Polyhalogen Compounds - Chemistry Class 12 - NEET PDF Download

Polyhalogen derivatives:

Trichloromethane (Chloroform), CHCl₃

Preparation

1. From methane - successive free-radical chlorination

Methane undergoes stepwise free-radical substitution with chlorine to give chloromethane, dichloromethane, trichloromethane (chloroform) and finally carbon tetrachloride. The sequence of transformations is:

CH₄ + Cl₂ CH₃Cl + HCl
                              Chloromethane

CH₃Cl + Cl₂ CH₂Cl₂ + HCl
                                  Dichloromethane

CH₂Cl₂ + Cl₂ CHCl₃ + HCl
                                    Trichloromethane

CHCl₃ + Cl₂ CCl₄ + HCl

The mixture of CH₃Cl, CH₂Cl₂, CHCl₃ and CCl₄ can be separated by fractional distillation. Control of reaction conditions (temperature, ratio of reactants, light) helps maximise the desired chlorinated product.

2. From chloral (and chloral hydrate)

Chloroform can be obtained by base treatment of chloral or chloral hydrate:

NaOH + CCl₃CHO → HCOONa + CHCl₃
                                      chloral

NaOH + CCl₃CH(OH)₂ → HCOONa + CHCl₃ + H₂O
Chloral hydrate               sodium formate Chloroform

(Here CCl₃CHO is chloral and CCl₃CH(OH)₂ is chloral hydrate; HCOONa is sodium formate.)

3. Laboratory method - from ethanol or acetone using bleaching powder

Bleaching powder (a source of Cl₂) reacts with water to give calcium hydroxide and chlorine:

CaOCl₂ + H₂O → Ca(OH)₂ + Cl₂

When ethanol is treated with the chlorine generated, it is oxidised and chlorinated stepwise to chloral, which on treatment with calcium hydroxide (or alkali) yields chloroform and calcium formate:

CH₃CH₂OH + Cl₂ CH₃CHO + 2HCl

CH₃CHO + 3Cl₂ CCl₃CHO + 3HCl

                                                              Chloral

Ca(OH)₂ + 2CCl₃ CHO 2CHCl₃ + (HCOO)₂Ca

                                                               Chloroform Calcium formate

Products: chloroform and calcium formate.

4. From carbon tetrachloride (partial reduction)

Partial reduction of carbon tetrachloride yields chloroform:

CCl₄ + 2[H] CHCl₃ + HCl (partial reduction)

5. Haloform reaction

Compounds with the CH(OH)-CH₃ or methyl ketone fragment when exhaustive halogenated in presence of base give the haloform (e.g., CHCl₃ from acetone or ethanol oxidised to acetaldehyde then to methyl ketone). The general haloform sequence involves halogenation at the methyl group followed by base-promoted cleavage to give the haloform and a carboxylate:

Typical mechanistic outline:

• Attack of nucleophile (base) on halogenated intermediate.
• Elimination of the leaving group leading to cleavage.
• Proton transfer and formation of the haloform and the corresponding carboxylate.

Comparative elimination (dehydrohalogenation, E2)

Problem. Compare rate of elimination (dehydrohalogenation in presence of alcoholic KOH), i.e., E2:

1. (a)  (b)  (c)  (d)

c > b > a > d

2. (a)  (b)  (c)

c > b > a

3. (a)  (b)  (c)

c > b > a

4. (a)  (b)  (c)

b > a > c

Dehalogenation (vicinal dihalide → alkene, E2)

Elimination of vicinal dihalides under strong base (intramolecular syn or anti elimination depending on mechanism and stereochemistry) gives alkenes. Typical schematics are shown:

Another example of dehalogenation (intramolecular) is shown:

Intramolecular (cyclic) elimination: E_c or E_i

• Leaving group and base are present in the same molecule.
• Reaction proceeds through a cyclic transition state.
• Overall it is a syn elimination (atoms eliminated from same face).
• Hofmann product is usually the major product because it is obtained from the least hindered site of the cyclic transition state.
• No rearrangement occurs in this mechanism.

Example of E_c/E_i: Pyrolysis of esters

Thermal elimination (pyrolysis) of suitable esters gives alkenes via a six-membered cyclic transition state (a classic example of an intramolecular elimination). Illustrations and mechanistic diagrams:

Additional steps and examples:

Physical properties of chloroform

Chloroform is a colourless, heavy liquid with a sweetish, somewhat sickly odour. It boils at 334 K (approximately 61 °C) and is slightly soluble in water. Chloroform is denser than water. Its volatility and potency as a central nervous system depressant historically made it useful as an anaesthetic, but safety concerns have reduced such use.

Chemical properties of chloroform

1. Oxidation on exposure to air and light - formation of phosgene

In presence of air and light, chloroform is oxidised to phosgene (COCl₂), a highly toxic gas:

2 CHCl₃ + O₂ 2 COCl₂ + 2 HCl

As formation of phosgene is dangerous, commercial and laboratory chloroform is stored in dark, completely filled bottles to exclude light and air. A small amount of ethanol (about 1%) is often added; ethanol reacts with any phosgene to form non-toxic diethyl carbonate, preventing accumulation of COCl₂:

COCl₂ + 2 C₂H₅OH → O=C(OC₂H₅)₂ + 2 HCl

Diethyl carbonate (non-poisonous).

2. Hydrolysis (in presence of alkali)

Chloroform is hydrolysed by alkali to give formate salts. For example, with aqueous KOH:

H - CCl₃ + (aq.) 3KOH  HCOOK

3. Reduction

Reducing agents (or nascent hydrogen generated from Zn + 2 HCl) convert chloroform to lower chlorinated methanes and ultimately to methane:

Zn + 2 HCl → ZnCl₂ + 2 [H]

CHCl₃ + 2 [H] → CH₂Cl₂ + HCl
                                Dichloromethane
                              (Methylene chloride)

CHCl₃ can be further reduced to CH₄:

CHCl₃ CH₄ + 3 HCl

4. Reaction with acetone (and related haloform formation)

Chloroform in presence of base reacts with ketones such as acetone to give chlorinated derivatives (haloform type sequences). An important laboratory reaction is formation of chloro-derived reagents and chlorinated products; illustrated schematics are:

(CH₃)₂C = O + CHCl₃  

                                                                     Chlroetone

Chloro-products such as chloroacetone (chlroetone) have been used historically; chloroacetone is an example of a chlorinated ketone with sedative/hypnotic action recorded earlier.

5. Nitration (reaction with nitric acid)

Chloroform reacts with nitric acid under certain conditions to give chloropicrin (CCl₃NO₂), a powerful lachrymator and insecticide (and historically used as a war gas):

2 CHCl₃ + HONO₂ → CCl₃NO₂ + H₂O

                                               (Chloropicrin)

Chloropicrin is used as an insecticide and fumigant; it is strongly lachrymatory and toxic.

6. Reaction with silver

Chloroform reacts under certain conditions with silver metal to produce characteristic transformations (illustrated):

7. Chlorination

Chloroform can be further chlorinated to give carbon tetrachloride:

CHCl₃ + Cl₂ CCl₄ + HCl

8. Reimer-Tiemann reaction (in presence of phenols)

Chloroform in strongly basic aqueous solution (NaOH) generates dichlorocarbene (:CCl₂), which reacts with activated aromatic substrates such as phenols to give ortho-formylation products (Reimer-Tiemann reaction). Schematic overall transformation:

+ CHCl₃ + 3 NaOH   + 2 NaCl + 2 H₂O

Uses of chloroform

• As a solvent for oils, fats, alkaloids, rubbers, and resins in laboratory and industrial operations.
• As a preservative for anatomical specimens (fixative solvent).
• As a reagent in organic synthesis (for generation of dichlorocarbene, haloform reactions, etc.).
• Historically used as an anaesthetic; modern use is limited due to safety concerns (toxicity and risk of phosgene formation).