Understanding Homologous Series
In organic chemistry, a homologous series is a group of compounds that have the same functional group and similar chemical properties but differ by a constant unit.
Key Characteristics of Homologous Series:
- Each member of the series can be derived from the previous one by adding a specific number of carbon (C) and hydrogen (H) atoms.
- The difference between successive members is usually a simple structural addition.
Why the Correct Answer is D: CH2
When considering the neighbors in a homologous series:
- Homologous Series Composition: The difference between any two adjacent compounds is consistent and can be denoted as a specific molecular formula.
- Example of Differences:
- If we take alkanes as an example, the general formula is CnH2n+2.
- The difference between two consecutive alkanes (like methane CH4 and ethane C2H6) is CH2.
Options Analysis:
- Option A: CH - This would imply a difference of one carbon and one hydrogen, which is not consistent with how compounds progress in a homologous series.
- Option B: CH3 - This represents a methyl group, but the addition of CH3 would not reflect the incremental increase between neighbors.
- Option C: CH4 - This indicates a complete molecule rather than a unit difference, making it unsuitable.
- Option D: CH2 - This is the correct choice. It signifies that each successive member of the series differs by one methylene group (–CH2–), which is the defining characteristic of how these compounds are structured and classified.
Conclusion
Thus, the correct answer is D: CH2, as it accurately represents the consistent and systematic difference in molecular structure between two neighboring compounds in a homologous series.