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Benzene 02.07.2020.Pdf Benzene Benzene is one of the most important organic compounds with the chemical formula C6H6. Benzene is the parent compound of the various aromatic compound. The benzene molecule is composed of six carbon atoms joined in a ring with one hydrogen atom attached to each. As it contains only carbon and hydrogen atoms, benzene is classed as a hydrocarbon. Benzene is a natural constituent of crude oil and is one of the elementary petrochemicals. Due to the cyclic continuous pi bonds between the carbon atoms, benzene is classed as an aromatic hydrocarbon, the second [n]-annulene ([6]-annulene). It is sometimes abbreviated PhH. Benzene is a colorless and highly flammable liquid with a sweet smell, and is responsible for the aroma around petrol (gasoline) stations. It is used primarily as a precursor to the manufacture of chemicals with more complex structure, such as ethylbenzene and cumene, of which billions of kilograms are produced annually. As benzene has a high octane number, aromatic derivatives like toluene and xylene typically comprise up to 25% of gasoline (petrol). Chemical formula C6H6 −1 Molar mass 78.114 g·mol Appearance Colorless liquid Odor Aromatic, gasoline-like Density 0.8765(20) g/cm3[2] Melting point 5.53 °C (41.95 °F; 278.68 K) Boiling point 80.1 °C (176.2 °F; 353.2 K Soluble Solubility in alcohol, CHCl3, CCl4, diethyl ether, acetone, acetic acid Molecular shape Trigonal planar Related compounds Toluene,Borazine Structure of Benzene The structure of benzene has been of interest since its discovery. Benzene is a cyclic hydrocarbon (chemical formula: C6H6), i.e., each carbon atom in benzene is arranged in a six-membered ring and is bonded to only one hydrogen atom. According to molecular orbital theory, benzene ring involves the formation of three delocalized π – orbitals spanning all six carbon atoms, while the valence bond theory describes two stable resonance structures for the ring. Benzene structure Properties of Benzene The various properties of benzene are mentioned below: Benzene is immiscible in water but soluble in organic solvents. It is a colourless liquid and has an aromatic odour. It has a density of 0.87g cm-3. It is lighter than water. Benzene has a moderate boiling point and a high melting point. (Boiling point: 80.5°C, Melting point: 5.5°C) Benzene shows resonance. It is highly inflammable and burns with a sooty flame. Lighter than water Non-polar Miscible with non-polar solvents like petroleum ether, CCl4 etc. M.P. and B.P. show the usual regular gradation Inflammable and burns with sooty flame Resonance of Benzene The oscillating double bonds in the benzene ring are explained with the help of resonance structures as per valence bond theory. All the carbon atoms in the benzene ring are sp2 hybridized. One of the two sp2 hybridized orbitals of one atom overlaps with the sp2 orbital of adjacent carbon atom forming six C-C sigma bonds. Other left sp2 hybridized orbitals combine with s orbital of hydrogen to form six C-H sigma bonds. Remaining unhybridized p orbitals of carbon atoms form π bonds with adjacent carbon atoms by lateral overlap. This explains an equal possibility for the formation of C1 –C2, C3 – C4, C5 – C6 π bonds or C2 – C3, C4 – C5, C6-C1 π bonds. The hybrid structure is represented by inserting a circle in the ring as shown below in the figure. Hence, it explains the formation of two resonance structures proposed by Kekule. Kekule structure of benzene Aromaticity of benzene Benzene is an aromatic compound, as the C-C bonds formed in the ring are not exactly single or double, rather they are of intermediate length. Aromatic compounds are divided into two categories: benzenoids (one containing benzene ring) and non- benzenoids (those not containing benzene ring), provided they follow Huckel rule. According to Huckel rule, for a ring to be aromatic it should have the following property: Planarity Complete delocalization of the π electrons in the ring Presence of (4n + 2) π electrons in the ring where n is an integer (n = 0, 1, 2, . .) Uses of Benzene Benzene is used in various industrial processes such as in the manufacture of lubricants, plastics, rubbers, dyes, synthetic fibres, etc. However, it has non-industrial uses too which are limited due to the reason benzene is toxic and carcinogenic. The different uses of Benzene are mentioned below. Benzene is used in the preparation of phenol. It is also used to prepare aniline used in dyes and in dodecylbenzene used for the detergents. In early times, benzene was used in degreasing of metal. It is used for manufacturing of nylon fibres. The main use of benzene is that it is used in the manufacture of other chemicals such as ethylbenzene, cyclohexane, cumene, nitrobenzene, alkylbenzene, etc. Benzene and derivatives of benzene Benzene derivatives have from one to six substituents attached to the central benzene core. Examples of benzene compounds with just one substituent are phenol, which carries a hydroxyl group, and toluene with a methyl group. When there is more than one substituent present on the ring, their spatial relationship becomes important for which the arene substitution patterns ortho, meta, and para are devised. For example, three isomers exist for cresol because the methyl group and the hydroxyl group can be placed next to each other (ortho), one position removed from each other (meta), or two positions removed from each other (para). Xylenol has two methyl groups in addition to the hydroxyl group, and, for this structure, 6 isomers exist. Representative arene compounds The arene ring has an ability to stabilize charges. This is seen in, for example, phenol (C6H5–OH), which is acidic at the hydroxyl (OH), since a charge on this oxygen (alkoxide –O−) is partially delocalized into the benzene ring Preparation of Aromatic Hydrocarbons:- Friedel-Crafts Alkylation Wurtz-Fittig reaction Reduction of acylbenzenes Hydrogenation of alkenylbenzenes 1) Friedel-Crafts Alkylation:- 2) Wurtz-Fittig reaction:- Reduction of acylbenzenes Hydrogenation of alkenylbenzenes General Chemical Properties A. Electrophilic aromatic substitution reactions 1. Halogenation: 2. Nitration: 3. Sulphonation: 4. Friedel-Crafts Alkylation: .
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