CHEM..ORGANIC..XI

Resonance  Effect  :  The  polarity  produced  in  the  molecule  by  the  interaction of  two
pi  bonds or between a pi bond and lone pair of  electron present on an adjacent atom.
There are two types of  resonance effect:
1)  Positive  resonance  effect  :  In  this  effect  the  transfer  of electr ons  is  away  fro m
an atom or substituent group attached to the conjugated system.
The  atoms  or  groups  which  shows  +R  effect  are  halogens, -OH  ,  -OR,-NH2
2)  Negative  resonance  effect  :  In  this  effect  the  transfer  of  electrons  is  towards
the  atom or substituent group attached to the conjugated system.
The atoms or groups which  shows  -R effect are –COOH , -CHO , -CN

Q.1. What effect does branching of an alkane chain has on its boiling point? 

Ans. As the branching increases, the surface area of an alkane approaches that of a sphere. Since a sphere has minimum surface area, therefore, Vander Waal forces of attraction are minimum and hence the boiling point of the alkane decreases with branching.

Q2. Among Cis and Trans structure of Hex-2-ene which has higher boiling point and why?

Ans: The dipole moment of a molecule depends upon dipole-dipole interactions. Since cis-isomer has higher dipole moment, therefore it has higher boiling point

Q3. Write IUPAC names of the following compounds )

(a) CH₃CH=C(CH₃)₂ (b) CH₃CH(CH₃)CH=CH₂

Ans. (a) 2-Methylbut-2-ene                  (b) 3-Methylbut-1-ene

Q.4. Out of benzene, m–dinitrobenzene and toluene which will undergo nitration most easily and why?(L-II)

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

                  

Toluene> benzene> m-dinitrobenzene.

Q5. Explain why (CH₃)₃C⁺ is more stable than CH₃CH₂⁺ and ⁺CH_3.  (L-I)

Ans. Greater the number of alkyl groups attached to a positively charged carbon atom, the greater is the hyperconjugation interaction and +I effect of methyl group greater is the stabilisation of the cation.Thus, CH₃)₃C⁺ is more stable than CH₃CH₂⁺ and ⁺CH_3.

Q6. Explain electromeric effect, resonance effect and hyperconjugation.

Ans: Electromeric effect is defined as the complete-electrons totransferoneof  o

the atoms joined by a multiple bond on the demand of an attacking reagent. It is represented by E

and the shifting of the electrons is shown by a curved arrow.

The resonance effect is defined as ‘the polarity produced in the molecule by the interaction of two-bndsπ or between-bondandlone pairaof electronsπ present on an adjacent atom’. The effect is transmitted through the chain.

Hyperconjugation involves delocalisation of σ electrons—Hbondof an ofalkyl groupC directly attached to an atom of unsaturated system or to an atom with an unshared p orbital. The—H σbondelectronsofthealkylgroupenterofinto partialC conjugation with the attached unsaturated system or with the unshared p orbital. Hyperconjugation is a permanent effect.