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last SG · 2015-08-19T22:54:00+00:00

Topic : atomic radius Hi! This is from singapore cambridge nov a level exam year 2010,Paper 1 qn 1Qn: which element has the largest atomic radius?BrKKrScSo...

UltimaOnline SG

ok thanks god for ur patience, benevolence and still not giving up when i ask qns (high tolerance)

2007 p3 qn 4c

https://www.dropbox.com/s/pelo2bzgf0k0ocz/20160925_235715.jpg?dl=0

what does he mean by since reagents used are non chiral, the reactions involved are identical?

That statement of CS Toh's, is an additional (and secondary) point : that because hot Al2O3 is a 'chirally blind' or non-stereospecific reagent, as opposed to say using the biological enzymes in your human body, hence whether you use enantiomer D or E, you end up with the same products F and G.

Flying grenade SG

what's the oxidation state of N in nitrobenzene?

some from internet says its +3 ,some say it's +5

help ultima

check my drawing of nitrobenzene pls ultima, thanks

https://www.dropbox.com/s/ai69tw8ehxq6701/20160925_205912-1.jpg?dl=0

update : i got +3. is it correct?

what about the oxidation state of N in the resonance hybrid?

Flying grenade SG

ohh yes ok i understand now !!! thanks !!!

a mixture of the cis and trans products F and G, which we can't tell the composition/%/major or minor product at this level

UltimaOnline SG

what's the oxidation state of N in nitrobenzene?

some from internet says its +3 ,some say it's +5

help ultima

check my drawing of nitrobenzene pls ultima, thanks

https://www.dropbox.com/s/ai69tw8ehxq6701/20160925_205912-1.jpg?dl=0

update : i got +3. is it correct?

what about the oxidation state of N in the resonance hybrid?

Your drawing and (thus) OS are wrong.

Either you show formal charges and don't show dative bond (the BedokFunland JC way used in University level), or you show dative bond and don't show formal charges (the Singapore JC way because most JCs don't teach H2 Chem students about formal charges, only H3 Chem, Olympiad Chem and BedokFunland JC students have this educational privilege).

Redraw your nitrobenzene correctly, using both alternative ways.

Draw at least the first couple of resonance contributors (ie. benzene ring pi electrons delocalizing into the NO2 group), then you'll see the answer clearer for yourself.

I won't spoonfeed you the final answer for OS : draw out the resonance contributors correctly, then work out the OS of N yourself (hint : if the OS of N differs in different resonance contributors, you can only conclude the OS of N in the resonance hybrid is somewhere in between the different OS obtained for individual contributors).

Flying grenade SG

eh? for nitro benzene , are the one +ve charge(on N) and one -ve charge(on one O atom) , delocalised by resonance between the ONO?

from the resonance structures i see on google image, seems like the charges are just delocalised by resonance into benzene ring, and not at the ONO?

UltimaOnline SG

eh? for nitro benzene , are the one +ve charge(on N) and one -ve charge(on one O atom) , delocalised by resonance between the ONO?

from the resonance structures i see on google image, seems like the charges are just delocalised by resonance into benzene ring, and not at the ONO?

If *positive* formal charges are delocalized *into* the benzene ring, obviously it means pi electrons are delocalized *out* of the benzene ring, isn't that obvious?

U go figure this out for urself. I'm going out now for makan, be back at 3am to look at this thread.

Flying grenade SG

I'm going to sleep alr. help look at my questions above, thanks God !

UltimaOnline SG

2008 p2 qn 5d

after nitrating via Esub, to place two NO2 groups on the molecule, how to make sure only one of the NO2 is reduced to NH2?

No way to make sure. You can only (and should) add on in your answer that limited amounts of reagent, limited duration, and not too high temperature used, to reduce only 1 of the 2 groups.

UltimaOnline SG

ok understood the use of Sn and HCl from chankimseng page 330.

*small* amt of conc HCl(and small amt of sn) should help in the above reaction. then there will be a mixture of products, some with the 4-nitro group being reduced, some with the 2-nitro group being reduced

if use excess/large amounts of HCl(and sn) , very high possibility that both NO2 groups are reduced.

Ok u figured out urself liao. Next time edit ur post lah. Waste my time type out answer, then turns out u also posted the answer in the next post liao.

UltimaOnline SG

to generate tri-nitro-substituted phenols we use conc HNO3. for monosubstitution, we use HNO3(aq)

how about for di-substituted nitrophenols?

UltimaOnline SG

eh? for nitro benzene , are the one +ve charge(on N) and one -ve charge(on one O atom) , delocalised by resonance between the ONO?

from the resonance structures i see on google image, seems like the charges are just delocalised by resonance into benzene ring, and not at the ONO?

Continue googling out the resonance contributors and hybrid. If you still can't figure out, go ask your school teacher or private tutor. Move on.

UltimaOnline SG

how to know and why is stage II an elimination reaction?

this resembles very much like a decomposition reaction

is Decomposition reaction acceptable?

If you're a BedokFunland JC exam-smart student, what would you write?

UltimaOnline SG

2007 / p2 / qn 4dii

this nucleophilic substitution can be either SN1 or SN2 right?

interesting, first time see this transition state in SN2

SeanChua's mechanism is exactly the same as CSToh's , for this qn

Wrong. The correct mechanism for nucleophilic acyl substitution is addition-elimination (H3 syllabus). While Cambridge *may* accept SN1 or SN2 (H2 syllabus) for this question, but if *you* know the correct (H3 level) version, you should certainly draw the correct version. That's what I always do, and I always get my A grade *guailan laughter*.

Flying grenade SG

the number of optical isomers in this molecule is 2^3 ?

is there a pair of cistrans isomer as well? i.e. 1 cis and 1 trans isomer

Flying grenade SG

what in the world is the yellow brown ppt??

https://www.dropbox.com/s/7oymrgijg8ok4kk/20160926_180017.jpg?dl=0

http://forums.sgclub.com/singapore/cuso4_ki_407273.html

this one only have brown soln due to I3- ions

Flying grenade SG

this qn came out in 2006 p2 qn 2e

and

2010 p3 qn 5e

https://www.dropbox.com/s/cd7ixziplyy6y3x/20160926_180400.jpg?dl=0

would u accept cs toh's answer(would u mark it correct)? instead of, or an additional point to the more complex answer of CuI precipitating discussed in Sgforums and in made easy inorganic book

https://www.dropbox.com/s/0fb9ue4ng4xsord/20160926_180413-1.jpg?dl=0

Flying grenade SG

pls look at my 2 qns at the btm of page 67 too, thank you !

Flying grenade SG

amide cannot be formed between carboxylic acid and amine right?

https://www.dropbox.com/s/h4xc7lpf2r5xcgh/20160926_182157.jpg?dl=0

https://www.dropbox.com/s/pb0hom468wft4kf/20160926_182206.jpg?dl=0

Flying grenade SG

2006 p3 qn 3

https://www.dropbox.com/s/n5545rg8gj6fiyj/20160926_184014.jpg?dl=0

https://www.dropbox.com/s/33jr12rs6io7pa2/20160926_184020.jpg?dl=0

how u know, and why is the rate of forward reaction small compared to the reverse reaction?? is it becos qn say 'the two gases react slowly'?

Flying grenade SG

https://www.dropbox.com/s/6ebkwa8la584e8e/20160926_185432-1.jpg?dl=0

small +ve E°value means redox reaction does occur, but is it occurs very slowly, or extent of forward reaction is very small, or both?

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