Minggu, 11 Oktober 2009

Chemical Reactions


Introduction

One of the greatest challenges that organic chemistry presents to students is the seemingly overwhelming number of reactions and reagents that must be learned. While it is true that organic chemists have developed thousands of reactions, it is also true that all of those reactions, every one, can be classified into one of three categories, substitution reactions, elimination reactions, and addition reactions.

Substitution Reactions

All substitution reactions result in the replacement of one atom or group by another. Figure 1 presents 4 specific examples of different types of substitution reactions. The bonds that are broken and formed during each reaction are shown in red. The atom or group that is displaced is highlighted in green, while the incoming atom or group is colored blue.

Figure 1

Substitution Reactions

While the reactions shown in Figure 1 involve a variety of structures and occur under a wide range of reaction conditions, in each case an atom or group attached to a carbon atom is replaced by another atom or group. Scheme 1 presents a general representation of all substitution reactions.

Scheme 1

A General Scheme for Substitution Reactions


Exercise 1 Complete the following table.

Reaction
G
X
Y
(1)
(2)
(3)
(4)

Elimination Reactions

Elimination reactions are processes in which two atoms or groups are removed from adjacent atoms with the formation of a multiple bond between those atoms. Figure 2 presents several examples.

Figure 2

Elimination Reactions

Scheme 2 depicts the minimal features of the various types of elimination reactions.

Scheme 2

A General Scheme for Elimination Reactions

Two features of Scheme 2 are worthy of mention. First, since the atoms or groups that are eliminated are attached to adjacent atoms, this type of reaction is often referred to as a 1,2-elimination. Second, as Equation 7 makes apparent, X may represent atoms other than C.

Addition Reactions

Addition reactions are the reverse of elimination reactions. Three examples are given in Figure 3. This type of addition is called a 1,2-addition reaction, implying that the atoms or groups are added to adjacent atoms.

Figure 3

Addition Reactions

Scheme 3 illustrates the essential features of reactions 8-10.

Scheme 3

A General Scheme for Addition Reactions

The implication of the notation G1/G2 is that the two atoms or groups that add to the multiple bond may be the elements of a single molecule as they are in reactions 8 and 9, or they may come from different molecules as they do in reaction 10. An organic chemist would say that Equation 10 described the addition of the "elements of" methane to the C-O double bond of acetaldehyde, the elements of methane being CH3 and H.

It is a relatively simple matter to classify any reaction when you see an equation like those shown in Figures 1-3. Mastering organic chemistry, however, requires that you be able to predict the outcome of a chemical reaction when all you are given is the starting materials.


Exercise 2 Enter the structure of the organic product formed in the following reaction.


In order to predict the outcome of a reaction, you should be able to answer the following questions:
  • What is the reactant?
  • What functional group is present in the reactant?
  • What is the nature of the reagent? Is it an acid? a base? an oxidizing reagent? etc.
  • Is there a catalyst? If so, what is it and what is its function?
  • What is the solvent?

Conventions

In order to discuss reactions intelligently, it is necessary to understand the vocabulary that organic chemists use. That vocabulary includes the following definitions:

  • reactant the organic molecule that is transformed from starting material to product in a chemical reaction
  • reagent the compound that effects the transformation of the reactant to the product
  • catalyst the compound or combination of compounds that is used to accelerate the rate of the reaction
  • solvent the medium in which the reactant, reagent, and catalyst are dissolved

Running a Chemical Reaction

In the laboratory, each reaction is unique. Still, most reactions involve the following operations.

  • Preliminary Planning This is the first stage of the process. Some of the activities included in this stage are writing an equation describing the expected reaction, calculating the molar ratios of reactants and reagents and the masses that correspond to those molar quantities, selecting a solvent , and choosing the appropriate apparatus.
  • Running the Reaction While this is the meat of the matter, it is perhaps the easiest part of the process. You mix the reactant(s), reagent(s), catalyst(s) and solvent(s) together. You may need to stir the reaction or heat it or cool it, but basically all you do is put the stuff together and let nature take its course.
  • The Work-Up This is the technically most demanding part of running a reaction. The objective is to separate the desired product from unreacted starting material, excess reagent(s), and any catalyst(s) that were used. Typical work-up involve extraction, distillation, crystallization, and chromatography.
  • Product Analysis and Identification Once you have isolated the desired product, it is necessary to determine its purity and establish its identity. To that end chromatography and spectroscopy work hand in hand. It is often the case that the desired product is accompanied by undesired side-products. Thin layer chromatography (TLC), gas chromatography (GC), and high pressure liquid chromatography (HPLC) all provide information about the number of components in the product mixture. Infra-red (IR) spectroscopy and nuclear magnetic resonance (NMR) spectroscopy are the two techniques most commonly used to characterize the structure of a compound once it has been obtained in pure form. Elemental analysis and mass spectrometry (MS) are used to determine the molecular formula and molecular mass, respectively.

In our discussion of host-guest chemistry, we considered the design and synthesis of a molecule that would mimic the catalytic acitivity of the enzyme carboxypeptidase. This link will take you to a page that presents an annotated experimental procedure used in one of the reactions involved in the synthesis of the target molecule.

The outcome of a chemical reaction depends upon a number of factors, some of which we can control, some of which we can't. One of those factors is energy. The relationship between the energy of the reactants and the products in a chemical reaction is described in terms of a reaction profile diagram.

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