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Paper No and Title Paper 1: - I (Nature of Bonding and Stereochemistry) Module No and Module 7: Title Module Tag CHE_P1_M7

CHEMISTRY Paper No. 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) Module 7: Annulenes

Table of Content

1. Learning Outcomes 2. Introduction 3. Annulenes 4. of annulenes 4.1 The case of [8] 4.2 The case of [10] annulene 4.3 The case of bridgehead [10] annulene 4.4 The case of [12] annulene 4.5 The case of [14] annulene 4.6 The case of [16] annulene 4.7 The case of some higher annulenes 5. Preparation of some annulenes 6. Summary

CHEMISTRY Paper No. 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) Module 7: Annulenes

1. Learning Outcomes

After studying this module, you shall be able to:

 Comprehend the structure of annulenes  Classify annulenes as aromatic, anti-aromatic and non-aromatic.  Understand some cases of higher annulenes and their substituted variants like bridgehead and heteroatom substituted annulenes.  Know the preparation of some annulenes.

2. Introduction

Dear students, now you are well versed with the conditions which are required to be fulfilled by the structure of a compound, for it to be classified as aromatic and anti-aromatic. A molecule is aromatic if all the following conditions are fulfilled:

1. It is cyclic, planar and has continuous delocalization of  electrons (electrons in p orbitals) with or without the participation of (s)/- charge/ + charge (i.e., having electrons or vacant p orbital).

2. The delocalized -electron cloud must contain a total of (4n+2) electrons, where n is a whole number (i.e., n =0,1,2,3 and so on). Putting n= 0 in (4n+2), we get 2 electrons, similarly putting n=1, we get 6 electrons; n= 2 gives 10 electrons; n= 3 gives 14 electrons and so on.

In this module, we shall discuss in detail about these classifications for a series of compounds which are conjugated monocyclic known as annulenes.

3. Annulenes

Annulenes are the completely conjugated monocyclic hydrocarbons containing an even number of . They have the general formula CnHn (when n is an even number) or CnHn+1 (when n is an odd number). Alternatively, they are represented by (CH=CH) n

CHEMISTRY Paper No. 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) Module 7: Annulenes

As per convention, annulenes with 7 or more number of carbon atoms are named as [n] annulene. That is, they are named by indicating the number of carbon atoms in the ring, in the square bracket before the root word annulene.

e.g., – [6] annulene – [8] annulene

You are already familiar with the first 3 members of the series, [4]-, [6]-, and [8]-annulene but you must have used their name as 1,3-, benzene and 1,3,5,7-cyclooctatetraene.

Of these we have seen that cyclobutadiene is anti-aromatic, benzene is aromatic and cyclooctatetraene is anti-aromatic.

Some other examples of annulenes are:

4. Aromaticity of Annulenes

Annulenes could be aromatic, anti-aromatic or non-aromatic. For example, [4] Annulene which is cyclobutadiene is anti-aromatic, [6] Annulene (Benzene) is aromatic and [8] Annulene i.e., cyclooctatetraene is non-aromatic.

CHEMISTRY Paper No. 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) Module 7: Annulenes

The behavior of the annulenes can be explained on the basis of Hückel’s rule which we have already studied in detail for these three cases of annulenes. Let us now take some higher annulenes to extend our understanding.

Some Higher Annulenes

CHEMISTRY Paper No. 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) Module 7: Annulenes

4.1 The case of [8] Annulene

Cyclooctatetraene can be assumed to have a planar cyclic which has 4n e where n = 2 as shown in the figure.

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°.

CHEMISTRY Paper No. 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) Module 7: Annulenes

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. 1.  2. 8  

Relating this with its reactions, we have the facts to justify its non-aromatic behaviour. It behaves

like a typical and undergoes addition reaction with electrophillic reagents like Br2 and HCl,

oxidation by K M n O4 and not substitution reactions like benzene does.

The non aromatic behaviour of cyclooctatetraene can also be explained by the following figure using MO theory. The polygon rule discussed earlier gives the energy levels of various molecular orbitals according to which there are three bonding, two non bonding and three anti bonding molecular orbitals.

Figure 1. MO diagram for cyclooctatetraene

Since n orbitals are half filled so it should be predicted to be antiaromatic. However cyclooctatetraene is not planar, but a tub shaped molecule. The p-orbitals of one sp2 hybridized carbon are is not coplanar with those of the neighbouring ones, therefore no effective overlapping

CHEMISTRY Paper No. 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) Module 7: Annulenes

of the orbitals take place for delocalization. The non coplanarity avoids anti aromaticity and in molecule remains as non aromatic.

4.2 The case of [10] Annulene

As discussed earlier also, although 10 electrons give an indication of aromaticity as per the (4n+2 electrons requirement for Huckel’s rule, but the fact is that it is non-aromatic. This is because, [10] annulene is unable to adopt the necessary planar configuration.

If one 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 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 angular strain, 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.

CHEMISTRY Paper No. 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) Module 7: Annulenes

Hence due to non bonded interactions between the internal protons, the molecule acquires non planar geometry, which in turn prevent the delocalization of  e s. Therefore [10] Annulene is non-aromatic through it has 1 0 e  available for delocalization.

4.3 The case of 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 [10] annulene. Here the bridgehead C’s are 9 and 10 which are still sp2 hybridized, but there are no hydrogen atoms creating strin. Hence, the 10 carbon atoms remain in plane, thereby fulfilling the conditions of cyclic, planarity, continuous delocalisation of pi electrons in this 10 e system which is hence aromatic.

Compare this with , a 10 system which is aromatic and focus only on the continuously delocalized 10π electrons around in a cyclic fashion on planar structure.

Similar to methylene as bridgehead on [10] annulene, its oxygen and 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.

CHEMISTRY Paper No. 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) Module 7: Annulenes

Syimilarly, 9,10 aza [10] annulene is aromatic fulfilling the conditions of cyclic, planarity, continuous delocalisation of 10 pi electrons.

4.4 The case of [12] Annulene

The structure of [12] Annulene is planar and shown in the figure below.

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 delocalisation of pi electrons and fulfilling the first condition. But the number of pi electrons continuously delocalised are 12 i.e 4n pi electrons,where n = 3. Since it is a 4n e system, it is anti-aromatic in nature.

4.5 The case of [14] Annulene

CHEMISTRY Paper No. 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) Module 7: Annulenes

This is a 14 e system i.e., a (4n + 2) 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 e from one of the  bond of CC are delocalized into

aromatic  system and the molecule becomes aromatic. The other pair of e does not interact with the delocalized system as it is at right angles to the conjugated system of e s.

Some more examples of stable bridged [14] annulene are shown below:

4.6 The case of [16] Annulene

CHEMISTRY Paper No. 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) Module 7: Annulenes

Let us check for the two broad conditions, the first condition is fully satisfied with the molecule being cyclic, planar and continuous delocalization of pi electrons. In the second condition, it is a 16 pi electrons system and thus a (4n) pi electron system where, n= 4. Hence, we can conclude that it is anti-aromatic. 1. ✔ 2. 16   In other words, [16] Annulene is a 4n e system so it is anti aromatic in nature.

4.7 The case of [18] Annulene

[18] Annulene is aromatic in nature. It is (4n + 2) system when n = 4. It has nine conjugated double bonds. The internal H’s do not pose steric hinderence, and the molecule is planar to allow complete delocalization of s.

4.8 The case of higher annulenes

CHEMISTRY Paper No. 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) Module 7: Annulenes

All the higher annulenes, do not have any steric hinderance of the internal Hydrogen atoms and thus they are all planar and satisfy the first condition of being yclic , being planar and having continuous delocalisation of pi electrons. In order to classify them , you just check whether it is a (4n + 2) e system or (4n) system and accordingly they shall be aromatic or anti-aromatic respectively.

Some examples:

 [20] annulene: (4n) system where n= 5; Hence, anti-aromatic.  [22] annulene: (4n + 2) system where n= 5; Hence, aromatic.  [24] annulene: (4n) system where n= 6; Hence, anti-aromatic.  [26] annulene: (4n + 2) system where n= 6; Hence, aromatic.

5. Preparation of Some Annulenes

1. Preparation of [14] annulene

Synthesis of [14] annulene can be done by coupling of a polyene in the presence of cupric acetate in . It results in a cyclic structure containing two groups which on further treatment with potassium t-butoxide in the presence of t-butyl alcohol and reduction with H2 – Pd/C gives [14] annulene.

Scheme 1.

2. Preparation of [18] annulene

The following scheme is used to synthesize [18] annulene. The synthesis involves coupling three moles of hexa-1,5-diyne in the presence of cupric acetate in pyridine to give a cyclic trimer(II).

CHEMISTRY Paper No. 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) Module 7: Annulenes

This cyclic trimer undergoes rearrangement in the presence of t-BuOK which gives a 1,7,13- tridehydro [18] annulene(III) which ultimately results [18] annulene on partial reduction.

Scheme 2.

3. Preparation of bridgehead annulene: 9,10 methane [10] annulene

The following scheme represents the synthesis of 9,10 methane [10] annulene.

CHEMISTRY Paper No. 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) Module 7: Annulenes

Scheme 3.

6. Summary

 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).  The first 3 members of the series, [4]-, [6]-, and [8]-annulene but you must have used their name as 1,3-cyclobutadiene, benzene and 1,3,5,7-cyclooctatetraene.  Annulenes could be aromatic, anti aromatic or non-aromatic.  [4] Annulene: anti-aromatic  [6] Annulene: aromatic  [8] Annulene: non-aromatic.  [10] Annulene: non-aromatic.  [12] Annulene: anti-aromatic.  [14] Annulene: anti-aromatic.  [18] Annulene: aromatic.

 The following bridgehead [10] annulenes are aromatic:  9,10 methane [10] annulene  9,10 oxa [10] annulene  9,10 aza [10] annulene

 Preparation of [14] annulene, [18] annulene and 9,10-methane [10] annulene from simple and easily available molecules.

CHEMISTRY Paper No. 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) Module 7: Annulenes