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In the reduction of an aldehyde (e.g. ethanal) to a primary alcohol, a reducing agent is needed: $$\ce{CH3CHO + 2[H] -> CH3CH2OH}$$ My book states that one hydrogen (the $\ce{H-}$ ion) is from the reducing agent, and the other hydrogen (the $\ce{H+}$ ion) is from the solvent.

However, elsewhere it more generally notes that "$\ce{[H]}$ denotes a hydrogen from a reducing agent".

Does the $\ce{[H]}$ notation refer only to hydrogen atoms from the reducing agent, or, as this example implies, can it also refer to hydrogen atoms from other sources (such as the solvent)?

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    $\begingroup$ Any element in square brackets indicates that it is in atomic state I.e it's a free atom. $\endgroup$ – JM97 Jan 3 '17 at 11:47
  • $\begingroup$ @JM97 I disagree. If it is a free atom, why not simply write it as a free atom? The brackets instead denote what OP wrote: [H] is any reducing agent. As such it is better to not include it in balancing reactions and instead write $$\ce{CH3CHO ->[\ce{[H]}] CH3CHO}$$ Whether in the end $\ce{H}$ comes from the solvent, or reducing agent directly, or the reducing agent via the solvent is hard to determine. $\endgroup$ – Linear Christmas Jan 3 '17 at 14:28
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    $\begingroup$ There's an old concept called a "nascent state" whereby an element produced in situ supposedly provides higher reactivity. For example, hydrogen generated in a reaction mixture was called "nascent hydrogen" and often denoted as $\ce{[H]}$, and was thought to be a better reducing agent than bubbling gaseous $\ce{H2}$ in the reaction mixture. To my understanding, however, the idea of nascent states is not well supported, and so is falling in disuse. $\endgroup$ – Nicolau Saker Neto Jan 3 '17 at 14:30
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    $\begingroup$ @NicolauSakerNeto The concept of a lone atom present on a surface in situ (an adatom is well established and frequently used, however, in electrochemistry and materials science at least. Perhaps also in catalysis, at active sites? I have less of a sense for 'nascent atoms' in homogeneous kinetics/catalysis, however. $\endgroup$ – hBy2Py Jan 3 '17 at 16:59
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    $\begingroup$ @hBy2Py I've really only read about this topic lightly, but what I gather is that by establishing the existence of some special nascent state for an element, one neglects a better microscopic description of the reaction system (e.g. surface catalysis by adsorbed single atoms/molecules, for which the surface is also vital, not just the element) and its mechanistic implications, and that's what's frowned upon. These more modern concepts have superseded the nascent state. $\endgroup$ – Nicolau Saker Neto Jan 3 '17 at 17:46

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