Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 328

Brief Organic Chemistry

Summer 2004

Lecture Notes: 11 June

© R. Paselk 2004


Organic Reactions: Alkenes

Common Types of Reactions in Organic Chemistry

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

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:

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:


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.


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).

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:

tertiary (3°) > secondary (2°) > primary (1°) > methyl


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