Aromaticity: Benzenoid and non-benzenoid compounds – generation and reactions.
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1.The correct order of pKa values for the compounds X, Y and Z is……
More acidic strength of compound less will be pka value
‘Y’ is most acidic due to presence of strong electron withdrawing and to gain aromatic character So, it has least pka value ‘Z’ is least acidic, due to more Annelation effect of two Benzene ring so, It have highest pka value therefore order of pka value Z > X > Y
2.The compound that is antiaromatic is
Only I one follows huckel rule and contain 4nπ electron system
II and IV are aromatic
III is non aromatic due to sp3 carbon.
3.The decreasing order of basicity of the following compounds is
(a) I > II > III > IV
(b) IV > I > II > III
(c) III > II > I > IV
(d) IV > III > II > I
Basicity depends on the ease of donating lone pair or greater the electron density greater will be the basic character
Pyridine is least basic as it is part of sp2 ring system.
Highest electron density will be of guanidine group. Hence C is the correct option
Refrerence : Organic Chemistry, Clayden.
4.The correct order of basicity for the following anions is
(a) II > III > I
(b) I > II > III
(c) II > I > III
(d) III > II > I
In case of ortho and para resonance as well as inductive effect both operate.
Thus, electron withdrawl will be more in the case of ortho and para substituted nitro phenol.
Thus, conjugate base of ortho and para will be less basic.
Meta substituted phenoxide (II) will be most basic.
Now, in between I and III, since H-bonding (intramolecular) occurs in the case of I, it is less
acidic as com-pared to para, thus it will be more basic as compared to III.
=> II > I > III.
5.Amongst the following, the compound which has the lowest energy barrier for the cis-trans isomerisation is:
Among all options, option (c) has gain aromaticity in both the rings. Therefore, the lowest energy barries for cis-trans isomerisation is option (c)
6.Among the following compounds, the one which has highest dipole moment is
The compound which have maximum charge separation, have highest dipole moment.
This compound is aromatic in nature, and have maximum charge separation. So, it have highest dipole moment.