Not only can lone pairs act as nucleophiles…. but π bonds can too! Alkenes are a lot more exciting than they’re often given credit for. That means that given a sufficiently frisky electrophile, they can donate their pair of π electrons to form a new sigma bond.
Like this!
However, there’s one little problem here. See that curved arrow? What does it really mean? If you weren’t given the product, would you be able to draw it, given that curved arrow?
See the problem here: Which atom of the alkene is actually forming the bond to hydrogen? When we were dealing with lone pairs, it was easy: atoms clearly “own” their lone pairs, and we can tell exactly which atom is forming a bond to which. With alkenes, it’s different: since they “share” that pair of electrons, we’re going to have to somehow show which atom gets the new atom and which is left behind as a carbocation.
Here’s the conventional way it’s done. If we want to show the bottom carbon forming the bond, the usual way to do this is to draw this loop like this, to show the “path” of the electrons coming in an arc from this direction. The carbon on the alkene “closest” to the hydrogen is the one that ends up bonded to it.
Similarly, if we wanted to show the left carbon forming the bond, we’d “arc” the bond like this:
One problem with this: it’s kind of a kludge. The curved arrow notation is limited in that all we can really do is decide where the tail should go (at the π bond, obviously) and where the head should go (to form the new bond). But the question of which carbon forms the bond is still ambiguous.
And if there’s one thing organic chemists hate, it’s ambiguity.
Give me clear definitions or give me death!
To try and deal with this issue, organic chemists have come up with two potential solutions. They’re worth looking at if you’re finding this issue confusing.
Modified Curved Arrow Convention #1: “Bouncy” Arrows.
Instead of showing the curved arrow as a big sweeping arc, one solution is to put an extra bounce into the arrow. The idea here is that we’re showing the pair of electrons travelling to the carbon in question, and from there moving on to form the new sigma bond. No more ambiguity here. [Literature reference]
This solves the ambiguity problem at the expense of putting in an extra hump in the arrow. Although it doesn’t seem like a big deal, the extra bounce has likely been the reason why this convention hasn’t taken off. However well intentioned, the trouble with a convention like this is humanity’s natural tendency towards laziness: taking the time to consistently draw an extra hump into the arrow – even if it takes only 5 seconds – represents extra work that is skipped unless absolutely necessary. Behavioral change is very difficult.
Modified Curved Arrow Convention #2: “Pre-bonds”.
Another way of dealing with this is to insert the equivalent of “training wheels” into our curved arrows. Since the curved arrow is itself ambiguous, to clarify things we put in a dashed line that precisely delineates where the new bond is forming. Then, we draw the arrow with the tail comingfrom the electron source (the π bond) and the head going to the new bond. We can put the arrow right on the dashed line itself. This has the advantage of not modifying the curved arrow convention itself, just adding in an optional “guide” that makes its application more clear. [For an application of this technique I recommend checking out Dr. Peter Wepplo's blog, where I first found this convention used]
If you find yourself confused following the movement of electrons in the reactions of alkenes with electrophiles, these supplementary conventions might be of use to you.
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