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Step-by-Step Solution
Step 1: Identify the Reaction
The process described (treatment of an alkene with ozone followed by reduction using Zn/H2O) is known as ozonolysis. In ozonolysis, the carbon–carbon double bond ($\text{C=C}$) of the alkene is cleaved, and each carbon of the double bond is converted into a carbonyl group (either aldehyde or ketone, depending on the substituents).
Step 2: Break the Double Bond
Consider the alkene: CH3–CH=CH–CH3 (2-butene). Upon ozonolysis, the double bond breaks at the following position:
CH3–CH(1) = CH(2)–CH3
Where carbon (1) and carbon (2) are the ones originally forming the double bond.
Step 3: Form the Carbonyl Products
Each of these carbons involved in the double bond becomes bonded to an oxygen (from the ozone). The products after the oxidative cleavage (and upon reduction) are determined by whether the carbon is primary, secondary, or tertiary:
Carbon (1) is secondary (it is attached to two other carbons), so cleavage forms an aldehyde or ketone. Here, it only has one substituent that differs from a hydrogen (CH3 and H), so it forms an aldehyde.
Carbon (2) is also secondary, similarly forming an aldehyde.
But for 2-butene, each half is the same. Consequently, the ultimate product from each side is the same aldehyde, acetaldehyde (CH3CHO).
Step 4: Confirm the Final Product (B)
After reductive work-up with Zn/H2O, the product obtained is CH3CHO (acetaldehyde). This matches the final compound B in the sequence.
Conclusion
The compound B obtained from the ozonolysis of CH3–CH=CH–CH3 under the given conditions is acetaldehyde (CH3CHO).
Hence, the correct answer is CH3CHO.
