An Overview of Alkene Chemistry
Alkenes - Structure & Reactivity
Before being able to understand the specific alkene reactions, we have to understand what an alkene is: how and why it reacts. An alkene is a hydrocarbon that has a carbon-carbon double bound. The double bond is sp2-hybridized, making alkenes have trigonal planar geometry. This grants the atoms a bond angle of 120 degrees.
The simplest example of a molecule in the alkene family would be ethane, C2H4, pictured below with its trigonal planar molecular geometry:
Here we have three sigma bonds from each carbon and a single unhybridized 2p orbital perpendicular to the plane of the molecule. Each carbon has sp3 orbital overlap with two other hydrogen atoms, forming single bonds with them. The two carbons also have sigma overlaps in addition to the overlap of their empty unhybridized 2p orbitals, netting one sigma bond and one π bond between them. Below is a visualization of the sigma and π bonds and orbitals in the molecule ethane, as well as a line-structure depiction showcasing stereochemistry.
As we can see, the unhybridized 2p orbital of the sp2 hybridized carbon atoms sits half on the top and half on the bottom of the plane of the molecule. The 2p orbitals of the carbons are coplanar, meaning that they sit on the same plane perpendicular to the plane of the molecule. This allows for the overlap of the 2p orbitals to form the π bond in ethane.
Compare this orbital configuration with that of the alkane family. Below the orbital structure of ethane, C2H6, is depicted.
As we can see, in alkanes there exists only sigma bonding interactions, making alkanes considerably less reactive than alkenes.
This orientation in space gives rise to the reactivity of alkenes. As we can see above, the now overlapped 2p orbitals of the carbon-carbon double bond are positioned in such a way that they are open for reaction. The hydrogen bonds to carbon are in the plane of the molecule and do not hinder the electrons in the overlapped 2p orbitals. This availability of the electrons allows them to react freely with other molecules in such a way that alkanes cannot. Hence, we can see why the reactivity of alkenes trump that of alkanes.
Another key component to understanding the reactions of alkenes are the terms nucleophile (nucleus-loving) and electrophile (electron-loving). A simple memorization of the definitions will not serve a student of organic chemistry well because their importance lies in their application. In terms of chemistry, the word nucleophile translates to a species that is attracted to positive charges. In the same vein, an electrophile is a species that is attracted to strong negative charges. In general, these two terms are depicted as such:
The nucleophile, abbreviated Nu, has a lone pair of electrons on its right side, indicating that it is a source of electrons and has some sort of abundance of them. The electrophile, simply abbreviated E, has a positive charge indicated on it, showing that it is positive-rich or partially positive. A nucleophile is a species that is attracted to positive charges, an electrophile is a species that is attracted to negative charges.
With these two terms lies the foundational principle of organic reactions. Most of the reaction mechanisms that come up in organic chemistry will begin with some nucleophile and electrophile. The nucleophile being electron-rich, an electron source, seeks out the electrophile, being electron-poor, an electron sink, and attacks it. Naturally this then leads to a product wherein a nucleophile is bound to an electrophile.
A smart way to begin looking at any reaction is to first identify: what is the nucleophile and electrophile? What makes this molecule a nucleophile and why does it want to attack this certain species? What makes this molecule partially positive and why does it attract an attack? When the nucleophile attacks the electrophile, what do the products look like and what do the charges look like? The applications of these questions and concepts will prove incredibly useful for understanding organic reactions and why they take place. It will make the process more about concepts and applications than simple memorization.
Let’s apply what we’ve learned so far with a simple reaction mechanism for an alkene. In this case, we’ll employ the alkene 2-butene.
2-butene has a π bond between carbon 2 and carbon 3, resulting in a carbon-carbon double bond between these atoms in the molecule. The π electrons in the carbon-carbon double bond are electron-rich, meaning that in this specific reaction, they are the source of electrons. In turn this would mean that the molecule 2-butene is our nucleophilic species for the reaction.
If an electrophile is introduced to the medium, the nucleophilic π electrons of the 2-butene double bond will reach out and attack the electron-poor species. This attack breaks the carbon-carbon double bond and forms a bond with the electrophilic species. Depicting this process would show an arrow coming from the carbon-carbon double bond and ending at the positive electrophilic species. This is an important notion in the process of organic reaction mechanisms. The arrows for these reactions always flow from an electron source to an electron sink. The arrow starts at an area of high electron density and ends at one with low electron density.
Never start an arrow at a positive species. These species get attacked and do not perform any attacks themselves because they lack any electrons. This is crucial to understand as a misplaced arrow can throw off an entire reaction mechanism. Once a correct arrow is drawn to emulate the movement of electrons, a reaction arrow is placed showing the products of the reaction.
As one can see, the skeletal structure of the molecule 2-butene has not changed, but is instead missing the double bond because it was broken when the π electrons attacked the electrophilic species. Recall that the π electrons existed on both carbons 2 and 3. It does not matter where the electrophile is put, which carbon it is bonded to. If, however, the molecule is a primary, secondary, or tertiary carbon, where one places the electrophile will make a difference. These results will be explored later in the text. If as above, the electrophile is connected to carbon 3 in the chain. The bond connecting the electrophile to carbon 3 is made out of the two electrons that did the attacking in the reaction. Herein lies another key concept to understand. Carbon 3 has the electrophile attached. It had four bonds in butene, and now it has four bonds as well. This is because there is an invisible hydrogen present in the reaction and when we depict structures in the line structure form, it cannot be easily seen. In carbon 2, however, having had only one hydrogen before the reaction took place (when the molecule still had a carbon-carbon double bond), it now has a formal charge of +1.
One can verify the formal charge by taking a swift count of the electrons and applying the formula for the formal charge, or one can try to understand what has happened. We started with a double bond on carbons 2 and 3. The π electrons moved to attack the electrophile and the electrophile bonded to carbon 3 atom, leaving the carbon 2 atom with nothing. The carbon 2 atom, now electron-deficient, has a positive charge, which means that it is waiting for something to come and fill those spaces for electrons.
This step of the reaction has left us with a carbocation intermediate. Now we must take a second to reevaluate our species.
We must decide what the new nucleophile is and what the new electrophile is in our new reaction step. Take a second to decide what you think the electrophilic and nucleophilic species will be in this new reaction step.
The electrophile will be the new carbocation intermediate that we formed in the last step of the reaction. This is because it has a positive charge, and is therefore an electron sink, and is seeking an electron source (another nucleophile). At this point, any random nucleophile present in the solution will react with the carbocation intermediate to continue the mechanism of this reaction. Once again, the electrons on the nucleophile will reach out and attack the electrophile carbocation to form a bond.
This nets us our final product of this reaction mechanism. The change in charge for the nucleophile and electrophile has not been accounted for here because that would depend on the specific atoms or molecules that are acting as those species.
The purpose of this general reaction mechanism has been to grant us a solid understanding of what an alkene is and how an alkene reacts.
