What are Annulenes..??
Annulenes are the completely conjugated monocyclic hydrocarbons containing an even number of carbon atoms. They have the general formula CnHn (when n is an even number) or CnHn+1 (when n is an odd number).
Some Examples Of Annulenes
Aromaticity Explanations: Annulenes could be aromatic, anti-aromatic or non-aromatic. For example, [4] Annulene which is cyclobutadiene is anti-aromatic, While [6] Annulene (Benzene) is aromatic and [8] Annulene i.e. cyclooctatetraene is non-aromatic compound.Lets take some higher Annulenes to understand how it will show aromaticity.
1. [8] Annulene Cyclooctatetraene can be assumed to have a planar cyclic conjugated system which has huckle electrons.This regular planar octagon has bond angles of 135° with large bond angle strain due to large deviation from sp2 bond angles of 120°. To overcome this strain molecule assumes a non-planar, tub-shaped geometry with angles C=C−C = 126.1° and C=C−H = 117.6°.Tub shaped structure is not planar, not aromatic and neither antiaromatic (as nonplanairty hinders delocalization). Hence it is non-aromatic. So, if we check for the two broad conditions, we can conclude it is non-aromatic because of both the first and second conditions not fully met with.
2. [10] Annulene 10-pi electrons give an indication of aromaticity as per the Huckel’s rule, but the fact is that it is non-aromatic. This isbecause, [10] annulene is unable to adopt the necessary planar configuration. If we looks at the structure of this molecule,If we draw the structure as in Figure (a) which seems to be planar, all the double bonds are cis (fig a) or if we consider it to be like Fig (b), then one double bond is trans and other four are cis. In both these situations, there is lot of angular strain in the ring.If we try making two double bonds trans and arrange all the atoms to have no angularstrain, as in Fig (c) , the planarity is lost as the two H atoms facing each other inside the ring pose steric hinderence to each other.Thus the molecule attains a puckered or non-planar structure where the two-half cyclic parts are angular to each other.
3. bridgehead [10] Annulene Interestingly, if two internal ‘H’ of [10] annulene are replaced by a methylene bridge above the molecule, the strain can be overcome, and it can acquire a flat geometry. Let us consider the case of 9, 10 methane [10] annulene. Here the bridgehead C’s are 9 and 10 which are still sp2 hybridized, but there are no hydrogen atoms creating strain. Hence, the 10 carbon atoms remain in plane, thereby fulfilling the conditions of cyclic, planarity, continuous delocalization of pi electrons in this 10-pi-e system which is hence aromatic. Compare this with naphthalene, a 10-pi-esystem which is aromatic and focus only on the continuously delocalized 10Ï€ electrons around in a cyclic fashion on planar structure.
Similar to methylene as bridgehead on [10] annulene, its oxygen and nitrogen analogous are also aromatic. For example, 9,10 oxa [10] annulene is aromatic fulfilling the conditions of cyclic, planarity, continuous delocalisation of 10 π electrons.
Similarly, 9,10 aza [10] annulene is aromatic fulfilling the conditions of cyclic, planarity, continuous delocalization of 10 pi electrons.
4. [12] Annulene[12] annulene is planar. The three H in-between the ring are far enough and do not create any strain for the planar arrangement. So this is a cyclic, planar system having continuous delocalization of pi electrons and fulfilling the first condition. But the number of pi electrons continuously delocalized are 12 i. e 4n-pi electrons, where n = 3. Since it is a 4n-pi-e system, it is anti-aromatic in nature.
5. [14] AnnuleneThis is a 14-pi-e system i.e., a (4n + 2) pi-e system and can be presumed to be aromatic. However, it was found not to undergo substitutive nitration or sulphonation reactions indicative of its non-aromatic behaviour. Let us understand why it is not aromatic?
As can be seen from the figure that ‘H’ present at the interior of the ring interfere with each other, and X ray analysis shows that the molecule is not planar. Also it was observed that Dehydro –[14] annulene formed by removal of two interfering hydrogen leads to formation of a triple bond, and a planar molecule. The two Electron from one of the pi bond of carbon-carbon triple bond are delocalized into aromatic pi system and the molecule becomes aromatic. The other pair of pi-e does not interact with the delocalized system as it is at right angles to the conjugated system of pi-e.
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