Which among the following reactions forms acid anhydride?

Which among the following reactions forms acid anhydride?
  1. $\mathrm{R}-\mathrm{COOH}$ treated with $\mathrm{NaHCO}_{3(\mathrm{aq})}$
  2. $\mathrm{R}-\mathrm{COOH}$ treated with $\mathrm{PCl}_5$
  3. $\mathrm{R}-\mathrm{COOH}$ (excess) heated in presence of $\mathrm{P}_2 \mathrm{O}_5$
  4. $\mathrm{R}-\mathrm{COOH}$ treated with $\mathrm{R}^{\prime}-\mathrm{OH}$ and heated

Asked in: MHT CET 2022 (06 Aug Shift 2)

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