In the hydration of alkenes, one H atom and one OH molecule are added across the C=C double bond. 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. However, a curious thing happens when we add HBr to an alkene in the presence of peroxides (ROOR). This reaction is a Markovnikov addition, and it gives a mixture of syn and anti products. In the hydrohalogenation of alkenes, one H atom and one halogen atom are added across the C=C double bond. 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. Regiochemistry: Markovnikov addition and anti-Markovnikov addition.Stereochemistry: syn addition, anti addition.Addition Reactions Stereochemistry and Regiochemistryīefore 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. Then the base would come and grab a beta-hydrogen. The main difference is that in the E1 reaction, the leaving group leaves first, forming a carbocation. The E1 reaction is similar to the E2 reaction. This results in the temporary negatively charged carbon sharing its electrons with the temporary carbocation to form a double bond. In an E2 reaction, the Lewis base grabs a beta-hydrogen and the leaving group leaves simultaneously. We have covered elimination reactions in detail in these articles: E2 Reactions, E1 Reactions, Understanding E1 vs E2 Reactions. Elimination ReactionsĬhemists utilize elimination reactions to form alkenes. Since the C=C double bonds of alkenes are electron-rich, they can function as bases or nucleophiles. This is because alkenes readily undergo addition reactions. We must also put a “cis”, “trans”, “E”, or “Z” in the front of our molecule name if necessary.Ĭis-pent-3-enal Alkene Reactions General Reaction TrendsĬhemists often use alkenes as the building blocks for the synthesis of other complex molecules. Finally, we attach the suffix “ene” to the end of our molecule name. We should assign to the double bond the lowest number possible. Next, we assign a number to the double bond and other substituents (if there are any) to indicate their position within the molecule. 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. Thus, Z is when Cl and C is on the same side, and E is when they’re on opposite sides. In this example, Cl has a higher priority than O, and C has a higher priority than H.
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