The New Clayden pp. 470 – 527

p. 473 — since when is the C – C bond length 1.47 Angstroms?  — I remember 1.54

p. 475 — Great to see actual physical evidence for the charge distribution in protonated benzene using 13C NMR (and on p. 485 for protonated toluene).

p. 479 — The fact that ortho and para positions are brominated in phenol, shows you that the symmetrical ring current in benzene isn’t the whole picture. The electron distribution is asymmetric as shown by the proton shifts in NMR (and elsewhere by the 13C shifts in NMR).  It would be interesting to see the bond lengths between the ring carbons (I couldn’t find it).

p. 481 — How does concentrated nitric acid oxidize phenol (and to what does it oxidize it?).

p. 482 —  Test question:  Assuming all 3 bromination reactions of benzene, anisole, and N,N dimethylanalinine are done at the same temperature and pressure, how much lower is the free energy of activation of the last compound relative to benzene.  I’m embarassed that the answer was NOT at the tip of my tongue.
In general, the book is silent about the absolute (free) energy of activation of any reaction, as is Anslyn.  Relative rates are easy to figure, if done at the same temperature, because everything cancels (RT, the Arrhenius constant etc.).  The answer is basically exp{ – (E_Activation_1/E_activation_2) }.  So for a difference of 10^9 the ratio of activation energies is 16 (exp(16) is 10^9.

p. 488 — What does FeBr3 look like?  In the mechanism FeBr5- is written down (but not drawn out).

p. 489 — Poor overlap of the p orbitals of the halogens with the p orbital of carbon is mimicked in the following table (however these are sigma bonds, but the effect on p orbital overlap should be the same).

          Bond Length    Strength (KiloCalories/Mole)
C – F       1.39            115
C – Cl      1.78              79
C – Br      1.93             67
C – I         2.14             58


p. 490 — The dissection of the conflicting inductive vs. the resonance effects of halogens on benzene reactivity and regioselectivity is positively talmudic.  Premeds had better get ready to do this sort of thing for the rest of their professional lives.  Strong therapies often work better, but carry a higher risk.  Where is the ‘best point’ which maximizes therapeutic effect and minimizes risk (remembering that people differ in the amount of risk they will accept, and also remembering that hard data to guide such a decision is often thin on the ground or nonexistent).
Just to show that figuring out how to direct substituent attachment to benzene isn’t a dead area, the 28 June ’12 Nature has an editorial (pp. 478 – 479) and a paper (518 – 522) on a new way to accomplish this.  The article mentions something called vicarious nucleophilic subsitution which you should wikipedia — probably too specialized for an introductory textbook.  Interesting, nonethless.  The work is elegant and clever, and I think even a neophyte studying Clayden for the first time would be able to understand large parts of it.  It’s another example of why I love organic.  You build molecules to test your ideas and in this case you build a molecular machine to carry out a reaction.

It’s also amusing, on reading the editorial and the paper, how little shrift organic chemists give to the Heisenberg uncertainty principle and quantum mechanics in general.  This work is all about moving atoms and groups around in space, with the only uncertainty being how to make the reaction work.  Ultimately, of course chemistry rests on QM — atoms of an element have the properties they do because their energy levels are fixed, and identical for all.

p. 491 — It would have been quite a digression for the text, but you should look up the structure of P2O5 (formed by burning elemental phosphorus).  It comes in several forms one of which looks like adamantane (P40xygen10) with oxygens where the secondary carbons would be linking the 4 phosphorus atoms  at what would be the 4 tertiary carbons of amantadine.  In addition there are 4 oxygens coming off the 4 phosphorus atoms.

p. 509 — “The litthium cuprates (R2CuLi) that are formed are not stable”  — what happens to them?

p. 512 — The cyanoimine shown in the inset doesn’t look ‘simple’ to me.  How in the world is it made?

p. 513 — Does the alpha effect explain why Br2 is a better nucleophile than Br- (if in fact it is).  Is this true for all halogens?

p. 517 — It is marvellous to have 13C-NMR to confirm the charge distributions we all ‘knew’ about 50 years ago, for the cyclohexadiene anion and cation.

p. 519 — An 80 fold increase in rate is only a 4 fold decrease in activation energy.  See p. 482 in these notes.

p. 523 — The benzyne intermediate and mechanism is fascinating.  50 years ago people talked about it, but the evidence for its existence was thin.  For some of the impressive things benzynes can do, look at the Wikipedia article.
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