Primary amide $RCONH_{2}$ shows two absorption bonds between 3400-3500 $cm^{-1}$ on treatment with $P_{2}O_{5}$, the compound formed absorbs at:

Question

Accepted Answer

When a primary amide ($RCONH_{2}$) is treated with phosphorus pentoxide ($P_{2}O_{5}$), it undergoes dehydration to form a nitrile ($RCN$). This transformation is a well-known reaction in organic chemistry where the amide loses water to form the corresponding nitrile.

1. **Explanation for Option 3 ($2256~cm^{-1}$):**
   - Nitriles are characterized by a strong absorption band in the infrared (IR) spectrum due to the carbon-nitrogen triple bond ($C \equiv N$). This absorption typically occurs around $2250~cm^{-1}$ to $2260~cm^{-1}$. Therefore, the compound formed after the dehydration of the primary amide by $P_{2}O_{5}$ is a nitrile, which absorbs at $2256~cm^{-1}$. This aligns perfectly with the given correct answer, Option 3.

2. **Explanation for why other options are incorrect:**
   - **Option 1 ($3500~cm^{-1}$):** This region is typically associated with O-H or N-H stretching vibrations. Since the amide is converted to a nitrile, there are no O-H or N-H bonds present in the nitrile, making this option incorrect.
   - **Option 2 ($3250~cm^{-1}$):** This region is also associated with N-H stretching vibrations, which are present in amides but not in nitriles. Since the amide is converted to a nitrile, this absorption would not be observed, making this option incorrect.
   - **Option 4 ($1650~cm^{-1}$):** This region is typically associated with C=O stretching vibrations found in amides. However, in the conversion to a nitrile, the C=O bond is lost, and thus this absorption would not be present in the nitrile, making this option incorrect.

In conclusion, the correct answer is Option 3 ($2256~cm^{-1}$) because it corresponds to the characteristic absorption of the carbon-nitrogen triple bond in nitriles, which is the product formed from the dehydration of the primary amide with $P_{2}O_{5}$.