The IUPAC name of the compound CH3CH(CH3)CHCH2 is

Analyze the question: We need the IUPAC name of CH3CH(CH3)CHCH2. Written out in full, the structure is CH3–CH(CH3)–CH=CH2. The formula as typed leaves out the double bond, but the only sensible reading is that the last two carbons are CH=CH2, since CH and CH2 would otherwise have unfilled valences.

Concept: To name an alkene:

1. Find the longest carbon chain that contains the double bond.
2. Number the chain from the end that gives the double bond the lowest possible number.
3. Name and locate the substituents using that numbering.

Solution:

Structure: CH3–CH(CH3)–CH=CH2

1. Longest chain containing the C=C: 4 carbons, so the parent is butene.
2. Numbering from the right, so the double bond gets the lowest number: • C1 = CH2 • C2 = CH (the double bond is between C1 and C2, so but-1-ene) • C3 = CH(CH3), which carries the methyl group • C4 = CH3
3. The methyl group is on C3.

Name: 3-methylbut-1-ene

If we numbered from the left instead, the double bond would be at position 3 and the methyl at 2. The double bond must get the lowest number, so we reject that direction.

Check the options:

• A (2-methylbut-1-ene) has the methyl on the double-bonded carbon, which is CH2=C(CH3)CH2CH3. This is not our compound.
• B (2-methylbut-2-ene) is (CH3)2C=CHCH3. This is not our compound.
• C (3-methylbut-1-ene) matches our structure.
• D (3-methylbut-2-ene) is not a valid name, because numbering would start from the other end to give 2-methylbut-2-ene.

Tip: Number for the double bond first, then for the substituents. A quick check is that the double bond position must be the lowest possible number. Also, a name like "3-methylbut-2-ene" is always wrong, since renumbering would give a lower locant for the substituent.