The incorrect statement for IUPAC system of nomenclature is

Options

A. In an organic compound, the longest carbon chain Is always selected for assigning the root word
B. there Is no campound with the name 3-ethyl pentane
C. out of `-NH_2` and `-OH` groups present in an organic camopund, `-NH_2` Is treated as substituent.
D. different alkyl groups are written alphabetically while writting the IUPAC name

Answer

To determine the incorrect statement regarding the IUPAC system of nomenclature, let's analyze the provided statements step by step.

### Step 1: Analyze the First Statement
**Statement:** In an organic compound, the longest carbon chain is always selected for assigning the root word.

**Analysis:** This statement is correct. In IUPAC nomenclature, the longest continuous carbon chain is chosen as the parent chain, and the root word is derived from the number of carbon atoms in this chain.

### Step 2: Analyze the Second Statement
**Statement:** There is no compound with the name 3-ethylpentane.

**Analysis:** This statement is incorrect. The structure of 3-ethylpentane can be drawn, and it is a valid compound. The longest chain is pentane (5 carbon atoms), and the ethyl group (2 carbon atoms) can indeed be attached at the 3rd carbon of the pentane chain. Therefore, this statement is false.

### Step 3: Analyze the Third Statement
**Statement:** Out of NH2 and OH groups present in an organic compound, NH2 group is treated as a substituent.

**Analysis:** This statement is correct. In the case of multiple functional groups, the one with higher priority (in this case, the OH group) is treated as the principal functional group, while the NH2 group is treated as a substituent.

### Step 4: Analyze the Fourth Statement
**Statement:** Different alkyl groups are written alphabetically while writing the IUPAC name.

**Analysis:** This statement is correct. When naming compounds with multiple substituents, the substituents are listed in alphabetical order regardless of their position on the carbon chain.

### Conclusion
The incorrect statement regarding the IUPAC system of nomenclature is the second statement: "There is no compound with the name 3-ethylpentane."

### Final Answer
**Incorrect Statement:** There is no compound with the name 3-ethylpentane.

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