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Give an account of the properties and uses of phenol

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Introduction

1. Give an account of the properties and uses of phenol. (1) Phenol is acidic and its conjugate base, phenoxide ion is stabilized by resonance. However, phenol is not acidic enough to liberate CO2 from phenol because an aqueous solution of carbon dioxide is a stronger acid than phenol. An aqueous solution of phenol has a pH value of 9. This means phenol reacts with NaOH but not NaHCO3. Phenol dissolves in NaOH(aq) to form sodium phenoxide, an ionic salt. (2) Phenol is corrosive and is a useful antiseptic. (3) Phenol is a colorless solid of low melting point. Its pink appearance is due to the presence of impurities. (4) When attached to a benzene ring, a hydroxyl group does not show the properties of an alcohol. The delocalization of electrons into the ring makes the -OH group inert towards replacement. On the other hand, the -OH group activates the ring toward electrophilic substitution at the ortho- and para-positions. Therefore, phenol reacts with (a) NO2+, (b) CH3+, (c) CH3CO+, (d) Br2(aq). Phenol is so reactive that it easily decolorizes bromine water in an electrophilic substitution as follows: C6H5OH + 3/2 Br2 --> C6H2Br3OH (2,4,6-tribromophenol) (5) In alkaline medium, phenol changes into phenoxide ion, a good nucleophile, C6H5O- easily undergoes reaction with i. Halogenoalkane to form an ether - C6H5O- + Cl-CH3 --> C6H5OCH3 + Cl- ii. Ethanoyl chloride to form an ester - C6H5O- + Cl-COCH3 --> C6H5OCOCH3 + Cl- (6) Phenol reacts with methanal to form a thermosetting plastic with high melting point. In the plastic, the phenol molecules are joined together by -CH2 units derived from methanal HCOH. ...read more.

Middle

Coloring agents are a mixture of dyes added to make the food more attractive. Caramel is a coloring agent added to oyster sauce. Yellow dyes are added to chicken feed to give chicken skin a yellow color. Beta-carotene is often added to butter and margarine to give their yellow appearance. About half of the 30 coloring agents are extracted from natural products like carrots, beets and grape skins. Tartrazine is an example of synthetic dye used in some fruit squash. Sulphur dioxide is used to bleach food products like cheese and bread where they act as preservatives. Sulphur dioxide dissolves in water and alkali to form the sulphite ion, responsible for the bleaching action. White bread is made from flour bleached with sulphur dioxide. If the pH of fruit juice is too high, citric acid and lactic acid are added to give it a good flavor. Phosphoric acid, tartaric acid and maleic acids are also used. If the pH of fruit juice is low, buffer salts such as sodium citrate or tartrate are used. The acidic additives act as preservatives to prevent the growth of micro-organisms and as antioxidants to prevent rancidity and browning in dressings. Nutrients include vitamin C (fruit juice) vitamin B (enriched flour), vitamin D (milk and margarine), ammonium ferric citrate (infant milk formulations and bread flour) Possible menace of food additives include (1) allergies such as rashes and stomach upsets, (2) hyperactivity in children caused by food colorings, (3) long term illness. MSG has been associated with the so-called 'Chinese restaurant syndrome', an unpleasant reaction characterized by burning sensation, thirst, headache, chest pain, vomiting and abdominal discomfort.. ...read more.

Conclusion

Measuring the initial rate at doubled [A], constant [B], the order of reaction in A is found. (2) Use excess reagent A, and measure the changes in [B] with time indirectly by finding the changes in concentration of some products. Titration and colorimetry are two common methods for finding concentration changes. For a iodine-producing or iodine-consuming reaction, the changes in [I2] is done by titrating the reaction mixture with standard thiosulphate solution and the reactant concentration can be calculated. The reaction mixture must first be quenched either by removing the H+ in the acidic medium or lowering abruptly the reaction temperature. For a reaction involving colored reactants or products, the progress of the reaction may be monitored with a colorimeter. The concentration of the colored species can be found by calibrating the colorimeter with a standard solution of the colored species. For example, a 0.01 M [I2] can be used to calibrate the colorimeter so that the exact [I2] in the reaction mixture would be calculated accurately. Colorimetry is a fast and effective technique and no quenching is required and any changes in concentration of a colored species are measured rapidly. Once the changing concentration of [B] is calculated, the order of reaction with respect to B could be determined by plotting ln [B] versus time or 1/{B} versus time. A linear ln [B] vs t plot indicates that the reaction is first order in [B]. The slope of this plot is -k. A linear 1/[B] vs t plot indicates that the reaction is second order in [B] and the slope of the graph is +k. The approach is repeated using excess reagent B, and measure the changes in [A]. The order of reaction with respect to [A] can be done by similar plots. 6. ...read more.

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