All organic compounds can be classified in two types; aliphatic and aromatic compounds. Aliphatic compounds are organic compounds with straight or branched carbon chain where C atoms are bonded with other atoms such as H, N, S and O atoms. They can have multiple bonds between carbon atoms and on the basis of the presence of multiple bonds; aliphatic compounds can be classified as alkane, alkene and alkyne.
Out of these three; alkene and alkyne are unsaturated aliphatic compounds as they have double and triple bonds between carbon atoms of parent chain.`
Cyclic organic compounds can be two types; alicyclic and aromatic compounds. Alicyclic compounds are like aliphatic compounds with single or multiple covalent bonds between carbon atoms. They are also named as cycloalkanes and cycloalkynes.
Another type of cyclic compounds is aromatic compounds. They are well known for their specific aroma and extra stability. Today we know a large number of aromatic compounds with their vast applications in different fields. Now question arise; how will we determine that compound is aromatic or not. All aromatic compounds must satisfy certain conditions;
- Planer structure
- 4n+2 pi electron system
The planarity and 4n+2 pi electron system causes delocalization of electrons that is responsible for the extra stability to the molecule. For example benzene is a planer cyclic structure with 6 C atoms in a ring. Here presence of 3 pi bonds in the cyclic ring makes the compounds more stable then cycloalkenes. Six pi electrons delocalized over 6 Carbon atoms of ring that forms two resonating structures of benzene as given below.
Some other aromatic compounds are given below.
All the given compounds above have benzene ring with some side chain or functional group such as âNH2, -OH, -CH3, -CHO etc. All of these aromatic compounds have a common benzene ring so they are also known as derivatives of benzene. Because of extra stability of aromatic ring, unlike alkenes aromatic compounds tend to give electrophilic substitution reactions as aromatic rings are electron rich and H of ring can be substituted with some other group.
Alkyl benzene derivatives are simplest derivatives of benzene in which some alkyl group is bonded with aromatic ring. For example C6H5-CH3 is methyl benzene, commonly known as toluene. Similarly C6H5-CH2-CH3 is ethyl benzene whereas isopropyl benzene is called as cumene.
Styrene is an aromatic compound in which vinyl group (-CH=CH2) is bonded with aromatic ring. It is one of the well known aromatic compounds and mainly used in the manufacturing of latex, synthetic rubber and resin. The polystyrene resins are used in the manufacturing of disposable cups, plastic packaging and containers, insulation, and other products.
This aromatic compound is also found in some plants and very often detected as air pollutant in urban air. The major sources of styrene in atmosphere are photocopiers and laser printers, packaging made of polystryrene and cigarette smoke. A low level of exposure does not affect much but long exposure can cause irritation of the eyes, breathing passages and injury to their nervous systems. The major application of styrene is in the production of polystyrene plastics and resins. The short term exposure of styrene causes irritation in mucous membrane, eye irritation and gastrointestinal effects whereas long term exposure results headache, fatigue, weakness, and depression and hearing loss. It is also present in alcoholic beverages, cranberry, vinegar, parsley, bilberry, currants, grapes, milk and dairy products. It is a flavoring ingredient for some of the food stuff. Polystyrene is also used in ion-exchange resin in food processing, to form adhesives, oatings and packaging materials. Since a vinyl group is bonded with benzene ring, styrene is also called as vinyl benzene. It is a colorless oily and volatile liquid with sweet smell at low concentration which turns less pleasant odor at high concentration. It acts as precursor for polystyrene and several other copolymers such as Buna-S. Some other industry uses of polystyrene are as fuels and fuel additives, intermediate in many organic reactions, in manufacturing of paint additives and coating additives, plasticizers, in processing aids etc. At the same time there are various consumer uses also such as in building and construction materials and as plastic and rubber products.
Todayâs life is directly or indirectly affected by different polymers as they are very convenient to use and very economical products. Some common polymers we used are polyethylene, polyvinyl chloride, polystyrene etc. Polystyrene which is mainly used to form styrene plastics is produced by addition polymerization of styrene.
Polymerization of styrene can occur with Radical Chain-Growth Polymerization mechanism. In this polymerization reaction, we require some radical initiator like peroxide or certain azo compounds.
First two steps are initiation steps for the formation of free radicals which further propagate the chain propagation steps. In chain propagation step, monomer units are added to free radicals to continue the reaction and increase the length of polymer chain. Polystryene can also prepare with Anionic Chain-Growth Polymerization in which reaction of cold THF solution of styrene with n-butyllithium causes polymerization to form polystyrene.
Styrene is an aromatic compound with structural formula C6H5-CH=CH2. Here one vinyl group is bonded with aromatic ring therefore it can be considered as the vinyl derivative of benzene. Since it is an aromatic organic compound which is composed of C and H only therefore it is a non-polar compound with less solubility in water approx 1 mg / mL. It boils at 293 F at 760 mmHg pressure. The relative density of styrene is 0.9060 (20/4℃), refractive index is 1.5469, viscosity is 0.762 cP at 68°F. It exists in liquid state at room temperature. Some of the properties of styrene are listed below.
| Properties of styrene
| Chemical formula
| Molar mass
|| 104.15 g/mol
|| colorless oily liquid
|| 0.909 g/cm3
| Melting point
| Boiling point
| Solubility in water
| Vapour pressure
|| 5 mmHg
Styrene butadiene rubber is also abbreviated as SBR. It is one of the cheaper synthetic elastomers. It acts as a substitute for natural rubber (NR). It is monomer of styrene with Butadiene monomer units. These monomer units polymerize with free radical polymerization or emulsion polymerization. The reaction can occur at temperature at 30 to 60Â°C or cold at temperatures near 0Â°C.Since it is a polymer of two different monomer units therefore it is also called as copolymer. It is one of highly random polymer with 10-25% styrene as monomer unit.
The presence of styrene lowers the price and enhances the wear and bonding properties with strength, blend and abrasion resistance properties of polybutadiene polymer. SBR has good fatigue resistance, low temperature properties, heat-aging properties and abrasion resistance which are advantage of natural rubber. SBR can exhibit thermal and oxidative degradation because of presence of double bonds in the polymeric backbone. Degradation of polymeric chain occurs through cross-linking that increases the stiffness. SBR has 25% styrene and 75% butadiene monomer units whereas with higher butadiene content produces Styrene-Butadiene Latex which is more elastic compare to SBR. Today SBR is marketed with the name of Buna-S. The polymerization of 1,3-butadiene with styrene occurs in the presence of Na as catalyst at high temperature to form Buna-S (SBR).