help wth organic chem qns

arigatoast SG

hi guys /gals, i got stumbled upon these kind of questions.. like how to count the number of geometric isomers.. and when the question involve optical and geo isomers together... help needed.. thanks

unclebutcher SG

if i’m not wrong first one got 4 chiral carbon and 2 cis trans. so 2 power 6. can’t rly rmb

unclebutcher SG

second one is 4/. cuz conc h2so4 is a dehydrating agent will remove the -OH group. then got 2 C=C double bond so 2 power 2

UltimaOnline SG

I recognize this question from a certain JC's 2007 prelim paper. Notice that the 2nd question is phrased ambiguously. There are several different constitutional / structural isomers that may be formed, each with its own set of stereoisomers (EZ/cis-trans and enantiomers/optical isomers). So arguably, none of the MCQ choices offered are correct. Ambiguous questions like these are a student's annoyance.

arigatoast SG

if i’m not wrong first one got 4 chiral carbon and 2 cis trans. so 2 power 6. can’t rly rmb

hi butcher appreciate yr help... but not the correct answer for both question.

arigatoast SG

if i’m not wrong first one got 4 chiral carbon and 2 cis trans. so 2 power 6. can’t rly rmb

hi butcher .. appreciate yr help.. but not the correct answer for both question.

arigatoast SG

for the 1st question... i manage to get 5 chiral centers and 2 C=C double bonds capable of cis trans. so 2 to power of 5 for optical... and for the 2 C=C double bonds how do i count?

2 power 5 + 2 power 2 ? or simply 2 power (5+2) ?

for the 2nd question... there are 2 possibilities for the product since the H2O can be removed in 2 ways ( the below and right of the OH on top)..

one possibility is remove OH and the H to the right side of OH .. and also the H2O at the bottom of the ring.. and u will get 2 double bonds and one chiral center ( the C atom on top of the ring) .. so 2 power 2 plus 2 power 1?

2nd possibility is remove OH and H below OH ( which is connected to the ring).. and of course the H2O at the bottom of the ring...so 2 power 2?

add up both cases.. i got 10.. but.. not one of the options..

even if i cal 2 power 3 plus 2 power 2 =12 also no correct le..=(

anibody can help? thanks

arigatoast SG

hi Ultimaonline... maybe if u just consider only optical and geometric isomers for A level syllabus... what will yr answer be?

UltimaOnline SG

Because the question is flawed or inadequate in phrasing (asking only for 'stereoisomers' implies no structural isomers are formed, a false assumption), hence there is no satisfactory answer. It may well be that all the available MCQ choices are wrong in the Prelim paper.

Notice that there are two possible structural isomers (the double bond at the bottom of the ring applies to both structural isomers, and isn't capable of cis-trans or E-Z, so ignore it in your counting), both of which have diastereomeric ("geometric") isomers - one is capable of cis/trans and E/Z, the other capable of E/Z only (many JCs wrongly teach students to artificially force cis/trans isomeric categorization for many alkenes that can actually only have E/Z. Technically, cis-trans isomers exist only when an alkene has two substituents and two hydrogens attached to the sp2 hybridized carbons).

After dehydration by excess conc H2SO4, only one of the two abovementioned resulting structural isomers have a single stereocenter / chiral center.

So if the question was reiterated or rephrased to "how many isomers in total (include both structural and stereoisomers) may be generated?", then the correct answer would be

structural isomer #1 - E - rectus

structural isomer #1 - E - sinister

structural isomer #1 - Z - rectus

structural isomer #1 - Z - sinister

structural isomer #2 - E

structural isomer #2 - Z

Alternatively,

structural isomer #1 - cis - rectus

structural isomer #1 - cis - sinister

structural isomer #1 - trans - rectus

structural isomer #1 - trans - sinister

structural isomer #2 - E

structural isomer #2 - Z

Or,

structural isomer #1 - cis - dextrorotatory

structural isomer #1 - cis - levorotatory

structural isomer #1 - trans - dextrorotatory

structural isomer #1 - trans - levorotatory

structural isomer #2 - E

structural isomer #2 - Z

Or,

structural isomer #1 - E - dextrorotatory

structural isomer #1 - E - levorotatory

structural isomer #1 - Z - dextrorotatory

structural isomer #1 - Z - levorotatory

structural isomer #2 - E

structural isomer #2 - Z

arigatoast SG

Thanks Ultimaonline for the long post. If the extra stuff u mentioned were inckluded in A levels, i think most of the JC pple will be turned off by chem bah... i think i will be turned off too.. haha.

the answer for the 2nd qns is 6.

for the 1st question... i manage to get 5 chiral centers and 2 C=C double bonds capable of cis trans. so 2 to power of 5 for optical... and for the 2 C=C double bonds how do i count?

2 power 5 + 2 power 2 ? or simply 2 power (5+2) ?

Thanks

d3sT1nY SG

the answer for the 2nd qns is 6.

for the 1st question... i manage to get 5 chiral centers and 2 C=C double bonds capable of cis trans. so 2 to power of 5 for optical... and for the 2 C=C double bonds how do i count?

2 power 5 + 2 power 2 ? or simply 2 power (5+2) ?

Thanks

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