What are organic halogen compounds?

Organic halogen compounds are organic molecules that contain one or more halogen atoms (F, Cl, Br, I) attached to a carbon atom. They can be classified into alkyl halides, aryl halides, and acyl halides.

What are aryl halides, and why are they less reactive in nucleophilic substitution?

Aryl halides are organic compounds where a halogen is directly bonded to an aromatic ring (e.g., chlorobenzene). They are less reactive due to: Partial double bond character (resonance effect). Sp² hybridization of carbon, making bond cleavage difficult. Lack of carbocation stabilization in the SN1 mechanism.

Organic Compounds Containing Halogen: Types, Preparation & Reactions

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JEE Chemistry

Organic Compounds Containing Halogens

Halogen-containing organic compounds are categorized into two main groups:

Alkyl Halides: These are aliphatic carbon chains where one or more hydrogen atoms are substituted by halogen atoms.
Example: Chlorobutane.
Aryl Halides: These are aromatic rings with halogen atom(s) directly attached to the ring.
Example: Chlorobenzene.

Methods of Preparation of Alkyl Halides

1. Grove’s Process

Involves replacing the -OH group in primary and secondary alcohols with a halogen (X) using zinc chloride (ZnCl₂).

R−OH + HX → ZnCl₂R−X + H₂O

Where:

R-OH = Alcohol (Primary or Secondary)
HX = Hydrogen halide (HCl, HBr, HI)
ZnCl₂ = Catalyst
R-X = Alkyl halide

The reaction follows the Sₙ2 mechanism when ZnCl₂ concentration is low.

2. Darzens Process

Involves the reaction of thionyl chloride (SOCl₂) with primary alcohols.
Can proceed with or without pyridine.

Reaction Steps:

With pyridine:

ROH + SOCl₂ → HCl + ROSOCl

HCl + C₅H₅N → C₅H₅NH⁺Cl⁻

ROSOCl + Cl⁻ → RCl + SO₂ (Sₙ2 mechanism)

Other Methods for Preparing Alkyl Halides

These reactions are fundamental in organic chemistry and widely applied in synthesis and industrial processes.

Using Phosphorus Halides:
ROH + PCl₅ → RCl + HCl + POCl₃
Halogen Addition to Alkenes:
R-CH=CH₂ + Br₂/CCl₄ → R-CH(Br)-CH₂Br
Photohalogenation:
CH₄ + Cl₂ + hv → CH₃Cl + HCl
Halogen Exchange (Finkelstein Reaction):
RCl + NaI → RI + NaCl
Borodine-Hunsdiecker Reaction:
RCOOAg + Br₂ → RBr + CO₂ + AgBr
Hydrohalogenation of Unsaturated Hydrocarbons:
Without peroxide:
RCH=CH₂ + HBr → RCH(Br)CH₃
With peroxide (Anti-Markovnikov’s Rule):
RCH=CH₂ + HBr + Peroxide → RCH₂CH₂Br

Methods of Preparing Aryl Halides

1. Halogenation

Reaction:
Ar-H+X₂+Lewis Base→Ar-X+HX
In this process, an aromatic hydrocarbon (Ar-H) reacts with a halogen (X₂) in the presence of a Lewis acid to form an aryl halide (Ar-X).

2. From Diazonium Salts

Aryl halides can also be synthesized from diazonium salts using the following reactions:

Schiemann Reaction:
C₆H₅N₂Cl+HBF₄→C₆H₅F
Sandmeyer Reaction:
C₆H₅N₂Cl + CuCl→C₆H₅Cl
Gattermann Reaction:
C₆H₅N₂Cl + Cu powder→C₆H₅Cl

Sₙ1 and Sₙ2 Mechanism Comparison

Feature	Sₙ1 (Unimolecular Nucleophilic Substitution)	Sₙ2 (Bimolecular Nucleophilic Substitution)
Steps	Two steps: (1) R-X → R⁺ + X⁻ (2) R⁺ + Nu⁻ → R-Nu	One-step: R-X + Nu⁻ → R-Nu + X⁻
Rate Equation	Rate = k [RX] (First-order reaction)	Rate = k [RX] [Nu⁻] (Second-order reaction)
Transition State (TS) of Slow Step	Formation of carbocation (R⁺) in the rate-determining step	Single-step reaction with a transition state involving both R-X and Nu⁻ (backside attack)
Stereochemistry	Leads to inversion and racemization	Results in inversion (backside attack)
Molecularity	Unimolecular (depends only on RX)	Bimolecular (depends on both RX and Nu⁻
Reactivity of Alkyl Halide (R-X)	3° > 2° > 1° > CH₃ (favored due to carbocation stability)	CH₃ > 1° > 2° > 3° (hindered by steric effects)
Determining Factor	Stability of carbocation (R⁺)	Steric hindrance in R group
Reactivity of Halide (X)	RI > RBr > RCl > RF	RI > RBr > RCl > RF
Effect of Solvent	Rate increases in polar protic solvents	Rate increases in polar aprotic solvents
Effect of Nucleophile	No effect on rate (nucleophile attacks in second step)	Stronger nucleophile increases the reaction rate (I⁻ > Br⁻ > Cl⁻ > RS⁻ > RO⁻)
Catalysis	Lewis acids (e.g., Ag⁺, AlCl₃, ZnCl₂) may promote ionization	None required
Competing Reactions	Elimination, rearrangement	Elimination

Reactions of Alkyl Halides

Hydrolysis:
RX + OH⁻ → ROH + X⁻
Williamson Synthesis:
R-ONa + R'X → R-O-R' + NaX
Reaction with Dry Silver Oxide:
2R-X + Ag₂O → R-O-R
Reaction with Sodio-Alkynides:
R-C≡C-Na + X-R → R-C≡C-R + NaX
Reaction with Potassium Cyanide:
KCN + R-X → RCN + KX
Reaction with Silver Cyanide:
AgCN + R-X → RNC + AgX
Reaction with Silver Nitrite:
AgNO₂ + R-X → RNO₂ + AgX
Reaction with Potassium Nitrite:
KNO₂ + R-X → R-O-N=O + KX
Friedel-Crafts Reaction:
R-X + C₆H₆ + AlCl₃ → C₆H₅-R
Malonic Ester Synthesis:
R-X + CH(CO₂C₂H₅)₂ → R-CH(CO₂C₂H₅)₂ + HX
Acetoacetic Ester Synthesis:
R-X + CH(CO₂C₂H₅)₂ → R-CH(CO₂C₂H₅)₂ + HX
Reaction with Ammonia:
R-X + NH₃ → R-NH₂ + HX
Wurtz Reaction:
2R-I + 2Na → R-R + 2NaI
Dehydrohalogenation:
CH₃CH₂CH₂Br + alco.KOH → CH₃-CH=CH₂ + KBr + H₂O
Reaction with Alcoholic AgNO₃:
R-X + AgNO₃ → R⁺ + AgX↓ + HNO₃

Haloform Reaction (Trihalide Reaction)

Preparation

Chloroform (CHCl₃) can be prepared from alcohols containing the –CH(OH)CH₃ group or from aldehydes and ketones with three α-hydrogen atoms, using halogen (X₂) and an alkali or Na₂CO₃.

Substitution / Elimination

CH₃X	RCH₂X (1°)	R₂CHX (2°)	R₃CX (3°)
Methyl	1°	2°	3°
Reaction Type	Bimolecular reactions only	Bimolecular and unimolecular	Sₙ1/E1 or E2
Reactivity	Gives Sₙ2 reactions	Mainly Sₙ2, but with hindered strong bases (e.g., (CH₃)₃CO⁻), E2 dominates.	Mainly Sₙ2 with weak bases (I⁻, CN⁻, RCO₂⁻), and E2 with strong bases (RO⁻).

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