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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. (7) Phenol reacts with stable diazonium salt formed from aromatic amine, forming an azo dye via a -N=N- link at the ortho- or para-position.
2. The preservation technique must not have a detrimental effect on the nutritive value of the food nor be potentially harmful, either immediately or cumulatively, to health. (1) Food preservation techniques involve the killing of micro-organisms, inhibition of microbial growth, or the retardation of chemical changes leading to food spoilage. Principles of food preservation include i. Removal of moisture – By making water unavailable for microbial growth and chemical processes, it is possible to preserve food. This technique includes drying and dehydration.
, ii. Altering temperature – Bacterial cells and spores are destroyed at high temperature, while at low temperatures, the growth of micro-organisms and the rate of reactions which cause food spoilage are retarded. These methods include heat treatment and freezing. iii. Changing pH – The use of vinegar in pickling and the production of lactic acid in yoghurt which retards the growth of potential spoilage organisms. iv. Use of osmotic pressure – Adding sugar or salt increases the solute concentration within the aqueous environment of the food.
The microbial cell loses water to the surrounding concentrated sugar or salt solution by osmosis. The micro-organisms cannot reproduce and multiply, and therefore would not cause food spoilage. v. Use of chemical additives – Nitrate and nitrite are used in meat curing for their antimicrobial properties. Anti-oxidants are added to certain food products such as potato crisps to reduce rancidity of fats and oils, thereby prolonging the shelf-life of these foods by chemical changes vi. Irradiation – ? -rays prevent spoilage from inside fruit and vegetable and the unbroken skin prevents aerial oxidation.
This method significantly increases the length of storage. (2) Food preservation technique – i. Heat treatment – Cooked food and milk sterilized by UHT. Heat kills micro-organisms, alters protein structure, destroys enzyme activity of micro-organisms in food. ii. Chilling and freezing at -20oC slow down microbial activities and chemical changes resulting in spoilage. Freezing promotes the retention of nutrients and does not destroy nutrients. iii. In canning, food is cooked under pressure in sealed containers. Cooking destroys enzymes and micro-organisms and most canned foods keep well for more than one year.
iv. Sugaring and salting remove water essential for enzyme action and microbial growth. v. Pickling in vinegar reduces growth of micro-organisms. vi. Chemicals such as nitrate and nitrite prohibit the growth of micro-organisms. vii. Benzoate, sulphite and vitamin C retard chemical spoilage in fruit juice. 3. Give an account of the uses of food additive. Food additives include monosodium glutamate, benzoic acid, vitamins, butylated hydroxytoluene(BHT). Food additives prevent oxidation, add flavor, texture and color, kill bacteria, increase vitamin content, etc
The chemical preservatives that are used as food additives are of two kinds: antimicrobial and antioxidant. Antimicrobial agents like the nitrates(III) and nitrates(V) of sodium and potassium have been added to cured meats like bacon, sausage, luncheon meat for a long time. Nitrates(III) are valuable in preventing the growth of Clostridium botulinum, the bacteria that produces deadly botulism poisoning in canned foods. Sulphur dioxide and sulphates(IV) are another group of antimicrobial preservatives that have been added to alcoholic beverages and dried fruits for centuries.
They have been included in fruit juices, jellies and jams. They prevent the growth of yeast and are useful as bleaches and antioxidants to prevent browning in alcoholic beverages dried fruits, fruit juice and vegetables. Antioxidants are used to oppose the oxidative decay of food due to microbial activity. Atmospheric oxidation is the chief cause of destruction of fats and oils in food. Fats and oils deteriorate rapidly by air oxidation, producing carboxylic acids, aldehydes and ketones, which together give spoiled fats their characteristic rancid taste and odor.
An antioxidant added to the food can hinder oxidation. BHA (butylated hydroxyanisole) and BHT (butylated hydroxytoluene) are common antioxidants used in food to retard this development of oxidative rancidity in unsaturated fats and oils. These phenols appear to work by donating the H-atom of the -OH group to the free hydroperoxide radical (ROO. ) involved in the autoxidation of fats and oils, thereby stopping the chain reactions in oxidative spoilage: AH + ROO. –> ROOH + A. where AH represents the antioxidant, and A.is a radical derived from the antioxidant, e. g. An effective natural antioxidant is vitamin E, which, like BHT and BHA, is a phenol.
Vitamin E, is more expensive than the synthetic antioxidants. Vitamin C (ascorbic acid) and sulphur dioxide. An acidic antioxidant like vitamin C can slow down the fast browning of a piece of apple exposed to air. Sulphur dioxide and sulphates(IV) also act as antioxidants (or reductants) which preferentially react with oxygen and prevent the oxidation of ethanol to ethanoic acid in wines.