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Chapter 17: Q78P (page 911)

Phenolphthalein, a common nonprescription laxative, is also an acid-base indicator that is colorless in acid and red in base. Phenolphthalein is synthesized by the acid-catalyzed reaction of phthalic anhydride with 2 equivalents of phenol.

(a) Propose a mechanism for the synthesis of phenolphthalein.

(b) Propose a mechanism for the conversion of phenolphthalein to its red dianion in the base.

(c) Use resonance structures to show that the two phenolic oxygen atoms are equivalent (each with half a negative charge) in the red phenolphthalein dianion.


Phthalic anhydride Phenolphthalein red dianion

Short Answer

Expert verified

(a)

Mechanism for the synthesis of phenolphthalein

(b)

Mechanism for the conversion of phenolphthalein to its red dianion

(c)

Two phenolic oxygen atoms are equivalent

Step by step solution

01

Phenolphthalein

Phenolphthalein is a chemical compound generally used as an indicator in acid-base titration which turns colorless in acidic solution and pink in basic solution.

02

Mechanism for the synthesis of phenolphthalein

(a) The mechanism for the synthesis of phenolphthalein is represented as follows:

Mechanism for the synthesis of phenolphthalein

(b) The phenolphthalein molecule is converted to red dianion ( two negative charge ions) in the presence of a base. This dianion form exists in numerous resonance structures by the delocalization of negative charge on ring and oxygen atom.

Mechanism for the conversion of phenolphthalein to its red dianion

(c) The resonance structure describes the delocalization of electrons or charge on an ion or a molecule. This structure helps in determining the reason for the stability of a molecule.

Resonance structures

Hence two phenolic oxygen atoms are equivalent in the dianion of phenolphthalein.

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Most popular questions from this chapter

In Chapter 14, we saw that Agent Orange contains (2,4,5-trichlorophenoxy) acetic acid, called 2,4,5-T. This compound is synthesized by the partial reaction of 1,2,4,5-tetrachlorobenzene with sodium hydroxide, followed by a reaction with sodium chloroacetate, ClCH2CO2Na.

(a) Draw the structures of these compounds, and write equations for these reactions.

(b) One of the impurities in the Agent Orange used in Vietnam was 2,3,7,8-tetrachlorodibenzodioxin (2,3,7,8-TCDD), often incorrectly called 鈥渄ioxin.鈥� Propose a mechanism to show how 2,3,7,8-TCDD is formed in the synthesis of 2,4,5-T.

(c) Show how the TCDD contamination might be eliminated, both after the first step and on completion of the synthesis

Which reactions will produce the desired product in good yield? You may assume that aluminum chloride is added as a catalyst in each case. For the reactions that will not give a good yield of the desired product, predict the major products.

Reagents Desired Product

(a) benzene + n-butyl bromide n-butylbenzene

(b) ethylbenzene + tert-butyl chloride p-ethyl-tert-butylbenzene

(c) bromobenzene + ethyl chloride p-bromoethylbenzene

(d) benzamide (PhCONH3) + CH3 CH2 CI p-ethylbenzamide

(e) toluene + HNO3, H2SO4 , heat 2,4,6-trinitrotoluene (TNT)

Indane can undergo free-radical chlorination at any of the alkyl positions on the aliphatic ring.

indane

  1. Draw the possible monochlorinated products from this reaction.
  2. Draw the possible dichlorinated products from this reaction.
  3. What instrumental technique would be most helpful for determining how many products are formed, and how many of those products are monochlorinated and how many are dichlorinated?
  4. Once the products have been separated, what instrumental technique would be most helpful for determining the structures of all the dichlorinated products?

What products would you expect from the following coupling reactions?

(a)

(b)

(c)

(d)

(e)

Predict the major products when the following compounds are irradiated by light and treated with (1) 1 equivalent of Br2 and (2) excess Br2.

(a) isopropylbenzene

(b) tetralin
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