Please identify the MAJOR contributing resonance structure (MOST significant) in this case: Note: Here MAJOR contributor = the MOST significant contributor. MINOR contributor = the LEAST significant. The INSIGNIFICANT contributors and all other INTERMEDIATE contributors are not considered. Review Chapter 2.8. CO: 2
Reactive Intermediates
In chemistry, reactive intermediates are termed as short-lived, highly reactive atoms with high energy. They rapidly transform into stable particles during a chemical reaction. In specific cases, by means of matrix isolation and at low-temperature reactive intermediates can be isolated.
Hydride Shift
A hydride shift is a rearrangement of a hydrogen atom in a carbocation that occurs to make the molecule more stable. In organic chemistry, rearrangement of the carbocation is very easily seen. This rearrangement can be because of the movement of a carbocation to attain stability in the compound. Such structural reorganization movement is called a shift within molecules. After the shifting of carbocation over the different carbon then they form structural isomers of the previous existing molecule.
Vinylic Carbocation
A carbocation where the positive charge is on the alkene carbon is known as the vinyl carbocation or vinyl cation. The empirical formula for vinyl cation is C2H3+. In the vinyl carbocation, the positive charge is on the carbon atom with the double bond therefore it is sp hybridized. It is known to be a part of various reactions, for example, electrophilic addition of alkynes and solvolysis as well. It plays the role of a reactive intermediate in these reactions.
Cycloheptatrienyl Cation
It is an aromatic carbocation having a general formula, [C7 H7]+. It is also known as the aromatic tropylium ion. Its name is derived from the molecule tropine, which is a seven membered carbon atom ring. Cycloheptatriene or tropylidene was first synthesized from tropine.
Stability of Vinyl Carbocation
Carbocations are positively charged carbon atoms. It is also known as a carbonium ion.
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Here we are looking at the resonance canonical forms the molecule 4-methoxybut-3-en-2-one.
This is an 
unsaturated ketone, where the C=O carbonyl double bond is in conjugation with the double bond and that can result in the movement of the lone-pairs on the O atom via the conjugation.
Since the initial structure is neutral , we must ensure that the resonance canonical forms must also have a net charge of 0, else the resonance rules are violated.
Also, the movements of the lone-pair electrons and the pi bond must be in continuous flow.
Keeping all these things in mind let us draw some viable resonance canonical forms for 4-methoxybut-3-en-2-one.
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