![]() ![]() There is still one unhybridized 2p z orbital on each carbon atom. Thus there is six sigma C-C bonds and six sigma C-H bonds. The third hybrid orbital of each carbon atom overlaps axially with the half filled 1s- orbital of the hydrogen atom to form carbon-hydrogen sigma bonds. Two out of the three sp 2 hybrid orbitals of each carbon atom overlap axially with sp 2 hybrid orbitals of the neighbouring carbon atoms on either side to form carbon-carbon σ bonds. The fourth unhybridized p-orbital having two lobes is lying perpendicular to the plane of the hybrid orbital. The three sp 2 hybrid orbitals are lying in one plane and oriented at an angle of 120°. The orbital structure of benzene: All the carbon atoms in benzene are sp 2 hybridised. As a result, the energy of resonance hybrid also decreases or its stability increases. As a result, each carbon-carbon bond length in benzene is 139 pm which lies between standard C – C bond length 154 pm and C=C bond length 134 pm.Įffect of resonance: Due to resonance, the -electron charge in benzene is distributed over a greater area. Benzene molecule is a resonance hybrid of the following two main contributing structures:ĭue to resonance in benzene, the carbon-carbon bonds in benzene acquire an intermediate character of carbon-carbon single and double bonds. The stability of benzene is explained in terms of resonance. Resonance in benzene: Benzene ring has three double bonds in it and is expected to be quite reactive. ![]() ![]() Different contributing structures are written by putting a double head arrow ( ↔ ) between them. ![]() The actual structure that is intermediate between all the contributing structures is called resonance hybrid. These different structures of the molecule are known contributing structures or resonating structures or canonical forms. The following compounds are not aromatic:Ĭycloheptatriene although obeys Huckel’s rule yet it is not aromatic as it is not planar and can not show resonance.Ī resonance may be defined as a phenomenon in which a single compound is supposed to be existing as a hybrid of two or more compounds differing in the distribution of electrons and not of atoms. (b) Fused ring systems: The polynuclear hydrocarbons such as naphthalene, anthracene and phenanthrene are also aromatic according to Huckel’s rule.Īromatic ions: Some cyclic ions also exhibit aromatic character. (a) Monocyclic systems: Some monocyclic systems having π-electrons (obey Huckel’s rule) possess aromatic character. Huckel’s rule can be applied successfully to polycyclic compounds, annulenes and also other non-benzenoid compounds. According to this rule, a cyclic compound will behave as aromatic compound if it contains (4n + 2) electrons, where n may be 0, 1, 2, 3 etc. (v) An essential criterion for the aromatic character is that the compound must obey Huckel’s rule. (iv) The molecules have delocalised electron cloud above and below the plane of the ring. (iii) They must resist addition reaction and take part in the electrophilic substitution reactions. (ii) Their molecular formulae suggest these compounds are highly unsaturated due to the presence of one or more double bonds in the ring but they must behave as saturated compounds. (i) The compounds must be cyclic in nature and have flat planar structure. In NMR spectra, different peaks give information about different atoms in a molecule according specific chemical environments and bonding between atoms.The compounds possessing aromatic character show the following characteristics: In many laboratories today, chemists use nuclear magnetic resonance to determine structures of important chemical and biological compounds. The MRI also clearly shows the spinal column and skull. \): 1H MRI of a human head showing the soft tissue such as the brain and sinuses. ![]()
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