Organic Compounds Containing Oxygen: Chemistry | JEE Main
A compound 'X' when treated with phthalic anhydride in presence of concentrated H2SO4 yields 'Y'. 'Y' is used as an acid/base indicator. 'X' and 'Y' are respectively:
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Correct answer
Carbolic acid (phenol) condenses with phthalic anhydride in the presence of concentrated sulfuric acid to yield phenolphthalein, a common acid-base indicator.
Option analysis
Why each option works or fails
A · Carbolic acid, Phenolphthalein
None. This is the correct choice. Phenol (commonly known as carbolic acid) reacts in a 2:1 ratio with phthalic anhydride via electrophilic aromatic substitution catalyzed by concentrated H2SO4 to form phenolphthalein.
B · Anisole, methyl orange
Assuming phthalic anhydride condensation produces azo dyes such as methyl orange from aromatic ethers like anisole. Methyl orange is an azo dye synthesized via diazotization of sulfanilic acid and subsequent coupling with N,N-dimethylaniline, not via phthalic anhydride condensation.
C · Salicylaldehyde, Phenolphthalein
Believing any substituted phenolic compound like salicylaldehyde condenses with phthalic anhydride to yield phenolphthalein. Phenolphthalein specifically requires unsubstituted phenol (carbolic acid) at the para position; salicylaldehyde introduces an aldehyde group that prevents the formation of phenolphthalein.
D · Toludine, Phenolphthalein
Confusing aromatic amines (toluidine) with phenols in the synthesis of phthalein indicators. Toluidine gives rhodamine- or fluorescein-type derivatives or sulfonated compounds under different conditions, whereas phenolphthalein specifically requires phenol.
Reviewed route
Solution
StepWorking
01identify
The reaction involves condensation of an aromatic substrate 'X' with phthalic anhydride in the presence of concentrated H2SO4 (a dehydrating agent) to produce an acid-base indicator 'Y'.
02mechanism
Two molecules of phenol (carbolic acid) condense with one molecule of phthalic anhydride at the para-position with elimination of one molecule of H2O catalyzed by conc. H2SO4.
03product
The resulting compound is phenolphthalein ('Y'), which is widely used as an acid-base indicator. Thus, 'X' is carbolic acid (phenol) and 'Y' is phenolphthalein.
✓verify
Phenolphthalein is an acid-base indicator (colorless in acid, pink in base), and carbolic acid is the common name for phenol. This matches Option (0).