A series of straight chain alkanes is given with one term missing. Choose the correct alternative from the options that will complete the series. ?, Hexane, Heptane, Octane

Introduction / Context:
This question is based on the systematic naming of straight chain alkanes in basic organic chemistry. The series lists Hexane, Heptane, and Octane, which are the alkanes with six, seven, and eight carbon atoms respectively. To find the missing term at the beginning of the list, we must recall the standard prefixes used in the IUPAC naming system and recognise the sequence of increasing carbon chain length.

Given Data / Assumptions:

• Given series: ?, Hexane, Heptane, Octane.
• Hexane is the alkane with 6 carbon atoms (C6H14).
• Heptane is the alkane with 7 carbon atoms (C7H16).
• Octane is the alkane with 8 carbon atoms (C8H18).
• The series is arranged in increasing order of the number of carbon atoms.

Concept / Approach:
Straight chain alkanes follow a clear naming pattern: Methane (1), Ethane (2), Propane (3), Butane (4), Pentane (5), Hexane (6), Heptane (7), Octane (8), Nonane (9), and so on. In the given series, names with prefixes Hex, Hept, and Oct appear consecutively. Therefore, the missing name immediately before Hexane should be the alkane with 5 carbon atoms, namely Pentane. Recognising the numeric prefixes Pent, Hex, Hept, and Oct is the key step in solving this question.

Step-by-Step Solution:
Step 1: Write the standard sequence of straight chain alkanes around the given range: Pentane (5), Hexane (6), Heptane (7), Octane (8), Nonane (9). Step 2: Observe that the series in the question lists Hexane, Heptane, and Octane, which correspond to 6, 7, and 8 carbon atoms respectively. Step 3: The missing term appears before Hexane, so it must be the alkane with 5 carbon atoms in the standard list. Step 4: The alkane with 5 carbon atoms is called Pentane, so Pentane is the correct term to complete the series.

Verification / Alternative check:
We can verify using the prefixes: Pent refers to five, Hex to six, Hept to seven, and Oct to eight. Since the series is clearly moving from lower to higher carbon numbers, there is no gap within the names provided. Checking the options, only Pentane fits exactly before Hexane within the well known sequence of alkane names. Benzene is not even an alkane, while names like Methane and Propane do not sit immediately before Hexane in that sequence.

Why Other Options Are Wrong:
Benzene is an aromatic compound and does not belong to the straight chain alkane series at all. Nonane represents the alkane with 9 carbon atoms and would appear after Octane, not before Hexane. Methane has only one carbon atom and is far earlier in the series, not just one step before Hexane. Propane has three carbon atoms and also does not lie adjacent to Hexane in the sequence. Hence, none of these can be the missing term directly preceding Hexane in an increasing sequence.

Common Pitfalls:
A frequent error is to answer based on familiarity with fuel related names such as Propane or Methane without recalling the full ordered list of alkanes. Some students may also confuse Benzene with alkanes because it is a familiar hydrocarbon name, but Benzene is structurally different and not part of this simple straight chain series. Always relate the prefix to the number of carbon atoms when tackling such questions.

Final Answer:
The missing term before Hexane in the straight chain alkane series is Pentane.