Most literature, formal and informal, imply after the Nitroalcohol is formed during a Nitroaldol condensation of a benzaldehyde and nitroalkane, a subsequent dehydration takes place to form the Nitroalkene. Other write ups on this reaction appear use Sodium Bisulfate to preserve the Hydroxyl group, although I do not know very much about that particular variation.
Curiously I stumbled upon this particular reaction using a phase transfer catalyst.Comprehensive Organic Reactions in Aqueous Media - 436 Page PDF
"The Henry (nitroaldol) reaction was reported under very mild reaction conditions in aqueous media using a stiochiometric amount of nitroalkane, and an aldehyde, in NaOH 0.025 M and in the presence of cetyltrimethylammonium chloride (CTACl) as cationic surfactant. Good to excellent yields of β-nitroalkanol are obtained. Under these conditions several functionalities are preserved, and side reactions such as retro-aldol reaction or dehydration of 2-nitroalcohols are avoided.
The relevant study:http://www.organic-chemistry.org/abstracts/literature/527.shtm
This last bit of the quote (in bold) had me wondering what causes the dehydration to the nitroalkene.... In the above study, does this simply not happen, due to the very small quantity of base? I read that alcohols are amphoteric... so this is my best guess. If there's not enough base to activate the hydroxyl group, then only very small quantities would dehydrate? If this is the case that could explain why yields were not 98%+ for the particular listed product. Assuming the PTC allowed for high reaction yields, the impurity could be the nitroalkene. The small quantity of base could still allow for some dehydration to occur.
Can anyone offer some knowledge? I haven't really learned much about base-catalyzed dehydration reactions... almost all of the ones I have encountered in literature have been acid-catalyzed and run at high temperatures, but this dehydration that happens during the henry reaction seems to even go over at room temperature.