In this topic, you will learn about arenes, their structure, rules of nomenclature, and the orientation of electrophiles.
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What are Arenes?
Arenes are cyclic aromatic hydrocarbons, containing alternating single and double-bond between the carbons. The system of two or more pi bonds separated by a single bond is known as the conjugate system. The presence of the conjugate system is a key feature that separates arenes from other cyclic structures.
Benzene (C6H6), is the simplest example of arene.
In benzene, the conjugate pi bond resonates freely in the cyclic ring and give rise to the resonance structures of benzene. The delocalized pi electrons moving from one carbon to another provide stability to the benzene ring.
Kekulé Structure of Benzene
Friedrich August Kekulé, a German scientist provided a good basis regarding the structure of benzene. In this structure, alternating single and double bonds between adjacent carbon form a six-membered hexagonal ring with a hydrogen atom bonded to each carbon. Kekulé purpose the idea of two structures where there is an oscillation between the two forms.
The oscillation of double bonds in benzene can be explained with the help of resonance. According to resonance theory, the combination of several possible structures represents the molecule. In benzene, the delocalization of electrons gives rise to two interconvertible structures known as resonating structures. From X-ray diffraction studies, the C-C bond length in benzene was found to be equal to 1.39 Angstroms which is the intermediate bond length between carbon-carbon single (1.54) and double (1.34) bonds. Thus, even though the two resonating structures are equivalent to one another, the true structure of benzene is given by the resonance hybrid.
Resonance of benzene
There are two theory that explains the occurrence of resonance in benzene.
Valence bond theory: Each carbon atom in benzene has three sp2 hybrid orbitals and one unhybridized 2pz orbital. Out of the three sp2 hybrid orbitals, two participate in C-C σ bond, while the remaining one forms C-H σ bond. Thus, there are a total of 12 σ bonds. The remaining unhybridized 2pz orbital forms π bond with the adjacent carbon through lateral overlap. There is an equal possibility of π bond formation by all 6 carbon atoms in the ring which explains the formation of two resonating structures proposed by Kekulé.
Molecular orbital theory: Similar to valence bond theory, 2pz orbitals are responsible for the formation of π bond. Here, the six unhybridized 2pz orbital overlap to generate 6 molecular orbitals (3 bonding and 3 anti-bonding). The like phase orbitals (bonding to bonding, and anti-bonding to anti-bonding ) combine to form a electron cloud above and below the ring.
Prediction of aromaticity in Arenes.
Huckel’s rule states that a conjugated ring system containing (4n + 2) π electrons will show aromatic character. (n is a positive integer) .
When n=1, the conjugated system has 6π electrons. Similarly, when n=2, the system has 10π electrons and when n=3, the conjugated system has 14π electrons.
So, the system of 6π , 10π ,14π electrons and so on are aromatic.
While naming substituted benzene the substituent becomes a prefix to the word root benzene.
Chlorine – Chloro+benzene= Chlorobenzene
Nitro group- Nitro+benzene= Nitrobenzene
For disubstituted benzene. The naming is done as, (position of substituent)+ di+ (substituent) + benzene.
Instead of using numbers for disubstituted benzene, ortho (o) ,meta (m) and para (p) can mark the positions.
- 1,2 positions – ortho (o) position
- 1,3 positions – meta (m) position
- 1,4 positions – para (p) position
Similarly for trisubstituted benzene.
For (ii) structure, aniline is used instead of benzene due to the presence of NH2 group.
Phenyl group is a cyclic group of atoms with the formula C6H5 and represented by the symbol Ph. The replacement of a hydrogen atom by a substituent in benzene forms a phenyl group (R-C6H5).
Benzene with a methylene (CH2) group is called benzyl. It is represented by a general formula. R−CH2−C6H5.
During nomenclature, both phenyl and benzyl are used as prefix.
The group present in the benzene ring decides the position of the entering groups. Chemists call this phenomenon the directive effect. The terms directors or directing groups describe the groups already present.
1. Ortho- and para-directors: They direct the electrophile toward ortho and para positions. -OH,-NH2, -Cl, -CH3 belong in this category. Ortho- and para-directors are electron-donating and ring activating ring-activating groups. (Except for halogen which is a ring deactivator) .
2. Meta-directors: They are electronegative atoms (electron-withdrawing group) that direct the electrophile towards meta position. They are meta-directing and ring-deactivating group. For example, -COOH, -CHO , – CN
Arenes Practice Problems
Why is cyclohexane non-aromatic:
What is the difference between phenyl and phenol?
Why are certain groups ortho and para directors while others are meta directors?
Arenes Practice Problems Solution
1: Cyclohexane consist of only C-C single bond, due to which there are no pi electrons present. The absence of pi electrons makes it non-aromatic.
2: The replacement of a hydrogen atom by a substituent in benzene forms a phenyl group (R-C6H5). Whereas, phenol is the replacement of hydrogen by a hydroxyl group (-OH).
3: The o/p directors are an electron donating group. They donate electrons to the ring which causes the electron density to be high at the ortho and para positions . This is why electrophiles prefer to attack at ortho and para positions.
Simillary, the meta directors are electron withdrawing groups. They withdraw electrons and deactivate the ring. In this case, the electron density is maximum at meta position (no positive charge at meta position) , thus the electrophile attack the meta position.