for my Q12, I was wondering why the dehydration cannot occur
with the H from the aldehyde (taking H from a C that is 'less rich'
in H).
Why is the reactivity of halogenating agents as such:
F>Cl>Br>I ?
I'll deal with your halogenating qn 1st :
Your qn is ambiguous, do you refer to halogenation via free
radical mechanism, or via nucleophilic (aliphatic or acyl)
substitution, or via electrophilic aromatic substitution, etc?
Never be ambiguous in your A level exam answers, if Cambridge asks
"what is this reaction?" you should specify "halogenation via
_________ (mechanism)".
So what exactly are you asking? Provide a context if relevant,
eg. a specific TYS or prelim paper qn, or a website link, or a page
number from a CS Toh book, or George Chong book, or Chan Kim Seng
book, etc.
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Edited : "for my Q12, I was wondering why the dehydration
cannot occur with the H from the aldehyde (taking H from a C that
is 'less rich' in H)."
Eh sial lah! From your statement above, it shows you're just
blindly memorizing and applying Markovnikov's and/or Zaitsev's
rules without a proper understanding of *how* or *why* they usually
work (and thus when they sometimes won't work)! You (ie. most
Singapore JC students) jialat liao lah, this is the kind of
teaching in Singapore JCs which illustrates the superiority of my
BedokFunland JC pedagogy : *Understanding* then applying (as
opposed to "blindly memorizing then applying" as taught in
Singapore JCs).
As to your Zaitsev qn : Ahhh ok, what you mean is you're
thinking of generating the ketene functional group
from the acyloin functional
group, well you could have just said so ;Þ
There are a couple of problems with your proposed reaction.
First of all, it's not even thermodynamically feasible to generate
a ketene (which is highly unstable and reactive) from an alpha-haloaldehyde/ketone,
much less from an alpha-hydroxyaldehyde/ketone (ie. the acyloin
functional group).
The dehydration of alcohols to alkene involves the the E2
mechanism (pri & sec alcohols) or E1 mechanism (sec & tert
alcohols) catalyzed by an acid (see Jim
Clark and
Khan Academy and
James Ashenhurst).
But when you have an acyloin functional group, it's more
kinetically and thermodynamically feasible to protonate the
carbonyl group rather than the hydroxy group, in which case
hydrolysis of the aldehyde into the germinal diol
occurs instead, which is unstable and simply dehydrates back into
the aldehyde.
But since it's not entirely impossible or inconceivable (just
less kinetically and thermodynamically favourable) for your
proposed mechanism pathway to occur (ie. to generate the ketene
functional group from the acyloin functional
group) as a minor pathway or product via the E2 mechanism (E1
is even less feasible because of the destabilizing inter-nuclei
repulsions between the partial and formal positively charged
adjacent C atoms in the hypothetical carbocation intermediate),
what happens then?
Under the reaction conditions to dehydrate an alcohol into an
alkene (ie. concentrated H2SO4 acid catalyst), the highly unstable
and reactive ketene product generated would be immediately
acid-hydrolyzed into a carboxylic acid (try drawing out the
mechanism for this reaction ; Cambridge asked data-based questions
about the reaction of ketenes in both the UK / CIE and Singapore A
level papers in recent years).
So the bottomline : good that you've considered the possibility*
of obtaining a ketene as an alternative product, but unless the
question gives you hints that the product is a ketene, you should
stick to the more syllabus-familiar product of a normal
alpha-carbonylalkene, rather than the more unstable ketene (H2 Chem
students are expected to be able to deduce its instability by its
structure). But nonetheless (as I said, good that you've
thought-out-of-the-box* and considered the ketene product), in the
A level exams, if you're not sure if the unusual product (here, the
ketene) is a viable alternative product (or even one required by
the Cambridge qn), then the exam-smart BedokFunland JC thing to do,
is to give both answers (ie. draw structures of both alternative
products), with labels or annotations as to which is likely to be
the more stable (ie. thermodynamic) product, and hence major
product.
* updated : actually it turned out you were just blindly
applying Markovnikov's / Zaitsev's rule, not creatively thinking
out-of-the-box... *sigh*
