Haloalkanes and haloarenes notes by rawat sir

JFC Haloalkanes and Haloarenes Notes by RAWAT Sir [M.Sc. Chemistry, 3 times NET (JRF), GATE ]
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1. Nature of C-X bond in alkyl halides: X is more electronegative than carbon. So, the C-X bond is polarized with C having
a partial positive charge and X having a partial negative charge
2. Preparation of Haloalkanes:
1. Sandmeyer’s reaction
Benzene diazonium salt with cuprous chloride or cuprous bromide results in the replacement of the diazonium group by –Cl
or –Br.
A place where you feel the CHEMISTRY

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2. Wurtz reaction
Alkyl halides react with sodium in dry ether to give hydrocarbons containing double the number of carbon atoms present
in the halide. 2 RX + 2 Na ether R-R +2 NaX
3. Wurtz-Fittig reaction -A mixture of an alkyl halide and aryl halide gives an alkylarene when treated with sodium in
dry ether.
4. Fittig reaction - Aryl halides when treated with sodium in dry ether give diphenyl.
3. Preparation of Haloarenes:
a. By Electrophilic substitution reaction:
b. Sandmeyer’s reaction:
c. Gattermann reaction:

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d. Balz – Schiemann reaction:
4. Physical properties of haloalkanes:
a. Solubility. Although haloalkanes are polar in nature, yet they are practically very slightly soluble in water. In order
for a haloalkane to dissolve in water, energy is required to overcome the attractions between the haloalkane molecules
and break the hydrogen bonds between water molecules. However Haloalkanes are not able to form hydrogen bonds with
water and therefore, less energy is released when new attractions are set up between the haloalkane and the water
molecules because these are not as strong as the original hydrogen bonds in water molecules. As a result, solubility of
haloalkanes in water is low.
b. Density: Simple fluoro and chloroalkanes are lighter than water while bromides and polychloro devrivatives are
heavier than water. With the increase in number of carbon atoms, the densities go on increasing. 4 With the increase in
number of halogen atoms, the densities go on increasing. The densities increase in the order: Fluoride < chloride <
bromide < iodide The density also increases with increasing number and atomic mass of the halogen.
c. Polarity and Boiling points:
B.P. is dependent on- Larger the chain higher will be the boiling point
More the hydrogen bonding MORE will be the B.P.
More branching more B.P.
Heigher the weight of halogen higher will be the B.P.
Molecules of organic halogen compounds are generally polar. Due to the polarity as well as higher molecular mass as
compared to the parent hydrocarbon, the intermolecular forces of attraction (dipole – dipole and van der Waals) between
the molecules are stronger in halogen derivatives of alkanes. As a result melting and boiling points of chlorides, bromides
and iodides are considerably higher than those of the parent hydrocarbon of comparable molecular mass. Amongst
themselves, the following trends are observed:
o For the same alkyl group the boiling points of alkyl chlorides, bromides and iodides follow the order RI > RBr > RCl > RF
where R is an alkyl group. This is because with the increase in the size of the halogen, the magnitude of van der Waals
force increase.
o In general, the boiling points of chloro, bromo and iodo compounds increase with increase in the number of halogen
atoms.
o For the same halogen atom, the boiling points of haloalkanes increase with increase in the size of alkyl groups.
o For isomeric alkyl halides, the boiling points decrease with branching. This is because branching of the chain makes the
molecule more compact and, therefore, decrease the surface area. Due to decrease in surface area, the magnitude of van
der Waals forces of attraction decreases and consequently, the boiling points of the branched chain compound is less than
those of the straight chain compounds.

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5. Physical Properties of Haloarenes:
a. These are generally colourless liquids or crystalline solids.
b. These are heavier than water.
c. Melting and boiling points of haloarenes: Melting and boiling points of haloarenes are nearly the same as those of alkyl
halides containing the same number of carbon atoms. The boiling points of monohalogen derivatives of benzene are in the
order: iodo > bromo > chloro > fluoro For the same halogen atom, the melting and boiling points increase as the size of
the aryl group increases. The melting point of para isomer is quite higher than that of ortho or meta isomers. This is due
to the fact that is has symmetrical structure and therefore, its molecules can easily pack tightly in the crystal lattice. As a
result intermolecular forces of attraction are stronger and therefore, greater energy is required to break its lattice and it
melts at higher temperature.
5. Chemical properties of haloalkanes:
a. Nucleophilic substitution reaction:
Mechanism:
SN1 SN2
First order reaction Second order reaction
Carbocation formation No such carbocation formation
Racemic mixture
Two step reaction
Electrons are donated into
empty p-orbital of carbocation
Inversion of configuration
One step reaction
electrons are donated into antibonding of leaving group
Rate =k [RX]
Faster in CH3X < 10
< 2o
< 30
Rate =k [RX] [Nu-
]
Faster in CH3X > 10
> 20
> 30
b.Elimination reaction: Dehydrohalogentaion ( - elimination):
When a haloalkane with β-hydrogen atom is heated with alcoholic solution of potassium hydroxide, there is elimination of
hydrogen atom from β-carbon and a halogen atom from the α-carbon atom. As a result, an alkene is formed as a product.
Zaitsev rule (also pronounced as Saytzeff) is followed.

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It states that “In dehydrohalogenation reactions, the preferred product is that alkene which has the greater number of alkyl
groups attached to the doubly bonded carbon atoms.”
c. Reaction with metals:
Reaction with Magnesium: Grignard’s reagent is formed:
→
Wurtz reaction: R-X + 2 Na + X-R → R-R + 2 NaX
6. Chemical properties of haloarenes:
Dow’s process:
Electrophilic aromatic substitution(EAS):
*Friedel craft alkylation and friedel craft acylation-

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7. Some important reactions for conversions:
*From Grignard’s reagent:
→
*Chloroform is slowly oxidised by air in the presence of light to an extremely poisonous gas, carbonyl chloride, also known as
phosgene. It is therefore stored in closed dark coloured bottles completely filled so that air is kept out.

Haloalkanes and haloarenes notes by rawat sir

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Haloalkanes and haloarenes notes by rawat sir