Haloalkane (Alkyl halide)

Polyhaloalkane: Chloroform

Laboratory Preparation of Trichloromethane (Chloroform):

Theory:
Chloroform is prepared in the laboratory by heating ethanol or acetone with an aqueous bleaching powder paste. Bleaching powder acts as oxidizing, chlorinating, and hydrolyzing agent.

\underset{Bleaching\ powder}{CaOCl_{2}}+H_{2}O\rightarrow Ca(OH)_{2}+Cl_{2}

1. From ethanol (ethyl alcohol):

i. Oxidation of ethanol into acetaldehyde:

ii. Chlorination of acetaldehyde into trichloroacetaldehyde (Chloral):

iii. Hydrolysis of chloral into chloroform:

2. From acetone (Propanone):

i. Chlorination of acetone into trichloroacetone:

ii. Hydrolysis of trichloroacetone into chloroform:

Physical Properties:

1. It is a colorless sweet-smelling liquid.
2. It is heavier than water.
3. It boils at 61 C and freezes at -63 C.
4. It is insoluble in water.
5. Vapour of chloroform causes unconsciousness.

Chemical Properties:

1. Action with air: Chloroform reacts with air in presence of sunlight to give carbonyl chloride or phosgene which is a highly poisonous gas.

To prevent the formation of phosgene gas:
i. It is stored in a dark bottle.
ii. Small amount of ethanol is added which converts phosgene into non-poisonous diethyl carbonate.

Test of purity of chloroform

Pure covalent contains three covalent C-Cl bonds which don’t ionize into Cl^– ion. Because of the absence of Cl^–, there is no reaction with AgNO₃. If white ppt. is obtained with AgNO3 solution, chloroform is impure otherwise it is pure.

\begin{align*} \underset{Impure}{CHCl_{3}}+AgNO_{3}&\rightarrow \underset{White\ ppt.}{AgCl}+ HNO_{3}\\ \underset{Pure}{CHCl_{3}}+AgNO_{3}&\rightarrow No\ reaction \end{align*}

2. Action with acetone: Chloroform reacts with acetone to give chloretone which is a hypnotic (sleep-inducing) drug.

3. Action with nitric acid: Chloroform reacts with concentrated nitric acid to give chloropicrin which is used as insecticides and as war gas or tear gas.

4. Action with silver: Chloroform is heated with silver powder to give acetylene gas.

5. Action with NaOH or KOH: Chloroform is heated with aqueous KOH or NaOH followed by acidification that gives formic acid.

6. Action with phenol: Chloroform is heated with phenol in presence of aq.KOH followed by acidification gives salicylaldehyde as the major product. This reaction is called the Reimer Tiemann reaction.

Here, p-Hydroxy benzaldehyde is formed as a minor product.

7. Action with primary amine: Chloroform is heated with primary amine in presence of alcoholic KOH to give isocyanide or carbylamine which is a bad-smelling compound. This reaction is known as carbylamine test or Isocyanide test since it is used to detect both chloroform as well as primary amine.

8. Reduction:

\begin{align*} \underset{Chloroform}{CHCl_{3}}+2[H]&\xrightarrow[]{Zn/HCl} \underset{Methylene\ chloride}{CH_{2}Cl_{2}}+ HCl\\ \underset{Chloroform}{CHCl_{3}}+6[H]&\xrightarrow[]{Zn/H_{2}O} \underset{Metane}{CH_{4}}+3HCl \end{align*}

Iodoform test:

Alcohol containing CH₃-CH-OH and aldehyde and ketone having CH₃-C=O structural unit is treated with iodine and aqueous NaOH to give a compound having yellow crystalline ppt with hospital smell known as Iodoform. This test is called the iodoform test.