Why is aromaticity stable

Ernest Z. Jan 6, Aromatic rings are stable because they are cyclic, conjugated molecules. In a cyclic conjugated molecule, each energy level above the first occurs in pairs.

A Frost circle reflects the pattern of orbital energies. You place the ring inside a circle with one of its vertices pointing downwards.

Then you draw a horizontal line though the vertices. This represents the orbital energy levels. Monocyclic compounds made up of alternating conjugated double bonds are called annulenes.

Benzene and 1,3,5,7-cyclooctatetraene are examples of annulenes; they are named [6]annulene and [8]annulene respectively, according to a general nomenclature system in which the number of pi-electrons in an annulene is designated by a number in brackets. Some annulenes are aromatic e. As shown in the following diagram, 1,3,5,7,9-cyclodecapentaene fails to adopt a planar conformation, either in the all cis-configuration or in its 1,5-trans-isomeric form.

The transannular hydrogen crowding that destabilizes the latter may be eliminated by replacing the interior hydrogens with a bond or a short bridge colored magenta in the diagram. Naphthalene and azulene are [10]annulene analogs stabilized by a transannular bond.

The bridged [14]annulene compound on the far right, also has aromatic properties. A modified [10]annulene, aromatic by nmr criteria, was prepared recently by chemists at California Institute of Technology.

Remarkably, this hydrocarbon is chemically unstable, in contrast to most other aromatic hydrocarbons. To learn more Click Here. A synthesis of barrelene bicyclo[2. Zimmerman Wisconsin , using a double Hofmann elimination. As shown in the following diagram, the chemical behavior of this triene confirmed it was not aromatic in the accepted sense of this term.

Bromine addition took place rapidly with transannular bond formation, in the same fashion as with norbornadiene bicyclo[2. Pyrolysis of barrelene gave the expected cycloreversion products benzene and acetylene. The heat of hydrogenation of barrelene reflects its thermodynamic stability.

Furthermore, the first double bond of barrelene is reduced with the release of An explanation for the lack of aromatic behavior in the case of barrelene may be found by comparing the orbital symmetry of the six component p-orbitals with those of benzene.

Benzene is an annulene in which all six p-orbitals may be oriented with congruent overlapping phases. The cylindrical array of p-orbitals in barrelene cannot be so arranged, as shown in the diagram on the right. There will always be one region a nodal plane in which the transannular overlap is incongruent. By clicking on this diagram , a Jmol model of barrelene will be displayed in a separate window. This model may be moved about for viewing. The p-orbitals of the double bonds may also be displayed.

As noted above, 1,3,5,7-cyclooctatetraene is non-planar and adopts a tub-shaped conformation. The compound is readily prepared, and undergoes addition reactions typical of alkenes. Catalytic hydrogenation of this tetraene produces cyclooctane. The simple C 8 H 6 hydrocarbon pentalene does not exist as a stable compound, and its hexaphenyl derivative is air sensitive.

Azulene is a stable blue crystalline solid that undergoes a number of typical aromatic substitution reactions. Other examples may be cited. Thus, all attempts to isolate 1,3-cyclobutadiene have yielded its dimer, or products from reactions with other compounds introduced into the reaction system. Cyclooctatetraene is a fascinating compound. To see more of its chemistry Click Here. Practice Problems. Return to Table of Contents. This page is the property of William Reusch. Comments, questions and errors should be sent to whreusch msu.

These pages are provided to the IOCD to assist in capacity building in chemical education. Recall the definitions of electrophile and nucleophile: Electrophile : An electron deficient atom, ion or molecule that has an affinity for an electron pair, and will bond to a base or nucleophile.

Nucleophile : An atom, ion or molecule that has an electron pair that may be donated in bonding to an electrophile or Lewis acid. Many functional groups have weakly electrophilic carbon atoms colored red in the following examples. These electrophilic functions may react with nucleophiles bases in two distinct ways:. Because these electrophilic reactants are weak, such reactions normally require strong nucleophiles or bases to proceed. Some confusion in distinguishing basicity base strength and nucleophilicity nucleophile strength is inevitable.

Since basicity is a less troublesome concept; it is convenient to start with it. Basicity refers to the ability of a base to accept a proton. Basicity may be related to the pK a of the corresponding conjugate acid, as shown below. The strongest bases have the weakest conjugate acids and vice versa. The range of basicities included in the following table is remarkable, covering over fifty powers of ten!

In an acid-base equilibrium the weakest acid and the weakest base will predominate they will necessarily be on the same side of the equilibrium. Learning the pK a values for common compounds provides a useful foundation on which to build an understanding of acid-base factors in reaction mechanisms. Nucleophilicity is a more complex property. It commonly refers to the rate of substitution reactions at the halogen-bearing carbon atom of a reference alkyl halide, such as CH 3 -Br.

Thus the nucleophilicity of the Nu: — reactant in the following substitution reaction varies as shown in the chart below:. The reactivity range encompassed by these reagents is over 5, fold, thiolate being the most reactive. Clearly, there are significant differences between these nucleophilicities and the basicities discussed above.

Some useful trends have been documented: i For a given element, negatively charged species are more nucleophilic and basic than are equivalent neutral species. Basicity varies in the opposite manner. For two or more molecules incorporating nucleophilic atoms of the same kind and charge, the stronger base is usually the stronger nucleophile.

Thus, 2,2,2-trifluroethoxide pK a 12 is a weaker base and nucleophile than ethoxide pK a A notable exception to this rule occurs when a vicinal adjacent atom carries a non-bonding electron pair.

Two common examples of this exception, called the alpha effect , are hydroxide ion pK a In each of these pairs the weaker base is the stronger nucleophile. Solvation of nucleophilic anions markedly influences their reactivity. The nucleophilicities cited above were obtained from reactions in methanol solution. Polar, protic solvents such as water and alcohols solvate anions by hydrogen bonding interactions, as shown in the diagram on the right. These solvated species are more stable and less reactive than the unsolvated "naked" anions.

Polar, aprotic solvents such as DMSO dimethyl sulfoxide , DMF dimethylformamide and acetonitrile do not solvate anions nearly as well as methanol, but provide good solvation of the accompanying cations. Consequently, most of the nucleophiles discussed here react more rapidly in solutions prepared from these solvents.

In reality , however, benzene has about bb -" Why are aromatic compounds stable? Truong-Son N. Jan 9, For instance, consider the hypothetical 1,3,5-cyclohexatriene, and compare its enthalpy of complete hydrogenation to that of benzene: This would indicate how much energy input it requires to break all the pi interactions in the system.

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