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Carbon monoxide ($\ce{CO}$) is considered to be a neutral oxide, not acidic or basic. But why? It reacts with hot concentrated sodium hydroxide solution to give a solution of sodium methanoate:

$$\ce{NaOH + CO -> HCOONa}$$

Doesn't the reaction of carbon monoxide with the basic hydroxide ion show that it must be acidic?

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    $\begingroup$ The idea of acidic/basic/neutral oxides is hardly used anywhere apart from high school, so I would say that the classification of CO as a "neutral oxide" has more to do with the particular syllabus you are taught rather than chemistry as a whole. $\endgroup$ – orthocresol Jan 7 '16 at 7:25
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Well, carbon monoxide can be created from formic acid by adding sulphuric acid which will dehydrate said formic acid:

$$\ce{HCOOH ->[\ce{H2SO4}] CO + H2O}$$

Therefore, we can imagine the reverse reaction theoretically, which would make carbon monoxide an acidic oxide. However, the forward reaction does not proceed easily and it needs both the high acidity of sulphuric acid and its strong dehydrative properties to actually work. And your question mentions using hot, concentrated sodium hydroxide to make the reverse one work.

Most oxides that are classified as acidic or basic either have a very electrophilic central atom (e.g. $\ce{CO2}$) which can be attacked by the weak nucleophile water (which in turn can then release an acidic proton), or they have a high charge density on the oxygen which allows it to abstract a proton from water directly. Carbon monoxide is neither. If you check out its molecular orbitals, you will notice that even though carbon is partially positive it has the largest HOMO contribution, meaning a proton would be more likely to attatch to the carbon side — which doesn’t want one at all. The LUMO is, luckily, also more carbon-centred, meaning nucleophilic attacks on carbon are possible. However, it is also degenerate due to the double bond so that an attack is not favoured.

Thus, the carbon monoxide molecule is one that won’t react with water at all and totally defies the concept of acidic/basic oxides.

Abbreviations:

  • HOMO is a widely used abbreviation for the Highest Occupied Molecular Orbital, i.e. the one with the highest energy that still contains electrons. It is usually the orbital that will attack nucleophilicly or that will be attacked electrophilicly.

  • LUMO is a widely used abbreviation for the Lowest Unoccupied Molecular Orbital, i.e. the virtual (unoccupied) orbital that has the lowest energy. When considering a nucleophilic attack, the attacking electrons will usually interact with the LUMO. Electrophiles attack with other molecules’ HOMO with their LUMO.

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