8 Overview Org React

The symmetrical cleavage is said to be homolytic, and the unsymmetrical cleavage is said to be heterolytic. We’ll develop this point in more detail later, but you might notice for now that the movement of one electron in a homolytic process is indicated using a half-headed, or ―fishhook,‖ arrow, whereas the movement of two electrons in a heterolytic process is indicated using a full-headed curved arrow.
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A mechanism describes in detail exactly what takes place at each stage of a chemical transformation—which bonds are broken and in what order, which bonds are formed and in what order, and what the relative rates of the steps are. A complete mechanism must also account for all reactants used, all products formed, and the amount of each. All chemical reactions involve bond breaking and bond making.
H These two reactants… H H C Cl C H H
H
Methane (an alkane)
+ Cl
Cl
Light
+
H
Cl
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H Chloromethane
(an alkyl halide)


Rearangement reactions occur when a single reactant undergoes a reorganization of bonds and atoms to yield an isomeric product. An example is the conversion of the alkene 1-butene into its constitutional isomer 2-butene by treatment with an acid catalyst:
H C C H
1-Butene
CH3CH2
Acid Catalyst
CH3 C H
2-Butene
H C CH3
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H
2. How Organic Reactions Occur: Mechanisms


Having looked at the kinds of reactions that take place, let’s now see how reactions occur. An overall description of how a reaction occurs is called a reaction mechanism.
H C C H H
Ethylene (an alkene)
These two reactants…
…add to give this product.
H H
H
Br C H
5
+
H
Br
H
C
H
Bromoethane (an alkyl halide)


Elimination reactions are, in a sense, the opposite of addition reactions and occur when a single reactant splits into two products. An example is the reaction of an alkyl halide with base to yield an acid and an alkene:
…gives these two products.
This one reactant…
H H C H
Br C H H Base H C H
Ethylene (an alkene)
H C H
6
+
H
Br
Bromoethane (an alkyl halide)



Substitution reactions occur when two reactants exchange parts to give two new products. An example is the reaction of an alkane with Cl2 in the presence of ultraviolet light to yield an alkyl chloride. A Cl atom from Cl2 substitutes for an H atom of the alkane, and two new products result: …give these two products.
A
B
A
+
B
A
B
A +
B
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Just as there are two ways in which a bond can break, there are two ways in which a covalent two-electron bond can form: A bond can form in an electronically symmetrical homogenic way when one electron is donated to the new bond by each reactant, or a bond can form in an electronically unsymmetrical heterogenic way when both bonding electrons are donated to the new bond by one reactant.
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When these themes are understood, learning organic chemistry becomes much easier and rote memorization is minimized. The aim of this book is to describe the themes and clarify the patterns that unify organic chemistry. We’ll begin with an overview of the fundamental kinds of organic reactions that take place, seeing how reactions can be described.
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1. Kinds of Organic Reactions



Organic chemical reactions can be organized broadly in two ways—by what kinds of reactions occur and by how reactions occur. Let’s look first at the kinds of reactions that take place. There are four general types of organic reactions: additions, eliminations, substitutions, and rearrangements.
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Homolytic bond breaking (radical) (one electron stays with each fragment) Heterolytic bond breaking (polar) (two electrons stay with one fragment)
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Addition reactions occur when two reactants add together to form a single new product with no atoms ―left over.‖ An example is the reaction of an alkene, such as ethylene, with HBr to yield an alkyl bromide:
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When two molecules come together, react, and yield products, specific bonds in the reactant molecules are broken, and specific bonds in the product molecules are formed. Fundamentally, there are two ways in which a covalent two-electron bond can break: A bond can break in an electronically symmetrical way so that one electron remains with each product fragment, or a bond can break in an electronically unsymmetrical way so that both bonding electrons remain with one product fragment, leaving the other fragment with a vacant orbital. 11