The New Clayden pp. 614 – 693

Standard stuff about aldol and Claisen condensations.  Important because they are a way of making carbon carbon bonds.  Not much to comment on here.  Well written and clear as a bell.  My main concern about reading this, is whether it is relatively obsolete given the importance of transition metal organic chemistry.  I’ll have to wait to chapter 39 which begins 400+ pages later to find out. 

 

p. 635 — How do you make hexamethyldisilazane?

 

p. 637 — The animation of the intermolecular aldol condensation is not to be missed.  You can rotate the animation every which way to watch what is going on.  What you cannot do is change the relative orientation of the parts of the molecule to each other — if you did the reaction wouldn’t work. 

 

p. 644 — Cute rationalization why carbonates are more electrophilic than esters. 

 

p. 655 — They spend so much time on carbonyls because it is (or was) the way chemists use to form carbon carbon bonds.  I wonder if this is still true now that we have metasthesis and transition metal chemistry.  This doesn’t come up for another 400 pages.  

p. 657 — Very nice to see actual bond strengths for a change.

p. 658 — Interesting that pKa’s are given in some places but not others.  The pKa of R-SH is 9 – 10 (vs. about 15 for R-OH).   All they said here is that thiols are ‘more acidic’ than alcohols.

 

p. 660 — Nice to see an issue which isn’t resolved (stabilization of carbanions by sulfur) put into an introductory textbook, which by it’s very nature, must present a lot of factual material as given (e.g. cut and dried).   The interesting stuff is always what is NOT understood.

p. 661 — “Ab initio calculations suggest that the C-S bond in -CH2SH is longer than in CH3SH”  — hasn’t anyone looked? 

p. 663 — “There are many more methods for hydrolyzing dithioacetals and their multiplicity should make you suspicious that none is very good.”  This is also (unfortunately) exactly the case in medicine when you see a large number of treatments for a disease.

 

p. 668 — Why is the Si – Si bond 2/3 the strength of the C – C bond.  This explains why we don’t have a silicon based life-form, but the fact itself could use some explanation (assuming there is one). 

 

p. 668 — “Bonds to electronegative elements are generally stronger with silicon than with carbon”  — this is because bonds between elements with greater electronegativity differences are stronger, and silicon is less electronegative than carbon.

 

p. 669 — The polarization of the C – Si with carbon being relatively negative to Si would have been a great time to bring in why tetramethyl silane (TMS) is used as an NMR reference standard — the most shielded carbon nucleus around. 

p. 669 — What is the geometry of the pentacovalent intermediate of Silicon?  Which d orbital does the fluoride go into, or does Si rehybridize in some way?

 

p. 673 — The claim is made that electrophilic aromatic substitution is ipso to SiMe3 due to the C-Si sigma bond overlaping with the p-orbitals of the aromatic ring.  What about the d orbitals?  Also how are Ar-SiMe3 compounds made?

 

p. 674 — A very slick mechanism for the retention of E and Z on electrophilic substitution on a vinyl silane.  Again, why are they assuming it’s only the C-Si sigma bond and not the Si d orbitals — I can see a role for them ‘steering’ the Si atom to the correct position. 

 

p. 674 — There should be a note pointing to methods for making vinyl silanes of the desired stereochemistry (p. 683).

p. 687 — The mechanism given for the Julia reaction on p. 687 can’t possibly work with the example given on p. 686 for the cyclohexyl starting material, as it has no alpha hydrogen.  Perhaps the base picks off the beta hydrogen. 

 

p. 691 — Aren’t the D orbitals of phosphorus involved in ylids?  They aren’t mentioned.
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