Hi Ultima, rlly appreciate what you are doing here! Hoping that
you can help me clarify some doubts:
1) Regarding the coordination number of a complex, I understand
that it is defined as the number of coordination bonds around the
central metal atom/ion. For the case of a porphyrin ring complexed
with Mg2+ in chlorophyll a, I understand that 2 of the N forms
ionic bonds while the other 2 N coordinate bonds with the Mg2+ ion.
In such a case, would the coordination number be 2 or 4? Also,
since complex is when the no. of coordination bonds exceed the O.S
of the central metal ion, would this compound even be a complex
since it only has 2 coordinate bonds?
2) Regarding optical isomerism in metal complexes, while i am
aware of the propeller chirality in compounds such as
(cr(cro4)3)3-, I was wondering if there would also be such
isomerism in tridentate or even hexadentate ligand complexes like
fe edta complex.
3) Regarding cyanohydrins, will basic hydrolysis of it yield
coo- or will the original aldehyde be produced instead (Oh-
extracts H from Oh in cyanohydrin which results in formation of c=o
bond again and the elimination of cn-)
4) For the amino acid tyrosine, the OH in the phenol is
apparently neutral and tyrosine is not ionised at physiological ph
of 7. However, we learnt that phenol usually exists as phenoxide at
ph7. Is there a particular reason why tyrosine is neutral at ph7
and if we are asked to circle an ionisable r grp at ph7, can we
circle the phenol in tyrosine?
5) When
asked for possible interactions between R groups, do we assume they
are protonated or not, because does assuming that they are not
protonated / charged rule out interactions like ionic
interactions?
This is especially so if the question does not give the amino acid
residues in their charged form yet it is quite obvious that
ionic
interactions are possible
6) Pertaining
to questions that ask for the reactions of period 3 elements with
oxygen and chlorine, do we give the reaction that least oxidised
the element or the reaction that most oxidises the element when it
is not specified. Eg
should we give the formation of p4o6 or p4o10 when p4 is burnt in
oxygen?
Thank you!
No problem, MagicLeprechaun.
Q1. In complex (no pun intended) and unusual cases such as these,
the proper chemist and exam-smart student would specify exactly
which type of bonds involved in calculating the coordination
number. Because different chemists and different authors have a
different take on whether ionic bonds should be included or not.
For instance, in a P2 or P3 qn, you should write, "coordination
number is 2 if we only count the dative covalent bonds;
coordination number is 4 if we also include the 2 ionic bonds." and
Cambridge would go, "now there's a smart student.". If it were an
MCQ however, then it would be the question's fault for not
specifying this, then the exam-smart student would deduce the best
option by eliminating more implausible options. But all things
considered, as far as the underlying definitional purpose of
"coordination" is concerned (as opposed to a shallow, dogmatic,
specific interpretation), it's actually better (more sensible)
chemistry to include *all* bonds (after all, all of the bonds
herein, whether ionic or dative covalent, are in truth part ionic,
part covalent anyway) that the central metal cation is coordinated
with. Concordantly, the coordination number of Mg2+ in chlorophylls
(ignoring any other ligands not included in the simplified diagrams
given in A level exams) is better regarded as 4 (ie. including
ionic bonds) instead of 2. And yes, the Mg-chlorin ring in
chlorophylls is considered a coordination complex.
Q2. Yes, it's possible, as long as the coordination complex is
overall tetrahedral or octahedral, and the mirror image is
non-superimposable (or more strictly, non-superposable) on the
molecule itself. But assuming you're a H2 Chem student (are you? or
do you take H3 Chem as well?) this is beyond H2 Chem syllabus and
shouldn't be your concern.
Q3. MagicLeprechaun and Twe375, because the nucleophilic addition
of CN- to a carbonyl compound is reversible (HCN is a slightly
stronger acid than H2O, so CN- conjugate base is slightly more
stable than OH- conjugate base because even though the negative
formal charge is on C which is less electronegative than O, but the
C atom being sp hybridized has high % s orbital character), hence
under alkaline hydrolysis conditions, you do get a mixture of both
(carbonyl and amide) products initially. But because the first
hydrolysis product of the nitrile group is an amide, which is
itself further hydrolyzed eventually to a carboxylic acid (acidic
hydrolysis) or carboxylate ion (alkaline hydrolysis), this pulls
the position of equilibrium eventually to the RHS, which is why for
the H2 Chem syllabus, you simply have to state that the final
product of cyanohydrin / hydroxynitrile hydrolysis is a
hydroxycarboxylic acid (acidic hydrolysis) or hydroxycarboxylate
ion (alkaline hydrolysis).
Q4. "We learnt that phenol usually exists as phenoxide at pH
7". This is not correct. The pKa of phenol is 9.95 (in aqueous
solvent), which means at pH 7, which is more acidic than the pKa
value of 9.95, the majority of phenol molecules exist as the
protonated conjugate acid form, instead of the deprotonated
conjugate base form. Notice that the pKa of the phenol side chain
in Tyrosine is 10.07, which reflects its slightly weakened acidity
(compared to phenol by itself), as a result of the benzylic sp3 C
atom which donates electrons slightly by induction (but only
slightly, because of the electronegative N atom of the alpha
amine). If by "ionizable" you mean a small percentage exists in
it's anionic deprotonated form at pH 7, that's still technically
correct, by definition of acid-base equilibria. But for A levels,
it wouldn't be acceptable, because if Cambridge asks the question,
it implies "circle the most ionizable group(s) at pH 7" and the
phenolic side chain of Tyrosine ranks a distant 3rd place behind
the the alpha COOH/COO- and alpha NH2/NH3+ groups, and Cambridge
would think, "this student's answer implies he/she doesn't know the
alpha groups ionize to a far greater extent than the R group, so
zero marks for him/her!".
Q5. Again, be exam-smart. If the pH is given, use the predominant
form (ie. either protonated or not at that pH). If the pH is not
given, then you should take the initiative to state "at
physiological pH, these groups exists as..." and label the R group
interactions accordingly. Or if there isn't an obvious predominant
form at the pH indicated, then you should (be exam-smart and) give
both answers : eg. as conjugate base form, NH2 can donate and
accept H bonds ; as conjugate acid form, NH3+ can donate H bonds
and participate in ionic bonding. If it's an MCQ, then (be
exam-smart and) eliminate the less plausible choices, and the
remaining option will clue you in on what the Cambridge question
setter wanted.
Q6. Again, be exam-smart. Give both eg. "in limited oxygen P4O6 is
formed ; in excess oxygen P4O10 is formed", if you're uncertain on
which one the question wanted. You won't be penalized because both
equations would be correct. Cambridge would accept the one they
wanted, and ignore the one they didn't want (in some cases,
Cambridge will accept either, eg. P4O6 or P4O10, unless the
question precluded one of them in its description). You'll only be
penalized if one of your answers contradict or nullify the other
point (in which case Cambridge would go "KNN this bugger trying to
cheat!"). But in the context of your question, and especially if
you're too short of time to write both answers, you should give the
form of the compound that is more thermodynamically stable under
standard conditions and thus more common, eg. SO2 instead of SO3,
and/or the form that assumes excess O2 and Cl2 is present and
available for complete reaction, ie. P4O10 instead of P4O6, and
PCl5 instead of PCl3.
