See image — GOC and Organic Chemistry Basics Chemistry Question

💡 Solution & Explanation

Concept: Canonical (resonance) structures must obey the following rules: (1) the atomic connectivity (sigma framework) must remain unchanged - only pi electrons and lone pairs can be delocalized; (2) the total charge must be conserved across all canonical forms; (3) every atom must have a valid valence (no atom should exceed its normal valence or have an impossible bonding situation); (4) charges should arise from electron pair movement (arrows), not from arbitrary placement. The parent species appears to be the protonated form of a conjugated aminodiene, specifically the cation formed when the nitrogen of but-1,3-dien-1-amine (H2N-CH=CH-CH=CH2) is protonated, giving the resonance-delocalized system: CH2=CH-CH=CH-NH3+. Step 1 - Evaluate Structure I: CH2=CH-CH=CH-⊕NH3 This is the starting (most stable) canonical form. The positive charge is on nitrogen (which bears the ammonium group). All bonds are normal, valences are satisfied, and the total charge is +1. This is a valid canonical structure. Step 2 - Evaluate Structure II: ⊕CH2-CH=CH-⊖CH-⊕NH3 This structure has a carbanion (⊖) on C4 and two positive charges (⊕ on C1 and ⊕ on N), giving a net charge of +1. Moving electron density along the conjugated chain can place a negative charge at an internal carbon while positive charges appear at terminal positions. Although unusual, charge-separated resonance structures with formal positive and negative charges on adjacent or nearby atoms within a conjugated system are permissible as canonical forms as long as total charge is conserved and valences are valid. Net charge = +1 - 1 + 1 = +1. Connectivity is unchanged. This can be considered a valid (though high-energy) canonical structure. Step 3 - Evaluate Structure III: ⊕CH2-CH=CH-CH=NH3 (where NH3 is doubly bonded to carbon, i.e., =NH with one extra H, written as =NH3? but actually =NH2 would be the imine-type) Structure III shows ⊕CH2-CH=CH-CH=NH3. The terminal nitrogen in =NH3 would require nitrogen to form a double bond to carbon AND bear three hydrogen atoms simultaneously - this means nitrogen would have 5 bonds (one double bond to C counts as 2, plus three N-H bonds), giving nitrogen a valence of 5. While nitrogen CAN have 5 bonds in some contexts, here a neutral nitrogen with a double bond and three hydrogens (=NH3) is impossible because nitrogen only has 5 valence electrons - it would need to form 5 bonds without any formal charge, which violates standard valence rules for a neutral atom. Additionally, the positive charge is on C1 (⊕CH2) and nitrogen appears neutral, but =NH3 on nitrogen is not a valid bonding arrangement (it would require a pentavalent neutral nitrogen). This makes Structure III an INVALID canonical structure because it violates nitrogen's valence rules - neutral nitrogen cannot simultaneously form a double bond to carbon and three bonds to hydrogen. Step 4 - Why other options fail: - Option (a) I is incorrect because Structure I is a perfectly valid canonical form. - Option (b) II is incorrect because Structure II, though charge-separated, maintains correct valences and conserves total charge. - Option (d) none of these is incorrect because Structure III is indeed invalid. Therefore, the correct answer is C.