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Organic Chemistry

Introduction to Arrow-Pushing

A Crystal Clear Chemistry Tutorial

The Octet Rule

The octet rule states that second-row elements of the periodic table cannot have more than eight electrons in their outermost energy level (their valence shell). Carbon cannot have more than four bonds, oxygen usually only can have two bonds, and nitrogen usually has only three bonds. For a more detailed explanation of this rule, see the article on the octet rule.

What the octet rule says, when applied to full-arrow pushing, is that for a fully bonded system, in order for a bond to form, a bond must usually break. Consider the reaction below. When the hydroxide ion's oxygen donates an electron pair to the carbon, if no bond breaks, there are too many (ten) electrons around carbon, violating the octet rule. Thus, another bond from carbon must break and take electrons away from carbon in order for hydroxide to bond to carbon. In this case, the bromine atom can take away an electron to form a noble gas configuration, and hydroxide can bond to the carbon to form methanol.

As practice, let us balance the formal charges on this reaction (the correct one). As we found above, the hydroxide ion's oxygen has a formal charge of -1. When the oxygen donates two electrons to form a bond to carbon, the arrow makes the oxygen atom positive by 1, which gives -1 + 1 = 0, a neutral charge. The arrow points to carbon, which makes its charge more negative, to -1. But we see in the diagram that there is still no negative charge on carbon! Why? Well, we haven't gotten to the second arrow yet. The second arrow adds a positive charge to carbon, since it starts at a carbon-bromine bond and is pointing away from carbon. Carbon starts out neutral, and the first arrow makes it more negative, while the second arrow makes it more positive. These all cancel out to give 0. Finally, the second arrow puts a negative charge on bromine to give a -1 formal charge. Thus, we see that overall, a negative charge has been taken from oxygen and given to bromine, and that the charges all balance out on both sides of the reaction (no charges have been created or lost).

When pushing arrows, be careful not to violate the octet rule. For fully bonded systems (where everything has a noble gas configuration), if one atom donates electrons to another atom to form a bond, then the second atom has to in turn give away electrons to another atom by breaking a bond. This rule does NOT apply to situations in which atoms don't have a noble gas configuration.

Nucleophiles and Electrophiles, Nucleofuges and Electrofuges

Nucleophiles (which means "nucleus loving") are those atoms (and often the molecules those atoms are in) that donate electrons to form bonds (they are at the start of the arrows). They are electron rich, and so they "love" nuclei, which are positively charged. Electrophiles are those atoms (and often the molecules these are a part of) which receive the electrons in bond-forming reactions. They are usually electron poor, which makes them "want" electrons -- hence, the name, which means "electron loving". Nucleofuges are the atoms and molecules which carry away the electrons in a bond-breaking reaction, and electrofuges are the atoms and molecules from which electrons are taken in a bond-breaking reaction. The terms nucleofuge and electrofuge aren't used very commonly, but you may come across them from time to time. Usually, nucleofuges are just called leaving groups.

In the bond forming reaction above between the hydrogen atom and the hydroxide ion, the hydrogen atom is the electrophile, and the hydroxide atom is the nucleophile. For a more in-depth discussion on identifying the electrophiles and nucleophiles in a reaction, see the tutorial on how to tell electrophiles from nucleophiles.

The Leaving Group

A very important concept in organic chemistry is the leaving group, which is sometimes called the "nucleofuge" (we recommend you stick with the easier to remember term leaving group). When a bond breaks, the electrons usually move together onto one atom or the other (the rare case where the bond is split equally between the two atoms is covered below with the single-barbed arrow pushing). For example, in reaction (2) that we talked about above (I show it below again), why don't the electrons go onto the hydrogen atom instead of the chlorine?

This is mainly because the electrons tend to move onto the atom that is more electronegative (sometimes, however, electrons can only move in one direction, especially if other electrons are forcing them to). This is only natural; the electrons go onto the atom that "wants" them more. Thus, since chlorine is more electronegative than hydrogen, the electrons move onto hydrogen.

In addition, electrons tend to want to go to atoms that are bigger. Large atoms tend to be "poofy", in that the outermost electrons are very loosely held to the atom, making it easy to squeeze an extra electron in if need be (there's enough space that no one will really notice that much, and there are so many electrons that a lot of the positive charge from the nucleus gets shielded from the outer electrons by the inner ones). Iodine, for example, is a large, "poofy" atom, and it can hold an extra electron (with a negative charge) quite easily, even though it is not much more electronegative than carbon. This is because the negative charge can get "smeared" all over the extremely large iodine atom, making the charge less concentrated and therefore more stable. This "poofiness" is known as polarizability, which justs talks about the fact that it's really easy to pull electrons around on a poofy molecule (since they're only loosely held). The combination of electronegativity and polarizability makes some atoms better leaving groups than others (that is, they more readily accept electrons to form negative ions). When bonds break, the better leaving group takes the electrons and floats away.

The nucleophile is the electron donor of a bond-forming reaction, and the electrophile, is the electron acceptor of a bond-forming reaction (it accepts the electrons from the nucleophile). A leaving group (or nucleofuge) is the electron acceptor of a bond-breaking reaction, while the electrofuge is the electron donor of a bond-breaking reaction (its electrons get taken away by the nucleofuge).

Single-Electron Movements and Half-Arrows

Remember back at the beginning when I talked about how electrons can either move as electron pairs or as single electrons? Usually, in organic chemistry, electrons move as pairs, and we use double-barbed full-arrows for that. In a few cases, however, single electrons move by themselves. This is most common in situations that involve metal atoms or light. Single electron movements are depicted with single-barbed half-arrows, like in the following reaction.

This kind of reaction, in which the bond is split equally into two single electrons, is called a homolytic reaction. The homo- prefix means something like "equal" or "same", and "-lytic" means breaking or cutting. Homolytic reactions are a subset of the category of radical reactions, which includes all single electron movement bond formations, as well. That is, any reaction involving single electron movements is a radical reaction, and that's what we'll call them here. (The name comes from the fact that a radical refers to unpaired single electrons.) As a side note, the reactions involving pairs of electrons (and depicted with normal full arrows) are called heterolytic reactions.

Radical reactions don't change formal charges. You can check and see that any neutral molecules splits into neutral parts with a radical reaction and combine into neutral whole molecules. Since only one electron in each bond counts for formal charge anyway, splitting the bond equally changes nothing.

The octet rule, however, still holds with radical reactions. The intuition is a bit different, since you can no longer have simple rules like "one bond formed gives one bond broken." You have to always keep track of electrons. If one more electron is being added to an atom with a full octet, then a bond must be broken. However, many radical reactions involve atoms without a full octet. Thus, the following two reactions demonstrate slightly different circumstances: the first shows a reaction between two atoms without full octets; the second shows a reaction involving an electron adding to an atom with a full octet, causing another bond to break.

You can see that in reaction (3), the half-arrows point to each other's end. This is usually the way a bond-forming reaction with half-arrows is written, since each bond is composed of two electrons (and each half-arrow represents one electron). Reactions that involve breaking a bond with the donation of a radical (a single electron) such as reaction (4) are often called radical abstraction reactions (to "abstract" in chemistry means to "remove"). Again, notice that there are no charges formed or destroyed, which is almost always the case with radical reactions.

Radical reactions, which involve the movement of single electrons, are depicted with half-arrows. Half-arrows do not change formal charges, but they obey the octet rule (as always).

With this, you should be all set to start learning some reactions in organic chemistry! Almost all of organic chemistry is organized in arrow-pushing notation because it works so well as a way to think about and predict reactions. Just remember the rules in the boxes and practice a lot. Make sure to look at the examples of what to do and what not to do. For more on nucleophiles and electrophiles, see the article on how to tell electrophiles from nucleophiles.