19.16 a-Halogenation of Carboxylic Acids: The Hell-Volhard-Zelinsky Reaction

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19.16a-Halogenation of Carboxylic AcidsThe
Hell-Volhard-Zelinsky Reaction
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a-Halogenation of Carboxylic Acids


X2
HX

• analogous to a-halogenation of aldehydes and
  ketones
• key question Is enol content of carboxylic
  acids high enough to permit reaction to occur
  at reasonable rate? (Answer is NO)

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But...
P or PX3


X2
HX

• reaction works well if a small amount
  ofphosphorus or a phosphorus trihalide is added
  tothe reaction mixture
• this combination is called the Hell-Volhard-Zelin
  sky reaction

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Example

Br2
PCl3
benzene80C
(60-62)
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Value
Br2
P
(77)

• a-Halogen can be replaced by nucleophilic
  substitution

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Value
Br2
P
(77)
K2CO3H2Oheat
(69)
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Synthesis of a-Amino Acids
Br2
(CH3)2CHCH2COH
PCl3
(88)
NH3H2O
(48)
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19.17Decarboxylation of Malonic Acidand Related
Compounds
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Decarboxylation of Carboxylic Acids
Simple carboxylic acids do not decarboxylatereadi
ly.

RH
CO2
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Decarboxylation of Carboxylic Acids
Simple carboxylic acids do not decarboxylatereadi
ly.

RH
CO2
But malonic acid does.
150C

CO2
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Mechanism of Decarboxylation of Malonic Acid

• One carboxyl group assists the loss of the other.

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Mechanism of Decarboxylation of Malonic Acid
One carboxyl group assists the loss of the other.

• This compound is the enol form of acetic acid.

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Mechanism of Decarboxylation of Malonic Acid
One carboxyl group assists the loss of the other.

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Mechanism of Decarboxylation of Malonic Acid
One carboxyl group assists the loss of the other.
These hydrogens play no role.

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Mechanism of Decarboxylation of Malonic Acid
One carboxyl group assists the loss of the other.
Groups other than H may be present.

R
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Decarboxylation is a general reactionfor
1,3-dicarboxylic acids
CO2H
H
(74)
(96-99)
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Mechanism of Decarboxylation of Malonic Acid
One carboxyl group assists the loss of the other.
This OH group plays no role.

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Mechanism of Decarboxylation of Malonic Acid
One carboxyl group assists the loss of the other.
Groups other than OH may be present.
R
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Mechanism of Decarboxylation of Malonic Acid
This kind of compoundis called a b-keto acid.
a
b

• Decarboxylation of a b-keto acid gives a ketone.

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Decarboxylation of a b-Keto Acid
25C

CO2