Bromobenzene is an important fine organic chemical product, which is generally used in the synthesis of flame retardants, and can also be used as a solvent, analytical reagent, and intermediate synthesis of pharmaceuticals and pesticides.
Methods for Preparing Bromobenzene
Bromobenzene can be directly substituted by benzene and bromine under iron-catalyzed conditions; it can also be prepared by diazotization reaction of aniline according to the Sandmeyer synthesis method.
- Direct Bromination of Benzene
This method uses benzene as raw material and iron bromide as catalyst for direct substitution.
The direct bromination of benzene has a short synthesis route and simple operation, but it is accompanied by the formation of by-product p-dibromobenzene, and the yield of monobromobenzene is not easy to control. Since low temperature is beneficial to inhibit the progress of the series of reactions, the control of temperature has become an important factor affecting the yield. After adding bromine, the temperature can be controlled so that the temperature is not too high.
- Preparation of Bromobenzene by Sandmeier Reaction
Nitrobenzene is prepared by nitration of benzene in the presence of mixed acid, then nitrobenzene is reduced to aniline with iron powder and hydrochloric acid under heating, then aniline is made into brominated diazonium salt with hydrogen bromide and nitrite at low temperature, and bromobenzene is produced by Sandmeier reaction in the presence of cuprous bromide.
Experimental Procedure
1. Operation Steps
(1) Feeding
In a 250mL three-port flask equipped with reflux condenser, mechanical agitator and drop funnel, 11.0g benzene and 0.2g iron filings were added first, then 1mL bromine was added from the drop funnel (containing 20g bromine), heated flask until hydrogen bromide was volatilized, and the rest bromine was added in 20min. Heat 45min under 60oC until the brown vapor of bromine disappears completely.
(2) Separation
After the iron filings were separated from the reaction mixture, they were washed with water in a separatory funnel, and the bromobenzene was subjected to steam distillation. When white p-dibromobenzene crystals appear in the condensing tube, replace the receiver and continue the distillation until no other substances are evaporated except water.
(3) Purification
After the first part of bromobenzene is separated from the water layer in the separation funnel, the 30min is dried with calcium chloride and then distilled. The fraction with boiling point below 140oC contains unreacted benzene, while the fraction with boiling point between 150~170oC is mainly bromobenzene. Re-distillation to collect the fractions of 152~158oC.
(4) Purification of by-products
The residue after distillation was poured into a ceramic dish while it was hot, and after condensation, it was combined with p-dibromobenzene obtained by steam distillation. After drying on a porous ceramic plate, add 1g of activated carbon and recrystallize with methanol (about 5mL of methanol is needed for each gram of p-dibromobenzene).
2. Calculate Yield
3. Matters Needing Attention
- The temperature setting should be reasonable, and high temperature is conducive to continuous reaction;
- Do not raise the temperature too much after dropping bromine to prevent the generated HBr from taking away the raw benzene;
- Add reducing agent to destroy excessive bromine after reaction.
4. Detection and Identification of Products
- Observation on the appearance and properties of bromobenzene and p-dibromobenzene.
- Determination of refractive index of bromobenzene.
- Determination of melting point of bromobenzene.
- Infrared spectrum determination.
