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It states that a compound is said to be aromatic. If it has (4n +2) p electrons which are de-localised where n = 0,1, 2, 3, ………

i. All aromatic compounds undergoes substitution reactions rather than addition reactions and this property is referred to as aromaticity or aromatic character. 

ii. The aromatic character of benzene is correlated to its structure. 

iii. Aromaticity is due to extensive cyclic delocalization of p electrons in the planar ring structure. 

iv. Three rules of aromaticity that is used for predicting whether a particular compound is aromatic or non-aromatic are as follows:

• Aromatic compounds are cyclic and planar (all atoms in ring are sp hybridized). 
• Each atom in aromatic ring has a p orbital. The p orbitals must be parallel so that continuous overlap is possible around the ring. 
• Huckel rule: The cyclic π molecular orbital formed by overlap of p orbitals must contain (4n + 2) p electrons, where n = integer 0, 1, 2, 3, … etc.

Huckel proposed that aromaticity is a function of electronic structure of an organic compound. A compound may be aromatic, 

if it obey the following rules: 

• The molecule must have a cyclic structure. 
• The molecule must be co-planar. 
• Complete delocalisation of it electrons in the ring.
• Presence of (4n + 2) it electrons in the ring where n is an integer (n = 0,1,2….), this is known as Huckel’s rule.

Aromatic compounds:

• Aromatic compounds contain higher percentage of carbon. 
• They bum with sooty flame. 
• They are cyclic compounds with alternate single and double bonds. 
• They are not attacked by normal oxidizing and reducing agents. They do not undergo addition reactions easily. They do not decolourise dilute alkaline aqueous KMnO₄ and Br₂ in CCl₄ , though double bonds appear in their structure. 
• They prefer substitution reactions.

Aliphatic compounds:

• Aliphatic compounds contain lower percentage of carbon. 
• They bum with non-sooty flame. 
• They are open chain compounds.
• They are easily attacked by oxidizing and reducing agents. 
• Unsaturated aliphatic compounds undergo addition reactions easily. They decolourise dilute aqueous alkaline KMnO and Br₂ in CCl₄ . 
• The saturated aliphatic compounds give substitution reactions.

(i) I₂ < Br₂ < Cl₂ < F₂

Bond dissociation enthalfy of I₂ is less as compared to that of Cl₂ and Br₂ which means that the homolysis of I₂ is the easier. However , HI formed in a reaction is a strong reducing agent and it makes the reaction reversible. Therfore, iodine does not reacts reversible. Therfore, iodine does not reacts with ethane. 

CH₃ – CH₃ +I₂ -------------> CH₃ – CH₂ – I + HI 

Ethane                                     Iodoethane

Correct option is: (A) hydrogenation

Alkanes can be prepared by hydrogenation of unsaturated hydrocarbons

Alkane \(\xrightarrow {H_2 /Catalyst} Alkane\)

(A) hydrogenation

i. True 

ii. False Toluene is an aromatic hydrocarbon.

iii. True

The repulsive interaction between the electron clouds, which effects stability of a conformation, is called torsional strain. Magnitude of torsional strain depends upon the angle of rotation about C-C bond. This angle is called dihedral angle or torsional angle. Of all the conformations of ethane, the staggered form has the least torsional strain & eclipsed form has the maximum torsional strain. 

The peroxide effect in anti-markovnikoff addition involves a free radical mechanism.

(a) Anthracene

(a) 2 – Methylpentane

Ethane reacts with HBr to form Bromo ethane.

(a) sp³ – s and sp³ – sp³

-CH₃ group is electron releasing while –NO₂ group is electron withdrawing. Therefore, maximum electro density will be in toluene followed by benzene and least in m-dinitrobenzene. Therefore, the ease of nitration decreases in the order:

Toluene> benzene> m-dinitrobenzene

• Benzene is highly unsaturated molecule but despite of this feature, it does not give addition reaction. 
• The actual structure of benzene is represented by the resonance hybrid which is the most stable form of benzene than any of its resonance structures. 
• This stability due to resonance (delocalization of π electrons) is so high that π-bonds of the molecule becomes strong and thus, resist breaking.

Thus, benzene does not prefer to undergo addition reactions.