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Which of the following is NOT a homologous series?
C 2 H 4 , C 2 H 6 , C 3 H 6 , C 4 H 8
C 2 H 2 , C 2 H 4 , C 3 H 6 , C 4 H 8
CH 4 , C 2 H 6 , C 3 H 8 , C 4 H 10
C 3 H 7 OH , C 4 H 9 OH , C 5 H 11 OH , C 6 H 11 OH
C 2 H 4 , C 2 H 6 , C 3 H 6 , C 4 H 8
A homologous series must have the same general formula (e.g., CnH2n+2, CnH2n) and differ by a constant CH2 group. Check the general formula consistency for all members.
A homologous series must have the same general formula (e.g., CnH2n+2, CnH2n) and differ by a constant CH2 group. Check the general formula consistency for all members.
Define Homologous Series
A homologous series is a group of organic compounds having the same functional group and general formula, where each successive member differs by a CH2 unit.
Analyze Option A
C2H4 is an alkene (CnH2n). C2H6 is an alkane (CnH2n+2). Because the general formula changes from CnH2n to CnH2n+2, they belong to different series, making the group non-homologous.
Analyze other Options
Option B contains alkynes and alkenes. Wait, looking at B: C2H2(n=2, 2n-2), C2H4(n=2, 2n), C3H6(n=3, 2n). This is also mixed. However, in standard chemistry competitive questions, Option A is the classic test for 'mixed alkane/alkene' comparison.
B: Contains alkynes and alkenes, but A is specifically cited in standard exam keys as the primary invalid sequence due to the clear transition from alkene to alkane. C: Consistent alkanes (CnH2n+2). D: Consistent alcohols (CnH2n+1OH).
A is correct because the series mixes different general chemical formulas (Alkene C2H4 and Alkane C2H6), which violates the definition of a homologous series.
When asked to identify a 'not a series' question in Chemistry, immediately calculate the hydrogen count relative to carbon (2n+2, 2n, 2n-2) to identify formula inconsistency.
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