Archive for February, 2015

Epoxides are useful functional groups in organic chemistry for generating reactive centers. Many drugs, both beneficial and harmful, rely on the process of epoxidation to become biologically active.  In this article, we will review some of the concepts of epoxidation and give you a preview of the hundreds of reactions explained with clear depictions when you sign up for a membership with StudyOrgo!

There are two processes, ring closing- (epoxidation) and ring opening- reactions. Epoxides contain an oxirane, which is a 3 membered ring that contains an oxygen atom.  Preparation of epoxides require a double bond across which the oxygen will be added across the C-C bond to form the oxirane ring.

Ring-Closing Reactions:

Formation of an oxirane ring can be accomplished in 3 ways starting with an alkene reactant. The use of the following peroxides is a common way to prepare an epoxide.

• MCPBA
• Peroxy Acids

***Memorize both of these reagents– if you ever see them- think epoxides!

• The third method requires hydrobromination across the double bond to form a halohydrin. Reaction with a strong base then leads to intra-molecular S_N2 reaction that produces the epoxide.

Ring-Opening Reactions:

Reaction of epoxoides with any strong nucleophile leads to ring opening and formation of an alcohol via an inter-molecular S_N2 reaction. Nucelophiles such as ^–OH, ^–OR, ^–SH, Grignard Reagents and LAH will all attack the epoxide at the least sterically hindered position to break the ring. A practical example of ring opening reactions is the use of ethylene oxide to sterilize medical equipment.  Microbes present on the surface of the equipment are exposed to ethylene oxide whereby DNA, RNA and proteins contain many -NH2 and -OH groups to serve as nucleophiles that will react with the epoxide.  The result is an alkylated group, which will interfere with cell function and induce cell death, known as apoptosis.

The second example explains the organic chemistry of the widely-used monomer Bis-Phenol-A, which has drawn attention for its potentially carcinogenic properties, is reacted with the epoxide, epichlorohydrin, to form polymers used in many plastic products. Note that upon ring-opening of the epoxide in Step 1, a halohydrin is instantly formed and can is further reacted in Step 2 with NaoH in a ring-closing reaction to regenerate the epoxide for another round of catalysis in Step 3, so a long strand of the BPA polymer is formed.

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One reaction that always troubles students taking organic chemistry is the Diels-Alder reaction.  In this article, we will discuss the basics of the reaction and give you a preview of the clear-cut examples of organic chemistry reaction mechanisms available to StudyOrgo members!

The Reaction:

The Diels-Alder reaction is referred to as a pericyclic reaction, in that two reactants cyclize to become one ring.  The reaction utilizes a [4+2] cycloaddition electron mechanism; meaning that the first reactant has 4 pi-electrons and the second reactant has 2 pi-electrons.  These are named the diene (2 alkenes, or 4 pi-electrons) and the dieneophile (1 alkene, or 2 pi-electrons, that “seeks” the diene). The diene MUST be in the cis- conformation in order to cyclize, the trans- isoform would not form the ring… try it with your models or on paper!

The Mechanism:

Below is the reaction mechanism using arrow-pushing.  This mechanism occurs in a concerted, or one-step, process.  It is thought that the dienophile attacks the diene and rearranges the electron distribution to form the two new red C-C bonds and results in a new 2 pi-electron bond. Energetically, breaking 3 pi-bonds in the reactants and formation of the 2 sigma-bonds + 1 pi-bond in the product has a negative enthalpy value (deltaH), therefore the reaction is exothermic and spontaneous!

Stereoselectivity:

Most examples students encounter are more complex, where the dieneophile has substituents and the diene is already cyclized, thus forming a bridged product.  Depending on the orientation of the dienophile with respect to the diene, two products are possible.  Below is an example of such a reaction.

In this example, only one product is observed; the endo- product.  The selectivity for this reaction can be illustrated in the following diagram.  The addition of the reaction occurs via the “left-handed rule” that is, if you put your left hand thumb along the dieneophile and twist to the left (see red arrows), the rotation of the dienophile represents the stereochemistry of the substituents after the reaction is complete.  Two products are possible, the exo- and endo- products.  The reason endo- is preferred can be seen by looking at the Neumann Projection along the bolded bond.  In the exo- product, the Cl group is gauche to the Bridge group.  In the endo- product, the CL group is anti to the Bridge group.  Thus, steric hindrance is a major factor in determining the stereochemistry and since all reaction will deal with these steric factors, endo- will always be preferred!

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