In 1988, Paula Abdul released her debut album, Forever Your Girl. The album was a smash hit with four billboard number-one singles, including "Opposites Attract". The song described a central theme in pop culture; that opposite people attract each other. The idea of opposites coming together is well-debated and opposes other popular idioms such as "the birds of a feather flock together". Although both expressions are quite common, which one is true? Do opposites attract or do they repel each other?
Well, in chemistry, opposite charges love to come together. This is the case with nucleophiles and electrophiles, which is evidence that Paula was right. Stick around and learn more, so that the next time this debate comes up, you can declare once and for all whether opposites attract.
- In this article, we will discuss what defines nucleophiles and electrophiles, and the effects these have.
- Then, we will discuss some characteristics of each, and how electron orbitals play a part.
- Finally, we will look at some types of each and some examples.
What are Nucleophiles and Electrophiles?
It is scientifically well-known that opposites tend to come together to balance each other out. Hot water will mix with cold water, air will try to fill a vacuum, and negative charges will go towards positive charges. The restoration of balance is a common theme in this universe, which is responsible for polar reactions.
Polar reactions, or ionic reactions, involve the participation of ions as reactants, intermediates, or products.1
- Ionic reactions occur when two species try to restore a balance in charge.
- When an atom or molecule is saturated with electrons, it is referred to as being negatively charged.
- Conversely, when a molecule has fewer electrons, it is known as being positively charged.
- When these charges, negative or positive, are centered in a specific area, they are described as polar.
Polarity is affected by many different phenomena, but a major one is chemical bonds. If an atom is pulling electrons closer to itself than the other atoms it's bonded to, it has a large electron density. If the pulling of these electrons occurs in one direction, it causes an increase in polarity.
Effects of Polarity in Reactions
When describing polar molecules, we use the general term Polarity, which is described in detail elsewhere. As a quick refresher, recall that covalent bonds are not always equal, as they are shown in Lewis diagrams. Electrons are typically centered on one spot more than another. Differences in Electronegativity affect how electrons are held, and often result in an imbalance in the bond.
In a typical bond line, electrons are not shown. However, it is easy to forget that electrons are not held at equal distances between the two atoms. In the H–F bond, fluorine is much more electronegative. Fluorine will thus preferentially hold the electrons a lot closer to it. This creates a polarity difference since there will be a directional difference in electron density.
Electron density is the number of electrons that are held in a specific area. A high electron density means that many electrons are congregated at a certain point. A point can be an atom, a molecule, or a series of molecules.
Just like how ions and cations will come together to balance charges, different electron densities also tend to come together to balance charges. It is important to note that this can happen in polar and nonpolar molecules. The takeaway here is that molecules will try to balance differences in electron density. But how do we know when there are differences in electron density?
It was mentioned that electronegativity differences contribute to differences in electron density. This is perfectly illustrated by the molecule methane, CH4. This molecule is nonpolar with 4 equal C–H bonds. Since carbon is the most electronegative, the electron density is mostly held on carbon, with 4 hydrogens donating electrons. However, replace one of the hydrogens with another atom, and the electron density changes. The image below is called an electrostatic potential map. Areas in red show a high electron density, and areas in blue show areas of low electron density. In other words, red areas are more likely to have electrons.
On the left, you can see that more electron density is held on the chloride than on the carbon. This is because chlorine is more electronegative (3.16) compared to carbon (2.55). In the diagram on the right, carbon is pulling electrons away from lithium, which is less electronegative (0.98). Just by replacing one bond in methane, there can be extreme differences in electron density. This example shows two very different carbon atoms. The carbon on the left is behaving as an electrophile, and the carbon on the right is a nucleophile.
A nucleophile is an atom or molecule which is an electron-rich centre. An electrophile is an electron-poor centre.
- A nucleophilic center can donate electrons to an area of less electron density.
- In contrast, an electrophilic center can accept electrons from an area of higher electron density.
In the example above, if the two molecules were to come into contact, the nucleophilic center (C–Li) would likely donate electrons to the electrophilic center (C–Cl), causing a chemical reaction. These reactions typically occur via the donation of an electron pair. So, the two electrons in the C–Li bond would be donated to the C atom of the C–Cl bond. This particular reaction would be known as an SN2 reaction, which is discussed in Organic Chemistry Reactions.
Nucleophiles and Electrophiles Characteristics
The characteristics which are attributed to nucleophiles and electrophiles are opposite in their behavior. As we saw before, a nucleophile has a high amount of electron density, while an electrophile has a low amount of electron density. Simply put:
- Nucleophiles have either full or partial negative charges
- Electrophiles have either full or partial positive charges
If you can remember that electrons are negative, then you can pick out where nucleophiles and electrophiles will exist. This is a simple and easy way to think about this concept, albeit, it is slightly more complicated than that. Without going too deep into it, we will take a brief look at how these species react with each other to form a new bond.
Orbitals of Nucleophiles and Electrophiles
Determining the identity of a molecule can be done by recognizing differences in electron densities around certain atoms. However, it is also important to understand what orbitals of nucleophiles and electrophiles look like. We will now take a brief look at Molecular Orbital Theory.
We discussed how a nucleophile will donate electrons to an electrophile. The electrons it uses for that reaction are located in the HOMO (highest occupied molecular orbital). The electrons which are highest in energy will always react first. They will try to overlap with the LUMO (lowest unoccupied molecular orbital) of the electrophile. If the orbitals are close enough in energy, they can interact and form a new bond. When orbitals overlap to form a bond, they will form a new orbital that is lower in energy than the previous two. This is why the bond stays together. If the product is higher in energy than the reactants, it will be unlikely to stay that way and will revert to its previous state.
Orbital energies are often the real explanation for why reactions occur. However, it is unlikely that you will be able to determine a molecule's orbital energies just by looking at it. That is why we have other arguments for whether a reaction will occur.
Types of Nucleophiles and Electrophiles
Now that we understand what they are, it may be useful to look at some types of nucleophiles and electrophiles. Earlier we discussed polar molecules and how these look in electrostatic potential maps. This is one way to identify whether a molecule is nucleophilic, electrophilic or neither. If a bond is polarized (electrons are pulled in one direction), it can have a big effect on how it behaves.
Polarizability is the ability of an atom to unevenly distribute its electrons. This typically happens in response to external forces, such as other atoms.
Larger atoms, like iodine, have numerous electrons held far away from the nucleus. Large atoms, like iodine, have many protons (53, to be exact). To balance this, iodine also has 53 electrons, which float around the nucleus. Since electrons repel each other, iodine's valence electrons are very far away from the nucleus. This means that they are held looser than the electrons in a smaller atom are, like fluorine.
As a result, they can be unevenly distributed a lot easier. Iodide anions can do this in the presence of electrophiles, and thus behave as nucleophiles. An atom's polarizability has a lot to do with its strength as a nucleophile, which will be explored a bit later.
Nucleophiles
Iodide anions are an example of a nucleophilic atom. Typically, nucleophiles are seen in molecules, like with the C–Li example. This is a nucleophile that is formed due to inductive effects. Essentially, the different pull of electrons between the two atoms creates a carbon nucleophile. Carbon is holding the electron pair so close to it, that it can donate it to something else without lithium having a say in the matter. This is observed anytime carbon is bonded to a metal.
Another type of nucleophile is one that uses lone pairs. Some atoms, like nitrogen and oxygen, carry lone pairs (non-bonded electron pairs) which can attack electrophiles.
In the example above, the nitrogen atom has a lot more electron density than the carbon atom. This means that it can donate its lone pair in a nucleophilic attack. Lone pairs are often used by nucleophiles since they are not already shared with another atom.
One final example of nucleophiles is observed with π-bonds. In a C=C bond, one of the electron pairs held between the carbons can be donated to an electrophile forming a bond between one or both of the carbons and the electrophile.
In this example, the π-bond acts as a nucleophile and attacks one of the bromine atoms. This results in the bromine forming a bond with both carbon atoms.
The example of the π-bond attacking bromine is a unique example. It is unique for a few reasons:
- Bromine is more electronegative than carbon
- Bromine has 3 lone pairs, so it would seem it has more electron density
- One carbon acts as a nucleophile and one acts as an electrophile.
Due to the first two reasons, this reaction wouldn't be expected to occur. So, what gives? The Br–Br bond, is a non-polar bond because each atom has the same electronegativity. However, bromine is a large atom, with numerous electrons held far away from its nucleus. This means that bromine can be easily polarized. When the bromine molecule is adjacent to a nucleophile, it becomes polarized. One bromine atom will momentarily shift electrons away from the other bromine atom. This creates a slight positive charge on a bromine atom, which allows it to be attacked by a nucleophile.
Once the π-bond attacks the bromine, one carbon is satisfied, but the other is down 2 electrons. So, the bromine atom will donate two of its electrons and attack the newly formed carbon electrophile. This reaction represents two sequential nucleophilic attacks to form a 3-membered ring featuring a bromonium cation.
Electrophiles
Electrophiles can be formed due to polarizability as well. However, this is observed in ways opposite to that of nucleophiles. Just as an anion may act as a nucleophile, cations may behave as electrophiles since they are missing electrons. Electrophiles are usually present as molecules, with carbon being a very common example. Carbon cations (known as carbocations) are electrophilic and will easily be attacked by nucleophiles.
The carbon has only 3 bonds, giving it a positive charge, and making it an excellent electrophile. The oxygen atom of the hydroxide ion subsequently donates a lone pair, forming CH3OH.
You can also find carbon behaving as an electrophile in examples such as the one shown with C–Cl. When carbon has a bond that is pulling electrons away from it, it craves electron density from some other source. This is often observed with C=O bonds. The more electronegative oxygen pulls electrons away from carbon with two bonds, leaving it especially electrophilic.
In this example, the carbon atom has 3 different bonds with oxygen. It has most of its valence electrons being pulled away from it, making it electrophilic. The ester (molecule on the left) is open to attack. Water, which is a weak nucleophile, can attack it to form acetic acid, a.k.a. vinegar. This is a classic example of an electrophilic reaction, which will come up a lot more in organic chemistry.
It is pretty clear at this point that nucleophiles and electrophiles are pretty diverse topics. We have explored how to identify each and what some examples look like. The next step is to look at what makes these strong or weak, which is covered in Nucleophilicity and Electrophilicity. After that, be sure to check out some Reactions of Nucleophiles and Electrophiles.
Now that you understand what nucleophiles and electrophiles are, do you think Paula Abdul was right? Do opposites attract?
Strong and Weak Nucleophiles and Electrophiles
To discuss the properties of strong and weak nucleophiles and electrophiles, we must discuss nucleophilicity.
Nucleophilicity is the rate of reaction for a nucleophilic attack on an electrophile.
So, the strength of a nucleophile is determined by how quickly it will attack an electrophile. This is why it can be tricky to predict the strength of a nucleophile that has not been experimentally observed. Predicting the strength of a nucleophile can be done by identifying a few criteria:
- Does the nucleophile have a negative charge?
- Is the nucleophile highly polarizable?
- Is the solvent affecting the nucleophile?
By using the above-mentioned list, we can reasonably predict the reactivity of a nucleophile. There are other factors which affect nucleophilicity, like R groups, and HSAB (hard soft acid base) theory, but those will be ignored for now. We will start our analysis with electrical charge.
Nucleophilicity
Nucleophilicity is heavily dependent on the charge of a molecule. A nucleophile with a negative charge will be dramatically more reactive than one with a neutral charge. Previously, we discussed how negative and positive charges will come together. Well, a nucleophile with a full negative charge can balance a positive charge a lot better than one with a partial negative charge.
Partial charges are a way to describe electron density on an atom. If an atom has a partial negative charge, it will have a higher electron density than one with a partial positive charge. In C=O, carbon will possess a partial positive charge and O will have a partial negative charge.
In the example, hydroxide reacts exponentially faster than water does. That is because the oxygen in hydroxide has a full negative charge compared to the partial negative charge in water. Consequently, anionic nucleophiles are often strong nucleophiles.
Along with anionic charges, polarizability also affects nucleophilicity. For example, halides can redistribute their electron density quite well, which allows them to act as strong nucleophiles. This is the case for I-, Br-, and Cl-. However, F- can behave as either a strong or a weak nucleophile. The behavioral changes of fluoride are purely dependent on the solvent.
Solvent Effects on Nucleophilicity
Differing solvents can play a big role in nucleophilicity, either activating or deactivating a nucleophile. Nucleophiles are often ions, as are the leaving groups in reactions. To stabilize the ionic species which are participating in the reaction, organic chemists use polar solvents to run the reactions in. This is crucial for the reaction, since these species are often not soluble in non-polar solvents.
There are many factors to consider when selecting a solvent for an organic chemistry reaction. The most important factor is solubility. If the reactants are not soluble in the solvent, they will never come in contact, and will never react. There are many others which are also important including boiling point, toxicity, whether it is easy to remove afterwards, and more.
Chemists need to ensure the reaction is producing the desired product. But they also need to make sure they can isolate the product after the reaction is finished. If their product is a solid, but their reaction mixture is a liquid, they will have to use laboratory techniques to acquire it. The choice of solvent can make this easy or extremely difficult.
When organic chemists choose a polar solvent to use in a reaction, there are two types of solvents which they consider:
- Polar protic solvents
- Polar aprotic solvents
A polar protic solvent has a hydrogen directly bonded to an electronegative atom. A polar aprotic solvent has all of its hydrogens bonded to non-electronegative atoms.
This may seem hard to visualize, so let's look at some examples of some common solvents.
Polar Protic | Polar Aprotic |
| Water | H-O-H | Acetone | CH3-CO-CH3 |
| Methanol | CH3O-H | Dimethyl Formamide | H-CO-N(CH3)2 |
| Ammonia | H2N-H | Dimethyl Sulfoxide | CH3-SO-CH3 |
| Acetic Acid | CH3-CO2-H | Acetonitrile | CH3-CN |
The protic solvents have at least one hydrogen bonded to an electronegative atom. In the aprotic solvents, there can be electronegative atoms, but they won't have any bonds to hydrogen. This has consequences for certain species in a solution.
Solvents will affect the rate of nucleophilic attacks in certain reactions. Essentially, some solvents will increase or decrease the strength of the nucleophile. But how do we go about predicting this behavior? Well, as you may have guessed, polar protic and polar aprotic solvents differ in how they interact with nucleophiles.
Solvation
To understand how nucleophiles will behave in solution, we must first discuss a phenomenon known as solvation. When water contains ionic compounds, such as NaCl, the ions are present as Na+ and Cl-. To help keep energies at a minimum, water molecules surround these ions.
Solvation occurs when solvent molecules surround and form a shell around a species in solution. This helps to lower the overall energies of the dissolved species.
The Na+ ions are surrounded by the partial negative oxygen atoms. Cations in solution are always well-solvated, which means that they have a tight shell of solvent molecules surrounding them. In water, the Cl- ions hydrogen bond with the water molecules, which helps to lower their energy.
You have likely heard all about ionic bonds and covalent bonds before. There is another type of bond called the hydrogen bond. When a hydrogen atom is bonded to an electronegative atom, like O, N, S, F, etc., it is polarized. Its electrons are not held close to it, so it is partially positive. When it is close to another molecule that has an electronegative atom, it can weakly interact with a lone pair on that atom.
The result is a bond that is depicted as O-H · · · O. In this example, the oxygen atom of the neighboring molecule forms a weak bond with the hydrogen. This interaction keeps the molecules closer together and makes the overall solvent interactions stronger than they would be otherwise. This is part of the reason that water has such a high boiling point.
H2O has two polarized hydrogen atoms which form hydrogen bonds with neighboring oxygen atoms of other water molecules. Compare this to methanol, CH3OH, which only has one polarized hydrogen. It forms less hydrogen bonds than water will. This results in a boiling point that is over 30 °C less than that of water.
Hydrogen bonding plays a pivotal role in nucleophile solvation. If an anionic nucleophile forms hydrogen bonds with the solvent, it limits the nucleophile's ability to move around and react. Since the nucleophile is surrounded by solvent molecules, it can't attack an electrophile as quickly. This results in a slower reaction rate, and ultimately, a weaker nucleophile.
So, in a polar protic solvent, which has potential to form hydrogen bonds, nucleophiles react slower. This is not the case in polar aprotic solvents, since they don't solvate nucleophiles as effectively.
Polar Protic Solvent | Polar Aprotic Solvent |
Polarized hydrogen atom | Non-polarized hydrogen atom |
Effectively solvates nucleophiles | Doesn't effectively solvate nucleophile |
Nucleophiles less active | Nucleophiles more active |
The effects that solvents have on nucleophile rates are most relevant with substitution reactions. In substitution reactions, the nucleophile typically plays a large role in determining the rate of reaction. In reactions where nucleophile strength is not a determining factor, the solvent interactions may be less important.
Electrophilicity
Identifying an electrophilic center is slightly more complicated than identifying a nucleophilic center. Mostly because doing so sometimes requires a more in-depth understanding of organic chemistry. For now, we will just review what we mentioned before. The two main factors which influence electrophilicity are charge and polarization.
If an atom has a positive charge, it will be lacking in electrons, and can then accept them from a nucleophile. To be a little more precise, it will have an empty electron orbital. Atoms with empty orbitals are higher in energy, and thus become a lot more reactive. The more unhappy an atom is, the more reactive it becomes. It becomes less picky with what it will react, and as a result, reacts quicker. This effect results in a greater electrophilicity.
This is also observed with partial positive charges. When an atom is sharing an uneven bond, with a more electronegative atom, it will have fewer electrons on it. The more positive an atom becomes, the more reactive it will become. So, the more uneven bonds that it has, the more unstable it will become. This is especially relevant with double or triple bonds to electronegative atoms, such as in C=O. The second bond to the same oxygen atom makes carbon more electrophilic than it would be with two bonds to two different oxygens.
The carbonyl moiety, C=O, represents a very common bond, and a very electrophilic carbon atom. This bond is observed in vinegar (acetic acid), acetone (nail polish remover), amino acids (essential nutrients), and many more. Without the carbonyl functionality, human life would not exist. The reason that C=O is more electrophilic than, say, O-C-O, is because of something called resonance.
Resonance is a way in which molecules can shift around their electrons. At all times, molecules are shifting electrons back and forth, and all around, in a dynamic equilibrium. Similar to the concept of polarizability, except not caused by outside forces. Resonance allows atoms to become more reactive than they might appear to be. For example:
This example shows that acetone has two different resonance structures. The one on the left is what would be typically drawn. However, the carbonyl carbon can shift its electrons onto oxygen, which forms a full positive charge on the carbon. This makes it more electrophilic than it would be otherwise. Although acetone is not a very reactive molecule, there are many carbonyls that form extremely reactive electrophiles.
There are many other determinants of electrophilicity, but we won't address those here. For now, just remember that cations, and polarizable atoms, can make strong electrophiles.
Polymer Reactions with Nucleophiles and Electrophiles
Polymers, or plastics, are produced on an incredible scale. They are essential to the way of life as we know it. The types of polymers that exist are as diverse as their use. As such, there are many different methods to synthesize these polymers to provide different properties. It may come as no surprise that nucleophiles and electrophiles can be used in polymer reactions. The synthesis of polymers is something that could occupy its own article, so we will just take a very brief look at them.
Anionic Polymerization
Have you ever used superglue? It is a liquid in its tube, but as soon as it is exposed to air, it begins to harden almost immediately. The reason this happens is that moisture (water, suspended in air) can act as a nucleophile to polymerize the monomer.
In a polymerization reaction, a monomer reacts with itself to form a repeating unit of that molecule. When many monomers link together, they become a polymer.
In anionic polymerization, a nucleophile, typically an anion, starts the cycle by attacking an electrophilic monomer. The electrophilic monomer becomes nucleophilic, which continues the propagation. This cycle happens very fast and very efficiently. It will continue until there is no more material to polymerize.
That is why superglue is so quick and strong. All the glue will polymerize from water in the air. Typically, the nucleophile will be OH- instead of H2O. In this example, water was depicted just for simplicity.
Anionic polymerization is often referred to as "living" polymerization. That means that the growing polymer chain will keep going until there is nothing left. It will continue to "live" in an active state. Living polymerization forms very long polymer chains, which are known to be stronger polymers.
Cationic Polymerization
Another type of polymerization, cationic polymerization, is the opposite of anionic polymerization because it uses electrophiles to grow the polymer chain. To help illustrate what this looks like, we will take a look at the polymerization of isobutylene.
Cationic polymerization typically forms softer polymers, with shorter chain lengths, than those formed by anionic polymerization. The polymerization of isobutylene is an incredibly useful and important reaction. Polyisobutylene, also known as butyl rubber, has various applications.2 Some of its uses include:
- Rubber Gloves.
- Chewing Gum.
- Footballs.
- Tires.
- Explosives.
It's hard to imagine that one polymer could have that many applications, and more! It goes to show how much we rely on polymers. They are essential to our society, with many pros and cons. Despite this, it is clear how important it is to understand some reactions of nucleophiles and electrophiles.
Cationic polymerization is typically "non-living" polymerization. This is because chain growth can stop at any time. As was shown in the example, the electrophile can be attacked by another monomer, which would continue growth. But instead, it can be deprotonated by a base, which forms a double bond and stops chain growth. This will "kill" the polymer chain, which results in polymer chains with an assortment of different chain lengths.
Polymers with shorter chain lengths are referred to as low molecular weight. They are typically softer plastics, like the rubber example. Polymers with longer chain lengths are referred to as high molecular weight, and they typically form harder plastics.
By now, hopefully, it is clear how important nucleophiles and electrophiles are. These reactions happen in our bodies, in the world around us, and in chemistry laboratories. Without them, life on earth as we know it would cease to exist. Next time you look at an organic molecule, can you pick out the nucleophilic and electrophilic centers?
Nucleophiles and Electrophiles - Key takeaways
A nucleophilic center is an atom or bond which has a high electron density and is characterized as having a full or partial negative charge.
An electrophilic center is an atom that has a low electron density and is denoted as having a full or partial positive charge.
Nucleophiles react by overlapping the HOMO (highest occupied molecular orbital) with the LUMO (lowest unoccupied molecular orbital) of the electrophile.
Nucleophiles can form due to inductive effects, polarizability, or π-bonds.
Electrophiles form by inductive effects or by polarizability.
References
- David Klein. Organic Chemistry. 3rd ed. 2017
- Butyl Rubber. Wikipedia. 2022.
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