Aldehyde 8:

Solution

In a balanced reaction sequence, the total number of electrons is preserved. In organic chemistry, one often focuses on the organic substrate and ignores electrons gained or lost by inorganic oxidants and reductants, respectively. When two reaction sequences start at the same point and end at the same point, then the total electron change for the organic substates must be the same. When benzaldehyde 1 is converted into its dithiane 2, two electonegative oxygens are exchanged for two sulfur atoms; no net change. [If you need help with oxidation levels, click here.] Likewise, the proton exchange 2 ---> 5 involves no net electron change. The S_N2 reaction of 5 with ethyl iodide simply substitutes one electronegative group, the iodine, for another, the carbanion 5. The step 3 ---> 4 is the reverse of 1 ---> 2. For the Grignard route, the conversion of ethyl iodide into its Grignard reagent is a two electron reduction. [If the Grignard reagent is protonated (hydrogen is more electropositive than carbon) then ethane is formed, which is at a lower oxidation level than ethyl iodide.] The conversion of alcohol 6 to ketone 5 is a two electron oxidation. The critical feature of both reaction sequences is the carbon-carbon bond forming process. That difference is illustrated here.