Destroyer Help Please: Ochem-83 and 104

[sweetdaises2000]

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So question 83 asks which reacts fastest with an SN1 Mechanism. Judging from the solutions, it seems we have to consider aromaticity when judging which reaction would be best? Is that right? So a simple secondary carbocation, is better than an antiaromatic compound? Also what makes choice B so horrible? Its the one with the benzene ring carbocation. And what is the hybridization of that carbon that has the positive charge? For choice E. could you please explain why this structure is aromatic? Is the carbon with the positive charge sp2 hybridized and there is just an empty p orbital there?Is that what makes it aromatic?
I'm just having trouble understanding the theory behind this.

Question 104- The solution says that the back has been switched-im guessing they mean the configuration of the atoms. How and why was this done? When they convert the Newman to the Fischer, I dont understand why the Cl is on the right side of the fischer and not the left. Please explain guys, I'd really appreciate it!


Thanks a bunch,
Abby

 

[lex1489]

83-- First, you can't perform Sn1 or Sn2 reactions to anything bound directly to a benzene. You can't perform Sn1 or Sn2 on ANYTHING that is sp2 hybridized, benzene especially since you'd end up with a sp hybridized cation that definitely mess up the ring (see that thread on the relatively stability of benzene cation. Long story short, methyl cation is more stable!).
Choice e is best because in this case, you have a ring that starts out with 6 pi e-, but there is a stupid sp3 hybridized carbon that doesn't want a part of the electrons' conjugation -- its shell is full. When Cl- leaves, an sp2 hybridized cation is left, leaving you with an incredibly stable (aromatic) ring with 4(1) + 2 pi electrons.

You definitely don't want to use choice c because removing Cl- would leave you with 4 pi electrons attempting to conjugate and instead bumping into each other and generally being unruly. The 4n intermediate is anti-aromatic and will decompose quickly or will never form in the first place.

104-- this one messed with me for a little while too. Think about what a newman projection actually implies.

.....CH3
.....-
.....-
CL>C<H
.....-
.....-
Cl>C<H
.....-
.....-
.....CH3

The dots are for spacing, dashes represent actual dashed lines or bonds going into the screen. carrots [<] represent bonds coming out of the screen.

With 4 carbons, you can kind of look at it like a table shape. The first carbon is the bottom of the back leg, second carbon is directly above that, third carbon is parallel to the floor straight in front of the second, and the forth is directly below the 4th carbon. Upside down this |_|

You want to make the fischer projection match this shape, because right now you have the back 3 carbons correct, but the 4th carbon is *above* the 3rd and not under it. So rotate the front circle of the fischer projection 180 degrees so the bottom 2 carbons are lined up, and you'll see the Cl is on the right side.