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Question: The IUPAC name of the compound $CH_3-CH-C-CH_2CH_3$ $\quad | \quad |$ $CH_3 \quad CH_2$...

The IUPAC name of the compound

CH3−CH−C−CH2CH3CH_3-CH-C-CH_2CH_3 ∣∣\quad | \quad | CH3CH2CH_3 \quad CH_2

A

2-methyl-3 ethyl-3-butene

B

2-ethyl-3-methylbut-1-ene

C

2-(1-methylethyl)but-1-ene

D

2-Isopropylbut-1-ene

Answer

2-ethyl-3-methylbut-1-ene

Explanation

Solution

To determine the IUPAC name of the compound, we first need to correctly interpret its structure.

The given structure is: CH3−CH−C−CH2CH3CH_3-CH-C-CH_2CH_3 ∣∣\quad | \quad | CH3CH2CH_3 \quad CH_2

This representation indicates that:

  1. The CH3 group below CH is a substituent on that CH. So, it's CH(CH3).
  2. The CH2 group below C indicates a double bond between C and CH2. So, it's C=CH2. (If it were a single bond, the carbon C would have only 3 bonds, which is impossible, or it would be written as CH2-C or C-CH2 in a linear fashion).

Therefore, the expanded structure of the compound is: CH3−CH(CH3)−C(=CH2)−CH2CH3CH_3-CH(CH_3)-C(=CH_2)-CH_2CH_3

Now, let's follow the IUPAC rules for naming alkenes:

  1. Identify the longest continuous carbon chain that includes both carbon atoms of the double bond. Let's number the double bond carbons as C1 and C2, ensuring the double bond gets the lowest possible numbers. The double bond is C=CH2. Let CH2 be C1 and C be C2.

    From C2, there are two branches extending:

    • Branch A: -CH2CH3 (an ethyl group, 2 carbons)
    • Branch B: -CH(CH3)CH3 (an isopropyl group, 3 carbons)

    Let's explore the possible parent chains containing the double bond:

    • Option 1 (Parent chain includes Branch A): If we include -CH2CH3 as part of the main chain, the chain would be CH2=C-CH2-CH3. Numbering: CH2=C-CH2-CH3 1 2 3 4 This is a 4-carbon chain, so the parent alkane is but-1-ene. The substituent on this chain is the -CH(CH3)CH3 (isopropyl) group, which is attached to C2. Name: 2-isopropylbut-1-ene (or 2-(1-methylethyl)but-1-ene). This chain has 1 substituent.

    • Option 2 (Parent chain includes Branch B): If we include -CH(CH3)CH3 as part of the main chain, the chain would be CH2=C-CH(CH3)-CH3. Numbering: CH2=C-CH(CH3)-CH3 1 2 3 4 This is also a 4-carbon chain, so the parent alkane is but-1-ene. The substituents on this chain are:

      • -CH2CH3 (ethyl group) attached to C2.
      • -CH3 (methyl group) attached to C3. Name: 2-ethyl-3-methylbut-1-ene. This chain has 2 substituents.

  2. Choose the parent chain based on the number of substituents. Both Option 1 and Option 2 give a 4-carbon parent chain (but-1-ene). When two chains of equal length are possible, the IUPAC rule states that the chain with the maximum number of substituents should be chosen as the parent chain. Option 1 has 1 substituent (isopropyl). Option 2 has 2 substituents (ethyl and methyl). Therefore, Option 2 is the correct choice for the parent chain.

  3. Name the substituents and arrange them alphabetically. The parent chain is but-1-ene. The substituents are ethyl at position 2 and methyl at position 3. In alphabetical order, ethyl comes before methyl.

  4. Construct the full IUPAC name. Combining the substituent names with their positions and the parent chain name: 2-ethyl-3-methylbut-1-ene

Comparing this with the given options:

  • ◯\bigcirc 2-methyl-3 ethyl-3-butene (Incorrect double bond position and numbering)
  • ◯\bigcirc 2-ethyl-3-methylbut-1-ene (Matches our derived name)
  • ◯\bigcirc 2-(1-methylethyl)but-1-ene (This corresponds to 2-isopropylbut-1-ene, which was ruled out as it has fewer substituents on the parent chain)
  • ◯\bigcirc 2-Isopropylbut-1-ene (Same as above, ruled out)

The final answer is 2-ethyl-3-methylbut-1-ene.

Explanation of the solution (minimal): The compound is CH3−CH(CH3)−C(=CH2)−CH2CH3CH_3-CH(CH_3)-C(=CH_2)-CH_2CH_3.

  1. Identify the longest carbon chain containing the double bond. Two 4-carbon chains are possible:
    • Chain 1: CH2=C−CH2−CH3CH_2=C-CH_2-CH_3. This chain has one substituent (isopropyl) at C2.
    • Chain 2: CH2=C−CH(CH3)−CH3CH_2=C-CH(CH_3)-CH_3. This chain has two substituents (ethyl at C2 and methyl at C3).
  2. According to IUPAC rules, if chains are of equal length, select the one with the maximum number of substituents. Chain 2 has 2 substituents, while Chain 1 has 1. Therefore, Chain 2 is the correct parent chain, but-1-ene.
  3. Number the parent chain to give the double bond the lowest possible locant (1).
  4. Identify and name the substituents: ethyl at C2 and methyl at C3.
  5. List substituents alphabetically: ethyl before methyl. The IUPAC name is 2-ethyl-3-methylbut-1-ene.