The New Clayden pp. 163 – 221

The illustrations in the book that I comment on can be reached on the web by substituting the page number I give for xx in the following

p. 167 — Two pages of talking about pKa and what it is without mentioning (or defining) the equilibrium constant.  Strange.  However, the book assumes a previous basic chemistry course, where this must have been introduced.

p. 170 — “We can’t use a stronger base than hydroxide, since, no matter what strong base we dissolve in water, we will only at best get hydroxide ions”.  It should be mentioned that the concentration of water is 1000/18 = 55.5.

p. 172 — The first example of the type of thinking required for organic chemistry (and medicine).  In the discussion of acidities, the increasing stability of negative ions correlates with both acidity and with electronegativities when comparing C, N, O, F.  However acidity does not correlate with electronegativity when you compare F, Cl, Br, I.  In the second case bond strength is more important.  So you are called in an individual case to reconcile two contradictory trends and figure out the appropriate balance for a given case.  

This is exactly the way it is in medicine.  For instance,  corticosteroids depress inflammation, so they help autoimmune disease, but too much suppression of inflammation decreases the resistance to infection, and the MD hopefully tries to balance the two effects.  Similar thinking is required for any medication with side effects (which is all of them).  This is one of the reasons I think pre-meds should take and pass organic chemistry — it’s the sort of thinking they’ll be forced to engage in for the rest of their professsional careers. 

p. 174 “As nitrogen is less electronegative than oxygen, you should expect amines to be less acidic and more basic than alcohols”   Can you reproduce the chain of logic from less electronegativity of nitrogen to the conclusions of this sentence.  

p. 175 — The interchangeable use of pKa and pkaH for nitrogen bases is quite confusing.  However it’s common usage and had best be learned.

p. 184 — The link to the Grignard addition website is interesting, because the C-MG bond and the Mg-Cl bonds are much longer than the C-C or the C-H bonds.  The spacefilling animation looks positively sexual, as do most of them.  Clearly more is involved in chemical reactions than blobs of electron density (which is what the spacefilling models actually show).  Possibly if the models could be shaded for regions of positive and negative charge, things might be clearer.  Perhaps this will be done in later illustrations.  Another parameter which would be interesting to see visualized is ‘squishiness’ or electron cloud polarizability (which has been measured see Anslyn & Dougherty p. 25). 

p. 185 — How does iodine or 1,2, di-iodoethane dislodge the MgO layer from metallic Mg? 

p. 185 — Cl, Br, I can be used in R-X for Grignard formation, but only Cl, Br for organolithium.  What’s wrong with iodine?  One guess is that Li is so much smaller than Mg so the sizes of the orbitals don’t match.  However, looking it up, both the atomic radii and ionic radii of Li and Mg are pretty much the same. 

p. 189 — Less electropositive seems to be a poor choice of words.  Positive numbers are usually ordered .98 < 1.31 < 1.65 (which are the electronegativities of Li, Mg, Zn).  More electronegative isn’t great either (recall that F, the most electronegative element has an electronegativity of 4.0).  This all goes back to Pauling’s original definition of electronegativity as the difference in dissociation energies between X – Y and X – X and Y – Y.

p. 195 — “Hiding behid these observations is the more fndamental idea that reduction requires the addition of electrons while oxidation requires the removal of electrons.”  Exactly the way to think of it, but not the way reduction and oxidation were presented in this book. 

p. 195 — The spacefilling models are closer to what a molecule actually ‘sees’, but notice how very much harder it is for the human eye to understand than the stick model, which basically abstracts true 3 dimensional molecular  geometry to a connected graph.  How do molecules know what to do?  See the comments on p. 184.

p. 200 — The notion of ‘good leaving group’ really covers a lot of ground.  If the sophisticate thinks of it, it might not matter that Cl- is of lower energy than RO-.  Why?  Think of water being of much lower energy than H2 and O2 separately.  The activation energy for bond breaking from the tetrahedral intermediate (already at higher energy) is equally important.   However, if reaction conditions are such that the activation energy for bond breaking (and formation) by Cl- and RO- is available for both, then equilibrium effects take over, and the stability of the leaving groups becomes more important.

p. 209 — Watching ester hydrolysis and formation frame by frame on their web site (and in 3d to boot) will really help understanding.  

p. 214 — A discussion of the orbitals involved in SOCl2 or PCl5 would have been nice, otherwise the reagents for acyl chloride formation are just magic.

p. 219 — How do you make Me NH 0Me?
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