Heterolysis

Parting is never easy.

I have seen or experienced several goodbyes and farewells: on a platform, when the train is about to leave and the train guard whistles impatiently, as if he wanted to tear the air apart; in a Paris Métro station, where the last kiss is overwhelmed by a mob of prospective passengers rushing to cut their way through the gates; at a laboratory party, where the departing researcher, flooded with presents and greeting cards by cheerful colleagues, is wondering whether the half-written papers will ever see the light, fearing that they could be buried in a drawer, never to be submitted for publication.

All human parting is heterolytic: there is no such a thing as a homolytic breakup. While one moiety is parting, the other is being parted, someone always retains more than the other. This is a nagging impression that is hard to convey in words, but I know that it somewhat has to do with charges and electrons; from an apparently harmonious, neutral molecule, two oppositely charged moieties arise. Electrostatic attraction would try and bind them back together, but the process goes on, eventually taking the fragments apart. Heterolysis, as far as etymology is concerned, has this flavour: “setting free by creating oppositeness”, and I believe that all the sorrow of parting lies in this oxymoron embodying freedom and estrangement at the same time.

If we turn to organic chemistry, heterolytic cleavage is at the heart of a model reaction mechanism of nucleophilic substitutions: a molecule, put in the right environment, cleaves at one of its bonds, and a “leaving group” parts. The molecule is sometimes strained, and the breakup releases this inner tension; in other cases, a weak link snaps. Against all odds, the molecule holds together as long as the conditions for splitting up are not met yet; then, once the dissociation does take place, the two fragments, one positively, one negatively charged, drift apart, separated by an insulating cushion provided by the solvent, a safety net that surrounds and entraps both fragments, while keeping them also apart. Yet again, one of the two moieties usually feels more at ease in this new environment, much as one would expect when people part: one fragment gets pats on the back, and friends say that it has made the right choice, suggesting that it should look elsewhere for a better match, while the other fragment is frightened and bewildered in the unfamiliar settings. In this intermediate stage, the reaction could still be reversible: after all, even the original togetherness, far from idyllic, is much more sustainable than this transitory tug-of-war between reconciliation and separateness. Nevertheless – alas! – reactions are (usually!) planned to move on; in other words, the chemist, unemotional Cupid,  has already provided that “better match”, which is ready to woo and to bind more suitably than its predecessor. As in real life, however, a life-changing breakup is often accompanied by a feeling of uncertainty, a blurring of one’s own self-image: which shape will have the new molecule-to-be? In actual facts, the positive fragment is thought of as a “flat” object, spinning like a tossed coin: which side will end face-up, which cheek will the suitor kiss first? This quest of one’s own new identity, this restless turning of the head to understand which way to go, all of this reminds me of a painting by Lorenzo Lotto, Triplice ritratto di orefice (“Triple portrait of a goldsmith”). The self is a superposition of coexisting selves (all the more so at the time of parting): Lotto instinctively knew how to depict this.

http://upload.wikimedia.org/wikipedia/commons/e/eb/Lorenzo_Lotto_070.jpg

Going back to chemistry, sooner or later the reaction is over and the chemist isolates and purifies the new molecules, while the departed fragment is disposed of in a chemical waste jerrycan, the mass grave of forgotten atoms. In spite of this sombre image, the loss inherent to heterolytic breakups sometimes leads to unexpected riches. Take hydrochloric acid, HCl, for example: when added to water, this molecule readily parts into H+ and Cl (the latter being also a component of table salt). The diminutive proton, H+, looks bare, devoid of the warmth of an electron cloud, lost to Cl. But water, surrounding and guiding the parting couple, comes to the rescue: the oxygen in H2O sports beautiful electron petals along with its twin hydrogen stem, and water gathers around H+ like a floral crown. Water befriends and embraces H+, cuddling it in a cosy nest of  -as it seems now- six water molecules (http://pubs.acs.org/doi/abs/10.1021/ed400559t). The proton is no longer such, but a new entity, H13O6+, arguably more contented than Cl.

He who keeps less gets more.


PS: For those who frown upon the ideas of carbocations floating around and “pure” heterolytic mechanisms in nucleophilic substitutions: http://pubs.acs.org/doi/abs/10.1021/ed075p1482

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