Common WashesSodium Bicarbonate and Sodium CarbonateBrine (Saturated \(\ce{NaCl}\))Drying Agents

Purpose of a Work-Up

When the goal of an experiment is to conduct a reaction and isolate the product, the general sequence of events is shown in Table 4.4.

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Table 4.4: Typical reaction sequence of events.
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Figure 4.38: Reaction ssteustatiushistory.orge for the synthesis of isoamyl acetate.

The \(\ce{^1H}\) NMR spectrum in Figure 4.39a was taken of the reaction mixture immediately after ceasing heating and before the work-up. As expected, a significant signal for acetic acid is seen at \(2.097 \: \text{ppm}\).

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Figure 4.40: Bottle of saturated sodium bicarbonate.

The following reactions occur between bicarbonate ion (1), carbonate ion (2) and acid \(\left( \ce{H^+} \right)\) during a wash:

\<\ce{HCO_3^-} \left( aq \right) + \ce{H^+} \left( aq \right) \rightarrow \ce{H_2CO_3} \left( aq \right) \rightleftharpoons \ce{H_2O} \left( l \right) + \ce{CO_2} \left( g \right) \tag{1}\>

\<\ce{CO_3^{2-}} \left( aq \right) + \ce{H^+} \left( aq \right) \rightarrow \ce{HCO_3^-} \left( aq \right) \tag{2}\>

The initial product of reaction (1) is carbonic acid \(\left( \ce{H_2CO_3} \right)\), which is in equilibrium with water and carbon dioxide gas. This means that solutions of bicarbonate often bubble during a neutralization wash in a separatory funnel. The product of reaction (2) is the bicarbonate ion, which can subsequently undergo reaction (1). This means that solutions of carbonate ion also often bubble during neutralizations.

Safety note: To prevent excess pressure form being generated by the release of carbon dioxide gas into a separatory funnel during neutralization, the layers should be gently swirled together before placement of the stopper. They should be vented directly after inversion, and more frequently than usual. Figure 4.41 shows a strongly acidic organic layer (top) in contact with an aqueous solution of \(10\%\) sodium bicarbonate (bottom). A vigorous stream of bubbles is seen originating from a small portion of organic layer trapped on the bottom of the funnel. The bubbling was even more vigorous when the layers were mixed together.

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If the aqueous layer is on the bottom of the separatory funnel, test an "aliquot" of the aqueous layer (or tiny sample) on litmus paper through the following method:

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Anhydrous calcium sulfate \(\left( \ce{CaSO_4} \right)\), can be purchased containing a cobalt compound that is blue when dry and pink when wet (this is then sold under the name Drierite, Figure 4.49c+d). In this way, blue Drierite can be used as a visual indicator for the presence of water.\(^8\)

The most common drying agents used to remove water from organic solutions are anhydrous sodium sulfate \(\left( \ce{Na_2SO_4} \right)\) and anhydrous magnesium sulfate \(\left( \ce{MgSO_4} \right)\). Many steustatiushistory.orgists consider \(\ce{MgSO_4}\) the "go-to" drying agent as it works quickly, holds a lot of water for its mass, and the hydrates are noticeably chunkier compared to the anhydrous form, making it easy to see when you"ve added enough. A drawback to using \(\ce{MgSO_4}\) is that it is a fine powder, and so the solutions must be subsequently filtered to remove the drying agent. Another drawback to \(\ce{MgSO_4}\) is that all fine powders heavily adsorb product on their surface (which is why they must be rinsed with solvent after filtration), and sometimes more granular drying agents are used to minimize the loss of product by adsorption.

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In some procedures \(\ce{Na_2SO_4}\) or \(\ce{CaCl_2}\) are used if they seem to work just as well as \(\ce{MgSO_4}\), or if the solution is incompatible with \(\ce{MgSO_4}\) (see Table 4.8). A procedural advantage to these drying agents is that their granules are not easily dispersed, allowing for the solutions to be easily decanted (poured). In many situations drying agents are interchangeable (see Table 4.8 for a survey of drying agents). However, it is most common for desiccators and drying tubes to use \(\ce{CaSO_4}\) or \(\ce{CaCl_2}\) (Figure 4.50), as they can be easily manipulated in their pellet or rock forms.