE TABLE OF PK_A VALUES).