Q.In which of the following reactions is a new carbon-carbon bond NOT formed? a) Aldol condensation b) Friedel-Crafts reaction c) Kolbe's reaction d) Wolff-Kishner reduction

Catalytic hydrogenation (reduction) of an aldehyde adds H2 across the C=O bond to give the corresponding primary alcohol.

Propanal is CH3-CH2-CHO. Catalytic reduction means addition of hydrogen gas in the presence of a metal catalyst (Ni, Pd or Pt):

CH3CH2CHO + H2 --(Ni/Pd/Pt catalyst)--> CH3CH2CH2OH

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Full reduction of a C=O group all the way to CH$_2$ (carbonyl → methylene) is the Clemmensen reduction, done with zinc amalgam and concentrated HCl.

Acetophenone (C$_6H_5$-CO-CH$_3$) needs its carbonyl oxygen completely removed to give ethylbenzene (C$_6H_5$-CH$_2$-CH$_3$) — this is a deoxygenation, not just a reduction to an alcohol.

$C_6H_5COCH_3 \xrightarrow[\text{conc. HCl}]{Zn(Hg)} C_6H_5CH_2CH_3$

Zn(Hg)/conc. HCl (Clemmensen reduction) is specifically used for this complete carbonyl-to-methylene reduction, and works well for acid-stable substrates like this aromatic ketone.

The other options are ruled out because they stop at the alcohol stage, not the alkane: …