Aromatic compound Huckel rule

German physicist Erich Hiickel used the molecular orbital theory to explain the stability of benzene and other aromatic compounds. Huckel s rule determines the number of ir electrons that give stability to an unsaturated planar ring according to the formula 4n -I- 2. Eor benzene and its analogs,... 

Section 11 23 Huckel s rule can be extended to heterocyclic aromatic compounds Unshared electron pairs of the heteroatom may be used as tt electrons as necessary to satisfy the 4n + 2 rule... 

We saw in Chapter 12 that aromaticity reveals itself in various ways Qualitatively aro matic compounds are more stable and less reactive than alkenes Quantitatively their heats of hydrogenation are smaller than expected Theory especially Huckels rule furnishes a structural basis for aromaticity Now lets examine some novel fea tures of their NMR spectra... 

The Huckel rule is strictly applicable only to monocyclic compounds, but the genera] concept of aromaticity can be extended beyond simple monocyclic compounds to include polycyclic aromatic compounds. Naphthalene, with two... 

We know that aromatic compounds, polynuclear and heterocyclic compounds follow Huckel s rule in which they have a (4n + 2) system of n electrons and the protons attached to such systems are extremely deshielded due to the circulating sextet of n electrons. As a result, the signals of aromatic protons appear at a very low field than those observed even for benzene. The tau value of benzene is T = 2.63. From this the aromaticity of a compound can be verified. 

While benzene was the first aromatic system studied, the formulation of HtickePs rule and the theory behind it created an impetus to prepare non-benzenoid species such as the tropylium cation and cyclopentadienyl anion that also obeyed Huckel s rule to see if these species were also aromatic. This required that the properties of aromatic compounds be defined. 

The requirements necessary for the occurrence of aromatic stabilisation, and character, in cyclic polyenes appear to be (a) that the molecule should be flat (to allow of cyclic overlap of p orbitals) and (b) that all the bonding orbitals should be completely filled. This latter condition is fulfilled in cyclic systems with 4n + 2n electrons (Huckel s rule), and the arrangement that occurs by far the most commonly in aromatic compounds is when m = 1, i.e. that with 6k electrons. IOti electrons ( = 2) are present in naphthalene [12, stabilisation energy, 255 kJ (61 kcal)mol ], and 4n electrons (n = 3) in anthracene (13) and phenanthrene (14)—stabilisation energies, 352 and 380 kJ (84 and 91 kcal) mor respectively ... 

D is correct. Anthracene is a larger version of benzene, a prototvpic aromatic compound. It satisfies Huckel s rule, which states that if a compound has planar, monocyclic rings with 4n + 2 n electrons (n being any integer, including zero), it is by definition an aromatic compound. Benzene houses six x electrons, a pair for each double bond (while anthracene has 14 n electrons). 

The inscribed polygon method is consistent with Huckel s 4n + 2 rule that is, there is always one lowest energy bonding MO that can hold two Jt electrons and the other bonding MOs come in degenerate pairs that can hold a total of four Jt electrons. For the compound to be aromatic, these MOs must be completely filled with electrons, so the magic numbers for aromaticity fit Huckel s 4n + 2 rule (Figure 17.11). 

What value of n in Huckel s rule applies to the aromatic compound shown above ... 

Aromatic compounds are planar, conjugated systems which have 4n + 2 electrons (Huckel s rule) (see Section 7.1). If, on deprotonation, the anion is part of an aromatic Tt-system, then the negative charge will be stabilised. Aromaticity will therefore increase the acidity of the compound. 

As discussed in the Introduction, Huckel s rule is an electronic criterion for aromaticity, and is based on the configuration of the rr electrons. Another characteristic of aromatic compounds is a relatively large HOMO-LUMO gap, which indicates the absence of high-energy, reactive electrons, in agreement with the reduced reactivity of aromatic compounds to electrophilic reagents. This facet of electronic configuration can be expressed in terms of hardness (see p. 96 for the definition of hardness in terms of DFT theory). ... 

The Huckel rule is strictly applicable only to monocyclic compounds, but the general concept of aromaticity can be extended beyond simple mono-cyclic compounds to include polycyclic aromatic compounds. Naphthalene, with two benzene-like rings fused together, anthracene, 1,2-benz-pyrene, and coronene are all well-known compounds, BenzoLa]pyrene is particularly interesting because it is one of the cancer-causing substances that has been isolated from tobacco smoke. 

An aromatic compound is a cyclic structure that contains continuous tt electron clouds above and below the plane of the molecule. The ir clouds must also contain a total of kn+2 tt electrons, where n can be any integer. This requirement is called the 4n+2 rule or Huckel s rule and is derived from quantum mechanics. Thus, to determine whether any given compound is aromatic, first inspect the structure to see if it is cyclic and planar. Then detennine the number ofTf electrons and see if all these satisfy Kuckel s rule. 

The 14 IT electrons satisfy Huckel s Rule for aromaticity (4n +2, n = 3). Also,note that in aceplieadylene there are no hydrogens crowded in the center of the ring system, and so the structure will be considerably stable. This aromatic compound has actually been made, and is a red solid that is stable enough to form a mononitro compound. 

According to Huckel s rule, aromatic compounds are cyclic, fully conjugated, planar, and have (4n+2) tt electrons. Antiaromatic compounds are cyclic, fully conjugated, planar, and have 4n tt electrons,... 

This chapter deals with heteromacrocycles rather than smaller-ring heterocycles. Traditionally, heterocycles are three- to eight- or nine-membered rings that contain one or more noncarbon heteroatoms. Many, if not most, of the known heterocycles are aromatic compounds in which the noncarbon atoms contribute electrons to meet Huckel rule requirements. The crown ether situation contrasts with that of traditional heterocycles. The bulk of crown ether compounds contain 12 or more members and are aliphatic rather than aromatic. Those heterocycles that have cation binding properties usually contribute one or at most two donors to a cation s primary solvation... 

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