Nitrite ester, fast reaction

Esters of alcohols and acids usually are not formed quickly and quite some effort has to be put in separating the ester from the other chemicals. Also, the reaction usually is incomplete. In this experiment, however, fast and complete formation of an ester is shown. The reason of this fast and complete reaction is that the ester is a gas at room temperature, and the ester only is sparingly soluble in the reaction mix. The ester bubbles out of solution and as such, the esterification reaction proceeds in one direction quickly. Normally it would be an equilibrium, but because the ester escapes as gas, the reaction is driven to one side. One of the reaction products is taken out of the equilibrium system, resulting in completion of the reaction.

The experiment is simple:

  • Take 1 ml of methanol, and mix this with 5 ml or so of dilute hydrochloric acid, or sulphuric acid (for both acids, anything near 10% acid by weight is suitable). Swirl the liquid, such that all is well mixed.
  • Add a small spatula full of solid sodium nitrite or potassium nitrite (a few 100's of mg) to the acid/methanol/water mix.

When this procedure is performed, then the ester vigorously bubbles from the solution. There is slight contamination with nitrogen oxides, but this only is in the order of 1% of all gas formed.

The gas produced in this reaction is colorless and flammable. The pure gas burns with a pale blue/grey flame. The experiment was done with sodium nitrite, and with potassium nitrite. After the production of the gas, at the moment that the bubbling ceases somewhat, the gas is ignited with a cigarette lighter. With sodium nitrite, a bright yellow/orange flame is produced. With potassium nitrite, a pale grey/blue flame is produced, sometimes with a weak orange hue. The two pictures show the burning gas. The left picture is the burning of the gas, produced from sodium nitrite, and the right picture is the burning of the gas, produced from potassium nitrite.

        

The bright yellow/orange color is due to the presence of sodium ions in the gas. Because of the vigorous bubbling, tiny droplets of liquid are sprayed into the gas mix above the liquid. When these droplets contain traces of sodium ions, then the flame color is bright orange. The coloring properties of potassium ions are much weaker, and hence, the flame of the gas, produced with potassium nitrite is much more pale.

Both the reaction with sodium nitrite and the reaction with potassium nitrite were recorded in a small AVI movie. These two movies can be downloaded by clicking the links below:

The main reactions, occurring in this experiment are the following:

1) Formation of nitrous acid from strong acid and nitrite. Nitrous acid is a weak acid, and this reaction is very strongly shifted to the right.

          H+ + NO2– → HNO2

2) Formation of ester in equilibrium reaction, HNO2 has structure H-O-N=O.

          CH3OH + H-O-N=O ↔ CH3O-N=O + H2O

The product CH3O-N=O is a gas, it escapes from the system, and hence the equilibrium is broken and the reaction proceeds quickly from left to right, until one of the reactants is used up.

Methyl nitrite, CH3O-N=O, should not be confused with its isomer nitromethane, CH3-NO2. Nitromethane is a colorless liquid, with a boiling point of 101 ºC.

 

There also is a weak side reaction, in which HNO2 decomposes, giving nitrogen monoxide and nitric acid. With oxygen from air, the nitrogen monoxide forms brown nitrogen dioxide, which can be seen in the videos. The combustion of the nitrite ester also produces some nitrogen oxide(s), which also can be seen in the videos.

 
 

 
   

 

   

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