The IUPAC name of the compound CH3CH(CH3)CHCH2 is 3-methyl but-1-ene.
To name this compound, we first locate the longest continuous carbon chain, which is four carbons in this case. The parent name is butene. We number the chain so that the double bond has the lowest possible number, which is at carbon 1.
The next step is to identify the substituents on the main chain. There is one methyl group attached to the third carbon atom (counting from the double bond), and no other substituents. Therefore, the name of the compound is 3-methyl but-1-ene.