Alkyl Halides and Aryl Halides
(saturated hydrocarbons) in which one or more hydrogen atoms are replaced by halogen atoms (F, Cl, Br, I). Haloarenes, on the other hand, are aromatic hydrocarbons where one or more hydrogen atoms of the aromatic ring have been replaced by halogen atoms.
Classification
Based on the number of halogen atoms:
-
Mono-Alkyl halides:
One halogen atom is attached (e.g.,
CH
₃
Cl - Methyl chloride
). Di-Alkyl halides: Two halogen atoms are attached (e.g.,
CH
₂
Cl
₂
- Methylene chloride
).
-
Tri-Alkyl halides and so on:
Based on the type of carbon to which the halogen is attached:
-
Primary (1°) Alkyl halides:
Halogen is attached to a primary carbon-e.g.,
CH
₃
CH
₂
Cl - Ethyl chloride
-
Secondary (2°) Alkyl halides:
Halogen is attached to a secondary carbon -e.g.,
CH
₃
CH(Cl)CH
₃
- Isopropyl chloride
-
Tertiary (3°) Alkyl halides:
Halogen is attached to a tertiary carbon e.g.,
CH
₃
C(Cl)(CH
₃
) CH
₃
- tert-Butyl chloride
-
Aryl halides:
These are derivatives of aromatic hydrocarbons. Example:
Chlorobenzene- C
₆
H
₅
Cl
Nomenclature
Alkyl halides:
The halogen atom is treated as a substituent on the alkane chain.
Example:
CH
₃
Br is named as Methyl bromide
.
Aryl halides:
Named as derivatives of the parent aromatic hydrocarbon. Example:
C
₆
H
₅
I is named as Iodobenzene.
Complex Alkyl halides: The longest chain containing the halogen atom is chosen as the parent chain.
Example:
CH
₃
CH(Cl)CH
₂
CH
₃
is named as 2-Chlorobutane.
For multiple halogen substituents of the same kind: Use di-, tri-, tetra - prefixes. Example:
CH
₂
Cl
₂
is named as Methylene dichloride
. For multiple different halogens: Alphabetize the halogens.
Example: CH₃CHClBr is named as 1-Bromo-2-chloroethane.
Methods of Preparation of Alkyl Halides and Aryl Halides
Alkyl halides:
From Alcohols:
Alcohols react with halogen acids to give Alkyl halides.
R−OH + HX → R−X + H
2
O
where X = Cl, Br, or I; R = alkyl group.
From Alkanes:
Alkanes can be halogenated in the presence of sunlight or ultraviolet light.
CH
4
+Cl
2
→
hv
CH
3
Cl + HCl
From Alkenes:
Alkenes react with halogens to form haloalkanes.
CH
2
=CH
2
+Br
2
→ CH
2
Br−CH
2
Br
Aryl halides:
From Phenol:
Phenols can be halogenated in the presence of Lewis acids.
C
6
H
5
OH + Cl
2
→
FeCl
3
C
6
H
4
Cl−OH
From Diazonium Salts:
Areniazonium salts react with halogen acids to yield haloarenes.
C
6
H
5
N
2
+
Cl
−
+ CuCl → C
6
H
5
Cl + N
2
Finkelstein Reaction:
Alkyl halides react with sodium iodide in acetone to give alkyl iodides.
R−X + NaI → R−I + NaX
Swarts Reaction:
Fluorination of alkyl halides using metallic fluorides like AgF, CoF₂ or HF.
R−Cl + AgF →R−F + AgCl
Sandmeyer Reaction:
Aromatic primary amines are converted to aryl halides using copper salts.
C
6
H
5
NH
2
+ Cl
2
+ CuCl → C
6
H
5
Cl + N
2
+ HCl
Also Check –
Ascorbic Acid Formula
Chemical Properties of Alkyl Halides and Aryl Halides
Alkyl halides:
Nucleophilic Substitution Reactions:
The halogen atom in Alkyl halides can be replaced by other nucleophiles.
R−X + OH
−
→ R−OH+ X
–
Elimination Reactions:
Alkyl halides can lose a molecule of HX to form alkenes.
CH
3
CH
2
Cl + KOH → CH
2
=CH
2
+ HCl
Reduction:
Alkyl halides can be reduced to alkanes using reducing agents.
CH
3
CH
2
Cl + H
2
→
Pd/C
CH
3
CH
3
Aryl halides:
Electrophilic Substitution Reactions:
Aryl halides undergo electrophilic substitution due to the presence of an aromatic ring.
C
6
H
5
Cl + NO
2
+
→ C
6
H
4
Cl−NO
2
Nucleophilic Substitution Reactions:
Due to the partial double bond character of the C-X bond in aryl halides (owing to resonance), these reactions are less common. However, under certain conditions, they can occur.
C
6
H
5
Cl + NH
2
−
→ C
6
H
5
NH
2
+ Cl
–
Reaction with Metals:
Aryl halides react with metals like magnesium in the presence of ether to form Grignard reagents.
C
6
H
5
Br + Mg
→
ether
C
6
H
5
MgBr
SN1 Reaction (Substitution Nucleophilic Unimolecular):
Mechanism:
Ionization:
The leaving group departs, forming a carbocation. Equation for 2° alkyl halide:
CH
3
CH(Br)CH
3
→CH
3
CH
+
CH
3
+ Br
–
Nucleophilic Attack:
The nucleophile can attack the carbocation from either side. Equation:
CH
3
CH
+
CH
3
+ OH
–
→CH
3
CH(OH)CH
3
Characteristics: Rearrangements to form a more stable carbocation can occur in SN
1
reactions.
SN
2
Reaction (Substitution Nucleophilic Bimolecular):
Mechanism: Simultaneous Attack and Departure: The nucleophile attacks the carbon atom bearing the halide while the leaving group departs in a single concerted step.
Equation for 1° haloalkane:
CH
3
Br + OH
−
→CH
3
OH + Br
–
Wurtz Reaction:
Alkyl halides react with sodium metal in dry ether to form alkanes.
2R−X + 2Na → R−R + 2NaX
Wurtz-Fittig Reaction:
Reaction between an aryl halide and an alkyl halide in presence of sodium.
R−X + Ph−X + 2Na → R−Ph + 2NaX
Fitting Reaction:
Aryl halides react with sodium metal in dry ether to form biphenyls.
2Ph−X + 2Na → Ph−Ph + 2NaX
Grignard Reaction:
Alkyl/aryl halides react with magnesium in dry ether to form Grignard reagents.
R−X + Mg → R−Mg−X
Frankland's Reaction:
Alkyl halides react with zinc metal to form alkanes.
2R−X + Zn → R−R + ZnX
2
E2 Elimination:
Alkyl halides react with a base to produce alkenes in a concerted process.
RCH
2
−CH
2
X + B
−
→ RCH=CH
2
+ XB
−
Characteristics:
Inversion of configuration if the carbon undergoing substitution is a chiral center. Alkyl halides and SN
1
/SN
2
Reactions:
Generally, aryl halides do not undergo SN
1
or SN
2
reactions due to the partial double bond character of the C-X bond (owing to resonance). However, under certain harsh conditions, especially with the presence of a strong base and high temperatures, they can undergo nucleophilic substitution. Example using strong base on chlorobenzene (though not a typical SN
2
mechanism):
C
6
H
5
Cl + 4NaOH → C
6
H
5
ONa + 3NaCl + 2H
2
O
The resultant phenoxide salt can be acidified to give phenol:
C
6
H
5
ONa + HCl → C
6
H
5
OH + NaCl
Also Read:
Acetamide Formula
