Phenols bromine water

Certain phenols give white bromo derivatives with bromine water. 

Action of bromine water. To a concentrated aqueous solution of the phenol or to the phenol itself, add bromine water gradually. At first the bromine is decolorised and then on adding an excess a white or yellowish-white precipitate of a polybromo-derivative is produced with all except catechol, hydroquinone, i- and 2 naphthol. 

Extract the acidified solution with ether, remove the ether and identify the phenol in the usual manner (see Section IV,114).f Add a few drops of bromine water or nitric acid to the aqueous layer and test for sulphate with barium chloride solution. 

Dissolve 0 -1 g. of the compound in 10-15 ml. of water and add bromine water until the colour of the latter persists. A white precipitate will form if a phenol is present. 

The influence of the OH-group on the reactivity of the benzene ring is of great importance. All substitution processes which are traced back to preliminary addition reactions take place much more easily with the phenols than with the hydrocarbons as far as possible the groups which enter occupy the o- and p-positions. A number of reactions illustrating this statement is discussed below and in subsequent parts of this book. It may be mentioned here that by the action of bromine water, o-o-p-tnbromophenol is at once precipitated from an aqueous solution of phenol. (Method for the quantitative determination of phenol.)... 

Experiment.—To an approximately 2 per cent phenol solution bromine water is added until the bromine ceases to be consumed. The colourless flocculent precipitate is separated at the pump and, after drying, is recrystallised from ligroin or dilute alcohol. Melting point 95°. 

Materials Required Thyroid gland dried 1.0 g anhydrous potassium carbonate 17.0 g bromine solution (9.6 ml of Br2 and 30 g of KBr in 100 ml DW) 7.0 ml dilute phosphoric acid (10% w/v) 42.0 ml starch iodide paper phenol solution (saturated solution of phenol in water) 5.0 ml potassium iodide solution (10% w/v in water) 0.01 N sodium thiosulphate solution starch solution. 

They react with a solution of bromine in carbon tetrachloride by sub stitution and an equivalent quantity of hydrogen bromide is evolved (compare addition with unsaturated compounds). When the test is conducted with bromine water and a dilute aqueous solution of a phenol, the sign of reaction is the separation of a sparingly soluble bromine substitution product. ... 

Phenol. — The 1 .1100 aqueous solution may exhibit a milky turbidity on the addition of bromine water, but should show no crystalline precipitate. Nor should the solution afford a violet color with ferric chloride. 

Exercise 26-4 Explain why benzenol with bromine gives tribromobenzenol readily in water solution but 2- and 4-monobromobenzenol in nonpolar solvents. Notice that 2,4,6-tribromobenzenol is at least a 300-fold stronger acid than phenol in water solution. 

Diazo 2-bromo-6-chloro-p benzoquinone [ called 6-Chlor-2-brom-p-chinon-diazid-(4) or 6-Chlor-2 -brom-4-diazo-phenol in Ger], yel-red ndls (from hot eth, benz or chlf), explodes violently on heating to 15 0° v sol in hot ale si sol in hot eth or chlf insol in w was obtd by addn of bromine water to an aq soln of 4-diaZo-6-chloro -phenol-2 sulfonic acid(Refs 2 3)... 

When treated with bromine water an aqueous solution of phenol gives an immediate precipitate of 2,4,6-tribromophenol (Section 9.6.6, p. 1251), owing to the powerfully activating influence of the negatively charged oxygen in the phenoxide ion. 

Bromine water. Many phenols (with the exception of those with strong reducing properties) yield crystalline bromo compounds on the addition of bromine water. Dissolve or suspend 0.25 g of the compound in 10 ml of dilute hydrochloric acid or of water, and add bromine water dropwise until decolourisation is slow a white precipitate of the bromophenol may form. 

Dissolve 1.0 g of the compound in 10-15 ml of glacial acetic acid, cautiously add a solution of 3-4 ml of liquid bromine in 10-15 ml of glacial acetic acid until the colour of bromine persists and allow the mixture to stand for 15-20 minutes. Pour into 50-100 ml of water, filter off the bromo compound at the pump and wash with a little cold water. Recrystallise from dilute ethanol. Alternatively dissolve 1.0 g of the phenol in water, ethanol or acetone and add slowly, with constant shaking, just sufficient of a bromine solution (prepared by adding 5g of bromine to a solution of 7.5 g of potassium bromide in 50 ml of water) to impart a yellow colour to the mixture. Allow to stand for 5 minutes. Add about 50 ml of water, and shake vigorously to break up any lumps. Filter and wash the bromo derivative with a dilute solution of sodium metabisulphite. Recrystallise from ethanol or from dilute ethanol. 

Write equations for the reaction of phenol with dilute aqueous nitric acid and with bromine water. 

Phenol reacts with three equivalents of bromine in CC14 (in the dark) to give a product of formula Cg OB. When this product is added to bromine water, a yellow solid of molecular formula C6H2OBr4 precipitates out of the solution. The IR spectrum of the yellow precipitate shows a strong absorption (much like that of a quinone) around 1680 cm-1. Propose structures for the two products. 

Place a drop of the test solution in mineral acid on a spot plate, introduce a drop of saturated bromine water followed by 2-3 drops 2m potassium hydroxide (the solution must be alkaline to litmus). Mix thoroughly, add a crystal of phenol, then a drop of the diphenylcarbazide reagent, and finally M sulphuric acid dropwise until the red colour (from the reaction between diphenylcarbazide and alkali) disappears. A blue-violet colouration is obtained. 

---