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Aromaticity

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Published in: Chemical | Chemistry
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Detailed explanation of aromaticity with examples. It is helpful for 11 and 12 grade students

Sivasankare G / Bangalore

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Qualification: M.Sc

Teaches: Computer, English, EVS, Science, Chemistry, JIPMER

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  1. Aromaticity Introduction: In the world of organic chemistry, there is a big community called hydrocarbons. This community people is majorly made up of carbons and hydrogens. There are two teams in this community one team is called aliphatic and another one is called aromatic. The main difference between these two are former one is an open structure and the aromatic is a closed structure. Today we are going to categorise aromatic and aliphatic people into teams. Mainly, We are going to form a team of aromatic people. For that few conditions or qualifications needs to be satisfied. What are aromatic compounds? Aromatic compounds are composed of carbons and hydrogens that are arranged in ring structures with delocalized pi electrons. Aromatic hydrocarbons are named so due to their pleasant aroma. Aromatic hydrocarbons are essentially cyclic structures. These are also planar structures. What are aliphatic compounds? Aliphatic hydrocarbons are also composed of carbon and hydrogen atoms arranged in straight chains, branched or non-aromatic ring structures. Carbon and hydrogen atoms are bonded to each other via covalent bonds. Aliphatic hydrocarbons can be found in three types as alkanes, alkenes, and alkynes. Conditions to be satisfied: 1. It must be cyclic. 2. Every atom in the ring must be conjugated andsp hybridised. 3. The molecule must have [4n+2] pi electrons (n= 0, 1 4. The molecule must be planar (usually true if conditions 1-3 are met, but there are rare exceptions) Molecule Benzene Cyclic? Yes Conjugated pi bond? Yes Number of pi electrons 6 Planar? yes Aromatic or not? Aromatic
  2. o Trophylium ion NH Pyrrole yes yes No yes yes yes yes yes yes yes No Yes yes yes No yes 6 (lone pair not involved in the conjugation) 6 6 (the lone involves in conjugation) 10 4 4 yes yes yes yes yes yes yes yes Aromatic Aromatic No (Aliphatic) Aromatic Anti aromatic Aromatic Non-aromati c Anti aromatic
  3. yes yes 6 yes Aromatic Molecular Orbital diagram for Aromatic compounds: anti-bonding orbitals o Energy bonding orbitals molecular orbital diagram for benzene Frost Circles: Cyclobutadiene (four electrons) (six electrons) Cyc I ooctatetracne (eight electrons) Antibonding orbitals — Nonbonding orbitals Bonding orbitals
  4. Time to ask "WHY?". 1. If lone pair of pyridine does not involve conjugation, but pyrrole does. why? (5 pyridine (arotnatic) 2m 2pyi unhybridized atomic orbitals sp-' one sp2 is of sp2• hybridized atomic Orbitals unhybridized p orbital The lone pair of electrons on nitrogen is in sp - hybridized orbital that is perpendicular to the ring pi system. The lone pairs must be in coplanar orbitals to delocalize. So the lone pair is not counted in Hükel's rule. pyrole The lone pair of electrons on nitrogen is in a pure p-orbital that is coplanar to the ring IT-system, so the lone pair is counted in Hükel's rule.
  5. 2. 3. 4. 5. What about compound having two lone pairs like furan? O furan The lone pair here is pure p-orbital coplanar to the ring IT-system (is counted in Hükel's rule). The second lone pair is in an sp -hybridized orbital perpendicular to the ring IT-system (not counted in Hükel's rule) What if all are satisfied, except pi-electron count? If all three are satisfied and it follows 4n pi electrons then that is "Anti aromatic" compounds which are highly unstable and reactive compared to others. Because while drawing MOT for antiaromatic compounds, the electrons lie in higher energy regions. Why only 4n+2? According to Hückel's Molecular Orbital Theory, a compound is stable if all of its bonding molecular orbitals are filled with paired electrons. This is true of aromatic compounds, meaning they are quite stable. With aromatic compounds, 2 electrons fill the lowest energy molecular orbital, and 4 electrons fill each subsequent energy level (the number of subsequent energy levels is denoted by n), leaving all bonding orbitals filled and no anti-bonding orbitals occupied. This gives a total of 4n+2 TT electrons. What is the limitation of Huckel's rule? Some compounds possess aromaticity but still, they do not follow Huckel's rule. Generally, these compounds are polycyclic. For example, Pyrene is a polycyclic aromatic compound and has 4 fused benzene rings. It has 8 pi — bonds, thus 16 pi electrons. Thus, it does not follow Huckel's rule but is still an aromatic compound. Because only 14 electrons are involved in the delocalisation, the bold lines show the path for delocalisation. Other exceptions are trans — bicalicene, coronene. Both of these are also polycyclic aromatic compounds that contain 8 and 12 pi — bonds respectively and do not follow Huckel's rule. But still, show aromaticity.
  6. pyrene Summary: Aromatic Cyclic Planar Conjugated Have delocalised pi electrons system and 4n+2 pi electrons Stable o Test yourself: Anti aromatic Cyclic Planar Conjugated Have delocalised pi electrons system and 4n pi electrons Highly unstable Non-aromatic Compounds that are lack one or more requirements to be aromatic May or may not have the delocalised pi-electron system unstable 1. Which of the compound below is Anti aromatic? a. Naphthalene b. [10] Annulene c. Imidazole d. Pentalene Answer: Option d
  7. Solution: a. b. c. d. Naphthalene 10 electrons, cyclic, planar - aromatic [10] annulene 10 electrons, cyclic, non-planar - non-aromatic Imidazole 6 pi electrons, cyclic, planar - aromatic Pentalene Pentalene 8 pi electrons, cyclic, planar - anti aromatic
  8. 2. Which compound below is either not aromatic or antiaromatic? Lone pairs are not shown. a. o b. c. d. o o o Ans: option d Solution: a. oxepine 8 pi electrons - anti aromatic b. I ,3-dioxoI-2-yIium 6 pi electrons Aromatic
  9. c. 1 H-borole 4 pi electrons anti aromatic d. o 4H-pyran Not conjugated - non-aromatic Time to think: Think about the property of Biphenyls and allenes. Are they aromatic or not?

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