Experiment 20 Chemical Properties of Alkanols and Phenol 24-09-2003

Alkanols and phenol are all from hydroxyl compound containing the functional group OH. In this experiment, the chemical properties of hydroxy compounds are studied and compared, the reactions involving cleavage of O-H bond including solubility, pH, reaction with sodium hydroxide, reaction with sodium and reaction with alkanoic acid.

The reactions involving cleavage of C-O bond including the reaction with phosphorus pentachloride (PCl5) and reaction with acidified potassium dichromate.

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Results:

AI Testing solubility and pH

Ethanol

Phenol

pH

Neutral with pH7

Acidic with pH4

Solubility

Soluble in water

Soluble in water

AII Reaction with NaOH

Ethanol

Phenol

Temperature

No change

Increase

AIII Reaction with sodium metal

Ethanol

Phenol

Pop Sound Test

+ve indicate there is H2 evolved

+ve indicate there is H2 evolved

pH

Alkaline with pH11

Alkaline with pH9

AIV Reaction with Alkanoic acid

Ethanol

Phenol

Smell

Sweet

No reaction and smell

BI Reaction with Phosphorus Pentachloride and silver nitrate solution

Ethanol

Phenol

White ppt

Clear with no reaction

BII Reaction with Acidified Potassium Dichromate

Colour change

Ethanol

Orange to green

Propan-2-ol

Orange to green

2 methylpropan-2-ol

No change

Phenol

Orange to Dark Brown

Conclusion:

Phenol is more acidic than ethanol.

Phenol reacts with sodium hydroxide but ethanol does not

Phenol reacts with sodium in a faster rate than ethanol does.

Ethanol reacts with ethanoic acid but phenol does not.

Ethanol reacts with phosphorus pentachloride but phenol does not.

Ethanol, propan-2-ol and phenol react with potassium dichromate but 2 methylpropan-2-ol does not.

Precautions:

In heating the mixture directly into the Bunsen flame, it should be noted that the mixture should not be boiled because concentrated H2SO4 is inside. Hot and concentrated sulphuric acid can be very dangerous. The mixture is cool down first because ethyl ethanoate is a volatile liquid. It will evaporate and escape easily in high temperature.

Discussion:

Both phenol and ethanol are soluble in water because they are polar. OH and the benzene or R group have different electronegativity. OH will be partially negative while the R group will be partially positive. Therefore they are soluble in water which is well known to be a polar solvent with H+ and OH-.

In experiment AI, phenol is more acidic than ethanol is due to the resonance stabilization of its conjugate base, phenoxide ion.

The electrons on the oxygen atom overlap with the delocalized electron cloud on the benzene ring. This result in a dispersion of electrons over the entire benzene ring and oxygen atom. Thus the O-H bond is weakened and phenoxide ion is stabilized even with negatively charge oxygen atom.

Therefore the H in phenol is easier to lose and thus it is more acidic as acidity depends on the concentration of H+.

For the reaction with NaOH, it is a neutralization reaction.

NaOH(aq) + ROH(aq) –> NaO-(aq) + R+(aq) + H2O(l)

Neutralization is an exothermic reaction

Phenol reacts with NaOH and the temperature increase while ethanol does not react with NaOH. The result is due to the different in acidic properties between phenol and ethanol.

For the reaction with sodium metal, both phenol and ethanol react and give out hydrogen indicated by pop sound test. It is a typical acid-metal reaction:

2Na(s) + 2ROH(aq) –> 2RO-(aq) + Na+(aq) + H2(g)

Phenol reacts in a faster rate than ethanol as seen in the rate of bubbles given out. It is also due to the acidic properties of phenol is stronger than ethanol.

After all the sodium metal had reacted, add water and test for the pH. Both sodium ethanoxide and sodium phenoxide are alkaline, but ethanoxide shows a higher pH than phenoxide. It may because the excess ethanol and phenol in the test tube will neutralize the alkaline. As ethanol is neutral and phenol is acidic, ethanoxide shows a higher pH than phenoxide.

The water should be added to the test tube only when the sodium metal had reacted and dissolved totally. It is because water reacts more vigorous with sodium than ethanol.

2Na(s) + 2H2O (l) –> 2NaOH (aq) + H2 (g)

In the reaction with alkanoic acid, ethanol reacts with alkanoic acid to form a sweet smell compound. In fact it is a condensation reaction called esterification and the sweet smell compound is called ester. H2SO4 is also added in this experiment because esterification (condensation) is acid catalyst.

CH3CH2OH (aq) + CH3COOH (aq) –> CH3COOCH2CH3 (aq) + H2O (l)

Mechanism:

In this experiment, the ethanol and ethanoic acid are both in small amount 1cm3, therefore the mixture can be heated directly in Bunsen flame. Normally, the mixture is refluxed with a small amount of concentrated H2SO4.

There may be excess acid inside the test tube and the strong acidic smell may cover the smell of ester. Therefore the reaction mixture is poured into a beaker of water to smell so that the ethanoic acid which is polar will miscible with the water and being diluted while the ester which is non-polar will be immiscible with water and float on top, then the sweet smell of ester can be easier to be detected.

On the other hand, phenol does not react with ethanoic acid to give esters. This is because the lone pair electrons on the oxygen atom of phenol partially incorporated into the delocalized system of the benzene ring, making it slightly electron deficient (i.e. less nucleophilic) and less likely to undergo the reaction.

Chloroalkanes are readily formed from the reaction of alcohols with phosphorus pentachloride at room temperature:

R-OH + PCl5 –> R-Cl + POCl3 + HCl

In the experiment:

CH3CH2OH (aq) + PCl5 (aq) –> CH3CH2Cl (aq) + POCl3 (aq) + HCl (aq)

In the experiment ethanol reacts with phosphorus pentachloride in silver nitrate to form a white ppt.

Silver nitrate is used to indicate for halide ions because the halide ions displace out will form silver halide with the Ag+ ions. As AgCl, AgBr, AgI are solid and are not soluble in water, they shows as precipitates.

Ag+ (aq) + Cl- (aq) –> AgCl (s)

Ag+ (aq) + Br- (aq) –> AgBr (s)

Ag+ (aq) + I- (aq) –> AgI (s)

Except phosphorus pentachloride, some other phosphorus halides also react with alcohol to yield haloalkanes:

In react to potassium dichromate, colour of ethanol and propan-2-ol turn from orange to green indicated that oxidization had taken place, the orange dichromate ions acts as a oxidizing agent reduced to green chromium III ions.

Primary alcohols are first oxidized to aldehydes and to carboxylic acid. In the experiment, ethanol is oxidized as follows:

CH3CH2OH –> CH3CHOH –> CH3COOH

Secondary alcohols can be oxidized to ketones and stop at ketones because further oxidation requires the breaking of strong C-C bond.

In the experiment, propan-2-ol is oxidized as follows:

For tertiary alcohols, like 2 methylpropan-2-ol in the experiment, are generally resistant to oxidation unless they are subjected to sever condition like strong oxidizing agent and heat. It is because any oxidation would immediately involve the cleavage of C-C bonds in alcohol molecules. In the experiment condition, only mild oxidizing agent Cr2O7- is used thus there is no reaction.

For the phenol, it turned dark brown because phenol is oxidized to benzoquinone which is brown in color.

Source of error and way of improvement:

Phenol is a solid while ethanol is in aqueous solution. The amount of them is difficult to compare. For a fair test, phenol and ethanol should be in the same concentration and state.

Phenol put in the air without cover will absorb water and dilute itself which will change the concentration of the reactants. Therefore phenol should be covered properly.