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• UltimaOnline

  10 Sep 16, 05:40

  Originally posted by Flying grenade:

  2014 qn

  The best answer (expected by Cambridge) is indeed an alkyne. Just that Cambridge will probably (for A level purposes) accept a diene as well.

  energy released in forming triple bond + 2 single bond greater than 2 double bond is it?

   

  i was stuck here for a while, dk which is acceptable 

  then i thought of there will always be a mixture % of products 

  
  Whether there'll be a mixture of products, or only 1 product, depends on other conditions and factors (eg. solvent, catalyst, pH, temperature, pressure, etc) beyond the A level syllabus. And it's not always just about bond strength enthalpy change alone which determines the product(s), but other factors are also relevant, eg. ring strain, angle strain, steric strain, torsional strain, induction, resonance, reagent mechanism, entropy, etc.

  All of which you needn't bother with, in the context of this A level question (ie. Cambridge doesn't expect or require you to think about all these factors). If (in the 2016 A level exam), you're not sure which product you'll get, be exam smart and give both products, with qualifications and explanations as to why each alternative product is possible or possibly more favored / stable.

• Flying grenade

  10 Sep 16, 06:01

  for the Sn1 mechanism and energy profile diagram, why is there energy given out from the first transition state (first hump) to the intermediate? (why the curve come down from the apex of first hump(when the bond just break right?) to the trough of the curve, intermediate ?)

  from the intermediate to the second transition state, i understand have activation energy so there is energy required 

• Flying grenade

  10 Sep 16, 06:16

  2014 p1 qn 29

  the typical alkyl group only donates electrons by induction, which is a much weaker effect and insufficient for halogenation via electrophilic aromatic substitution.
  understood

  
  Interestingly for this particular question, the alkyl group actually withdraws electrons both by induction (due to higher % s character of the sp2 C atom of the alkene group),

  ohhh ok i understood alr , sorry for posting, i got it after posting 

   

  as well as by resonance delocalization of electrons over to the electron-withdrawing COOH group as there is conjugation throughout the entire molecule from one end to the other.

  understood 

• UltimaOnline

  10 Sep 16, 12:32

  Originally posted by Flying grenade:

  for the Sn1 mechanism and energy profile diagram, why is there energy given out from the first transition state (first hump) to the intermediate? (why the curve come down from the apex of first hump(when the bond just break right?) to the trough of the curve, intermediate ?)

  from the intermediate to the second transition state, i understand have activation energy so there is energy required 

  
  Because the intermediate is more stable than the transition state (which by definition are always the most unstable), obviously.
• Sw@gD@ddy

  11 Sep 16, 15:20

  So 2 months out till A levels, any tips on an effecient study plan to stick to? Like should I just keep doing papers? is there value in doing topical? Cus I find myself constantly stuck in the B range ... and its damn difficult to hit that glorious A

• UltimaOnline

  11 Sep 16, 17:00

  Originally posted by Sw@gD@ddy:

  So 2 months out till A levels, any tips on an effecient study plan to stick to? Like should I just keep doing papers? is there value in doing topical? Cus I find myself constantly stuck in the B range ... and its damn difficult to hit that glorious A

  
  Keep spamming Prelim papers. Focus more on your weakest topics, skip your strongest topics and easy questions. And track and increase your speed (often, students know how to do the questions, they just don't have the time to do them) until you're fast enough to finish Prelim papers (specifically top JCs) within the time allocated.
• Sw@gD@ddy

  11 Sep 16, 18:58

  Would it be wise to skip doing paper 1 and leave it till when im done with all papers? Thats the advice Im getting which I feel is slightly dubious!

  And I should I bother doing the top tier JC prelims or stick to mid tier JC prelim papers! 

  
  

  Thanks so much, the A level grind is real 

  
                   Originally posted by UltimaOnline:

  Keep spamming Prelim papers. Focus more on your weakest topics, skip your strongest topics and easy questions. And track and increase your speed (often, students know how to do the questions, they just don't have the time to do them) until you're fast enough to finish Prelim papers (specifically top JCs) within the time allocated.

   

• Sw@gD@ddy

  11 Sep 16, 19:41

  Oh and a chem question, how come adding HCL will protonate an alchol causing it to be more soluble in water? I thought alchohol is considered neutral?

• UltimaOnline

  11 Sep 16, 21:08

  Originally posted by Sw@gD@ddy:

  Would it be wise to skip doing paper 1 and leave it till when im done with all papers? Thats the advice Im getting which I feel is slightly dubious! And I should I bother doing the top tier JC prelims or stick to mid tier JC prelim papers! Thanks so much, the A level grind is real.
  
  Oh and a chem question, how come adding HCL will protonate an alchol causing it to be more soluble in water? I thought alchohol is considered neutral?

   

  
  Paper 1, up to you. No big deal either way. Yes, just keep spamming top tier JC Prelim papers.

  Relative to more acidic HCl, alcohols are basic. Relative to more basic species (eg. NH2-), alcohols are acidic. As Einstein figured out, everything is relative, including morality.

• Sw@gD@ddy

  11 Sep 16, 23:07

  Wow thanks that was deep, one last question how can an alkene exhibit geometrical isomerism with more than 2 different distinct R groups ??? ( E.g CH3, H, CH3CH2 ) like how do you tell ?

• UltimaOnline

  11 Sep 16, 23:13

  Originally posted by Sw@gD@ddy:

  Wow thanks that was deep, one last question how can an alkene exhibit geometrical isomerism with more than 2 different distinct R groups ??? ( E.g CH3, H, CH3CH2 ) like how do you tell ?

  
  Geometrical isomerism consists of cis–trans isomerism and E-Z isomerism. If there are only 2 R groups (need not be the same as each other), then cis–trans isomerism can be used. If there are 3 or 4 R groups, then E-Z isomerism needs to be used.
• 8truthseeker8

  12 Sep 16, 00:30

  Hi, can I ask two questions? I searched online for the answers but I cant seem to find a consensual answer and also the answers were not available.

  First, why are geminal diols unstable? I know they undergo dehydration but why?

  Second, why does Fehling's not react with aromatic aldehydes? I search online apparently its because of a step in the mechanism, but I cannot find the mechanism for oxidation using Fehling's anywhere online?

• UltimaOnline

  12 Sep 16, 01:31

  Originally posted by 8truthseeker8:

  Hi, can I ask two questions? I searched online for the answers but I cant seem to find a consensual answer and also the answers were not available.

  First, why are geminal diols unstable? I know they undergo dehydration but why?

  Second, why does Fehling's not react with aromatic aldehydes? I search online apparently its because of a step in the mechanism, but I cannot find the mechanism for oxidation using Fehling's anywhere online?

  
  Because of thermodynamically favorable positive entropy change, which shifts the position of equilibrium (ie. it's an equilibrium, you'll still get some germinal diols).

  As for why Tollen's can oxidize benzaldehydes while Fehling's can't, there are several reasons for this.

  For A level purposes, the more relevant concept is the more positive reduction potential of Ag+ to Ag, compared to Cu2+ to Cu+.

  At the same time, note that benzaldehyde is more resistant to oxidation compared to aliphatic aldehydes, as illustrated by the greater magnitude of oxidation state difference between the reactant (C atom's OS approximately 0.5 in the resonance hybrid for benzaldehyde, versus C atom's OS of +1 in aliphatic aldehydes) and the product (OS of +3 in the carboxylate ion).

  Oxidizing benzaldehydes are not a problem for stronger oxidizing agents K2Cr2O7 and KMnO4 (with more positive reduction potentials), which are (by virtue of higher oxidation states and charge densities of the Mn and Cr atoms) able to utilize a stronger ionic oxidation mechanism, as opposed to a weaker free radical oxidation mechanism for Fehling's and Tollen's, in which the more positive reduction potential for Ag+ to Ag over Cu2+ to Cu+ just sufficiently overcomes the larger thermodynamic barrier for the oxidation of benzaldehyde (as opposed to aliphatic aldehydes) via the weaker free radical mechanism (as opposed to the stronger ionic mechanism).

• 8truthseeker8

  12 Sep 16, 13:18

  Hi,

  Um about that, even if delta S +ve, then shouldnt that apply to vicinal diols, or any diol for that matter? And also delta H for the reaction is +ve since C-O & O-H bonds broken to form C=O overall BE is +ve.

  Also, do you happen to have a link on where I can read up about free radical oxidation mechanism? Or is it just an abstract idea of the metal accepting e-s? I tried googling but found nothing.

  Lastly, does it matter if you spell Fehlings' or Tollen's like this? Must it always be Fehling's and Tollens'?

  Thank you so much :D

   

• UltimaOnline

  12 Sep 16, 14:56

  Originally posted by 8truthseeker8:

  Hi,

  Um about that, even if delta S +ve, then shouldnt that apply to vicinal diols, or any diol for that matter? And also delta H for the reaction is +ve since C-O & O-H bonds broken to form C=O overall BE is +ve.

  Also, do you happen to have a link on where I can read up about free radical oxidation mechanism? Or is it just an abstract idea of the metal accepting e-s? I tried googling but found nothing.

  Lastly, does it matter if you spell Fehlings' or Tollen's like this? Must it always be Fehling's and Tollens'?

  Thank you so much :D

   

  
  Vicinal diols can't dehydrate via the same low Ea mechanism as geminal diols. The dehydration of vicinal diols require the same high Ea mechanism as any other alcohols.

  No link, sorry. You can explore these matters in more detail after your A levels (assuming you're a Sg A level student, are you?), especially if you continue on to study Chemistry in the Uni levels. Right now (assuming you're a Sg A level student), you should focus more on doing Prelim papers, and worrying less about the details of beyond syllabus concepts.

  Cambridge won't penalize you over minor spelling details such as these, no worries.

• 8truthseeker8

  12 Sep 16, 15:48

  Hahah yeah, because Im very confused when the teachers tell us that Fehlings cant oxidise aromatic aldehydes then im like wondering why hahah then i cant even find the answer

• Flying grenade

  12 Sep 16, 19:48

  YJC 2016 block test 

  https://www.dropbox.com/s/2lhrq1h7es3o1ly/20160912_194000-1.jpg?dl=0

  one part of the question asks for the major product when Br2(aq) is added to Limonene

   

  is the Major product two OH groups added, because the OH- nucleophile is in large excess? if it's not the major product, i dont understand why all the OH- will let the Br- nucleophile attack the carbonation first, before attacking the carbocation itself?

   

  for the substitution for one Br and one OH, which nucleophile attack the more stable, tertiary carbocation first? the Answer given is that the OH is atttached to both the tertiary carbon in Limonene ,while the Br to the primary carbon, for the electrophillic addition of Br2(aq) to the two alkene in Limonene 

  if the Answer given is correct, which is OH attach to tertiary carbon instead of Br, then i don't understand why the remaining of OH- will let the Br nucleophile attack the primary carbon, without attacking the carbocation itself?

• Flying grenade

  12 Sep 16, 19:56

  is it the first step the Br2 polarised undergo heterolytic fission , the delta+ of Br act as an electrophile, attack the C=C, so its the Br that  must be attached to the tertiary carbon? 

  so Br attached to primary carbon as the Major product is wrong?

   

   

  is two OH(forming a diol) electrophillically added to the alkene, a minor product?

• UltimaOnline

  12 Sep 16, 20:08

  Originally posted by Flying grenade:

  YJC 2016 block test 

  https://www.dropbox.com/s/2lhrq1h7es3o1ly/20160912_194000-1.jpg?dl=0

  one part of the question asks for the major product when Br2(aq) is added to Limonene

   

  is the Major product two OH groups added, because the OH- nucleophile is in large excess? if it's not the major product, i dont understand why all the OH- will let the Br- nucleophile attack the carbonation first, before attacking the carbocation itself?

   

  for the substitution for one Br and one OH, which nucleophile attack the more stable, tertiary carbocation first? the Answer given is that the OH is atttached to both the tertiary carbon in Limonene ,while the Br to the secondary carbon, for the electrophillic addition of Br2(aq) to the two alkene in Limonene 

  if the Answer given is correct, which is OH attach to tertiary carbon instead of Br, then i don't understand why the remaining of OH- will let the Br nucleophile attack the secondary carbon, without attacking the carbocation itself?

  
  U coconaden. U made at least 5 errors here. Still think u're ready for A grade?

  OH- ion is *not* present at all, let alone in large excess.

  And the only way u can get 2 OH groups added across a double bond to generate a vicinal diol is using KMnO4.

  Instead of asking "why all the OH- will let the Br- nucleophile attack the carbonation first, before attacking the carbocation itself?", u might as well ask "why all the Br- will let the OH- nucleophile attack the carbonation first, before attacking the carbocation itself?".

  First of all, it's not OH- nucleophile, it's H2O nucleophile. Secondly, nobody's letting anybody do anything. It's whoever is in that place at that time with the means, gets there first.

  And ur last statement "if the YJC Answer given is correct, which is OH attach to tertiary carbon instead of Br, then i don't understand why the remaining of OH- will let the Br nucleophile attack the secondary carbon, without attacking the carbocation itself?" shows u're woefully inadequate when it comes to understanding and drawing mechanisms. The only way u can answer ur qn is by correctly drawing out the full mechanism. That's something (if u google out the mechanism and still dun understand it) u'll need ur school teacher or private tutor to go thru with u step-by-step.

• UltimaOnline

  12 Sep 16, 20:10

  Originally posted by Flying grenade:

  is it the first step the Br2 polarised undergo heterolytic fission , the delta+ of Br act as an electrophile, attack the C=C, so its the Br that  must be attached to the tertiary carbon? 

  so Br attached to secondary carbon as the Major product is wrong?

   

   

  is two OH(forming a diol) electrophillically added to the alkene, a minor product?

  
  Just ask ur school teacher or private tutor to go thru the damn mechanism with u step-by-step, I won't entertain any more qns from u on this matter here, coz it's annoyingly inefficient to textually discuss mechanisms on the forum.
• Flying grenade

  12 Sep 16, 20:31

  the given YJC answer is correct, for the qn above.

• Flying grenade

  12 Sep 16, 20:42

  eh i got it alr, SWEE. i am happy

  Thank you so much George Chong and Ultima, you all are my saviors  !!

• Flying grenade

  12 Sep 16, 20:44

  thanks U.O. for being legit, selecting and introducing me these life-saving books, these books and your real teachings are legit , which are lifesavers. 神

• Flying grenade

  12 Sep 16, 20:48

  

• Flying grenade

  12 Sep 16, 21:00

  Originally posted by UltimaOnline:

  
   U made at least 5 errors here. Still think u're ready for A grade?

  
  

  nope. i didn't think that way nor am i complacent, hence i work hard 

  hence need seek ur reliable help in a bid to rectify my errors 

  corrected one error above, should be 1° carbocation, not 2°. I've edited the posts above

  thanks for your other replies and explanations *thumbs up*