Saturday 28 May 2011

Chemistry : Important organic substances

Important organic substances
-          They are useful since they can have different properties, namely strong, light, head-wearing, good insulators of heat and electricity, flexible, waterproof, can be moulded easily, can be colored easily, can be transparent (that’s not easy for metallic surface), chemically inert and cheap. (Each types of plastic include different properties from the above, not all)
-          Polymer is a compound which consists of very large molecules formed by joining together many small molecules repeatedly.
-          Polymerization is the process of repeatedly joining together many small molecules (monomer) to form very large molecules.
-          Polymers can occur naturally, like wool and silk. Man-made polymers are known as plastics. Regenerated polymers are modified from natural polymers while synthetic polymers are made by chemical reactions.
-          Repeating unit is the smallest part of polymer while the whole polymer can be obtained by repeating it. For example, the repeating unit of polyethene  is .
-          Addition polymerization is the reaction where monomers join together repeatedly to form polymer molecules and no atoms are lost from the monomers. They are done usually by the breaking of C=C bond in the monomers are bonding between the monomers.
Example: polystyrene, note that styrene is the common name of ethylbenzene.

-          Common addition polymers include polypropene (PP), polyvinyl chloride (PVC), polystyrene, Perspex and polyethene.
-          Polyethene can be classified into low density polyethene (LDPE) and high density polyethene (HDPE). LDPE has low density since the polymer chains are separated and have weaker intermolecular forces; LDPE are light and flexible which can be used as wrappings; HDPE has high density since the polymer chains are tangled each other, hence higher intermolecular force and higher density; HDPE are harder so it can be made into bottles.
-          Condensation polymers formed when monomers join together repeatedly to form polymer molecules with some small molecules eliminated. For example, nylon by adding diamine and dioic acid, note that –OH in dioic acid and –H in amine has been eliminated.
-          Nylon has hydrogen bond between the polymeric chains, hence it has high m.p. and tensile strength.
-          Typical condensation polymer includes polyester (dioic acid + ethylene glycol), nylon, urea-methanal and phenol-methanal.
-          Thermoplastic are softened upon heating, can be remoulded and hardened when cooled down. Most plastics are thermoplastics.
-          Thermosets, can’t be remoulded once it’s cooled. It’s due to the cross linkage between polymeric chains. For example, in urea-methanol, the repeating unit includes –NH- group. When methanal is added, the two H from two –NH- from two polymeric chains and the O in methanal eliminates in forms of water, when a methyl group links the two polymeric chains, which is known as the cross linkage. Typical thermosets includes urea-methanal (white) and phenol-methanal (black). They are often used as the case of electrical sockets.
-          It causes environmental problem since it’s non-biodegradable. We can make it biodegradable or photodegradable, or reduce the use of plastics by the 5Rs, etc.
Aspirin, IUPAC name: 2-ethanoloxybenzoic acid
It’s used to reliving pain/inflammation/fever or reducing risk of heart attack. The adverse effect is the irritation on stomach.
Synthesis: It can be prepared salicylic acid (2-hydroxybenzoic acid), forming ester with methanol. So we can use alkaline hydrolysis (back titration) to determine the % content / mass of aspirin contained in a tablet by C9H8O4 + 2NaOH → CH2COONa + C7H5O3Na + H2O, where C7H5O3Na is sodium 2-hydroxybenzoate.
Detergents: cleaning agents
Water has a high surface tension so it beads up on the surface, wetting up the agents slowing and hence can’t clean well. Detergents contains a hydrophobic hydrocarbon tail (alkane chain) and a hydrophilic anionic head (O-Na+).
1)       Soapy detergents are made from natural fats and oils. The anionic head is carboxylate, like –COO-K+.
2)       Soapless detergents are made from chemicals (petroleum). The anionic head is sulphate like –SO4-Na+.
Detergents as a wetting agent: the anionic head sticks on the water surface and separate the water molecules, then the surface tension decreases and the water spreads (instead of staying as beads) and spreads.
Detergents as an emulsifying and cleaning agent: originally there’s no interaction between water and the grease, the hydrophobic tails dissolves in the grease while the hydrophilic anionic head dissolves in water. Water keeps on attract the anionic tails and lifts the grease off the fabric. Upon stirring, the grease breaks up into tiny droplets and these droplets can’t combine again due to electrostatic repulsion among the anionic heads of the detergents particles. That explains the emulsifying effect and cleaning principle of detergents.
Soapy detergents form ppt when applied in hard water (which contains a greater amount of dissolved Ca2+ and Mg2+ ions due to displacement in the carboxylate ions. The insoluble white ppt accumulated is called scums. However, soapless detergents easily cause allergic, and it’s non-biodegradable.
Fats and lipids

Lipids are in forms of triglyceride, which is a condensed product from glycerol (1,2,3-propantriol) and fatty acids (a long chain carboxylic acid). Upon hydrolysis it becomes glycerol and fatty acids. So we can apply alkaline hydrolysis such that fatty acids become sodium carboxylate, which is a detergent. Therefore by heating oil and conc. NaOH we can obtain soapy detergents. We add conc. NaCl to lower the solubility of soap in water and a “soap cake” floats on the surface.
Soapless detergents can be prepared by alkylbenzene from petroleum. Upon sulphonation (conc. H2SO4 (aq) + excess SO3 (g)), SO3H group is added on the para- position. Adding NaOH would give SO3-Na+ and it can be soapless detergents.
Unsaturated fats means that it contains C=C bond (unsaturated compounds), which has a lower melting point (packed less regularly), usually liquid state in room temperature. So we hydrogenate the unsaturated fats to produce margarine. However, the by-product with trans- C=C bonds can’t be digested and accumulates in our body, which is known as trans fats, increasing risk of heart diseases.
Simple carbohydrates are known as sugars or saccharides. Monosaccharides are carbohydrates like glucose and fructose that can’t be cleaved into smaller carbohydrates by acidic hydrolysis, disaccharides can be broken down into two monosaccharides by hydrolysis while polysaccharides can be broken into more monosaccharides.
Monosaccharides can be classified into aldoses (containing aldehyde group) or ketoses (containing a ketone group). Glucose is a aldohexose with three structure, one is open-chain while another two is in cyclic form. The figure shows the open structure of glucose. The cyclic form is given by linking the first and last carbon by the O instead of the carbonyl group.

Proteins and polypeptides
Amino acids are in forms of H2NCHRCOOH, where R differs for different amino acids.
It condenses into a dipeptide by eliminating H2O from –COOH + H2N- to –CONH- + H2O.
Hydrolysis can be carried out: acidic hydrolysis gives Cl-H3N+CHRCOOH while alkaline hydrolysis gives H2NCHRCOO-. In living organisms the process are done by enzymes instead of chemicals.

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