Ebrahim Palkhi, IB1                                                                                                                                TM2/LB3

Lab 11: Investigating the Chemistry of Alcohols

1)To  2 cm3 of ethanol in a clean dry test tube, drop in a small piece of sodium metal. Repeat with diethylether.

Observations for ethanol:

  • Sodium floats on the surface of ethanol as it reacts
  • Effervescence( fizzes) seen and the reaction was relatively fast( slower than the reaction of sodium with water).
  • The reaction is exothermic, and the test tube becomes hot to the touch with time.
  • Bubbles are also seen as a gas is released. This gas burns with a pop sound when a glowing splint is inserted in the test tube.
  • A colourless solution is formed which has a similar smell to that of turpentine.

Inference:

Alcohols are very weak acids( generally weaker than water). As such they react with alkali metals like sodium, but they do not show any other reaction characteristics of acids.

If a small piece of sodium is dropped into some ethanol, it reacts steadily to give bubbles of hydrogen gas and leaves a colourless solution of sodium ethoxide, CH3CH2ONa. Sodium ethoxide is known as an alkoxide.

If the mixture resulting from the reaction between sodium and ethanol is carefully evaporated to dryness, the  white solid product is sodium ethoxide.

2CH3CH2OH(l)  + 2Na(s)→ CH3CH2O-Na+(s)  +H2(g)

Nuclear substitution reactions break the polar C—OH bond of ethanol. The Polar O—H bond is broken when an Ethanol is reduced by sodium. Hydrogen gas is slowly released, forming a solution of the product alkoxide ion RO- in ethanol. This forms the solid sodium ethoxide when evaporated.  

The general equation between an alcohol and sodium, forming an alkoxide and hydrogen gas is as follows:

2ROH(l) +2Na(s)2Na+RO-(s)   + H2(g)

2)To 1 cm3  of ethanol in  a clean dry test tube, add half a small spatula of PCl5(s).

Observations

  • An immediate reaction is seen.
  • effervescence occurs(fizzes seen)
  • the test tube becomes hot as the reaction is exothermic
  • the gas produced turns blue litmus red.

Inference:

Phosphorus pentachloride(PCl5) is a chlorinating agent like Phosphorus trichloride( PCl3) and Sulphur dichloride oxide(thionyl chloride-SCl2O). A chlorinating agent is a substance that replaces a hydroxyl group—OH with a chlorine atom—Cl.

PCl3  and SCl2O are liquids and the reaction is carried out by heating the alcohol and the chlorinating agent under reflux. However the reaction of an alcohol( in this case ethanol)  with solid PCl5 occurs at room temperature.. The reaction is as follows:

CH3CH2OH(l)  + PCl5(s) → CH3CH2Cl(l)    + PCl3O(l)    + HCl(g)

A chloroalkane ( in this case chloroethane),  phosphorus trichloride oxide and hydrogen chloride gas is formed.

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The production of HCl gas on adding phosphorus pentachloride is a useful test for the alcohol( presence of hydroxyl group) since the blue litmus experiences a colour change from red to blue due to the formation of an acidic gas( hydrogen chloride).

Nb: Phosphorus pentachloride contains the ions PCl4+ and PCl6-. The PCl4+ ion performs a similar role to the H+ ion in the process of nucleophilic substitution By HBr. It weakens the carbon-oxygen bond and assists in the nucleophilic substitution reaction. The PCl4+ ions and the H+ ion both act as Lewis acids, accepting a lone pair from the oxygen atom in the ...

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