A homologous series is a series of organic compounds which belong to the same family (i.e. possess same functional group) and show similar chemical properties. The members of this series are called homologous and differ from each other by the number of `CH_(2)`, units in the main carbon chain
Which of the following is not the property of a homologous series?

Options

A. They show similar chemical properties.
B. They differ by 14 units by mass.
C. They all contain double bond
D. They can be represented by a general formula.

Explanation

To determine which property is not characteristic of a homologous series, we need to analyze the properties of a homologous series of organic compounds. Here’s a step-by-step solution:

### Step 1: Understand the Definition of a Homologous Series
A homologous series is a group of organic compounds that have the same functional group and similar chemical properties. The members of this series differ from each other by a constant unit of `CH₂`.

**Hint:** Recall that functional groups define the chemical behavior of compounds in a homologous series.

### Step 2: Identify the Key Properties of a Homologous Series
1. **Same Functional Group:** All members have the same functional group, which leads to similar chemical properties.
2. **Gradation in Physical Properties:** There is a gradual change in physical properties (like boiling point and melting point) as the molecular size increases.
3. **Difference by `CH₂`:** Successive members differ by one `CH₂` unit, resulting in an increase in molecular mass by 14 atomic mass units.
4. **General Formula:** They can be represented by a general formula (e.g., alkanes are represented by CₙH₂ₙ₊₂).

**Hint:** Think about what makes a property characteristic of a series and what might be an exception.

### Step 3: Analyze the Options Given
The question asks which of the following is NOT a property of a homologous series. Let’s consider the options:
- **Option 1:** They show similar chemical properties.
- **Option 2:** They differ by `CH₂` units.
- **Option 3:** They all contain double bonds.
- **Option 4:** They can be represented by a general formula.

**Hint:** Look for the option that contradicts the definition or properties of a homologous series.

### Step 4: Determine the Incorrect Property
From the analysis:
- Options 1, 2, and 4 are true characteristics of a homologous series.
- **Option 3** states that all members contain double bonds, which is not true. Members of a homologous series can have single, double, or triple bonds depending on the specific compound.

### Conclusion
The property that is NOT characteristic of a homologous series is that they all contain double bonds.

**Final Answer:** The option that is NOT a property of a homologous series is: **They all contain double bonds.**

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