12th Class Chemistry Chapter 9 Aromatic Hydrocarbons Short Questions Answer
1.What do you mean by term aromatic?
Benzene and many derivatives of benzene had aroma type smell.The word aroma means fragrant.So All these compounds were called aromatic by Kekule.This thing enabled the scientists to distinguish between derivatives of benzene from aliphatic compounds.
2.What are nonbenzoid aromatic compounds?
Those aromatic compounds whose stabilities are just like benzene or derivatives of benzene but they do not have the benzene ring in them.The best examples are pyridne pyrrol furan thiophene and their derivatives.
3.What are the characteristics of aromatic compounds?
They are cyclic in nature planar unsaturated give substitution reactions rather than addition and they have low heat of hydrogenation and heat of combustion.
4.Give three examples of condensed aromatic hydrocarbons adn give their names?
Following are three examples:
5.Write the reactions which give the evidence for the ring structure of benzene?
When H2 and CI2 react separately with benzene cyclohexane and benzene hecachloride are produced which are cyclic compounds.We get only one mono-substituted derivative and three di-substituted derivatves of benzene.
6.What informations do we get from X-ray analysis of benzene?
It shows tha tbenzene is six membered planar molecule.All the bond lengths of C-C and C-H are all equal with the values of 139 pm and 109 pm respectively.
7.There are three alternate double bonds in the bonzene ring but why all the carbon-carbon bonds are equal length?
Since there is conjugated system of double bonds so resonance starts and six π electrons are completely delocalized.In this way the status of all the C-C bonds become equal and they are 139 pm in length.
8.Which orbitals in benzene give stability to benzene but not to alkenes?
9.Write down the resonace structures of benzene and indicate their relative contributions towards the actual structure of benzene?
10.How do you justify that 150.5 kj mol-1 is the resonance energy of benzene?
If each double bond of benzene ring behaves independly benzene should have been 358.5 kj mol-1 more unstable than cyclohexane.Experimentally it is 208 kj mol-1 more unstable.The difference of these two values is 150.5 kj mol-1 which is the amount of energy evolved due to stability of the benzene.That is why benzene is less reactive than ethene.
11.How hexane and heptane can give benzene and toluene respectively?Or What is aromatization?
12.Prepare maleic acid from benzene?
13.What is Wurtz-Fitting reaction?
14.What do you mean by electrophilic substitution reactions of benzene?Give general mechanism?
15.What is sigma complex and how does it get the stability?
16.What is the role of FeCI3 and AICI3 in electrophilic substitution reaction of benzene?
FeCI3 and AICI3 are Lewis acids.They generate the electrophiles which can attack the benzene ring and give electrophilic substitution reactions.
FeCI3 +CI2……………. FeCI4– + CI+
FeCI3 +CI2……………. FeCI4– + CI+
17.What are major products when chlorine reacts with toluene in the presence of sunlight?
18.Give the mechanism of nitriation of benzene?
19.What happens when benzene is heated with conc.H2SO4 at 80ᵒC?Why oleum is required for the sulphonation of benzene?
20.Give the general mechanism of Friedal Craft’s alkylation?
21.How benzene can be converted to acetophenone?Give its mechanism;
When CH3COCI is reacted with benzene in the presence of AICI3,acetophenone is obtained:
22.What are those reactions which show that benzene is unsaturated hydro carbon?
23.How does ozone affect benzene to give glyoxal?
24.How toluene can be converted to benzoic acid?
25.Nitration of toluene gives ortho and para nitrotoluene while the bromination of mitrobenzene gives m-bromonitro benzene.Give reasons?
26.If the groups like OH-Sh and – NH2 are present on the benzene ring the ring is activated and ortho para products are obtained.Give reasons?
27.If-COOH or –CN groups are present on the benzene ring they deactivate the ring and meta products are obtained.Give reasons?
Both these groups are meta-directing with deactivation of the benzene ring.Both these groups have multiple bonds in them which are in conjugation with the benzene ring.The conjugated multiple bonds create electron deficiencies at o-and p-positions and the ring becomes overall inactive.
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