Reactions of Alcohols and the Lucas Test

1Destroying sodium safely

Scraps of sodium left over from experiments must never go into a sink: with water, sodium can melt, catch fire and explode. A technician destroys them by dropping them, a few at a time, into ethanol in a fume cupboard. The sodium fizzes steadily, giving hydrogen and a solution of sodium ethoxide, but far more gently than with water. Ethanol behaves like a very weak relative of water, H–O–H with one H swapped for an ethyl group, and many of its reactions break the O–H bond just as water's do. Others break the C–O bond instead.

2Reactions of the O–H bond

• With sodium: $2{\mathrm{C}}_2{\mathrm{H}}_5\mathrm{O}\mathrm{H}(\mathrm{l})+2\mathrm{N}\mathrm{a}(\mathrm{s})\to 2{\mathrm{C}}_2{\mathrm{H}}_5{\mathrm{O}}^{-}\mathrm{N}{\mathrm{a}}^{+}(\mathrm{a}\mathrm{l}\mathrm{c})+{\mathrm{H}}_2(\mathrm{g})\backslash mathrm\{2C\_2H\_5OH(l)\; +\; 2Na(s)\; \backslash rightarrow\; 2C\_2H\_5O^-Na^+(alc)\; +\; H\_2(g)\}$. The fizzing (a gas that burns with a squeaky pop) is a test for an –OH group in a dry compound. Alcohols are weaker acids than water: they do not react with sodium hydroxide or with carbonates, and they do not change the colour of litmus. Methanol reacts fastest, then primary, secondary and tertiary alcohols, because alkyl groups push electron density onto the oxygen of the alkoxide ion and make it less stable.
• Esterification (Unit 8): heat with a carboxylic acid and a few drops of concentrated sulfuric acid: $\mathrm{C}{\mathrm{H}}_3\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{H}(\mathrm{l})+{\mathrm{C}}_2{\mathrm{H}}_5\mathrm{O}\mathrm{H}(\mathrm{l})\rightleftharpoons \mathrm{C}{\mathrm{H}}_3\mathrm{C}\mathrm{O}\mathrm{O}{\mathrm{C}}_2{\mathrm{H}}_5(\mathrm{l})+{\mathrm{H}}_2\mathrm{O}(\mathrm{l})\backslash mathrm\{CH\_3COOH(l)\; +\; C\_2H\_5OH(l)\; \backslash rightleftharpoons\; CH\_3COOC\_2H\_5(l)\; +\; H\_2O(l)\}$. Acyl chlorides react faster and completely at room temperature.

3Reactions of the C–O bond

• Substitution by a halogen (Unit 4): HX, $\mathrm{P}\mathrm{C}{\mathrm{l}}_5\backslash mathrm\{PCl\_5\}$, $\mathrm{P}\mathrm{C}{\mathrm{l}}_3\backslash mathrm\{PCl\_3\}$ or $\mathrm{S}\mathrm{O}\mathrm{C}{\mathrm{l}}_2\backslash mathrm\{SOCl\_2\}$. The steamy HCl fumes given off with $\mathrm{P}\mathrm{C}{\mathrm{l}}_5\backslash mathrm\{PCl\_5\}$ are a second test for –OH.
• Dehydration (Unit 3) depends on the conditions. Excess concentrated sulfuric acid at 170 °C gives the alkene. Excess ethanol at about 140 °C gives an ether, as one molecule of water is removed from two alcohol molecules: $2{\mathrm{C}}_2{\mathrm{H}}_5\mathrm{O}\mathrm{H}\to {\mathrm{C}}_2{\mathrm{H}}_5\mathrm{O}{\mathrm{C}}_2{\mathrm{H}}_5+{\mathrm{H}}_2\mathrm{O}\backslash mathrm\{2C\_2H\_5OH\; \backslash rightarrow\; C\_2H\_5OC\_2H\_5\; +\; H\_2O\}$.

Figure 3.1: The main reactions of ethanol. Conditions are shown in orange.

4The Lucas test

Figure 3.2: The Lucas test. The cloudiness is the insoluble chloroalkane forming; the faster it forms, the more stable the carbocation.

The alcohol is shaken with Lucas reagent at room temperature. The reaction is SN1: zinc chloride helps the –OH group leave, a carbocation forms, and chloride ions bond to it. The chloroalkane does not dissolve in the reagent, so it shows as a cloudiness or a separate oily layer. Tertiary carbocations form fastest, so tertiary alcohols react at once; primary alcohols, whose carbocations are very unstable, hardly react at room temperature. The test works only for alcohols that dissolve in the reagent, roughly those with up to six carbon atoms.

Table 3.1: Reactions compared for the three classes