Dear students in organic chemistry Some diff bw Benzene and phenyl is given below Benzene Benzene only has carbon and hydrogen atoms arranged to give a planar structure. It has the molecular formula of C6H6. Its structure and some of the properties are as follows. Benzene structure was found by Kekule in 1872. Because of the aromaticity, it is different from aliphatic compounds. Molecular weight: 78 g mole-1 Boiling point: 80.1 oC Melting point: 5.5 oC Density: 0.8765 g cm-3 Benzene is a colorless liquid with a sweet odor. It is flammable and evaporates quickly when exposed. Benzene is used as a solvent, because it can dissolve a lot of non-polar compounds. However, benzene is slightly soluble in water. The structure of benzene is unique compared to other aliphatic hydrocarbons; therefore, benzene has unique properties. All the carbons in benzene have three sp2 hybridized orbitals. Two sp2 hybridized orbitals of a carbon overlap with sp2 hybridized orbitals of adjacent carbons in either side. Other sp2 hybridized orbital overlaps with the s orbital of hydrogen to form a σ bond. The electrons in p orbitals of a carbon overlap with the p electrons of carbon atoms in both sides forming pi bonds. This overlap of electrons happens in all the six carbon atoms and, therefore, produces a system of pi bonds, which are spread over the whole carbon ring. Thus, these electrons are said to be delocalized. The delocalization of the electrons means that there aren’t alternating double and single bonds. So all the C-C bond lengths are the same, and the length is between single and double bond lengths. Because of the delocalization benzene ring is stable, thus, reluctant to undergo addition reactions, unlike other alkenes. Phenyl Phenyl is a hydrocarbon molecule with the formula C6H5. This is derived from benzene, therefore, has similar properties as benzene. However, this differs from benzene due to lack of a hydrogen atom in one carbon. So the molecular weight of phenyl is 77 g mole-1. Phenyl is abbreviated as Ph. Usually phenyl is attached to another phenyl group, atom or molecule (this part is known as the substituent). The carbon atoms of phenyl are sp2 hybridized like in benzene. All the carbons can form three sigma bonds. Two of the sigma bonds are formed with two adjacent carbons, so that it will give rise to a ring structure. The other sigma bond is formed with a hydrogen atom. However, in one carbon in the ring, the third sigma bond is formed with another atom or molecule rather than a hydrogen atom. The electrons in p orbitals overlap with each other to form the delocalized electron cloud. Therefore, phenyl has similar C-C bond lengths between all carbons, regardless of having alternating single and double bonds. This C-C bond length is about 1.4 Å. The ring is planar and has a 120o angle between bonds around a carbon. Due to the substituent group of phenyl, the polarity and other chemical or physical properties change. If the substituent donate electrons to the delocalized electron cloud of the ring, those are known as electron donating groups (E.g. -OCH3, NH2). If the substituent attracts electrons from the electron cloud, it is known as electron withdrawing substituent. (E.g. -NO2, -COOH). Phenyl groups are stable due to their aromaticity, so they don’t easily undergo oxidations or reductions. Further, they are hydrophobic and non-polar.
Posted on: Tue, 09 Jul 2013 02:40:57 +0000
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