Introduction

The question asks for the number of structural isomers for the molecular formula C₆H₁₄. Structural isomers are molecules that have the same molecular formula but different structural formulas (different arrangements of atoms).

The molecular formula C₆H₁₄ fits the general formula for alkanes, CₙH₂ₙ₊₂, where n=6 (C₆H₂(₆)₊₂ = C₆H₁₄). Therefore, we need to find all the possible non-cyclic, saturated hydrocarbon structures with 6 carbon atoms.

Step-by-step Derivation of Isomers

We will systematically determine the isomers by considering the longest possible carbon chain (the parent chain) and the possible arrangements of substituent groups.

Step 1: Parent chain with 6 carbons

There is only one way to arrange 6 carbon atoms in a continuous, unbranched chain.

1. CH₃-CH₂-CH₂-CH₂-CH₂-CH₃
   • Name: n-hexane

Step 2: Parent chain with 5 carbons (Pentane)

Now, we consider a main chain of 5 carbons. The 6th carbon atom must be a substituent, which will be a methyl group (-CH₃).

The pentane chain is C-C-C-C-C. We can number the carbons 1 to 5.

• Placing the methyl group on carbon 1 or 5 would result in n-hexane (it would just extend the chain), so these are not new isomers.
• Placing the methyl group on carbon 2: 2. CH₃-CH(CH₃)-CH₂-CH₂-CH₃ * Name: 2-methylpentane
• Placing the methyl group on carbon 3: 3. CH₃-CH₂-CH(CH₃)-CH₂-CH₃ * Name: 3-methylpentane
• Placing the methyl group on carbon 4 is equivalent to placing it on carbon 2 (just numbering from the other end of the chain), so it's not a new isomer.

Step 3: Parent chain with 4 carbons (Butane)

Next, we consider a main chain of 4 carbons. This leaves two carbon atoms to be placed as substituents. These can be two separate methyl groups (-CH₃) or one ethyl group (-CH₂CH₃).

• If we add an ethyl group to the butane chain (C-C-C-C), placing it on carbon 1 or 4 would extend the chain to n-hexane. Placing it on carbon 2 or 3 results in 3-methylpentane, which we have already found. So, no new isomers with an ethyl group.
• Let's add two methyl groups.
  • Both methyl groups on the same carbon: They cannot be on the end carbons (1 or 4) as that would create a pentane chain. We must place them on an inner carbon (2 or 3).
    • Placing both on carbon 2: 4. (CH₃)₃C-CH₂-CH₃ * Name: 2,2-dimethylbutane
    • Placing both on carbon 3 is equivalent to placing them on carbon 2 due to symmetry.
  • Methyl groups on different carbons: We can place them on carbons 2 and 3.
    • Placing one on carbon 2 and one on carbon 3: 5. (CH₃)₂CH-CH(CH₃)₂ * Name: 2,3-dimethylbutane
    • Placing them on (1,2), (1,3), or (1,4) would result in isomers we have already identified (e.g., 1,2-dimethylbutane is 3-methylpentane).

Step 4: Parent chain with 3 carbons (Propane)

If we take a 3-carbon chain, we have 3 carbons left to place as substituents. This is not possible without creating a longer chain that we have already considered. For example, trying to create 2,2-dimethylpropane as the core and adding a methyl group would result in 2,2-dimethylbutane.

Conclusion

By systematically analyzing the possible carbon skeletons, we have found a total of 5 distinct structural isomers for C₆H₁₄.

The 5 isomers are:

1. n-hexane
2. 2-methylpentane
3. 3-methylpentane
4. 2,2-dimethylbutane
5. 2,3-dimethylbutane

Thus, the total number of structural isomers is 5. This corresponds to option C.