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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

The more u spam (ie. one shot post many qns), the less willing I am to look at ur qns, and thus the longer u'll have to wait.

As it is, I'll only look at ur qns and reply them only after midnight. If u spam some more qns in the meantime, maybe I'll reply u only 24 hrs later, 36 hrs, 48 hrs, etc.

Of course, if anyone else (in the hundreds of lurkers observing my Chem threads, and not all of you are JC students) volunteers to reply your questions in the meantime, pls go right ahead.

UltimaOnline SG

wow! also form diol(deduced as can form 1,2dibromocyclohexane)? same as cold alkaline kmno4

Temperature factor outweighs pH factor for this reaction. Singapore JCs didn't teach u students that, huh.

UltimaOnline SG

for option A, we didnt learn that using HBr,heat , to a bromohydrin, can form bromo alkane. can we use this reaction ,write it in A levels?

*facepalm* It's alkene, not halohydrin. And if u still say u didn't learn that, too bad for u, I'm pretty sure the rest of the Sg JC cohort did.

UltimaOnline SG

2008/p1/q26

cs toh writes NaCN is in the form of NaCN(aq)

so for the nu add of HCN to ketone to form cyanohydrin, reagents and conditons are HCN, trace KCN, 10-20°C

the NaCN is in (aq)form? not (s) form?

NaCN solid

https://en.m.wikipedia.org/wiki/Sodium_cyanide

or is it becos NaCN and KCN readily fumes in moist air(does it) ?

ok. Miss Angeline tan writes that the catalyst is CN-(aq)

in 10-20°C, HCN is in liquid state

*facepalm*

When u wanna add flavoring to ur food and u want the flavoring and food to mix well (for nice taste reaction to occur), would u just dump in a solid chunk of salt, or sprinkle soya sauce solution nicely spread out over the food?

UltimaOnline SG

2008/p1/qn 40

i thought amino is used to refer to an amine group in an amino acid?

molecule in option 2 and 3 aren't amino acids right?

*faceplam's faceplam*

U coconaden!!! Those are just the R group of amino acids lah!!!

Flying grenade SG

*Those are just the R group of amino acids lah

ok yes i see now. so all the 3 options can form H-bonding between R groups. i understand liao. thanks ultima

Flying grenade SG

*facepalm* It's alkene, not halohydrin. And if u still say u didn't learn that, too bad for u, I'm pretty sure the rest of the Sg JC cohort did.

alkene + HBr(g) r.t. yields bromoalkane. i know this

see option A, 1st step cyclohexene + br2(aq) yields bromohydrin. thereafter 2nd step, HBr(g) + heat can yield 1,2dibromocyclohexane?

Flying grenade SG

*facepalm*

thank you ! You are God !

i think there's a probablity that not many ppl know this unless they came across this qn.

no source(books or online) writes nacn(aq)

Unless it is implied? everyone is supposed to know nacn(aq) for the nu add?

haha, thank u ultima for all ur help !

Flying grenade SG

Temperature factor outweighs pH factor for this reaction. Singapore JCs didn't teach u students that, huh.

i see. then solvent not important mah. can just anyhow use a acidic, neutral, or alkaline medium. Cold will suffice, then why want teach us cold acidified or cold alkaline

oh is it because with cold alkaline, higher chance to get diol compared to acidic conditions? alkaline conditions provide larger chance for weaker oxidation , i.e. acidic conditions always more conducive for oxidation to happen compared to alkaline?

UltimaOnline SG

alkene + HBr(g) r.t. yields bromoalkane. i know this

see option A, 1st step cyclohexane + br2(aq) yields bromohydrin. thereafter 2nd step, HBr(g) + heat can yield 1,2dibromocyclohexane?

Fair enuff, my bad, I misread.

It's still in the H2 syllabus. Replacing OH with Br using HBr. Cambridge can ask u to draw the mechanism for this, but not many H2 students know how to draw it out. Do u ?

UltimaOnline SG

i see. then solvent not important mah. can just anyhow use a acidic, neutral, or alkaline medium. Cold will suffice, then why want teach us cold acidified or cold alkaline

No, it's not so simple. But A levels is simplified anyway (then what's O levels? let's not go there). So for A level purposes, if asked for reagents & conditions, go ahead and write "cold, alkaline, dilute KMnO4(aq)", but just be aware that the most important factor is temperature. Beyond this, is beyond A levels, don't confuse yourself by going there.

UltimaOnline SG

thank you ! You are God !

i think there's a probablity that not many ppl know this unless they came across this qn.

no source(books or online) writes nacn(aq)

Unless it is implied? everyone is supposed to know nacn(aq) for the nu add?

haha, thank u ultima for all ur help !

As I said many times liao, don't OCD so much, don't worry so much, don't bother so much. Just write the reagents & conditions as given in trusted sources like CS Toh, Chan Kim Seng, or George Chong. And Cambridge isn't *that* anal, in this case they wouldn't even give a damn (ie. Cambridge will just ignore) whether u include the state symbol or not.

Flying grenade SG

becos in this qn, need to deduce homogenous catalyst.

i was unsure nacn is s or aq state.

Flying grenade SG

oh ! question : in solid state , no nucleophilic reaction can happen? since cn- is locked?

(understand ur food analogy, reagents in aq state can be better spread out throughout the reactants compared to solid)

this applies for most reactions? no rxn between 2 molecules in their solid state

what about amine(gas) react with carboxylicacid(l) to form amide(s)

UltimaOnline SG

oh ! question : in solid state , no nucleophilic reaction can happen? since cn- is locked?

(understand ur food analogy, reagents in aq state can be better spread out throughout the reactants compared to solid)

this applies for most reactions? no rxn between 2 molecules in their solid state

what about amine(gas) react with carboxylicacid(l) to form amide(s)

Obviously.

Low yield, requires very high Ea, not an acceptable method for A level synthesis.

Flying grenade SG

Ester(l) + amine(g) -> amide(s) . this is in ks chan book too, not only in M.S. notes

acylhalide(l) + amine(g) -> amide(s)

Flying grenade SG

why high ea for carboxylic acid and amine

magnitude of delta +ve on C atom of carboxyl group not large enough?

UltimaOnline SG

Ester(l) + amine(g) -> amide(s) . this is in ks chan book too, not only in M.S. notes

acylhalide(l) + amine(g) -> amide(s)

Duh. Notice the reactants are not (s). So why does it surprise u these reactions would work?

UltimaOnline SG

why high ea for carboxylic acid and amine

magnitude of delta +ve on C atom of carboxyl group not large enough?

Flying grenade SG

formation of amide from carboxylic acid and amine is a reversible reaction

http://www.chemguide.co.uk/organicprops/amides/preparation.html

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