Reactions of Aromatic Compounds - Nucleophilic Aromatic Substitution Ar-SN

Dehydroarenes (Arynes)

By particularly strong bases, such as sodium amide or overheated sodium hydroxide solution, the halogens of halobenzenes can be substituted, even though the halobenzene does not contain any further activating substituents. It must be noted that in contrast to electrophilic aromatic substitution it is not the hydrogen but rather the halogen atom that is exchanged for the base. However, the base (nucleophile) is introduced to the position that was formerly occupied by the halogen atom (ipso attack) and to the position that is in the ortho relation to it in exactly the same amounts (cine substitution, cine comes from kinesis, which is Greek for "movement"). The fact that the nucleophile (base) is introduced to two different positions can be proven by isotopic labeling experiments, in which the halobenzene's halogen-carrying carbon is exchanged for 13C (detection by 13C NMR) or radioactive 14C (detection by a scintillation counter). Due to a small kinetic isotope effect, these isotopic labeling experiments yield the two products only in approximately the same amounts.

Fig.1
Substitution of chlorobenzene's chlorine by the strong base amide.

Halobenzenes that do not contain a hydrogen in an ortho position do not react with strong bases in such a substitution reaction at all.

An addition-elimination mechanism, such as that of the electrophilic aromatic substitution, cannot account for such experimental results. Instead, an elimination-addition mechanism has been suggested. That is, in the first reaction step, the nucleophile does not add to the aromatic compound, but acts as a base by abstracting a proton from the position ortho to the halogen. The resulting anion immediately expels the halide anion. All in all, hydrogen halide has been eliminated from the halobenzene. The elimination yields a compound that formally contains a carbon-carbon triple bond. This compound is called an aryne or a dehydroarene, respectively (the unsubstituted aryne is called benzyne).

Fig.2
Elimination-addition mechanism with an intermediate aryne.

The elimination of hydrogen halide from a halobenzene can only be achieved through the application of an extremely basic nucleophile. The aryne is an extremely good and reactive electrophile (see below). Consequently, in the second reaction step - that is, in the addition - the base acts as a nucleophile and is added to the aryne. Last but not least, the resulting aromatic anion is protonated. If the aryne does not contain any directing substituent, the nucleophile is obviously added to either of the carbons of the "triple bond".

Fig.3

LUMO of the aryne: The predominant localization of the LUMO's orbital lobes at the "triple bond's" carbons leads to the conclusion that a nucleophilic attack at these particular positions is considerably favorable.

Fig.4

HOMO of the aniline anion: The particularly large orbital lobe of the HOMO at the negatively charged carbon is proof that this carbon can easily take up a proton.

Fig.5

Electrostatic potential surface of the aniline anion: The highly negative potential at the negatively charged carbon is clearly perceptible. This carbon can easily take up a proton from ammonia. Due to strong, electric repulsion, an additional nucleophilic attack by an amide anion is impossible.

The new bond of the aryne cannot be a conventional triple bond, as a linear geometry with sp-hybridized carbons cannot be achieved in a six-membered ring. Rather, it is in fact a distorted triple bond among more $sp2$-hybridized carbons. Thus, the aromatic π system, which consists of the overlapping p orbitals of the six $sp2$ ring carbons, should be largely intact. The two remaining electrons of the "triple bond" occupy the two $sp2$ orbitals at the "triple bond" carbons that are not involved in any σ bond. However, these $sp2$ orbitals cannot form a real π bond, as they are not parallel to one another. Thus, they cannot effectively overlap. Therefore, arynes' "triple bonds" are highly reactive.

Fig.6
Overlapping of π orbitals in arynes.
Fig.7

HOMO (-1) of benzyne: The original aromatic system of benzene consisting of the overlapping, parallel p orbitals of the six $sp2$ ring carbons represents the HOMO (-1) of benzyne. It hardly differs from the benzene's HOMO.

Fig.8

HOMO of benzyne: The $sp2$ orbitals of the "triple bond" carbons that, as a result of hydrogen halide elimination, are no longer involved in σ bonds represent the lion's share of the benzyne's HOMO. However, they cannot effectively overlap. Thus, this π bond is only very weak.

Fig.9

The benzyne's HOMO (solid surface) and HOMO (-1) (grid surface) are superimposed in this illustration.

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