Pentane (C5H12) can exist in how many distinct structural isomeric forms?

Introduction / Context:
Structural isomerism is an important concept in organic chemistry, where compounds with the same molecular formula can have different arrangements of atoms, leading to different physical and chemical properties. Pentane has the molecular formula C5H12 and is the simplest alkane that exhibits more than one structural isomer. This question asks how many distinct structural isomers pentane has, which tests your understanding of basic alkane isomerism.

Given Data / Assumptions:
- The compound under discussion is pentane, with formula C5H12.
- Structural isomers refer to compounds with the same molecular formula but different connectivity of carbon atoms.
- Only constitutional isomers are considered, not stereoisomers, because alkanes of this type have no chiral centres in their simplest forms here.
- Standard naming conventions for alkanes are assumed, such as n-pentane, isopentane, and neopentane.

Concept / Approach:
To find structural isomers of pentane, we must arrange five carbon atoms in different ways while maintaining the valency rules of carbon. One arrangement is a straight chain of five carbons, called n-pentane. A second arrangement has a four carbon chain with a one carbon side group, giving 2-methylbutane, often called isopentane. A third arrangement has a three carbon chain with two one carbon side groups on the central carbon, giving 2,2-dimethylpropane, also known as neopentane. No other arrangement of five carbon atoms produces a distinct connectivity pattern, so there are exactly three structural isomers for pentane.

Step-by-Step Solution:
Step 1: Draw the straight chain structure with five carbons in a row: CH3–CH2–CH2–CH2–CH3. This is n-pentane. Step 2: Rearrange the chain to four carbons with one methyl group attached to the second carbon: a CH3 group on a four carbon backbone. This gives 2-methylbutane. Step 3: Form a three carbon chain with two methyl groups attached to the central carbon, giving 2,2-dimethylpropane. Step 4: Check that any other drawings can be rotated or flipped to become one of the three already found, meaning no further distinct connectivities exist. Step 5: Conclude that there are three structural isomers of pentane.

Verification / Alternative check:
Organic chemistry textbooks list the structural isomers of alkanes and explicitly mention that C5H12 has three isomers: n-pentane, isopentane, and neopentane. For C4H10 there are two isomers, and for C3H8 there is only one. This pattern is often presented in tabular form to show the growing number of isomers with increasing carbon count. These authoritative listings confirm that three is the correct number of structural isomers for pentane.

Why Other Options Are Wrong:
Two structural isomers: This number applies to butane, C4H10, not to pentane.
One structural isomer: This is true for propane, C3H8, but not for pentane, which is more complex.
Four structural isomers: This overcounts, because any additional structure proposed for pentane turns out to be identical to one of the three known isomers after rotation or reflection.

Common Pitfalls:
Students sometimes miscount isomers because they treat rotated or flipped versions of the same structure as different, or they forget to check for symmetry that makes two drawings equivalent. Another frequent error is to assume that the number of isomers always increases by one when moving from one alkane to the next, which is not a reliable rule. The safest approach is to systematically construct and check possible structures or to memorise the isomer counts for the first few alkanes, including the fact that pentane has three structural isomers.

Final Answer:
Pentane, C5H12, can exist in Three structural isomers: n-pentane, 2-methylbutane, and 2,2-dimethylpropane.