Fun and Miscellaneous
The 8 Types of Arrows In Organic Chemistry, Explained
Last updated: February 12th, 2020 |
To my knowledge there are 8 different types of arrows you meet in organic chemistry. Here’s a little guide to them.
1. The forward arrow, otherwise known as the “reaction arrow”. The purpose of this arrow is to show action. Now, “BH3, then NaOH/H2O2” might not exactly seem like your idea of “action”, but put yourself in the alkene’s position – it’s double bond is being ripped asunder in order to form new bonds to boron and hydrogen, and then with the addition of H2O2, the bond to boron is replaced with one to oxygen. That’s a pretty eventful day in the life of an alkene.
There isn’t any hard and fast rule about what is supposed to go above or below the arrow, although reagents tend to go above and solvents tend to go below. You’ll often see a sequence of reactions placed over the arrow and numbered “1) , 2), 3), etc. These represent individual steps that could often be shown with individual arrows of their own, but they are placed in series here over the arrow to save space.
2. The Equilibrium arrow. This shows a reaction that is reversible, usually in the context where the reversibility is being highlighted (such as in a reaction mechanism). To further highlight the position of an equilibrium, you may also occasionally see one of the arrows being longer than the other, showing that the equilibrium favors the starting materials or products.
3. The Resonance arrow. Not to be confused with the equilibrium arrow, this double-headed arrow shows two (or more) species that are resonance structures of each other. That is to say they differ in the arrangements of their electrons and nothing else. Although it’s a separate discussion, it’s important to note that the molecule *does not* shuffle back and forth between these forms, but instead the “true picture” of the molecule is a combination or hybrid of these structures.
4. The Dashed Arrow. This is often used to show a speculative or theoretical transformation, where conditions might have yet to be discovered. Alternatively in a test situation it’s a way of visually depicting the question, “How would you do this?”
5. The Curved Arrow (double headed). The curved arrow formalism is such an important tool that entire books have been devoted to it (for instance, I highly recommend checking out “Arrow Pushing in Organic Chemistry” by Daniel Levy). The point of the curved, double-headed arrow is to show the movement of an electron pair. They start at the tail and end up at the head. Hugely important for following how mechanisms work. Deserving of a series of posts in itself.
6. The Curved Arrow (single headed). The single-headed (or “single-barbed”) arrow depicts the movement of a single electron. Useful in discussions of radical chemistry mechanisms in particular. Identical in all other respects to the double-headed arrow.
7. The Broken Arrow. This is used to show reactions that don’t work: fluorine is a bad leaving group in nucleophilic substitutions, for example. And quinine can definitely not be synthesized from the oxidation of aniline.
8. The Retrosynthesis Arrow. The open arrow here doesn’t actually show a “reaction”, per se, but instead more of a mental exercise. The retrosynthetic arrow is meant to depict the process of breaking down a complex molecule to simpler starting materials. This is useful as a planning device to highlight a key strategy used for building a molecule.
This list should hopefully help demystify some of the arrows you’ll encounter in organic chemistry, although I must admit they’re not very useful in understanding diagrams like this one. If someone could explain what the dashed line here is supposed to mean, I’d appreciate it.
27 thoughts on “The 8 Types of Arrows In Organic Chemistry, Explained”
This non-chemist football fan welcomes confirmation that grid-iron is a trivial game for intellectual neophytes. If you are aiming for 1st Class Honours you will need to understand passing processes of realistic complexity beyond the 2 or 3 interactions of grid-iron (self-identified I note as the limit of its passing complexity). Time to join the real world and start watching real professionals playing the real passing game in the EPL.
What about “footy”? Where does that rank in the hierarchy? 😊
Can you add something about the curved arrows representing reagents and byproducts? They’re so trivial I get why you didn’t include them, but they’re *also* so trivial that I simply can’t find anything online to verify my guess on what they mean.
i looked at that and said thats confusing this has nothing to do with science then looked again and sure enough there was the coolest play of a life time
Can a single headed arrow ( arrow for irreversible reactions ) be used for net forward reaction in a reversible reaction??
Lazy people (like me sometimes) often use a single double-headed arrow in a reaction scheme to indicate a forward reaction in a reversible reaction. Mechanisms with reversible steps, on the other hand, should be drawn with equilibrium arrows.
Can the double headed arrow also be used to indicate electrophilic attack from a cation? Obviously the notation here would perfectly suffice, where the nucleophile’s electrons move toward the cation; however, I was told by a doctor student that it’s a matter of preference and one could also have the double-headed arrow pointing from the lewis acid to the lewis base.
I think he also mentioned some alternative meaning behind the single-headed arrow as well.
Any experience with this perspective? Thanks!
Er, the mechanism involved was acetonitrile nucleophilically coordinating to a molecule, and the following resonance structure leaving an sp2 carbocation out of the former sp-carbon. This carbocation he wanted to draw with an arrow toward I believe a reduced, tetracoordinated diboron moiety.
Love the article (as a chem major). As for the football play, this play is what is known as the quarterback option. In a quarterback option the positions of the backs are not indicative of any specific play to the defense and gives the quarterback the option (to use the above play as your example) to pitch to the halfback (throw the ball backwards, #1), to hand the ball off to the halfback (the halfback advances while as if going on a passing route and then, as the corner back is drawn back by the route-running wide receiver, cuts back to receive a hand-off from the then dropped-back quarterback in what is known as a “play-action”; #2), or to pass to the half-back (the halfback complete his route running a curl to the flat(the parts of the field outside the hashmarks) in a play known as a screen #3). (also, I played noseguard, why do i know any of this stuff, lol)
The specific play to be run would be said in the huddle or announced via audible (the Blue 42, Blue 42, set, hut you always hear in movies and stuff). It sacrifices the strength of a specific attack (such as using all your guys to block one runner) but may confuse the defense.
I am in your debt, sir.
Damn, great comment Jackie! Thank you very much!
Great, but I have been trying to figure out wth is happening above and sometimes below the arrows. It seems like it can mean various things and I am also curious why there are sometimes numbers and meh.. Sometimes whatever *pops bubblegum*
My source is the section “2.10.1 Other symbols and conventions in chemistry” in Quantities, Units and Symbols in Physical Chemistry, IUPAC Green Book, 3rd Ed., 2nd Printing 2008: IUPAC & RSC Publishing; Cambridge. Cohen, E. R.; Cvitas, T.; Frey, J. G.; Holmström, B.; Kuchitsu, K.; Marquardt, R.; Mills, I.; Pavese, F.; Quack, M.; Stohner, J.; Strauss, H. L.; Takami, M.; Thor, A. J.
But I found this online: http://goldbook.iupac.org/C01034.html
There is a small typo and above arrows/harpoons are interchanged with the lower ones. You give the correct position for the harpoons.
Nice post. Only some minor comments.
The standard symbol, recommended by the IUPAC, for reversible reactions is the double arrow (Latex code: \rightleftarrows).
The Equilibrium ‘arrow’ (really made of harpoons. Latex code: \rightleftharpoons) is used only for reversible reactions at equilibrium.
Thanks. Do you have a link?
Where did you find that football diagram? That looks like something that me and some friends tried back in the third grade (the coolest play EVER!).
Ha! I googled “Statue of Liberty play”. One of my favorites from Madden….
1. I like “bouncing curved arrows” for electrophilic addition to alkenes; as described in a recent J. Chem Ed article.
2. The dashed curved arrows are for when my cats are told “Quit Begging” (QB) with a squirt gun and run off to hide under the bed. (HB).
I saw that article and thought it was a really good idea. It’s pretty intuitive for multiple-step processes, especially for things like addition-elimination reactions on carbonyls, and it also makes it more clear which carbon gets attached to the electrophile in reactions of alkenes. That’s one of the weaknesses of the conventional arrow notation.
If I was your cat I’d want to alert the defence so they can call a safety blitz and break up the play.
Don’t squirt your cats! How would you like being squirted with water?
Cats are worthless vermin anyways
Heh – those are passes, of course! In this play, the quarterback receives the ball, pitches it back to the halfback who runs towards the line of scrimmage as the quarterback falls back, then the halfback pitches it back to the quarterback and heads off down the field doing a buttonhook pattern and the quarterback throws a forward pass to him. By now, even the commentators are lost!
Nice blog – I’m really enjoying your explanations.
Thank you, sir, for making me a less ignorant football fan. I am in your debt.
Oh, I thought it was a diagram from biochemistry :)