Radical chain reactions occur in three stages: During propagation, a bromine atom attacks ethane, producing a radical, which then reacts with another bromine molecule to produce ethyl bromide:. The sum of the two propagation steps corresponds to the balanced chemical equation for the overall reaction. There are three possible termination steps: Because radicals are powerful nucleophiles and hence highly reactive, such reactions are not very selective. For example, the chlorination of n-butane gives a roughly Because radicals are highly reactive, radical reactions are usually not very selective.
Oxidation—reduction reactions, which are common in organic chemistry, can often be identified by changes in the number of oxygen atoms at a particular position in the hydrocarbon skeleton or in the number of bonds between carbon and oxygen at that position. An increase in either corresponds to an oxidation, whereas a decrease corresponds to a reduction. Conversely, an increase in the number of hydrogen atoms in a hydrocarbon is often an indication of a reduction. We can illustrate these points by considering how the oxidation state of the carbon atom changes in the series of compounds, which is shown in part a in Figure The number of oxygen atoms or the number of bonds to oxygen changes throughout the series.
Organic chemistry
Hence the conversion of methane to formic acid is an oxidation, whereas the conversion of carbon dioxide to methanol is a reduction. Also, the number of hydrogen atoms increases in going from the most oxidized to least oxidized compound. As expected, as the oxidation state of carbon increases, the carbon becomes a more potent electrophile.
Thus the carbon of CO 2 is a stronger electrophile i. Similarly, in compounds with a carbon—nitrogen bond, the number of bonds between the C and N atoms increases as the oxidation state of the carbon increases part b in Figure We therefore expect the carbon of a nitrile to be a rather strong electrophile.
Write an equation to describe each reaction. Identify the electrophile and the nucleophile in each reaction.
Use the mechanisms described to show how the indicated products are formed from the reactants. The carbon bonded to chlorine is an electrophile because of the highly polar C—Cl bond. Thus the alkene is the nucleophile, and the proton of the acid is the electrophile. In each reaction, identify the electrophile and nucleophile. There are common patterns to how organic reactions occur.
Reaction Guide — Master Organic Chemistry
In a substitution reaction, one atom or a group of atoms in a substance is replaced by another atom or a group of atoms from another substance. Bulky groups that prevent attack cause the reaction to be sterically hindered. In an elimination reaction, adjacent atoms are removed with subsequent formation of a multiple bond and a small molecule.
An addition reaction is the reverse of an elimination reaction. Radical reactions are not very selective and occur in three stages: Oxidation—reduction reactions in organic chemistry are identified by the change in the number of oxygens in the hydrocarbon skeleton or the number of bonds between carbon and oxygen or carbon and nitrogen.
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Substitution Reactions In a substitution reaction , one atom or a group of atoms in a substance is replaced by another atom or group of atoms from another substance. A typical substitution reaction is reacting the hydroxide ion with methyl chloride: Note In nucleophilic substitution reactions, the nucleophile must possess a pair of electrons and have a greater affinity for the electropositive species than the original substituent. They have the general form Figure Note Elimination reactions are similar to cleavage reactions in inorganic compounds.
Addition Reactions A reaction in which the components of a species A—B are added to adjacent atoms across a carbon—carbon multiple bond is called an addition reaction. Note An addition reaction is the reverse of an elimination reaction. Radical Reactions Many important organic reactions involve radicals, such as the combustion of fuels. The overall reaction is as follows: Note Because radicals are highly reactive, radical reactions are usually not very selective.
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Example 6 Write an equation to describe each reaction. Substitution results in the formation of a new C—C bond: Summary The common classes of organic reactions—substitution, elimination, addition, oxidation-reduction, and radical—all involve reacting electrophiles with nucleophiles.
Conceptual Problems Identify the nucleophile and the electrophile in the nucleophilic substitution reaction of 2-bromobutane with KCN. Identify the nucleophile and the electrophile in the nucleophilic substitution reaction of 1-chloropentane with sodium methoxide. Do you expect an elimination reaction to be favored by a strong or a weak base? Structure and Reactivity Sketch the mechanism for the nucleophilic substitution reaction of potassium cyanide with iodoethane.
Sketch the mechanism for the nucleophilic substitution reaction of NaSH with 1-bromopropane.
Sketch the mechanism for the elimination reaction of cyclohexylchloride with potassium ethoxide. Now, we will transition into dissecting the reaction scheme. I discovered that there is a reliable pattern to the way chemists write them. This first scenario has one thing above the arrow and one thing below the arrow. In this case, we have more than one thing above and below the arrow; however, the same rule applies. This reaction means there are two individual steps we need to consider.
What we will need to in this scenario is look at what comes first above the arrow and what comes first below the arrow. The above example is more than likely much more complex that you will see on your exam; however, it is much easier to figure out than what you might first think.
I will discuss in the next section Organic Chemistry Simplified Part 6 how to put all this information together to help you solve questions on an exam or homework. The reaction schemes that I have put together in this post do not have the complete list of reagents for some examples. I will go over these differences in the next post to further explain.
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