Question

An optically active alkene having molecular formula C$_8$H$_{16}$ gives acetone as one of the products on ozonolysis. The structure of the alkene is

• A.
• B.
• C.
• D.

Hint:

Using a simple frame or just bolding for the box Key Points: Ozonolysis: Cleaves C=C double bonds. $R_1R_2$C=CR$_3R_4 \xrightarrow{\text{O_3; \text{reductive workup R_1R_2$C=O + O=CR$_3R_4$. To yield acetone (CH$_3$COCH$_3$), one side of the alkene must be =C(CH$_3$)$_2$. Optical Activity: Requires the molecule to be chiral (non-superimposable on its mirror image). Usually presence of one or more chiral centers (carbon atom bonded to four different groups) and lack of internal symmetry planes. Molecular Formula: Check if the sum of carbons and hydrogens matches the given formula (C$_n$H$_{2n$ for simple alkenes).

Answer 1

The Correct Option is B

We need to find an alkene that meets three criteria: (A) Molecular formula is C₈H₁₆. (B) Gives acetone (CH₃COCH₃) upon ozonolysis (followed by reductive workup, usually implied). (C) Is optically active (possesses a chiral center and is not a meso compound). Let's analyze each option:

• (A) 2,3-Dimethylhex-2-ene:
  Formula: C₈H₁₆ (Correct).
  Ozonolysis: Cleaves the double bond. C(CH₃)₂= gives acetone. =C(CH₃)CH₂CH₂CH₃ gives 2-pentanone. So, it yields acetone.
  Optical Activity: No chiral center. Carbon atoms are either sp² hybridized or have two identical groups attached (e.g., CH₃ groups on C2, H atoms on CH₂s). It is optically inactive.
• (B) 3,4-Dimethylhex-2-ene:
  Formula: C₈H₁₆ (Correct).
  Ozonolysis: C(CH₃)₂= gives acetone. =CHCH(CH₃)C₂H₅ gives 3-methylpentan-2-al. So, it yields acetone.
  Optical Activity: The carbon atom at position 4 (C4) is bonded to four different groups: H, CH₃, C₂H₅ (ethyl), and the C=CH(C(CH₃)₂) group.
  Thus, C4 is a chiral center. The molecule is optically active.
• (C) 3,4-Dimethylhex-3-ene:
  Formula: C₈H₁₆ (Correct).
  Ozonolysis: Cleaves the double bond. C(C₂H₅)(H)= gives propanal. =C(CH₃)CH(CH₃)₂ gives 3-methylbutan-2-one. Does not yield acetone.
• (D) 3-Ethyl-2-methylpent-2-ene:
  Formula: C₈H₁₆ (Correct).
  Ozonolysis: C(CH₃)₂= gives acetone. =C(C₂H₅)₂ gives 3-pentanone. So, it yields acetone.
  Optical Activity: No chiral center. The molecule is optically inactive.

Only option (B) satisfies all three conditions: has the formula C₈H₁₆, yields acetone upon ozonolysis, and is optically active due to the chiral center at C4.