Alkenes are among the most common and important molecules in organic chemistry and biochemistry. In this article, you will learn the structure of alkene, its nomenclature, and some of its reactions.
Topics Covered in Other Articles
Alkenes are hydrocarbons (compounds consisting entirely of carbon and hydrogen) that contain a carbon-carbon double bond (C=C, where one bond is a sigma bond, and the other a pi bond). The carbon-carbon double bond is the functional group of an alkene.
An alkene is said to be substituted when the hydrogen atoms at each end of the C=C bond are replaced by alkyl (or alkyl halide) groups.
Quick Facts on Alkenes
- Structure: A carbon double bonded to another carbon.
- General formula: CnH2n, where n ≥ 2
- Acidity: More acidic than alkanes
- Solubility: Insoluble in water but soluble in nonpolar organic solvents like diethyl ether
- C=C bond length: 1.33 Å
- C=C bond strength: 636 kJ mol-1
- IR Spectroscopy: Alkenes that possess a Csp2–H bond (i.e. any alkenes that are not tetrasubstituted) will have a peak at 3100 cm-1
We have covered this concept extensively in these articles: Structural Isomers, Geometric Isomerism, Cis Trans Isomers, Stereoisomers and Chiral Centers.
Thus, the following information will just be a brief refresher.
Due to the rigidity of the double bond (its inability to rotate), substituted alkenes (i.e. alkenes where the hydrogen atoms at each end of the C=C bond are replaced by non-hydrogen substituents) can exist as a pair of stereoisomers. Chemists use cis/trans or E/Z terminology to describe these stereoisomers.
Chemists use cis/trans terminology for disubstituted alkenes, where the substituents are connected to different ends of the C=C bond.
- The cis isomers have the same group on the same side of the double bond.
- The trans isomers have the same group on the opposite sides of the double bond.
Chemists use E/Z terminology for trisubstituted or tetrasubstituted alkenes.
To use the E/Z terminology, we first assign priority to the substituents using the Cahn-Ingold-Prelog rules (the higher the atomic number of the elements in the substituent, the higher the priority). We then turn our attention to each end of the C=C bond and compare the priority of the pair of substituents there with each other.
- The E isomers have substituents with higher priority on opposite sides of the double bond.
- The Z isomers have substituents with higher priority on the same side of the double bond.
To name an alkene (a pure alkene, without any other functional groups), we first identify the parent, which is the longest chain that contains the C=C double bond. Next, we assign a number to the double bond and other substituents (if there are any) to indicate their position within the molecule. We should assign to the double bond the lowest number possible. Finally, we attach the suffix “ene” to the end of our molecule name. We must also put a “cis”, “trans”, “E”, or “Z” in the front of our molecule name if necessary.
Moreover, if an alkene takes a form of a cyclic molecule, then we would put a “cyclo” in front of the parent chain.
IUPAC also recognizes some common names for common alkenes: ethylene, propylene, styrene, etc.
If the alkene is just a substituent of a molecule (i.e. there exist higher priority functional groups), then we will put an “en” between the parent chain and the functional group. We must also remember to put a number in front of “en” to indicate position (only this time the double bond doesn’t get the lowest number possible) and any stereoisomerism indication in the very front of the molecule name if necessary.
General Reaction Trends
Chemists often use alkenes as the building blocks for the synthesis of other complex molecules. This is because alkenes readily undergo addition reactions. Since the C=C double bonds of alkenes are electron-rich, they can function as bases or nucleophiles.
Chemists utilize elimination reactions to form alkenes. We have covered elimination reactions in detail in these articles: E2 Reactions, E1 Reactions, Understanding E1 vs E2 Reactions.
In an E2 reaction, the Lewis base grabs a beta-hydrogen and the leaving group leaves simultaneously. This results in the temporary negatively charged carbon sharing its electrons with the temporary carbocation to form a double bond.
The E1 reaction is similar to the E2 reaction. The main difference is that in the E1 reaction, the leaving group leaves first, forming a carbocation. Then the base would come and grab a beta-hydrogen.
Stereochemistry and Regiochemistry
Before looking at the addition reactions of alkenes, we suggest you to get familiar with the terminology chemists use to describe the stereochemistry and regiochemistry of a reaction.
- Stereochemistry: syn addition, anti addition
- Regiochemistry: Markovnikov addition & anti-Markovnikov addition
Hydrogenation of Alkenes
In the hydrogenation of alkenes, two hydrogen (H) atoms are added across the C=C double bond with the help of a metal catalyst like Pt, Pd, or Ni. This reaction is a syn addition.
Hydrohalogenation of Alkenes
In the hydrohalogenation of alkenes, one H atom and one halogen atom are added across the C=C double bond. This reaction is a Markovnikov addition, and it gives a mixture of syn and anti products.
However, a curious thing happens when we add HBr to an alkene in the presence of peroxides (ROOR). The product is still a mixture of syn and anti, but the reaction becomes an anti-Markovnikov addition, where Br ends up on the less substituted carbon.
Hydration of Alkenes
In the hydration of alkenes, one H atom and one OH molecule are added across the C=C double bond. This reaction can be Markovnikov or anti-Markovnikov, depending on our reagents.
As its name suggests, this reaction involves adding water to an alkene in the presence of an acid. This reaction is Markovnikov and gives both syn and anti products.
This hydration reaction involves two steps. The first step, hydroboration, is to add borane (BH3) with a stabilizing solvent like tetrahydrofuran (THF) into the alkene. The second step, oxidation, is to add hydrogen peroxide (H2O2) and a hydroxide (OH–) source like water or NaOH. This reaction is anti-Markovnikov and only gives syn products.
Dihydroxylation of Alkenes
In the dihydroxylation of alkenes, two OH molecules are added across the C=C double bond, effectively turning the alkene into a diol. Since we are adding the same thing (i.e. OH) to both carbons, the regiochemistry is irrelevant. In contrast, stereochemistry is a topic of great interest. The reaction can be syn or anti, depending on our reagents.
To achieve an anti dihydroxylation of an alkene, chemists treat that alkene first with peroxy acids (like MCPBA) and then with water under acid-catalyzed conditions. The peroxy acid turns the alkene into an epoxide, and the water opens up said epoxide.
To achieve a syn dihydroxylation of an alkene, chemists treat that alkene first with osmium tetroxide (OsO4) and then with aqueous sodium sulfide (Na2SO3) or aqueous sodium bisulfide (NaHSO3).
Another method is to treat the alkene with cold potassium permanganate (KMnO4) and a hydroxide source.
An alkene conjugated to a carbonyl will react with an electrophile to add a carbon-carbon bond.
Ozonolysis is one of the many reactions chemists use to cleave the C=C bond. To perform ozonolysis, chemists first bubble ozone into the alkene solution and then add into the solution a reducing agent like dimethyl sulfide (DMS). The result is a C=C bond split into two C=O bonds.