Nucleophiles and Electrophiles
Nucleophiles and Electrophiles
To be a little more technical, the negative molecules are called nucleophiles and the negative spots are called nucleophilic sites. Nuclei are positively charged and “philic” means “lover of”. Opposites attract, so negative things love positive things; they’re nucleophilic.
Positive molecules are called electrophiles. Positive spots are called electrophilic sites. Electrons are negatively charged, so electrophiles are lovers of negative things. Negative things are called nucleophiles, positive things are called electrophiles.
4. Circle the appropriate word in each of the following sentences.
a) Negatively charged molecules are called (electrophiles/nucleophiles).
b) Positively charged molecules are called (electrophiles/nucleophiles).
c) A molecule that is attacked by a negatively charged molecule is called a(n) (electrophile/nucleophile).
Finding Nucleophilic Sites
Let’s first address how we recognize the most negative spots (nucleophilic sites) in real molecules. Perhaps the most obvious situation is if an atom has a formal negative charge. Examples of this are oxides, amide ions, carbanions, and hydrides. Remember that arrows always show where the negative electrons move. In the following figure, the arrows originate at the negative, nucleophilic sites.
The charges are sometimes drawn on the atoms. When they are, that helps us determine where to start our arrows. But in fact, this negative charge is always balanced out by a counterion. The counterion is a positive cation, usually a metal cation. This metal cation is typically drawn attached to the most negative, nucleophilic site. This can make it a little trickier recognizing the negative atom. Remember that anytime you see a metal in a molecule, the atom attached has a negative charge. A positive metal has a negatively charged atom attached to it! It is this negative atom that will go out and attack in our mechanisms. Our arrow will start there. Here are some examples of how these compounds may be written with the metal. Remember that arrows are pointing away from the most negative, nucleophilic sites.
If you don’t see a formal negative charge on an atom, or if you don’t see a metal to tip you off to the location of a negative atom, look for other locations of nice, juicy negative electrons. The best place to look? Look for double or triple bonds as they contain one or two π bonds respectively. You can also look for lone pairs of electrons. This is a little tricky for beginning students because chemists are lazy when it comes to drawing structures. We quite often do not draw in all of the lone pairs of electrons. It is assumed you will know how many lone pairs there are on each atom. Remember the normal bonding patterns and you will be all right. So, if you have a nitrogen, oxygen, or sulfur atom, be sure to draw in the lone pairs of electrons.
Sometimes, you’ll have two sites to choose from. Let’s consider lithium acetylide. . .
In this molecule, we see the lithium metal atom, which we remember is positively charged. This means that the carbon atom attached to the lithium has a partial negative charge. We can think of it as having a lone pair of electrons on it. But, we also notice that there is a pi bond that is also available for bonding. When you see a molecule with a metal written, our mechanism arrow will almost always start at the atoms attached to it. So, typically, carbon-metal bonds are more negative (more nucleophilic) than pi bonds.
In summary, there are four main locations to look for negatively charged sites on molecules.
How to find negative sites
1. Formal negative charges
2. Bonds between nonmetal and metal atoms
3. Lone pairs of electrons
4. Pi bonds
5. Draw arrows for the following molecule starting at the most negative spots (nucleophilic sites) and pointing towards the positive charges.
If you struggled at all or missed any of the problems in the previous exercise, go back and reread the beginning of this chapter once again. You should make sure you understand what is going on before proceeding. If you were successful in the previous exercise, great! Let’s learn to recognize the positive spots (electrophilic sites) in molecules.
Finding Electrophilic Sites
The first and most obvious electrophiles are atoms with a positive charge explicitly written on them. A carbocation (positively charged cation on a carbon atom) is a prime example. Carbocations have a carbon atom with three bonds and an empty p orbital.
Other sources of atoms with a positive formal charge, which may seem “hidden”, are the acids. Recall that acids are a source of protons, H+. So, HCl, H2SO4, H3PO4 are all sources of the electrophilic H+.
If a carbon or hydrogen is attached to a more electronegative atom, the carbon or hydrogen atom has a partially positive charge. Alkyl halides, carbonyls, and proton acids are also electrophiles.
You may remember from general chemistry the strange chemical behavior of boron (B) and aluminum (Al). They are atoms that are electron deficient because they normally have six electrons in their Lewis structures. They have an incomplete octet. The B and Al atoms contain a partial positive charge because they normally lack negative electron density around them.
To summarize how to find positively charged electrophilic sites, look for the following.
1. Atoms with a formal positive charge written on them.
a. Sometimes the positive formal charge is written right on the atom. Hooray for those times!
b. Strong acids are a source of H+.
c. If you cannot find a positive formal charge, it might help to draw valid resonance forms until you find some good sites.
2. Atoms attached to electronegative elements.
3. Al or B atoms
6. Draw an arrow from the negative electrons to the most positive spot (most electrophilic site) in the following molecules.
7. Draw arrows from the most negative, nucleophilic sites on one molecule on the left towards the most positive, electrophilic sites in the molecule on the right.
One arrow or more?
In the previous exercise, we drew one arrow from the nucleophile to the electrophile. This is an important first step for our mechanism. Only one arrow is drawn in each step unless it is necessary to draw more. If only one arrow is needed, then draw the product and move on to the next step of the mechanism. More arrows can be drawn in subsequent steps, but for goodness sakes, draw only the necessary arrows in each step!
The second step of arrow drawing is learning whether a second arrow or more is needed. One common mistake of novice organic chemistry students is drawing too many arrows for no reason. A mechanism is a step-by-step story. There are two reasons to draw more arrows. One reason to draw more than one arrow is if you know that the reaction is concerted (that two or more things happen at the same time). If two things happen in the same step, they should be drawn in the same step. If two things happen sequentially, one after the other, they should be drawn in two steps, one after the other.
Another reason to draw more arrows in one step is if after the first arrow has moved two electrons, an atom has more than an octet of electrons. For example, if an atom will have ten electrons, then some electrons need to be immediately removed in order to maintain an octet. In the following figure, after the first two electrons are moved, the atom receiving electrons has an octet, so no more arrows are needed. Notice, only one arrow is used.
There are several things about the above mechanism that I want you to observe.
1. The arrow shows where the two red electrons moved. The electrons from the lone pair on the oxygen atom in the methoxide reactant move to become the bond between the carbon and oxygen atom in the product.
2. The carbon atom in the product has eight electrons around it (an octet). The carbon atom is happy, so only one arrow was required. If the carbon atom had more than eight electrons around it, some electrons would need to be removed with a second arrow.
3. Notice how the formal charges changed in the above reaction. The oxygen atom started with one bond and three lone pairs of electrons giving it a -1 formal charge. The carbon atom started with three bonds and no lone pairs of electrons giving it a +1 formal charge. After the oxygen atom donated two electrons to the carbon atom, we see in the product that both atoms have a normal bonding pattern. They have a zero formal charge in the product.
How about the following reaction? Observe what the product looks like after one arrow has moved two electrons
Do you see a problem with the product? Of course, there are five bonds around a carbon atom! Ten electrons around the carbon atom break the octet rule. This molecule needs to get rid of two electrons from that carbon. A better mechanism would be the following with two arrows happening at the same time, a concerted mechanism.
Using this two-arrow, concerted, mechanism we avoid the unsavory 10-electron species from above. Now, the carbon atom in the product only has its octet. So, a rule to follow is that if electrons are coming to an electrophile and it makes more electrons than an octet at an atom, then electrons MUST move away from this atom. If this is not the case, use only one arrow.
A similar situation occurs with acids. Consider what the following reaction looks like after one arrow has moved two electrons.
Do you see the problem? A hydrogen atom with two bonds in organic chemistry is not going to happen. Let’s please have only one bond for every hydrogen atom. In order to avoid a situation like this, we again need two arrows.
8. Draw in either one or two arrows, as appropriate, to complete the mechanism.
9. Draw in the product or resonance form that is made after pushing the electrons that the arrows show. It is recommended that you draw in all lone pairs of electrons and hydrogen atoms near where the arrows are. This will help you identify formal charges in the products.
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10. Let’s put it all together. This time, draw the mechanism arrow(s) as well as the product that is formed.
Answers
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a) Nucleophiles
b) Electrophiles
c) Electrophiles
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Notice in all of these, when there is a metal atom in the compound, the bond/atom next to it is the most negative site. Your arrow will start there! Pi bonds (double/triple bonds) are usually good locations for arrow beginnings, but not compared to spots next to very positive metal atoms!
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