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GS Free Sample Test Section III Question 30

GS Free Sample Test Section III Question 30

Postby viv.goldstandard » Thu Sep 24, 2020 8:48 pm

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viv.goldstandard
 
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Re: GS Free Sample Test Section III Question 30

Postby HunterEd » Mon Feb 22, 2021 9:16 pm

Hi,

The explanation in point 5 for why option C is the exception reads- "That would only be possible if C6 had a + charge and C4 had a - charge"

Shouldn't it be the other way round (C6- & C4 +), or am I missing something?

Thanks,
Hunter.
HunterEd
 
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Re: GS Free Sample Test Section III Question 30

Postby goldstanda3269 » Tue Feb 23, 2021 8:51 pm

HunterEd wrote:Hi,

The explanation in point 5 for why option C is the exception reads- "That would only be possible if C6 had a + charge and C4 had a - charge"

Shouldn't it be the other way round (C6- & C4 +), or am I missing something?

Thanks,
Hunter.


Good question!

Neither!

For the double bond to flip positions from C5=C6, like the starting material, to answer choice C’s C4=C5, it would take a special trick that comes up on GAMSAT occasionally: a hydride shift.

H must change positions from C4 to C6, because, of course, C can only bond 4 times so if a saturated C is to have a double bond, something must go. Please do not think that this is too complex for GAMSAT because hydride and/or methyl shifts have been topics in the past but were explained in the passage. You did not need to know anything about hydride shifts for this question. You just needed to follow the electrons; however, I want to emphasize this point in case you encounter this in the future.

One more point about GAMSAT: The most ‘famous’ GAMSAT-style question about resonance (you will see it multiple times in ACER's practice materials), whenever there is a unit with many questions, will have an answer choice where 1 atom inexplicably changes position (like a shift). Resonance illustrates the movement of electrons but NEVER the movement of atoms.

For more: HEAPS-3 or GAMSAT-level practice questions in Chapter 4 Alkenes, Chapter 5 Alcohols (GAMSAT Masters Series).
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Re: GS Free Sample Test Section III Question 30

Postby HunterEd » Wed Feb 24, 2021 9:21 pm

Understood! - thank you for taking the time to explain it with a bit of context.
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