The IUPAC name of `CH_(2)=CH- underset(CH_(2)-CH_(2)-CH_(3))underset(|)CH-CH_(2)-CH_(3)` is-

Explanation

To determine the IUPAC name of the compound `CH₂=CH-CH₂-CH₂-CH₃` with a substituent `CH-CH₂-CH₃`, we will follow these steps:

### Step 1: Identify the Longest Carbon Chain
First, we need to identify the longest continuous carbon chain that includes the double bond. In this case, the longest chain consists of 6 carbon atoms.

### Step 2: Number the Carbon Chain
Next, we will number the carbon chain starting from the end closest to the double bond to give the double bond the lowest possible number. The numbering will be as follows:
1. C1: CH₂ (part of the double bond)
2. C2: CH (part of the double bond)
3. C3: CH₂
4. C4: CH₂
5. C5: CH₃
6. C6: CH (substituent)

### Step 3: Identify and Name the Substituents
Now, we need to identify the substituents on the main carbon chain. Here, we have an ethyl group (C2H5) attached to C3.

### Step 4: Combine the Information
Now we can combine the information to write the IUPAC name. The main chain is a hexene (due to the double bond) and the substituent is an ethyl group at C3.

### Step 5: Write the Final IUPAC Name
The final IUPAC name is:
- 3-Ethylhex-1-ene

### Summary of the Steps:
1. Identify the longest carbon chain.
2. Number the carbon chain from the end nearest to the double bond.
3. Identify and name the substituents.
4. Combine the information to form the IUPAC name.