Question

Why is 2 Ethylpentane incorrect?

Answer 1

The image shows a new chain which is actually 6 carbons long (hexane), and there's now a methyl group on the third carbon (3-methyl). Together, this is 3-methylhexane. Because of this, '2-ethylpentane' isn't a real structure, since drawing it will leave you with 3-methylhexane as shown in the image.