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Chemistry 2131:
Organic Chemistry for the Life Sciences (3)

Alkenes, and an Introduction to Reaction Mechanisms

• the properties of alkenes are rather similar to those of alkanes. There are only dispersion forces holding them together in the liquid and solid states. These are of similar strength to the dispersion forces of alkanes. Thus, melting and boiling points are low and increase with molecular size, since the strength of dispersion forces increase with molecular size.
• one property of the cycloalkenes is that they cannot have trans double bonds until they are at least 8 carbons in size. cis-cyclooctene has been observed, but it is highly strained. You can demonstrate this by trying to make cyclohexene or other small cycloalkenes. Trying to join the ring causes significant stress on the double bond. This angle strain results in a weaker double bond, and a generally less stable molecule
• there is a section on terpenes that we will come back to after looking at the reactions of alkenes.

2. Classes of Organic Reactions:

• we will start soon to look at organic reactions. There are many different reaction types possible. Fortunately these can be grouped or classed into similar reaction types.
• the first reaction mechanism that we will look at is an addition reaction. As the name suggestsm the reaction involves the addition of two molecules or atoms. They are of the type A + B -> C.
• the reverse reaction is called elimination, and has the general formula A -> B + C.
• another common reaction type is a substitution reaction. This type of reaction occurs when one atom or group of atoms is swapped for another, so they have the general formula, A-B + C-D -> A-C + B-D. This is a very common kind of reaction.
• finally we have rearrangement reactions. These are reactions of the type A -> B. These are the isomerization reactions. For example, cis-2-butene to trans-2-butene.

3. Terminology of Reaction Mechanisms:

• you should become familiar with the terminology of organic reactions. Reactions occur when molecules collide, bonds are broken and new bonds form.
• we can look at reactions from the perspective of the energy of the process. This is well described in the text pages 98-100. To summarize you can think of the progress of a reaction by plotting the potential energy of the system versus the reaction coordinate. This latter term refers to the progress of the reaction. So, you start with the reactants, go through the transistion state, and on to the products.
• if the energy of the products is greater than the energy of the reactants, the reaction is called endothermic, and the prodcut is less stable than the reactants were. This process requires a source of energy such as heat or light...
• if the products are of lower energy than the reactants, then the reaction is called exothermic, and the products are more stable than the reactants. It is important to remember that these terms refer only to the initial and final energy levels and not to what happens in between.
• reactions can proceed smoothly, or go through intermediates. These intermediates are seen as brief troughs in the progress of the reaction, they are more stable than the transition states, but not as stable as the final product.
• that is the energetic approach, but what about the actual reaction process, the breaking and making of bonds? The way we try to figure out and understand these processes is through electron pushing. Before actually pushing electrons around we have to learn some terms.
• first of all, lets look at how bonds break. There are two ways for a covalent bond to break. If it breaks symmetryically, and one electron from the bond goes to each participating atom, this is called homolytic bond cleavage. For this kind of process one uses fish-hook arrows, which depict the flow of one electron. Following homolytic bond cleavage, we are left with two radicals, chemical species containing an odd number of valence electrons. These are unpaired electrons. Radicals are very reactive.
• the more common way for bonds to break is for both electrons to go to one atom. This asymmetrical bond cleavage is called heterolytic. To depict the flow of electrons in this case, double-headed arrows are used.
• Since there are two ways to break bonds there are two ways to make them. If one electron comes from each atom we call this homogenic bond formation. On the other hand, if both electrons come from one atom it is called heterogenic bond formation. Both types occur in biology.
• we will start with the more important type of reaction, ones involving heterolytic bond breakage and heterogenic bond formation. These are often called polar reactions.
• an important aspect of these reactions is bond polarity and the electronegativity of the atoms involved.
• in general, reactions occur between electron-rich atoms and electron-poor atoms. We refer to electron-poor atoms as electrophiles because they would like to get electrons from other atoms. These are either cations or partially positively charged.
• we refer to electron-rich atoms as nucleophiles. These can either be anions or partially negatively charged.
• another way to look at it is that nucleophiles form bonds by donating a pair of electrons whereas electrophiles form bond by accepting electrons.