Yes. I'm from Vietnam. I will have a test to NTU
soon. I asked my teacher about this but he is too [email protected]@ that why
i have to ask someone else. Are you the teacher in NTU [email protected]@
I find this website while I'm dead because of those A
level stuff and your answer is quite clear.. The content and the
demonstration of my book is rather different to our book so I
sometimes stuck . However, your answers help me a lot.
So you're taking a private NTU entrance exam? So you're
not taking the
Singapore A Levels H2 Chemistry exam? And you've to self-study
everything (the entire A level syllabus and more) within a few
weeks (when's your NTU entrance exam?) on your own? (Did you
previously study Chemistry in Vietnamese language, and now you've
mentally converting everything to English?) Tough. My sympathies
and empathies.
(Edit : No, I'm not a NTU teacher, hehe it's ok it's true
that many NUS/NTU lecturers are lazy hehe. I'm an A level
Chemistry private tuition teacher, specializing in
Singapore-Cambridge A level H2 Chemistry. So if you take Chemistry
at Uni level after you enter NTU, I won't be able to help you with
your Uni level questions here on the HomeWork Forum, as all Chem
questions beyond A levels should be asked at The
Chemical Forums instead).
Ok, the answer to your questions are :
iv) With 1-bromopropane, methoxide will undergo SN2, while
t-butoxide will undergo E2.
b) Guys are negative, Girls are positive. Guys like Girls.
Chemistry is about Love.
1 - Deprotonate the ketone (the alpha protons are slightly
acidic, because the enolate conjugate based is stabilized by
having its negative formal charge delocalized by resonance over
both the C and the O atoms) by use of a suitable strong base (eg.
lithium diisopropylamide, sterically hindered and negative formal
charge on the N atom).
2 - Give the enloate ion guy (who shoots out his balls or
electron pair, from the less electronegative C atom, rather than
the more electronegative O atom) nucleophile, a suitably attractive
n-pentylhalide girl electrophile, for him to make sweet SN2
love to.
3 - Use a hydride ion nucleophile guy (eg. from LiAlH4 in dry
ether) to attack and accompany the lonesome carbonyl C atom girl
electrophile (who has a partial positive charge due to both
induction and resonance). The resulting alkoxide ion may be
protonated (ie. give the O- guy a H+ girlfriend) via
hydrolysis (the H2O molecule may be thought of as a H+ girl with a
OH- boyfriend; the H+ proton girl can transfer over to
change boyfriends) to generate a secondary alcohol (ie. the
new couple).
4 - Dehydrate the secondary alcohol (using excess concentrated
sulfuric(VI) acid, at 170 deg C; alternatively use aluminium oxide
at 400 deg C, or phosphoric(V) acid at 250 deg C) to obtain the
alkene.
Although 2 different alkene products may be generated (since two
different alpha protons may be abstracted during the elimination
mechanism), the major product will be our desired alkene, because :
the more stable alkene is the alkene with the greater number of
alkyl groups bonded to its sp2 hybridized C atoms, ie. the more
heavily substituted alkene. This is in turn due to the
mechanism : the transition state (for E2) or intermediate (for E1)
may be conceptualized as a carbocation, the greater the number of
electron donating (by induction and hyperconjugation) alkyl groups
present, the more stable the transition state or intermediate, the
lower the activation energy required, the faster the rate of
reaction, and hence we get more of the product, which we
accordingly label as the 'major product'.
Mechanism given here by good ol Jim Clarke, whom, together with Rod
Beavon, are the two world's best A Level Chemistry websites that
even Cambridge uses in setting their own A level exam papers :
http://www.chemguide.co.uk/mechanisms/elim/dhethanol.html
5 - Oxidize the alkene using cold, dilute KMnO4 to obtain the
diol. If the temperature is not kept cold, there may be
sufficient activation energy to allow oxidative cleavage to
occur; pH is less important than temperature, but an alkaline pH is
preferred if one wishes to obtain the diol, and acidic pH
if one wishes to obtain oxidative cleavage. The reason for
this is due to the different activation energies for various steps
of the mechanism for the 2 pathways (dihydroxylation vs oxidative
cleavage) in different pH environments.
To gain a better understanding of the underlying mechanisms for
the various reagents in Organic Chemistry, the following pdf e-book
is highly recommended (especially if you will be taking
Chemistry at Uni level in NTU, and not just preparing
yourself for the A level NTU entrance exam) :
http://masterorganicchemistry.myshopify.com/
