Synthesis of Benzoic acid from Benzyl chloride


Aim: To synthesize Benzoic acid from Benzyl chloride by an oxidation reaction

Reference: F. G. Mann and B. C. Saunders, Practical Organic Chemistry, 4th Ed, 1974, P. No. 239-240

Requirement:
Apparatus: Beaker, Stirrer, Filtration assembly, RBF, Condenser, etc.
Chemicals: Anhydrous sodium carbonate, potassium permanganate, benzyl chloride, sodium sulfite

Theory/Principle:
When an aromatic compound having an aliphatic side chain is subjected to oxidation, fission of the side chain occurs between the first and second carbon atoms from the benzene ring, the first carbon atom thus becoming part of a carboxyl (-COOH) group. Such oxidations are frequently important for determining the position of a side chain relative to other substituents in the benzene ring. The oxidation is usually carried out with a mixture of potassium permanganate and sodium carbonate in aqueous solution, or alternatively with dilute nitric acid. The reaction is quite slow if the side chain is a simple alkyl group. The side chain containing a chlorinated alkyl group is more susceptible to oxidation. 
Hence in comparison to toluene, benzyl chloride more rapidly oxidizes in the presence of an aqueous oxidizing agent. Here benzyl chloride is first hydrolyzed to benzyl alcohol, and then undergoes oxidation of primary alcohol to the corresponding carboxylic acid

Fig. 1 Synthesis of Benzoic Acid from Benzyl chloride

Procedure:
  1. To 40 ml of water contained in a 500 ml bolt-head flask, add in turn 1 g of anhydrous sodium carbonate, 2 g of potassium permanganate, and finally 1 ml of benzyl chloride.
  2. To 40 ml of water contained in a 500 ml bolt-head flask, add in turn 1 g of anhydrous sodium carbonate, 2 g of potassium permanganate, and finally 1 ml of benzyl chloride.
  3. Fit the flask with a reflux water-condenser, and boil the mixture gently for 1-1.5 hours, i.e., until the reaction is complete and the liquid running down from the condenser contains no oily drops of unchanged benzyl chloride.
  4. During this boiling, the permanganate is slowly reduced, and manganese dioxide separates as a dark brown precipitate.
  5. Now cool the flask, and add concentrated hydrochloric acid (about 5 ml.) cautiously until the mixture is strongly acid, and all the benzoic acid has been precipitated.
  6. Then add a 20% aqueous solution of crystalline sodium sulfite (about 10 ml.) slowly with shaking until the manganese dioxide is completely dissolved and only the white precipitate of benzoic acid remains.
  7. When the mixture is quite cold, filter off the benzoic acid at the pump, and wash well with water.
  8. The benzoic acid is obtained as colorless needles, having m.p. 121°


Synthesis of m-Dinitrobenzene from Nitrobenzene

Aim: To synthesize m-Dinitrobenzene from Nitrobenzene by nitration reaction

Reference: F. G. Mann and B. C. Saunders, Practical Organic Chemistry, 4th Ed, 1974, P. No. 160.

Requirement:
Apparatus: Beaker, Stirrer, Filtration assembly, RBF, Condenser, etc.
Chemicals: Fuming nitric acid, sulphuric acid, nitrobenzene

Theory/Principle:
           Here nitration is occurring on nitrobenzene. It is an electrophilic aromatic substitution in the presence of NO2, which is a strong electron-withdrawing group and it directs the incoming substituents to the meta position. Here nitronium ions act as the electrophile which is generated from fuming nitric acid in the presence of conc. sulphuric acid. Nitration is an electrophilic substitution reaction on an aromatic ring.
Fig. 1 Synthesis of m-Dinitrobenzene from Nitrobenzene 
Fig. 2 Mechanism of Reaction
Procedure:

  1. This preparation must be performed in a fume-cupboard because nitrous fumes are evolved during the nitration. A ground-glass flask and air condenser should preferably be used
  2. Place 1.5 ml of fuming nitric acid in 100 ml. flask and add carefully with shaking 2.0 ml. (3.7 g) of concentrated sulphuric acid and then some fragments of unglazed porcelain.
  3. Fit a reflux air-condenser securely to the flask, and then add slowly down the condenser 1.2 ml of nitrobenzene: do not add more than 0.3 ml. of the nitrobenzene at a time, and after each addition shake the flask to ensure thorough mixing of the contents.
  4. Now heat the flask on a boiling water-bath for 1 hour, both the flask and the condenser being securely clamped in position if joined by a cork, which the acid fumes evolved may attack and weaken.
  5. Shake the flask vigorously from time to time throughout this period of heating.
  6. Finally, pour the mixture carefully with stirring into an excess of cold water (about 300 ml), whereupon the heavy oily dinitrobenzene will rapidly solidify.
  7. Filter the crystalline material at the pump, wash thoroughly with water to remove all acid, and then drain as completely as possible.
  8. Thus obtain crude m-dinitrobenzene, m.p. 90°.

Benzene and its derivatives

  1. Analytical, synthetic and other evidence in the derivation of the structure of benzene, Orbital picture, resonance in benzene, aromatic characters, Huckel’s rule
  2. Reactions of benzene - nitration, sulphonation, halogenation reactivity, Friedelcrafts alkylation- reactivity, limitations, Friedel crafts acylation.
  3. Substituents, the effect of substituents on reactivity and orientation of monosubstituted benzene compounds towards electrophilic substitution reaction
  4. D. Structure and uses of DDT, Saccharin, BHC, and Chloramine

Method of Recrystallization


Aim: Recrystallization of Benzanilide or Phenyl Benzoate or P-Bromoacetanilide

Reference: F. G. Mann and B. C. Saunders, Practical Organic Chemistry, 4th Ed, 1974, P. No. 13-15.

Requirement:
Apparatus: Conical flasks, Filter funnel, Buchner flask, Buchner funnel, etc.
Chemicals: Benzanilide or Phenyl Benzoate or P-Bromoacetanilide, Ethanol

Theory/Principle:
Organic compounds that are solids at room temperature are usually purified by recrystallization. The general technique involves dissolving the material to be recrystallized in a hot solvent (or solvent mixture) and cooling the solution slowly. The solid that crystallizes out from the solution is very pure material. During the recrystallization process, solid impurities (such as dust, filter paper, etc.) that do not dissolve in hot solution are normally eliminated through filtration. The dissolved impurities remain in the cold solution while the pure compound recrystallizes out of the solution. The process of recrystallization relies on the property that for most compounds, as the temperature of a solvent increases, the solubility of the compound in that solvent also increases
The general procedure for recrystallization is as shown in the flow chart below and the steps in the recrystallization of a compound are:
1.        Find a suitable solvent for the recrystallization
2.        Dissolve the impure solid in a minimum volume of hot solvent
3.        Use decolorizing carbon (if necessary)
4.        Remove any insoluble impurities by filtration
5.        Slowly cool the hot solution to crystallize the desired compound from the solution

Fig.1 Method of Recrystallization

Procedure:

  1. Weigh about 1.0 g sample into a 100 mL conical flask. Add 10 ml of ethanol and anti-bumping granules (3-5 pieces). 
  2. Heat the mixture on a hot plate until the solvent boils. Add successive small volumes of ethanol (2-3 mL) if required and continue boiling until compound has been dissolved (apart from insoluble impurities).
  3. If the solution is coloured, remove the solution from the hot plate. 
  4. Cool the solution to room temperature and add decolourising charcoal (0.2-0.3 g) if required.
  5. Mix thoroughly and boil the mixture for several minutes.
  6. Filter the hot mixture of sample through a filter paper into the preheated conical flask/beaker. 
  7. If crystallization occurs on the filter paper, add a minimum volume of hot ethanol to re-dissolve the crystals, and allow the solution to pass through the funnel. 
  8. After filtration, boil the filtrate to produce a more concentrated solution.
  9. Cover the conical flask with a watch glass and allow the solution to cool to room temperature, then in an ice-bath after the crystallization has occurred. 
  10. When all the crystals of compound have crystallized out, filter the crystals through a Buchner funnel at the suction pump.
  11. Wash the crystals with a little cold ethanol and dry. 
  12. Weigh the pure compound recovered, calculate the percentage yield, report melting point and compare with melting point of the crude sample.


Synthesis of P-Bromoacetanilide from Acetanilide


Aim: To synthesize P-Bromoacetanilide from Acetanilide by halogenation (Bromination) reaction

Reference: G Joshi and S Adimurthy, Environment-friendly bromination of aromatic heterocycles using a bromide-bromate couple in an aqueous medium, Ind. Eng. Chem. Res., 2011, Vol.50, P. No. 12271–75,

Requirement:
Apparatus: Beaker, Stirrer, Filtration assembly, conical flask etc.
Chemicals: Acetanilide, potassium bromate and potassium bromide, Oxalic acid

Theory/Principle:
Bromination is an electrophilic substitution reaction on an aromatic ring. Substituents already present on benzene nucleus determine the position and extent of substitution of the new incoming groups. These substituents are generally classified as strongly activating (e.g. -NH2), moderately activating (e.g.-NHCOCH3) and deactivating (e.g. -NO2). Bromination of acetanilide provides a good example to study orientation of the incoming electrophile on a moderately activated aromatic nucleus. The traditional experiment involves the use of Br2-AcOH and however, liquid bromine is extremely corrosive and is hazardous to handle. To avoid risks in using liquid bromine, the methods to generate bromine in situ have been developed.

Fig. 1 Traditional method of Bromination

In acidic medium, KBrO3-KBr is known to release bromine according to Fig. 2
Fig. 2 In-situ release of Bromine molecule

The liberated bromine reacts with the aromatic substrate as shown in fig. 3

Fig. 3 Synthesis of P-bromoacetanilide from Acetanilide

       Acetanilide undergoes bromination, with the formation of a mixture of o- and p-bromoacetanilide. The ortho compound is formed in only small amount, however, and being more soluble in ethanol than the para compound can be readily eliminated by recrystallization.

Fig. 4 Mechanism of Reaction
Procedure:

  1. A mixture of acetanilide, potassium bromate and potassium bromide (1:1:1 molar ratio) is placed in a conical flask fitted with a cork.
  2. The required volume of water (10 ml for 0.01 mol of acetanilide) is added and the mixture is gently stirred on a magnetic stirrer.
  3. An aqueous solution of oxalic acid or citric acid (0.01 mol in 10 ml of water) is added in small portions over a period of 15 minutes.
  4. The reaction mixture acquires a distinct yellow colour due to slight excess of bromine, indicating that no more bromine is required for the reaction.
  5. To ensure completion of bromination, the reaction mixture is stirred for another 15 minutes and the crude product is filtered at the pump, wash well with cold water, and drain
  6. The p-bromoacetanilide is obtained as colourless crystals, m.p. 167°.

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