Why is But-3-ene incorrect?

Answer 1

Since the prefix "but" stands for 4 carbons, it is not possible to have but-3-ene because that says that the double bond is located between carbon number 3 and 4. This would then be the same as counting from the other end and getting but-1-ene. Since you always count so as to get the lowest possible number, the compound would be named but-1-ene, and nto but-3-ene.

C-C-C=C can be written as C-C-C=C

1..2..3..4..............................4..3..2..1...

but-3-ene............................but-1-ene...

incorrect..............................correct.....