Thank you very much, I understood your explanation. For the same
question (2014 AJC Prelim, paper 2 Qn 5), for option B, there won't
be bromination then? Since it is COOCH3, I infer that it is also
electron withdrawing but still not sufficient for bromination.
For 2014 AJC Prelim, paper 3 Qn 2ciii, may I ask how is T
formed? What type of reaction could possibly occur to form the
alkene by-product? At first I thought that it could be SN1
mechanism since it is secondary alkyl halide. But it will not be
able to form an alkene, especially with the removal of O, since O
is needed for attacking the alkyl halide.
For 2014 AJC Prelim, paper 3 Qn3d, I had trouble deducing
Geraniol. I could deduce N,P,Q and R, but I wonder why Geraniol
doesn't have 2 terminal alkene. Since heating geraniol with excess
KMnO4 will give N and P, and we see that 2 carbon has been removed,
so why is there no 2 terminal alkene present? 2 CO2 could be
removed. Instead, the product is a diol. Are diols colourless
gases?
Perhaps Supercat has difficulty taking a photo of the qns or
uploading them. But as Flying Grenade said, everyone asking H2 Chem
qns here, do try to take a photo and upload the qns whenever
possible, for the sake of sharing and helping other JC students
here, coz not everyone has access to all the Prelim papers.
2014 AJC Prelim, paper 2 Qn 5 : Correct, ester group is
electron-withdrawing by both induction and resonance, hence the
benzene ring is deactivated, no halogenation via electrophilic
aromatic substitution occurs.
2014 AJC Prelim, paper 3 Qn 2ciii : E2 reaction occurs in
competition with SN2. While the Singapore A level H2 Chem syllabus
simplifies the matter of whether nucleophilic aliphatic
substitution (ie. SN1 vs SN2) or elimination (ie. E1 vs E2) occurs
as simply a matter of solvent (ie. aqueous vs ethanolic), but in
practice (ie. real-life and Uni level Chem), all 4 compete against
each other, and there are other factors involved in the 4-cornered
fight : SN1 vs SN2 vs E1 vs E2.
While E1 and E2 mechanisms are not required for the Singapore A
level H2 Chem syllabus, but nonetheless as evidenced by the AJC
Prelim paper (and all Singapore JC Prelim papers and significantly
tougher recent Singapore-Cambridge A level H2 Chem papers since
2010), it pays for students gunning for A grade to have a deeper
understanding of Chemistry beyond the basic A level H2 syllabus.
Which is the advantage that H3 Chem, Olympiad Chem, and
BedokFunland JC Chem students have over H2 Chem only students.
2014 AJC Prelim, paper 3 Qn3d : You're right that the 2 missing
C atoms are eliminated in the form of CO2(g). But you're incorrect
that 2 terminal alkene groups must therefore be present in
geraniol. Because if that were the case, how would heating geraniol
with acidified KMnO4 yield N and P and CO2(g)? The given formula of
geraniol precludes the acidic hydrolysis of ester or amide group as
a possible explanation.
Concordantly, the student is expected and required to deduce and
elucidate that the functional group in question (that results in 2
C atoms eliminated as CO2) is either the =CHCH2OH group, or the
=CHCH= group, either of which is oxidized by KMnO4 with oxidative
cleavage into ethandial, which is further oxidized to ethandioic
acid, which is further oxidized to 2 moles of carbonic(IV) acid,
which exists in equilibrium with, and hence decomposes into, 2
moles of CO2(g) + H2O(l), with thermodynamically favorable positive
entropy change, and with the position of equilibrium shifting to
the RHS as predicted by Le Chatelier's principle (since CO2(g)
leaves the aqueous reaction mixture).
“Once you eliminate the impossible, whatever remains, no matter
how improbable, must be the truth.” - Sherlock Holmes (Parody1 Parody2 Parody3)