IIT-JEE 2012 · Paper 1, Chemistry question 25

Crossed aldols. Acetaldehyde has three α-hydrogens. Each is replaced in turn by $\ce{CH2OH}$ through an aldol addition to formaldehyde (which has no α-H and can only act as the electrophile):
$$\ce{CH3CHO ->[HCHO] HOCH2CH2CHO ->[HCHO] (HOCH2)2CHCHO ->[HCHO] (HOCH2)3CCHO}.$$

Final step. $\ce{(HOCH2)3CCHO}$ has no α-H, so with the fourth HCHO and conc. NaOH it undergoes a crossed Cannizzaro reaction to pentaerythritol, $\ce{C(CH2OH)4}$, and formate. That step is not an aldol.

Number of aldol reactions: 3.

(C)