Chlorobenzene
Definition and meaning of Chlorobenzene in chemistry.
Chlorobenzene is an aromatic organic compound made by replacing one hydrogen atom on a benzene ring with a chlorine atom, giving the formula C6H5Cl. It is a colorless, volatile liquid with a mild, almond-like smell.
In more detail
Industry produces chlorobenzene through electrophilic aromatic substitution. Benzene reacts with chlorine gas in the presence of a Lewis acid catalyst such as FeCl3, which polarizes the bond between the two chlorine atoms so the aromatic ring can attack one of them. This reaction releases hydrogen chloride gas as a byproduct.
The chlorine atom attached to the ring behaves differently than chlorine attached to a simple alkyl chain. One of chlorine's lone electron pairs delocalizes into the aromatic ring through resonance, giving the carbon-chlorine bond partial double bond character. This extra bond strength makes chlorobenzene far less reactive toward nucleophilic substitution than an ordinary alkyl chloride.
Converting chlorobenzene into phenol therefore requires forcing conditions. Historically, chemists used the Dow process, which combines concentrated sodium hydroxide with chlorobenzene at about 350 degrees Celsius and high pressure. Chlorobenzene remains an important industrial solvent today and a key starting material for making phenol, aniline, and several pesticides.
Its lower reactivity compared with alkyl chlorides also makes it a useful, relatively stable solvent for reactions that would otherwise attack a more reactive chlorinated compound.
Key facts
| Formula | C6H5Cl |
|---|---|
| Molar mass | 112.56 g/mol |
| Boiling point | 131.7 °C |
| Field | Organic Chemistry |
| Production method | Electrophilic aromatic substitution with FeCl3 catalyst |
Treating benzene with Cl2 gas over an FeCl3 catalyst at room temperature replaces one ring hydrogen atom with a chlorine atom. This reaction produces chlorobenzene along with hydrogen chloride gas, following the general pattern of electrophilic aromatic substitution used throughout industrial organic synthesis. Chemists must handle the resulting chlorobenzene carefully in a fume hood, since it is flammable and moderately toxic if inhaled or absorbed through the skin.
Frequently asked questions
Why doesn't chlorobenzene undergo nucleophilic substitution as readily as alkyl halides?
The chlorine lone pair conjugates with the ring's pi electron system. This gives the carbon-chlorine bond partial double bond character, making it shorter and stronger than a normal single bond, so typical substitution reactions are strongly disfavored without harsh conditions.
What is chlorobenzene used for?
It serves as an industrial solvent and as a key intermediate in manufacturing phenol, aniline, and various pesticides, most notably DDT historically.
Is chlorobenzene the same as benzyl chloride?
No. Chlorobenzene has chlorine bonded directly to the aromatic ring, while benzyl chloride has chlorine bonded to a carbon atom attached to the ring. This difference makes benzyl chloride far more reactive in substitution reactions.
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