{"id":457,"date":"2014-12-23T17:06:58","date_gmt":"2014-12-23T17:06:58","guid":{"rendered":"http:\/\/www.studyorgo.com\/blog\/?p=457"},"modified":"2014-12-23T17:06:58","modified_gmt":"2014-12-23T17:06:58","slug":"aromaticity-rules-and-definition","status":"publish","type":"post","link":"https:\/\/www.studyorgo.com\/blog\/aromaticity-rules-and-definition\/","title":{"rendered":"Aromaticity Rules and Definition"},"content":{"rendered":"

Students often mention to us they are confused about the rules of aromaticity and how best to study for difficult examples.\u00a0 We at StudyOrgo.com have developed a clear and \u201cget-to-the-point\u201d presentation of the basics of aromaticity.\u00a0 This is just a sample of the clear-cut explanations available on our website to our members.
\n<\/span><\/p>\n

Aromatic Compounds<\/h2>\n

Aromatic compounds are unusually STABLE<\/strong> and have important chemical and synthetic uses.\u00a0 In fact, nucleic acids and amino acids that make up every cell rely heavily on the use aromatic compounds.\u00a0 But, what makes a compound aromatic?\u00a0 A short list of rules, discovered by Eric Huckel in the 1930\u2019s, lists the properties of aromatic compounds.<\/p>\n

The Huckel aromaticity rules are:<\/p>\n

    \n
  1. Molecule is cyclic<\/li>\n
  2. Have one p orbial per atom of the ring<\/li>\n
  3. Be planar, in an sp2 hybridized orbital, over every atom of the ring<\/li>\n
  4. Have a closed loop of 4n+2 pi-bond electrons, where n is equal to any integer (0,1,2,3,\u2026)<\/li>\n<\/ol>\n

    But like most natural phenomenon, there exists a rule exactly the opposite.\u00a0 Molecules that have an unusual INSTABILITY<\/strong> to them are anti-aromatic compounds.\u00a0 They have similar rules to aromaticity, including:<\/p>\n

      \n
    1. Molecule is cyclic<\/li>\n
    2. Have one p orbial per atom of the ring<\/li>\n
    3. Be planar, in an sp2 hybridized orbital, over every atom of the ring<\/li>\n
    4. But, anti-aromatic compounds have a closed loop of 4n pi-bond electrons.<\/strong><\/li>\n<\/ol>\n

      Below are the pi-orbital diagrams of benzene, the most identifiable aromatic compound.\u00a0 Each of the three double bonds contributes 2 pi-electrons over 6 atoms, for a total of 4*1+2=6 electrons, in a ring, in a pi-orbital that is planar.\u00a0 Therefore it is aromatic.\u00a0 In contrast, hexatriene meets all of these criteria as well, but is not in a closed ring.\u00a0 Hexatriene is therefore non-aromatic.\u00a0 Finally, cyclobutadiene is the most identifiable anti-aromatic compound which is different only in that it has 4*1=4 pi-electrons, in a ring, in a pi-orbital that is planar.<\/p>\n

      \"_pi-orbital-diagrams-of-benzene\"<\/p>\n

      Heterocyclic Aromatic Compounds<\/span><\/h2>\n

      The diversity of compounds relies on using atoms other than carbon, however.\u00a0 What about when atoms with lone pairs of electrons are involved?\u00a0 A good rule of thumb is that if the atom is already participating in the pi-bond forming in the ring, then the lone pair of electrons are perpendicular to the ring and therefore are NOT participating to aromaticity.\u00a0 A good example of this is pyrimidine, where both nitrogens are already contributing to the pi-bond ring and therefore, the lone pairs of electrons are not accessible.<\/p>\n

      \"pyrimidine\"<\/p>\n

      However, there are many molecules where lone pair DO participate to aromaticity.\u00a0 Below are several examples.\u00a0 Take furan for example; oxygen has two lone pairs of electrons.\u00a0 One of them is in a geometry parallel to the pi-bond system.\u00a0 Therefore, these electrons DO participate in the pi-bond system and add 2 electrons the pi-bond count resulting in 4*1+2=6 electrons, therefore furan is aromatic.\u00a0 Imidiazole is molecule that has two nitrogen atoms; one nitrogen participating in pi-bonding and not contributing lone pairs, while the other is not participating in pi-bonding but contributes electrons the pi-bond count.\u00a0 The 4*1+2=6 electron count for imidazole renders it aromatic.<\/p>\n

      \"furan\"<\/p>\n

      Aromatic Hydrocarbon Ions<\/span><\/h2>\n

      Sometimes, carbocations and carbanions are produced in chemical reactions.\u00a0 If these species are created in a cyclic conjugated system, it is possible that they can contribute to the formation of a Huckel aromatic compound, which gives the molecule added stability and special reactivity.
      \nFor example, cyclopentadiene is not aromatic because of the sp3 hybridized carbon at position 5 on the ring.\u00a0 However, in the presence of strong base, cyclopentadiene can be deprotonated and cyclopentadienyl anion is generated.\u00a0 The lone pair of electrons assumes a sp2 hybridized orbital, making the molecule planar, adding 2 more electrons to the ring to give 4n+2 pi-electrons and creating the 5th pi orbital necessary to complete Huckel\u2019s Rule and results in an aromatic ion.<\/p>\n

      \"reating<\/p>\n

      Another example is formation of a carbocation, a common intermediate in substitution and elimination reactions.\u00a0 Deprotonation of cycloheptatriene, a non-aromatic compound, at the sp3 hybridized position creates a sp2 hybridized orbital and, although this carbon\u2019s pi-bond orbital is empty (a carbocation), it completes the 7th pi orbital necessary to complete the ring and maintains a 4n+2 electron count.\u00a0 This carbocation, called tropylium ion, is now aromatic.<\/p>\n

      \"tropylium-ion\"<\/p>\n","protected":false},"excerpt":{"rendered":"

      Students often mention to us they are confused about the rules of aromaticity and how best to study for difficult examples.\u00a0 We at StudyOrgo.com have developed a clear and \u201cget-to-the-point\u201d presentation of the basics of aromaticity.\u00a0 This is just a sample of the clear-cut explanations available on our website to our members. Aromatic Compounds Aromatic […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_monsterinsights_skip_tracking":false,"_monsterinsights_sitenote_active":false,"_monsterinsights_sitenote_note":"","_monsterinsights_sitenote_category":0,"footnotes":""},"categories":[1],"tags":[],"class_list":["post-457","post","type-post","status-publish","format-standard","hentry","category-organic-chemistry"],"aioseo_notices":[],"_links":{"self":[{"href":"https:\/\/www.studyorgo.com\/blog\/wp-json\/wp\/v2\/posts\/457","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.studyorgo.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.studyorgo.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.studyorgo.com\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.studyorgo.com\/blog\/wp-json\/wp\/v2\/comments?post=457"}],"version-history":[{"count":3,"href":"https:\/\/www.studyorgo.com\/blog\/wp-json\/wp\/v2\/posts\/457\/revisions"}],"predecessor-version":[{"id":465,"href":"https:\/\/www.studyorgo.com\/blog\/wp-json\/wp\/v2\/posts\/457\/revisions\/465"}],"wp:attachment":[{"href":"https:\/\/www.studyorgo.com\/blog\/wp-json\/wp\/v2\/media?parent=457"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.studyorgo.com\/blog\/wp-json\/wp\/v2\/categories?post=457"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.studyorgo.com\/blog\/wp-json\/wp\/v2\/tags?post=457"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}