In some variations of the organic reduction by DIBAL-H, I've seen hexane used, as in this equation from this website;

organic reduction by DIBAL-H

I've looked it up, but I haven't seen any explanation for why hexane's used there. It's also seen in the action of DIBAL-H on alkyl nitriles to give aldehydes with ammonia.

Can anyone help me with why it's used in both cases?

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    $\begingroup$ It violently reacts with water and is soluble in hydrocarbons like hexane. Hexane is just a solvent here $\endgroup$ Apr 27 '21 at 5:07
  • $\begingroup$ @SafdarFaisal: that works for the first case, thanks. For the alkyl nitrile reaction, apparently there's this idea of an acid being formed instead if you don't use hexane first or something. Can you elaborate how that works? It wasn't very clear where I got it from. $\endgroup$
    – harry
    Apr 27 '21 at 5:17
  • 2
    $\begingroup$ Hydrolysis of alkyl nitriles lead to formation of acids, hexane prevents that side reaction from happening since there is no water to hydrolyse $\endgroup$ Apr 27 '21 at 5:22
  • $\begingroup$ You should give a reference for your second case. $\endgroup$ Apr 27 '21 at 5:22
  • 1
    $\begingroup$ Also, reduction by DIBAL-H is not a type of Rosenmund reduction. $\endgroup$ Apr 27 '21 at 5:34

Unlike other reducing metal hydrides (e.g., $\ce{NaBH4}$ and $\ce{LiAlH4}$), diisobutylaluminum hydride (DIBAL-H) is a liquid at room temperature and dissolve in many hydrocarbons such as toluene and hexanes, which also have very low freezing points. For example, hydrocarbons toluene and hexanes both have freezing points around $\pu{-95 ^\circ C}$. Thus, they are good solvents for reaction involving reduction by DIBAL-H at low temperatures such as the example given in the question ($\pu{-70 ^\circ C}$). As shown in the equation, the low reaction temperatures help stop the reduction at aldehyde level. At room temperature with excess DIBAL-H, the reduction may continue further to give corresponding alcohols or even corresponding alkanes.

DIBAL-H reduces alkyl or aryl nitriles to their corresponding imines at low temperatures and resultant imines converted to corresponding aldehydes upon acid work-up. The same principle mentioned above apply here as well. Here, at high temperatures, imines reduced further to give corresponding amines.

Note: To see the mechanism of the reduction, see here.


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