aromatic hydrocarbons:

Introduction:

We have discussed the chemistry of aliphatic hydrocarbons. Now we come to an additional class of compounds, namely, aromatic hydrocarbons.

Early in the growth of organic chemistry, organic compounds were arbitrarily classified as either aliphatic or aromatic. The meaning of the term, "aliphatic" means fatty. The aliphatic compounds were so named since the 1^st members of this division to be studied were the fatty acids.

Additionally to the aliphatic compounds, there was a huge number of another kind of compounds that were as well attained from natural sources, for example resins, balsams, aromatic oils, and so on. The structure of such compounds was unknown but they had one thing in ordinary, a pleasant odour. Therefore, such compounds were arbitrarily classified as aromatic compounds (Greek: aroma 'fragrant smell'). At present the word aromatic is utilized for benzene and related compounds. So the original meaning of the word aromatic (fragrant) has no consequence any longer.

Benzene, the easiest of the aromatic compounds, was isolated via Michael Faraday in the year 1825 from the gas attained through pyrolysis of whale oil. Later, in the year 1845, Holman determined benezene in coal tar that encloses benzene and a lot of of its derivatives.

Many compounds isolated from natural sources and many synthetic drugs are aromatic in nature. The local anaesthetic procaine and the tranquilizer diazepam (valium) are a few instances. Benzene is carcinogenic and injurious to health. Prolonged revelation leads to bone-marrow depression. Benzene as a solvent should, hence, be utilized suspiciously, avoiding vanishing in the open or inhaling its vapour.

Keeping in vision the significance of aromatic compounds, we shall learn the chemistry of benzene and its derivatives in this chapter.

Isolation of Benzene:

Coaltar was once the chief source of benzene and its derivatives. Nowadays, benzene and its derivatives can be removed from petroleum in that they take place logically. They are as well arranged from the non-aromatic constituents of petroleum that is now the main source. The most vital this process is hydroforming or catalytic changing

Hydroforming or Catalytic Reforming:

This process is depending on dehydrogenation, cyclisation and isomerisation reactions. The aromatic compounds so attained have similar number of carbon atoms as the aliphatic starting substances. Hydroforming is carried out under high pressure and at temperatures of around 750-820 K in the occurrence of platinum catalyst. Subsequent are several significant instances of hydroforming:

As we know that a mixture o-, m- and p- xylenes is referred to as xylene.

Several hydrocarbons are divided via a selective solvent procedure, but since, benzene is attained in much smaller amount than toluence and the xyleness, and such are changed into benzene via heating through hydrogen under pressure in the presence of a metal oxide catalyst. This procedure is termed hydrodealkylation.

Spectral properties:

The presence of an aromatic ring in a complex is measurable via UV spectroscopy. Aromatic compounds demonstrate a series of absorption bands through fairly intense absorption near 205 nm and a less intense absorption in the 255-275 nm range. As the conjugation rises, λ max as well enhances.

The ir spectrum is fairly helpful for recognizing the presence of aromatic compounds. The ir spectrum provides a weak absorption band near 3030 cm^-1 for aryl C - H extending vibration. Absorption due to C = C stretching in benzene provides a series of 4 bands, commonly between 1450 and 1600 cm^-1.

Since of a huge deal of overlapping of the several bands in the region of 1225-970cm^-1 this region isn't extremely useful for recognition purposes.

The nmr spectrum is a useful tool for the formation determination of benzene and its derivatives. Because all the six hydrogen atoms in benzene are the same, the nmr spectrum provides only one single at δ 3.27 ppm.

Recall that olefinic protons show at higher field values, usually at about δ 5.0 ppm. Electron-withdrawing substituents on the ring shift the absorption of adjacent protons additional downfield, whereas electron-releasing collections shift absorption up field from that of the unsubstituted benzene.

The mass spectrum of benzene provides important molecular ion peak (M⁺). As well M + 1 and M+ 2 peaks, due to ¹³C and ²H are examined. Benzene illustrates prominent peaks at m/z 78 (C₆H₆)⁺, m/z 77 (C₆H₅) ⁺, m/z 53 (C-₄H₅)⁺, m/z (C₄H₃)⁺, m/z 50 (C₄H₂)⁺ and m/z 39 (C₃H₃))⁺. All such as well take place in the mass spectra of nearly all benzene derivatives.

Structure of Benzene:

Molecular Orbital Theory gives an explanation of benzene. According to this theory, benzene is a planar flat balanced molecule having the shape of a regular hexagon. The C - C - C bond angle has a value of 1200. All carbon atoms in the molecule are sp² hybridized. 2 orbitals of the sp² hybridized carbon atom overlap through the other 2 orbitals of the adjacent carbon atom effecting in the formation of 2σ bonds. The 3^rd orbital of each carbon atom overlaps and structures an σ bond through 1s orbital of hydrogen atom. Therefore 6 carbon-carbon σ bonds are formed. Each carbon atom still has a p orbital perpendicular to the plane of the ring. The p orbital has 2lobes one above and the further below the plane of the ring nd since every p orbitals are the same, they overlap equally well through both the neighboring p orbitals effecting in a delocalized doughnut shaped π orbital cloud above and beneath the ring. The picture that appears out of this conversation is given below in Fig.

The benzene ring is a cyclic conjugated system and is generally symbolized as a regular hexagon through a circle inside the ring. This provides a thought of delocalization of π -electrons.

Resonance and Aromaticity:

We have already learned the basic idea of resonance in chapter 7 of this course. Now, we will talk about the resonance consequence in aromatic compounds. The structures of a huge number of organic compounds can be written through the assist of simple bond diagrams, for example, ethane as CH₂=CH₂ ethyne as HC≡ CH, etc. There are, though, many compounds for which simple bond diagrams don't precisely explain such molecules, one of the instances being benzene. The structure of benzene (Fig) gives the impression that it is a cyclic compound of 6 carbon atoms enclosing 3 single and 3 double bonds. If this were so, we would suppose 2 values of carbon-carbon bond lengths, via, one for single bonds (almost 154 pm as in ethane) and the other for double bonds (nearly 133 pm as in ethane). Experimental evidence through X-ray diffraction studies shows that all the six carbon-carbon bonds in benzene are equivalent and have a length of 139 pm,  that  is in between 133 and 154 pm. The illustration of this is as follows:

X-ray studies provide the bond lengths and bond angle.

The heats of hydrogenation of Cyclohexane and benzene computed experimentally are given below:

The heat evolved whenever hydrogen is added to cyclohexene (having one C = C bond) is 121 kJ mol^-1. The imagined value of the heat evolved when hydrogen is added to benzene (having 3 C=C bonds) should be 3 x 121 kJ mol^-1 = 363 kJ mol^-1, but the experimental value is 209 kJ mol^-1. We can infer that benzene is more stable (having lower energy content) than the hypothetical molecule having 3 isolated C=C bonds 363 - 209 = 154 kJ mol^-1.

This energy variation is termed the resonance energy and is dependable for the stability of benzene compared to other unsaturated compounds that lack resonance stabilization.

Fig: Energy diagram for the hydrogenation of cyclohexatriene (hypothetical) and benzene

We can't write down a single structure for benzene that would encompass all its properties rather it is considered to be the resonance "hybrid" of the subsequent hypothetical structures I-V

Such structures are termed resonance structures or contributors or canonical forms. The 2 "Kekule" forms, I and II, are of lower energy (more stable) than the 3 "Dewar" forms, III to V. Structures I and II could be expected to "contribute" more to the hybrid than either III, IV or V, hence, the properties of benzene would be expected to resemble more closely to either I or II than to III, IV or V. Because I and II have similar energy, each would contribute to the hybrid via   similar amount. The character of resonance, double-headed arrow (↔) doesn't point out equilibrium. The canonical structures I-V are hypothetical and don't have any physical subsistence. Such structures fluctuate in their electronic arrangement and occur due to shift of π electrons within the molecule.

Aromaticity

Maybe we have been wondering whether other cyclic compounds through π electron might as well be considered aromatic, several of such systems are certainly aromatic, but not all of them. What structural traits are needed for a molecule to be aromatic?

A German Physicist, Enrich Huckel in the year 1931, proposed the Huckels rule. According to this rule an aromatic molecule must be a cyclic conjugated species having (4n + 2)π - electrons where n is an integral (n = 0, 1, 2, 3 .....). This means that only the ring through 2, 6, 10, and 14 ... electrons might be aromatic but a ring with 4, 8, or 12 π electrons might not be aromatic. Huckel rule is as well applicable to ionic species. Let us look at several of the verification sustaining the Huckel rule.

1) Cyclobutadiene

no. of π  - electrons = 4

4 + 2 electrons are needed for aromaticity. Cyclobutadiene has 4π -electrons, therefore cyclobutadiene isn't aromatic as it has 4π -electrons and is extremely unbalanced.

2) Benzene

No. of π -electrons = 6

Here the 4n + 2π rule can be applied as it has 6 π -electrons which are needed for a single ring system. It is an admirable instance of an aromatic system.

3) Cyclooctatetraene

It doesn't have 4n + 2 π electrons and therefore isn't aromatic.

Another cause why cyclooctatetraene isn't aromatic is that it isn't even entirely conjugated. It is a tub formed molecule and the neighboring orbitals containing the π -electron don't have the necessary geometry for proper overlap.

cyclooctatetrane

From the above instance, it is clear that a flat planar geometry is needed for proper overlap effecting in delocalization of π -electrons which are a needed circumstance for aromaticity.

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