Reactions of Alcohols, Chemistry tutorial

Introduction:

Alcohols are neutral to litmus. They experience diverse reactions based on the position of cleavage of the bond, RO-H or R-OH making an alkoxide ion or making a carbonium ion correspondingly.

Acidity of Alcohols:

Alcohols have no effect on the pH paper or sodium hydroxide. The measurement of pH value of ethanol by a pH meter symbolizes that ethanol is neutral.

Alcohols are in reality amphoteric in nature. As an acid, it ionizes to make an alkoxide ion (RO-) and hydrogen ion, H+ in the presence of base.

ROH ↔ RO- + H+

The order of acidic strength of alcohol is as: 1° alcohol > 2° alcohol > 3°alcohol.

In the presence of an acid, the alcohol might function as a base; it can accept a proton as illustrated below:

ROH + H+ ↔ ROH+

The order of basic strength of alcohols is as: 3° alcohol > 2° alcohol > 1° alcohol

Water is a weak acid, though the acidic strength of alcohols is much weaker than that of water. For illustration, considering the equilibrium for the ionization of water and ionization of ethanol:

H2O + H2O ↔ Ho- + H3O+           Kα = 1.0 x 10-18

                 Hydroxide ion

CH3CH2OH + H2O ↔ CH3CH2O¯ + H3O+         Kα = 1.0 x 10-16

                                 Ethoxide ion

This represents that water is a stronger acid than ethanol.

The Aromatic alcohols, for illustration C6H5CH2OH, behave in a similar manner as tertiary aliphatic alcohol due to the electron donating properties of the benzene ring.

Reactions of Alcohols:

1) Combustion:

Alcohols generally burn in the air to give carbon (IV) oxide and water. For illustration: If ethanol burns in air by a pale blue flame to give carbon (IV) oxide and water.

C2H5OH + 3O2 → 2CO2 + 3H2O

2) Reactions based on position cleavage of bond in alcohol:

Apart from the combustion, alcohols experience many other reactions and the kind of reaction based on the position of cleavage of the bond. There are two likely position of cleavage, giving two classes of reactions.

i) Cleavage of R-OH

For illustration: R-OH → R+ + OH-

ii) Cleavage of RO-H

For illustration: RO-H → RO¯ + H+

Either of these reactions might yield the substitution or elimination product and generally a mixture of the two depending mostly on the nature of the alkyl group to which the hydroxyl group is attached.

The preferred position of cleavage for 1°, 2°, and 3° alcohols illustrated below:

1109_Position of cleavage for alcohols.jpg

Fig: Position of cleavage for alcohols

In A, the lower +I (I signifies inductive effect) effect from one alky group favors the formation the alkoxide ion, RCH2O- whereas in C the greater +I effect from the three alkyl groups stabilizes the intermediate carbonium ion, R3C+ .

Reactions involving R-OH cleavage:

The order of reactivity is as shown: 3°alcohol > 2°alcohol > 1° alcohol.

a) Halogenations:

The reaction is a nucleophilic substitution reaction.

i) Chlorination (Lucas test for alcohol):

The reaction takes place whenever concentrated hydrochloric acid is placed to absolute alcohol and then refluxed in the presence of the anhydrous zinc chloride as catalyst.

R-OH + HCl → (ZnCl2 cat - reflux) → R-Cl + H2O

            Conc.

For illustration:

CH3CH2OH + HCl → (ZnCl2 cat - reflux) → CH3CH2Cl + H2O

The order of reactivity is as shown: 3° > 2° > 1°alcohols.

An insoluble haloalkane will be made and a cloudy mixture appears. The time it takes for the cloudiness to come out and for separation into two different layers points out the kinds of alcohol.

For 3° alcohols, two different layers are made instantly, 2° alcohols forms two different layers in around 10 minutes whereas for 1o alcohols, a cloudy look is observed after refluxing for few hours.

The reaction is supposed to occur through the SN1 method as concentrated hydrochloric acid is utilized.

1699_SN1 mechanism.jpg

Fig: SN1 mechanism

The stable the carbonium ion, the faster will be the reaction rate and the faster is the time for appearance of cloudiness.

The stability of the carbonium ion is in the below order:

R3C+ > R2C+H > RC+H2

b) Bromination and iodination:

Different hydrogen halides will necessitate various reagents, for illustration:

i) Use of NaX:

CH3CH2OH → (NaI, Conc H3PO4/ reflux) → CH3CH2I + H2O

CH3CH2OH → (NaBr, Conc H3PO4/ reflux) → CH3CH2Br + H2O

The order of reactivity is as: HI > HBr > HCl

ii) By using the phosphorus halides (PCl5, PCl3, PBr3 or PI3)

For illustration, use of phosphorus (V) chloride for chlorination.

This reaction can be taken out by adding solid phosphorus (V) chloride directly to the alcohol in a dry test-tube.

For illustration:

CH3CH2OH + PCl5 → CH3CH2Cl + POCl3 + HCl

Whenever phosphorus (III) chloride is utilized rather than phosphorus (V) chloride, the reaction is less vigorous. 

3CH3CH2OH + PCl3 → 3CH3CH2Cl + H3PO3

iii) By using sulphur dichloride oxide (thionyl chloride, SOCl2)

Alcohol is refluxed by sulphur dichloride oxide (that is, thionyl chloride) in the presence of a small quantity of pyridine, C5H5N that serves to absorb the hydrogen chlorides as it is formed.

For illustration:

CH3CH2OH + SOCl2 → (with pyridine) → CH3CH2Cl + SO2 + HCl

iii) By using phosphorus trihalide:

Whenever alcohols react by phosphorus trihalide under reflux, the product is a haloalkane.

Phosphorus trihalide can be made simply by adding red phosphorus and a halogen to the reaction mixture under reflux. The products are obtained via distillation at the end of reaction.

ROH → (red P/X) reflux → RX + H3PO3

Alcohol                          Haloalkane

X = Br or Iodine

For illustration:

CH3CH2CH2OH → (red P/Br) reflux → CH3CH2CH2Br + H3PO3

Propan-1-ol                                          1-bromopropane

c) Dehydration:

Dehydration of the alcohols can be carried out by utilizing dehydrating agents like concentrated tetraoxosulphate (VI) acid or aluminum oxide to provide the corresponding alkenes.

Whenever alcohol vapor is passed over the aluminium oxide catalyst heated to 350°C or by heating to 170°C with surplus concentrated tetraoxosulphate (VI) acid, the corresponding alkene is made.

For illustration, the hydration of ethanol provides ethene.

CH3CH2OH → (with surplus H2SO4 at 170°C) → CH2 = CH2 + H2O

CH3CH2OH → (with Al2O3 at 350°C) → CH2 = CH2 + H2O

Mechanism:

2313_Mechanism of hydration of ethanol.jpg

Fig: Mechanism of hydration of ethanol

The reaction above can only occur with alcohols which have at least one hydrogen on the adjacent carbon. For illustration, methanol can't experience dehydration.

If the reaction comprises secondary or tertiary alcohols having more than three carbon atoms, the possibility of forming more than one alkene can yield from the dehydrated reaction. For illustration, the dehydration of butan-2-ol provides but-1-ene and but-2-ene

1869_dehydration of butan-2-ol.jpg

Fig: Dehydration of butan-2-ol

The alkene generated in greater abundance is pointed out by Saytzeff's rule.

Saytzeff's rule defines that the alkene formed preferentially is the one having the higher number of alkyl groups. In the dehydration of butan-2-ol, the predominant product is but-2-ene.

d) Formation of ethers:

Whenever an excess alcohol is heated by concentrated tetraoxosulphate (VI) acid at a lower temperature of around 140°C ether is formed rather than alcohol. The alcohol experiences intermolecular dehydration forming ether. For illustration:

Mechanism:

1214_Formation of ethers.jpg

Fig: Formation of ethers

Reactions involving cleavage of RO-H:

The order of rate of reaction follows:

1o alcohol > 2o alcohol > 3o alcohol

a) Reaction with reactive metals:

Alcohols react by reactive metals such as sodium and potassium to form metal alkoxides and hydrogen gas.

For illustration:

2CH3CH2OH + 2Na → 2CH3CH2ONa + H2

                              Sodium ethaoxide

The metal alkoxides are stronger bases than the sodium hydroxide; they are generally employed in the organic reactions needing a strong base in a non-aqueous solvent.  

This reaction is alike to the reaction of water with reactive metals, though alcohols react more slowly by reactive metals than water.

2H2O + 2Na → 2NaOH + H2

                 Sodium hydroxide

b) Esterification:

This is the reaction in which an alcohol reacts by carboxylic acid in the presence of concentrated tetraoxosulphate (VI) acid to form esters. For illustration: the reaction of ethanol by ethanoic acid to form ethyl ethanoate and water.

275_Esterification process.jpg

Fig: Esterification process

Esterification reaction is instead slow reaction even with heating. The reaction is reversible and the yield poor.

The forward reaction is termed as esterification whereas the backward reaction is known as acid-catalyzed hydrolysis of ester. Whenever excess acid or alcohol is added to the reaction mixture, the equilibrium will shift to the right hand side.

The formation of an ester can as well be accomplished by reacting with:

  • Acyl chloride
  • Acid anhydride

2471_Formation of an ester.jpg

Fig: Formation of an ester

c) Oxidation of alcohols:

Alcohols can be oxidized via making use of common oxidizing agents like acidified potassium tetraoxomanganate (VII) or acidified potassium heptaoxodichromate (VI) to give the corresponding carbonyl compound or the carboxylic acid.

Primary alcohols are readily oxidized on heating to aldehydes however in the presence of excess of the oxidizing agent the reaction carries on further to outcome the carboxylic acid.

For illustration:

CH3CH2OH → [O](From KMnO4 or K2Cr2O7) → CH3CHO → [O] → CH3COOH

Secondary alcohols are oxidized to the corresponding ketone.  Ketones are quite resistant to the oxidation and don't experience further reactions unless they are subjected to severe conditions. In that situation they are oxidized to a carboxylic acid- a mixture of smaller carboxylic acids will be formed as it comprises breaking a carbon-carbon linkage.

1896_Oxidation of alcohols.jpg

Fig: Oxidation of alcohols

Tertiary alcohols are as well resistant to oxidation. They are not oxidized at all in neutral or alkaline media.

In the acidic medium, tertiary alcohols undergo dehydration to the alkene that can be readily oxidized to the diol and further oxidation outcomes the oxidation products of primary alcohols.

1026_Tertiary alcohols undergo dehydration.jpg

Fig: Tertiary alcohols undergo dehydration

d) Triiodoform Formation (Iodoform Reaction):

Some alcohols with the structure react by an Iodoform reagent (that is, iodine in the presence of sodium hydroxide), a mild oxidizing agent to give sodium carboxylate and triodomethane (as well termed as Iodoform).  

The product, triiodomethane appears as the yellow precipitate in the reaction mixture.

408_Triiodomethane.jpg

Fig: triiodomethane

The reaction is a helpful reaction for confirming the presence of the group in an alcohol and as well a helpful means of synthesizing carboxylic acids that will have one carbon less than the parent alcohol.  

Illustration:

67_Triiodoform Formation.jpg

Fig: Triiodoform Formation

Difference between Primary, secondary and tertiary alcohol:

1) Lucas Test: To the unknown alcohol, add Lucas reagent that is, a mixture of concentrated hydrochloric acid and anhydrous zinc chloride in ratio 1:1 at room temperature. The tertiary alcohols reacts by Lucas reagent generating instant turbidity in the reaction mixture, the secondary alcohols give turbidity in 5 to 10 minutes, and the primary alcohols don't provide turbidity at all at room temperature.

2) Oxidation Test: The product formed from the oxidation of alcohols can be employed as a test for alcohols.

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