Question

evaluate the number of carbon for the ffg. pairs by writing G if greater, L if less than or E if equal
1. butene E sym dimethylethylene
sol'n: butene has 4 carbons and sym dimethylethylene also has 4 carbons so it's equal. Am i right?
2. methylbutane L isopentane
methylbutane has 6 carbons? and isopentane 7? How can i compute the numbers of carbons? please help me

Answer 1

For all four of these, it is only the prefix for the longest chain and number of substituents that matters in determining the number of carbons.

In #1, butene has the prefix "but-", so therefore has four carbons in the longest chain.
Dimethylethylene also has four carbons, one from each methyl group ("di" - two, "methyl" - one carbon) and two from the ethylene backbone ("ethyl" - two carbons).

In #2, methylbutane has five carbons, one from the methyl group, and four from the butane chain.
Isopentane also has five carbons. The prefix "iso-" refers to a configuration of the molecule, and does not change the number of carbons present, just their arrangement. The second prefix "pent-" means five carbons.