Understanding Propanenitrile Chemical Formula

The question asks for the correct chemical formula of Propanenitrile. To determine this, we need to understand the IUPAC naming convention for nitriles and apply it.

Nitrile Nomenclature Explained

Nitriles are organic compounds characterized by the presence of a cyano group, which has the functional group structure $R-C \equiv N$. In IUPAC nomenclature, the name of a nitrile is derived from the alkane that has the same number of carbon atoms as the longest carbon chain containing the cyano group. The carbon atom of the cyano group is included and numbered as the first carbon atom in the chain.

Breaking Down "Propanenitrile"

• The prefix "Propane" indicates a parent chain of three (3) carbon atoms.
• The suffix "nitrile" indicates the presence of the $-C \equiv N$ functional group.

Following the IUPAC rules, the carbon atom within the $-C \equiv N$ group is counted as part of the main three-carbon chain. Therefore, Propanenitrile has a structure where the cyano group is attached to a two-carbon chain ($C_2H_5$).

The structure can be represented as:

$CH_3CH_2C \equiv N$

Deriving the Molecular Formula

By counting the atoms in the structure $CH_3CH_2C \equiv N$:

• Carbon atoms: 1 (from $CH_3$) + 1 (from $CH_2$) + 1 (from $C \equiv N$) = 3 carbons ($C_3$)
• Hydrogen atoms: 3 (from $CH_3$) + 2 (from $CH_2$) = 5 hydrogens ($H_5$)
• Nitrogen atoms: 1 nitrogen ($N_1$)

So, the molecular formula is $C_3H_5N$.

Analyzing the Options

Let's examine each option provided:

• Option 1: $CH_3CN$ - This formula represents a molecule with 1 carbon in the methyl group and 1 carbon in the cyano group, totaling 2 carbons. This corresponds to Ethanenitrile (commonly known as Acetonitrile), with the formula $C_2H_3N$.
• Option 2: $C_2H_5CN$ - This formula explicitly shows an ethyl group ($C_2H_5$) attached to the cyano group ($-CN$). The structure is $CH_3CH_2CN$. This combination has a total of 2 + 1 = 3 carbon atoms, 5 hydrogen atoms, and 1 nitrogen atom. This correctly represents Propanenitrile.
• Option 3: $C_3H_7CN$ - This formula suggests a propyl group ($C_3H_7$) attached to the cyano group. The structure would be $CH_3CH_2CH_2CN$ (Butanenitrile) or a branched isomer. This molecule contains a total of 3 + 1 = 4 carbon atoms, not 3. Therefore, it represents Butanenitrile ($C_4H_7N$).
• Option 4: $C_3H_5N$ - As derived above, this is the correct molecular formula for Propanenitrile ($CH_3CH_2CN$).

Conclusion on Correct Formula

Both Option 2 ($C_2H_5CN$) and Option 4 ($C_3H_5N$) relate to Propanenitrile. Option 4 presents the overall molecular formula, while Option 2 represents the structure as an ethyl group ($C_2H_5$) bonded to the nitrile functional group ($-CN$). In many contexts, representing the formula as $R-CN$ is common practice to emphasize the functional group. Given the options, $C_2H_5CN$ explicitly describes the composition in terms of the alkyl group attached to the nitrile functional group, which is a standard way to represent Propanenitrile.