Haloalkanes and Chloroform: Class 12 Organic Chemistry Notes

If an unsymmetrical alkene is taken then two possible
products are formed.

The Formation and stability of these two products can be explained by following two rules:

[A] Markovnikov’s rule:

According to this rule “when an unsymmetrical alkene reacts with an unsymmetrical reagent than the positive
part of reagent goes to that double bonded carbon
containing greater number of the hydrogen atoms”. For example

[B] Peroxide effect (Anti- Markovnikov’s rule)

According to this rule “when an unsymmetrical alkene reacts with the unsymmetrical reagent in presence of
organic peroxide
(R-O-O-R)then the positive part of the reagent goes to that double bonded carbon containing less number of
the hydrogen atom”. For example,

Anti-Markovnikov’s reaction

It is also called the Kharasch effect.

HCl and HI do not give Markovnikov’s addition, why?

Ans: H-Cl is highly polar and hence does not undergo hemolysis
easily. HI undergoes homolysis to give iodine free radicals
which instantly combine to give I2.

3. From Alcohol

Generally, haloalkanes are prepared by the reaction of alcohols with haloacids or phosphorous halide, or thionyl chloride.

(a) Reaction with halogen acid(HX)

The
chloroalkane
is prepared by the reaction of alcohol with HCl in presence of anhydrous
zinc chloride (ZnCl₂).

The mixture of conc. HCl and anhydrous ZnCl₂ is called Lucas
reagent.

A reaction from 1,2,3 degree alcohol to respective Chloro substituted product.

(b) Reaction with a phosphorous halide (PX₅ or PX₃)

The haloalkanes are prepared by the action of
alcohol with PX₅ or PX₃.

Since PBr₃ and PI₃ are unstable compounds. So they are prepared in the reaction mixture (In-situ form)
by the action of red phosphorous with Br₂ or I₂.

The chloroalkanes are prepared by heating alcohol with SOCl₂ in presence of pyridine. Only chloroalkane is prepared
by this method. From this method, pure chloroalkane can be
prepared because SO₂ and HCl evolved as gases.

Darzen’s Reaction or Reaction of alcohol with thionyl chloride.

Physical properties of haloalkanes

• Lower members of haloalkane methyl chloride and methyl bromide are
  colorless gases, higher are colorless and sweet-smelling liquids and
  next higher are colorless solids.
  

• They are insoluble in water and soluble in almost all organic
  solvents like ether, alcohol, etc.
  

• They burn with green-edged flame in the air.
  

• The boiling point of haloalkanes is higher than corresponding
  parent alkanes.
  

• The boiling point of haloalkane having the same alkyl group is
  RI>RBr > RCl due to the large size of the halogen atom.
  

• Branched-chain haloalkane has a lower boiling point than
  straight-chain haloalkane due to its spherical nature.
  

• The B.P. increase as the increase in the alkyl group.
  

Chemical properties of haloalkanes

The haloalkanes are more reactive than alkanes due to the presence of polar C-X bonds. The polarity arises due to the difference in electronegativity
value between carbon and a halogen atom.

[A] Nucleophilic substitution reaction

The nucleophile is electron-rich species having lone pairs of electrons or negative charges and can attack
to electron-deficient center. When a
nucleophile
is substituted by another nucleophile then the reaction is called a
nucleophilic substitution reaction.

Nuc: + R-LG → R-Nuc + LG:

Nuc– nucleophile LG– Leaving group

Example: R-Br + OH⁻ → R-OH + Br⁻ 

Here, the existing nucleophile has been substituted by an
incoming nucleophile.

The alkyl halides undergo nucleophilic substitution reaction (SN- reaction ) by following two mechanisms.

S stands for Substitution

N stand for Nucleophile

The number represents Kinetic order

Alkyl halide ionizes to give carbocation. Then step is slow and hence it is the rate-determining step.

The rate-determining step of SN1 Reaction

In the second step, the nucleophile attacks the carbonium ion to give t-butyl
alcohol. (ionic reaction, fast)

Formation of product in SN1 Reaction

Similarly, NO₂⁻has two attacking sites.

–NO₂ & –ONO
R-X+CN⁻ ⟶ R-CN (Attack by carbon)
R-X+CN⁻ ⟶ R-NC (Attack by nitrogen)

2.
Reaction with alcoholic NaCN or
KCN

When haloalkane is heated with an alcoholic solution of NaCN or KCN
then alkane nitrile (Cyanides) are formed. This reaction is largely
used to increase the number of carbon atoms during organic
conversion.

Alkane nitrile(Cyanides) is a beneficial chemical that gives
various products when treated with different reagents.

(a) Reduction Reaction

A reaction in which an organic nitrile is reduced by nascent
hydrogen (e.g. from sodium in ethanol) to a primary
amine is REDUCTION REACTION.

b) Complete hydrolysis

c) partial hydrolysis

3.
Reaction
with
alcoholic AgCN

When haloalkane is heated with an alcoholic solution of AgCN then
alkyl isocyanide is formed.

Here, AgCN is a covalent compound. So it does not dissociate
easily. Therefore the lone pair of electrons in the nitrogen
atom attacks haloalkane to form isocyanides.

Similarly, isocyanide forms different compounds as:

(a)Reduction

(b)Acidic hydrolysis

4. Formation of amines

When haloalkane is heated with alc. ammonia then amines are
formed.

5. Reaction with sodium alkoxide(R-ONa) (Williamson’s ether
synthesis/ Formation of ether)

When haloalkane is heated with
sodium alkoxide
then ether is formed. This reaction is called Williamson’s
etherification reaction.

Both symmetrical and unsymmetrical ether can be prepared
by this method.

6. Formation of thioether

Thioether is (R–S–R”) analog of ether.
Name sulfides like ethers, replacing ”sulfide” for “ether” in the
common name, or “alkylthio” for “alkoxy” in the IUPAC system.

7. Carbylanime

When primary animes are heated with alcoholic
potassium hydroxide
(KOH) and chloroform forms a product which is a foul-smelling substance called carbylanime
reaction.

8. Reaction with aqueous NaNO₂ or KNO₂

When haloalkanes are heated with aqueous NaNO₂ or KNO₂ solution
then alkyl nitrite is formed.

Here, NaNO₂ is an ionic compound. Hence Na−O bond breaks easily and
negatively charged oxygen attacks haloalkane to form alkyl
nitrite.[Na-O-N=O]→[Na⁺+ ONO⁻]

9. Reaction with alcoholic silver nitrite (AgNO₂)

When haloalkanes are heated with alcoholic AgNO₂ solution then
nitroalkanes are formed.

Here, AgNO₂ is a
covalent compound. Hence the lone pair of electrons on the nitrogen atom attacks an
alkyl group of haloalkane to form
nitroalkane.[Ag-O-N=O]→[Ag−NO₂]

[B] Elimination reaction (ᵦ- elimination reaction)
Dehydrohalogenation reaction

When haloalkane is boiled with an alcoholic solution of NaOH or KOH
then alkene is formed. In this reaction, one hydrogen and halogen
atom are removed from adjacent carbon. So this reaction is called
dehydrohalogenation reaction.

This reaction is also called β-elimination or
1,2-elimination.

Saytzeff’s rule

If
dehydrohalogenation
of haloalkane gives two or more alkenes then alkene containing a
greater number of the alkyl group on double bonded carbon is the
major product. This rule is called Saytzeff ’s rule.

Saytzeff’s rule

Q. Convert 1-bromopropane to 2-bromopropane.

[C] Reaction with metal

Reaction with sodium (Wurtz’s reaction)

When alkyl halide is heated with sodium metal in presence of
dry ether, then alkane having a double number of carbon atoms is formed.
Therefore it is used to increase carbon length.

Reaction with sodium ( Wurtz’s reaction )

[D] Reduction reaction of Haloalkane

1. Catalytic reduction

2. Reduction with metal hydride

3. Reduction with metallic solution

(b) Hydrolysis of tri chloroacetone

At last, tri chloroacetone is hydrolyzed with calcium hydroxide to
form chloroform.

Physical properties of Chloroform

• It is a colorless mobile oily liquid.
  

• Its boiling point is 61 ̊C and melting point is -60 ̊C.
  

• It is heavier than water having an sp. gravity of 1.48.
  

• It has a characteristic sweet smell and taste.
  

• It is insoluble in water and soluble in almost all organic
  

• solvents like benzene, ether, etc.
  

• It is a good solvent for dissolving fats, oils, resins, waxes,
  etc.
  

• The Vapour of chloroform causes temporary unconsciousness when
  taken in small amounts. So, it is used as an anesthetic drug. *Why it happen?
  

Chemical properties of chloroform

1. Action with air (Oxidation)

When chloroform is exposed to air in presence of sunlight, a
highly poisonous
phosgene gas (Carbonyl chloride) is formed.

Therefore, the following precautions are taken while storing the
chloroform.

• It is always stored in a dark brown colored bottle to protect it
  from sunlight.
  

• It is completely filled up in the bottle to keep out air.
  

• A small amount (1%) of ethyl alcohol is added to the bottle of
  chloroform. The ethyl alcohol reacts with phosgene gas formed
  during storage to give non-toxic harmless diethyl carbonate.
  

2. Reaction with silver powder

When chloroform is heated with silver powder then acetylene is
formed.

3. Reaction with conc.HNO₃(Nitration)

When chloroform is treated with conc. nitric acid then chloropicrin is formed which is used as an insecticide and war gas (Tear gas).

4. Reaction with acetone

When chloroform is the heated presence of aqueous NaOH or KOH
then with acetone in a crystalline solid of chloretone is formed
which is used
hypnotic drug
(sleep-inducing).

5. Reduction

i)Acidic medium(Zn/HCl)

When chloroform is reduced in an acidic medium, methylene chloride
is formed.

Purity test of chloroform

6. Reaction with silver nitrate (AgNO₃)

i) With Pure Chloroform

Pure chloroform does not give white ppt. with AgNO₃ solution
because the C-Cl bond in chloroform is a strong covalent bond and
can not give Cl⁻ ion.

ii) With impure Chloroform

But, impure chloroform gives white ppt. of AgCl with AgNO₃ due to
the presence of Cl⁻ ion after the
oxidation
of chloroform in the air.

Read More: Class 12 Chemistry

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Haloalkane and Chloroform

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