Question

what is the iupac name of this compound?
h₃c
|
c=c
| |
h h
ch₂ch₃
options: cis-3-pentene, cis-2-pentene, ethylpropene, 1-methyl-2-ethylpentene, trans-3-pentene

Explanation:

1. First, identify the parent chain: The longest carbon chain with the double bond is 5 carbons (pentene).
2. Number the chain to give the double bond the lowest number: The double bond is between carbon 2 and 3 (since numbering from the left, the double bond starts at C2; numbering from the right would start at C3, but we choose the lower number for the double bond, so the double bond is at C2 - C3? Wait, no, let's re - examine. The structure is \(H_3C - C(H)=C(H)-CH_2CH_3\). Let's number the carbons: C1: \(H_3C\), C2: \(C(H)\) (attached to C1 and double - bonded to C3), C3: \(C(H)\) (double - bonded to C2 and attached to \(CH_2CH_3\)), C4: \(CH_2\), C5: \(CH_3\). Wait, no, the parent chain is 5 carbons (pentene). The double bond is between C2 and C3? Wait, no, let's count the carbons in the parent chain. The carbon atoms are: \(H_3C\) (C1), \(C\) (C2) with \(H\), \(C\) (C3) with \(H\), \(CH_2\) (C4), \(CH_3\) (C5). Wait, no, the correct way is to find the longest chain with the double bond. The double bond is between two carbons. Let's list the atoms: The first carbon is \(H_3C\), the second is \(C\) (bonded to \(H_3C\), \(H\), and double - bonded to the third carbon), the third carbon is \(C\) (double - bonded to the second carbon, bonded to \(H\) and \(CH_2CH_3\)), the fourth carbon is \(CH_2\), the fifth is \(CH_3\). So the parent chain is 5 carbons (pentene). Now, the double bond is between C2 and C3? Wait, no, when numbering, we want the double bond to have the lowest possible number. Let's number from the left: C1: \(H_3C\), C2: \(C\) (with \(H\)), C3: \(C\) (with \(H\) and \(CH_2CH_3\)), C4: \(CH_2\), C5: \(CH_3\). The double bond is between C2 and C3. Now, check the configuration: the two \(H\) atoms are on the same side of the double bond (cis - configuration). So the name should be cis - 2 - pentene. Let's check the other options:

• cis - 3 - pentene: If we number from the right, the double bond would be at C3, but we number to give the double bond the lowest number, so it's at C2, so this is wrong.
• ethylpropene: The parent chain should be pentene (5 carbons) not propene (3 carbons), so this is wrong.
• 1 - methyl - 2 - ethylpentene: The parent chain is not correctly identified (the longest chain is 5 carbons, and there is no need for these substituents as the parent chain is 5 carbons), so this is wrong.
• trans - 3 - pentene: The configuration is cis (same side for \(H\) atoms) and the double bond number is wrong, so this is wrong.

Answer:

B. cis - 2 - pentene