GOC & Aromaticity - 1 - GATE Chemistry MCQ

Nucleophilicity increases with the increase in the size of atom in the same period and decreases with the increase in electronegativity.

Basicity: Electron pair donating ability

Delocalised lone pair present on nitrogen which has the lowest basicity.

Pyridine has poor basic character. In pyridine, lone pair present on the nitrogen atom are perpendicular
to the orthogonal orbital, containing sp² hybrid orbital.

 (Aziridine having more s% character than pyrrolidine. so, pyrrolidine is more basic than Aziridine).

So, the correct order of pk_a value.

III>II>IV>I

• Resonance involves the displacement of lone pair and double bonds of the molecules without changing the position of the atoms.
• Tautomerization involves the change in position of lone pair and double bond with atom.

• Electron withdrawing group increases the acidic character.
• Electron donating group decreases the acidic behaviour.
  
• Higher the K_a, lower will be pk_a, higher will be acidity ⇒ acidic behaviour 
• Nitro Benzoic acid is more acidic as compared to acetic acid, which in turn is more acidic as compared to phenol.
• Thus the order of acidic behaviour will be:
  III > IV > I > II
  Hence, the correct order of pk_a will be:
  III < IV < I < II

• If the intermediate free radical which is formed via homolytic cleavage is more stable, the bond breakage is more feasible, and thus bond dissociation energy will be less.
• Since (C) forms, stabilized conjugated free radical, its homolytic bond dissociation energy will be least. 
• Followed by the same approach, the order will be  A > B > D > C. Hence, the correct option is (c).

There are three major factors contributing to the overall dipole moments:

(i) The σ bond dipole associated with C-0 and C—N bonds.

(ii) The π bond dipole associated with the delocalisation of π electrons from the heteroatom to the ring.

(iii) The dipole moment associated with the unshared electron pair (for 0) or N—H bond (for N).

• All these factors have a greater moment towards rather than away from the heteroatom for furan than for pyrrole.
• For pyrrole, the C-N π dipole should be larger and the N—H moment in the opposite direction from furan. These two factors account for the reversal in the direction of overall dipole moment.
• In furan and thiophene, the dipole moment is influenced by resonance in the same way as in the pyrrole. However, the effect of resonance is not sufficient enough to exceed the effect of heteroatom electronegativity. Thus, the direction of dipole moment will be: 

• As we know that rotation around double bond requires breakage of laterally overlapped bond. Hence, it is not energetically favourable to rotate around double bond as compared to single bond.
• In structure given in option (b) and (d). There is considerable double bond character around 1-2 bond due to delocalisation.
• (a) will remain in energetically favourable uncharged form, because there no driving force like aromaticity to push it in charged form.

Lone pair not available for delocalization into benzene ring.

More stable olefin will have lesser heat of hydrogenation and lesser heat of combustion.

It is found that among P, Q, R and S, P and S are antiaromatic i.e.,

• v_co, stretching frequency ∝ strength of bond. Thus, the species, in which there is more delocalisation, C-O bond have considerable single bond character and thus have a lower value of stretching frequency.
• Hence, among A, B, C and D, B will have a maximum value of v_ca, Since, its — C=0 bond is intact.
• On the other hand, C is an aromatic compound thus, it must have charge separation, which leads to &localisation. Which in turn makes C = 0 bond, considerable single bond character.
• Thus, C will have a minimum value stretching frequency. 

• Rate of sololysis in aqueous ethanol (weak nucleophile) will proceed preferentially by S_N 1 mechanism. since, rate of S_N 1 reaction ∝ stability of carbocation.
• Here, stability order of carbocation will be in the order of :

From the above reaction sequence, it is clear that A and B not an aromatic compound, whicle C which is ferrocene is a well know aromatic compound

Generally, cis acid are more acidic than trans acid, if group are small.

(Y) due to the presence of methyl group ortho to carboxylate ion. which are perpendicular to phenyl ring. So. carboxylate group not in conjugate with phenyl ring. Hence. carboxylate negative ion having more conjugation. So. (Y) is more acidic in nature 

Presence of electron releasing group, decreases the stability of negative charged conjugate base. So, less acidic

B, E, F, G , J, I are anti-aromatic in nature

B, D, H aromatic in nature

No such driving force for delocalization present in (d) and (e)

Electrophilic Aromatic Substitution of the methyl group with lewis acid leads to toluene. Through nitration, the nitro group is added then the reduction of nitro group leads to the amine.  

H₂O = pK_a value 15.74 (more pk_a value than H₂0 means less acidic than H₂0)

Compounds C,E,F,G having more pK_a value than H₂0