Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 328	Brief Organic Chemistry	Summer 2004
Lecture Notes: 11 June	© R. Paselk 2004
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Organic Reactions: Alkenes

Common Types of Reactions in Organic Chemistry

There are a number of types of reactions which commonly occur in organic chemistry:

• Addition reactions: two molecules combine to give a single product.

  

• Rearrangement reactions: Get a reorganizations of a single reactant, such as cis-trans isomerization.
• Elimination reactions: A single reactant splits to give two products:

  

• Substitution reactions: Two reactants exchange parts to give two new products:

  

We will begin our discussion of reactions in orgnaic chemistry using the alkenes as an example to review mechanisms etc. and to apply the general concepts of reactivity, bond breaking/making etc. to organic systems.

Mechanisms of Organic Reactions

Reaction mechanisms: Want to look at the details of what happens during a reaction - how and why does it occur? Let's look at some terms which describe reactions and reaction processes. Can have radical reactions (involving reactants with unpaired electrons in the valence shell) or polar reactions involving electron pair donation:

• Bond breaking:
  • Homolytic: bond is broken symmetrically, one electron goes with each product. Get radical formation.
  • Heterolytic: both electrons of a covalent bond go with one product. Get polar groups (often results in ion formation).
• Bond making:
  • Homogenic: one electron donated to the new covalent bond by each reactant (radical).
  • Heterogenic: a polar attack with both electrons contributed by one reactant (polar or ionic)

Polar reactions in organic chemistry occur when carbon is attached to an electronegative element, in which case the carbon picks up a partial positive charge (it becomes electrophilic ), or when it is attached to a "electropositive" element such as most metals to give an organometallic bond (carbon becomes nucleophilic ). Nucleophiles are electron rich reactants which can donate electron pairs; whereas electrophiles are electron poor reactants which can accept electron pairs.

The Mechanism of HCl Addition to Ethylene:

Rates, Equilibria, Intermediates, and Reaction CoordinateDiagrams

The reaction of HCl and ethylene can be visualized as follows:

• Initially we see an attack by the electron rich pi-bond of ethylene on the partial positive proton of HCl (recall that the pi bond is not only electron rich, it is also sterically exposed).
  • The electrons of the H-Cl bond are displaced to the chlorine to give a chloride ion.
  • The proton carbon bond formed with the ethylene takes the electron pair of the pi bond to give a new sigma bond and converting the carbon from an sp² to an sp³ hybridization.
• The other carbon is left electron deficient with an empty pi orbital to give a positively charged carbocation intermediate (notice that its hybridization is unchanged - sp²).
• Finally the chloride ion attacks the carbocation, inserting an electron pair into the empty p orbital, resulting in a change of hybridization around the carbon to sp³, and yielding the ethylene chloride (Chloroethane) product.

Rates vs. Equilibria and Reaction Coordinate (Reaction Progress vs. Energy) Diagrams

We have now seen, on a molecular level, how this model alkene reaction occurs. Next we can take a brief look at the kinetics and thermodynamics of this reaction. A reaction progress (reaction coordinate) diagram for the reaction is given below:

The reaction coordinate (x-axis) indicates how far the reaction has progressed (you can think of it as a time axis for a single molecular process). Keep in mind that the diagram describes a microscopically reversible process. That is we can begin with reactants and go to products, or we can begin with products and go to reactants - the path taken (and described in the diagram) is exactly the same in either direction. Let's interpret this diagram.

• The free energy, ΔG, describes whether the reaction is favorable as written - that is will there be more product at equilibrium. When ΔG is negative, that is the product energy is less than the reactant energy, then the equilibrium favors the product. In our example we see that the product is indeed favored, and energy will be given off by the reaction (its exergonic). (Note that you text uses ΔH as an approximation of ΔG. This is reasonable for most organic reactions because more of them have a much larger heat component to the free energy (ΔG) than they do entropy. But there are reactions that get cold instead of hot {e.g. cold packs}, that is they have a positive ΔH but a negative ΔG.)
• The activation energy, E_act, describes how fast the reaction will go. E_act is the difference in energy between the reactant (remember, we choose what the reactant is, that is, which way we write the equation) and the highest energy transition state (highest peak on the diagram). Note that the same transition state will determine the reaction rate in either direction. The lower the activation energy the larger the portion of molecules will have sufficient energy to get over the barrier, and therefore the faster the reaction. As temperature rises more molecules have more kinetic energy, and therefore will cross the barrier and therefore equilibrium will be achieved more quickly. (Note that the position of the equilibrium is determined by the differing specific rates in the two directions, that is how hard it will be for a molecule to get over the activation energy barrier.)
• Note that there is no necessary relationship between the rate of a reaction and the position of the equilibrium (e.g. at room temperature it will take a very long time indeed for gasoline to be converted to water and carbon dioxide, but it will eventually go completely - the equilibrium overwhelmingly favors the products, but the rate is very slow).
• Note that is this reaction there are two transition states and an intermediate - the carbocation. Formation of this intermediate is the slow step in this reaction - there is a large energy barrier. The diagram also shows that this intermediate is unstable - there are very small energy barriers to convert it to either the reactant or the product. Note that the diagram indicates that good portions of the intermediate will go in both directions!

Alkene Chemistry

Addition of HX to alkenes. We have already seen this reaction which gives alkyl halides. However we only looked at the simplest case, in which the alkene is symetrically substituted. It gets a bit more complicated with other alkenes.

Thus for unsymmetrical alkenes a single product generally predominates, instead of getting equal amounts of two molecules: 1-methylcyclohexene and HBr gives 91% 1-bromo-1-methyl-cyclohexane; 1-hexene and HI gives essentially pure 2-iodohexane, etc.

So what's going on? Have to look at intermediates. (Such a deal, these things are actually useful to understand what's going on.) Let's go back to our previous example:

In this example the two alkene carbons are identical, so the carbocation can form equally on either one, but what if one could stabilize this positive charge? Then we might see a difference in products. This is what happens when add alkyl groups to the alkene carbons - alkyl groups such as methyl etc. are more electron rich than hydrogens and can donate electrons to the carbocation, stabilizing it.

Thus the addition of HX to alkenes with differing alkyl groups on the two sides of the double bond are regioselective (selection of one direction of bond making or breaking over others).

• The reaction selects the orientation of the addition product.
• Since the carbocation is favored on the carbon with the most alkyl group we would then predict that
  • the proton will go to the carbon with the most hydrogens and
  • the halide will go to the carbon with the most alkyl groups.

This property is given a special name: Markovnikov's Rule: In the addition of HX to an alkene, the H attaches to the carbon that has fewer alkyl substituents, and the X attaches to the carbon that has more substituents.

As we might extrapolate from Markovnikov's Rule and the last example, the stability of carbocations increases with the number of alkyl substituents:

Hydration of Alkenes (addition of water to alkenes). This reaction should be very similar to the addition of HX, we have simply changed the nucleophile from :X^- to :OH₂. In this reaction however, water is not nearly as strong a Brønsted acid, nor is it as good a nucleophile, and an acid catalyst is needed. Note as is always the case, the catalyst is intimately involved in the reaction, but is regenerated, so at the macroscopic scale it is not changed:

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Last modified 11 June 2004