Question

The lowest alkane which has ethyl group as substituent has IUPAC name 

(a) 2-ethylpropane 

(b) 2-ethylbutane 

(c) 3-ethylpentane

(d) none of these

Answer 1

Correct option is (c) 3-ethylpentane

To determine the IUPAC name of the lowest alkane that has an ethyl group as a substituent, we can follow these steps:

Step 1: Identify the Alkane with the Ethyl Group

The ethyl group (C₂H₅) can only be a substituent on a carbon chain. The simplest alkane that can accommodate an ethyl group must have at least 4 carbon atoms in the main chain because an ethyl group itself has 2 carbon atoms.

Step 2: Consider the Possible Alkanes

The alkanes we can consider are:

- Propane (C₃H₈)

- Butane (C₄H₁₀)

- Pentane (C₅H₁₂)

Step 3: Analyze Each Alkane

1. Propane (C₃H8):

If we try to attach an ethyl group to propane, we would need to have a total of 5 carbon atoms (3 from propane + 2 from ethyl). This means we cannot have an ethyl group as a substituent on propane.

2. Butane (C₄H₁₀):

If we consider butane, we can attempt to place the ethyl group. However, if we try to place the ethyl group on butane, we would again exceed the number of carbons in the longest chain. The longest chain remains butane (4 carbons), and adding an ethyl group would not yield a valid structure.

3. Pentane (C₅H₁₂):

Now, if we consider pentane, we have 5 carbon atoms. We can place the ethyl group on the third carbon of pentane. This gives us a structure where the main chain remains pentane, and we have an ethyl substituent.

Step 4: Naming the Compound

When we place the ethyl group on the third carbon of pentane, we name the compound as "3-ethylpentane". This is the correct IUPAC name for the compound.

Conclusion

The lowest alkane which has an ethyl group as a substituent has the IUPAC name 3-ethylpentane.