See image — GOC and Organic Chemistry Basics Chemistry Question

💡 Solution & Explanation

Cross-conjugation is a special type of conjugation where a conjugated system branches at a central atom such that two separate conjugated pathways share one atom but the two pathways are not conjugated with each other through that atom in the normal linear sense. In other words, a central sp2 carbon is simultaneously part of two different pi systems, but the two terminal pi systems are not directly conjugated with each other—they are conjugated only through the central atom. Step 1 – Understand cross-conjugation: A cross-conjugated molecule contains a central carbon that is doubly bonded and also carries a substituent that is itself part of another pi system, creating a branched pi network where three pi bonds meet at one carbon but no single continuous conjugation path connects all three. Step 2 – Analyze each option: (a) CH2=CH-CH=CH-CH2: This is simply 1,3-pentadiene (linear conjugation). It is a normally conjugated diene, not cross-conjugated. (b) CH2=CH-C(=CH2)-CH2CH3: The central carbon bears a vinyl group (CH2=CH-) and a vinylidene (=CH2) plus an ethyl group. The ethyl group is saturated, so the two pi bonds (CH2=CH- and =CH2) share the central sp2 carbon. However, this is actually a cumulated or cross system with only two pi bonds meeting at the central carbon along with a saturated substituent. This is closer to a simple conjugated system rather than a true cross-conjugated system because there are only two pi-bond pathways meeting at the central carbon. (c) CH2=CH-CH(-CH=CH2)-CH=CH2: The central carbon here is sp3 (it bears four single bonds: two vinyl groups and two CH connections). Since the central carbon is sp3, the two vinyl groups are not conjugated with each other at all—there is no cross-conjugation, just two isolated double bonds. (d) CH2=CH-C(=CH2)(-CH=CH2): The central carbon is sp2 and is simultaneously involved in three pi systems: it is doubly bonded to CH2 (one pi bond), it is attached to CH=CH2 (vinyl, extending pi system one way), and it is attached to CH2=CH- (vinyl, extending pi system another way). The central carbon is part of all three pi bonds meeting at it. The two vinyl arms (CH2=CH- and CH=CH2) are each conjugated through the central carbon's double bond (=CH2), but the two vinyl groups are not conjugated directly with each other—they are cross-conjugated. This is the classic definition of a cross-conjugated system: three pi bonds meeting at a single carbon where no single linear conjugation pathway connects all three terminal carbons. Step 3 – Confirm answer: Option (d) has a central sp2 carbon bearing three pi-bond-containing groups arranged in a branched (cross-conjugated) topology, which is the hallmark of cross-conjugation. Therefore, the correct answer is D.