Chemistry Chemical Reactions-2

Topics Covered :

● Reactions Involving Cleavage of `C-O` Bond in Alcohols
● Reaction with Hydrogen Halides
● Reaction with Phosphorous Trihalides
● Dehydration
● Oxidation

Reactions involving cleavage of carbon – oxygen C–O bond in alcohols :

The reactions involving cleavage of `color{red}(C–O)` bond take place only in alcohols.

`color{red}("Note ")` : Phenols show this type of reaction only with zinc.

Reaction with hydrogen halides :

Alcohols react with hydrogen halides to form alkyl halides.

`color{red}(ROH + HX → R- X +H_2O)`

`=>` The difference in reactivity of three classes of alcohols with `color{red}(HCl)` distinguishes them from one another (`color{green}("Lucas test")`).

● Alcohols are soluble in Lucas reagent (conc. `color{red}(HCl)` and `color{red}(ZnCl_2)`) while their halides are immiscible and produce turbidity in solution.

● In case of tertiary alcohols, turbidity is produced immediately as they form the halides easily.

● Primary alcohols do not produce turbidity at room temperature.

Reaction with phosphorus trihalides :

Alcohols are converted to alkyl bromides by reaction with phosphorus tribromide (Refer Unit 10, Class XII).

Dehydration :

`=>` Alcohols undergo dehydration (removal of a molecule of water) to form alkenes on treating with a protic acid e.g., concentrated `color{red}(H_2SO_4)` or `color{red}(H_3PO_4)`, or catalysts such as anhydrous zinc chloride or alumina (See fig.1).

Ethanol undergoes dehydration by heating it with concentrated `color{red}(H_2SO_4)` at `443 K`.

`color{red}(C_2H_5 OH underset(443 K) overset(H_2SO_4)→ CH_2 = CH_2+H_2O)`

Secondary and tertiary alcohols are dehydrated under milder conditions. For example

`color{red}(CH_3 overset(OH)CHCH_3 underset(440K) overset(85 % H_3PO_4)→CH_3- CH= CH_2+H_2O)`

`color{red}(CH_3 - underset ( underset (CH_3) (|)) overset ( overset(CH_3) (|))C - OH underset (358K) overset(20 % H_3PO_4)→ CH_3 - overset(overset(CH_3)(||))C - CH_3+H_2O)`

Thus, the relative ease of dehydration of alcohols follows the following order :

Tertiary > Secondary > Primary

The mechanism of dehydration of ethanol involves the following steps:

Mechanism : See fig.2.

`color{red}("Note ")` ● The acid used in step 1 is released in step 3.

● To drive the equilibrium to the right, ethene is removed as it is formed.

Oxidation :

`=>` Oxidation of alcohols involves the formation of a carbonoxygen double bond with cleavage of an `color{red}(O-H)` and `color{red}(C-H)` bonds. See fig.1.

`=>` Such a cleavage and formation of bonds occur in oxidation reactions. These are also known as dehydrogenation reactions as these involve loss of dihydrogen from an alcohol molecule.

● Depending on the oxidising agent used, a primary alcohol is oxidised to an aldehyde which in turn is oxidised to a carboxylic acid. See fig.2.

`=>` Strong oxidising agents such as acidified potassium permanganate are used for getting carboxylic acids from alcohols directly. `color{red}(CrO_3)` in anhydrous medium is used as the oxidising agent for the isolation of aldehydes.

`color{red}(RCH_2OH overset(CrO_3)→ RCHO)`

`=>` A better reagent for oxidation of primary alcohols to aldehydes in good yield is pyridinium chlorochromate (PCC), a complex of chromium trioxide with pyridine and `color{red}(HCl).`

`color{red}(CH_3-CH = CH - CH_2OH overset (P C C)→ CH_3-CH = CH- CHO)`

`=>` Secondary alcohols are oxidised to ketones by chromic anhyride `color{red}(CrO_3)`.

`color{red}(undersettext(Sec- alcohol)(R- underset ( underset (OH) (|))CH-R') overset(CrO_3)→ undersettext(Ketone) ( R - underset( underset(O)(||))C-R'))`

`=>` Tertiary alcohols do not undergo oxidation reaction.

● Under strong reaction conditions such as strong oxidising agents (`color{red}(KMnO_4)`) and elevated temperatures, cleavage of various `color{red}(C-C)` bonds takes place and a mixture of carboxylic acids containing lesser number of carbon atoms is formed.

`=>` When the vapours of a primary or a secondary alcohol are passed over heated copper at `573 K`, dehydrogenation takes place and an aldehyde or a ketone is formed while tertiary alcohols undergo dehydration. See fig.3.