2.1. The Main Types of Organic Reactions. Addition, Substitution, and Elimination Reactions

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1 The Main Types of rganic Reactions Figure 2.1(A) shows raw fruit. The crisp, sharp-tasting fruit becomes soft and sweet when it is cooked. Figure 2.1(B) shows a chemist accelerating the tranformation of ethanol into ethanoic acid, by adding potassium dichromate and sulfuric acid. What do these reactions have in common? They are both examples of organic reactions. In this section, you will take a quick look at the main types of organic reactions. You will concentrate on simply recognizing these types of organic reactions. In the next section, you will examine the reactions of specific functional groups and learn how to predict the products of organic reactions. 2.1 Section Preview/ Specific Expectations In this section, you will describe different types of organic reactions, including addition, substitution, elimination, oxidation, and reduction predict the products of organic reactions, including addition, substitution, elimination, oxidation, and reduction communicate your understanding of the following terms: addition reaction, substitution reaction, elimination reaction, oxidation, reduction, condensation reaction, hydrolysis reaction A B Figure 2.1 rganic reactions take place all around you, not only in a science lab. Addition, Substitution, and Elimination Reactions Addition reactions, substitution reactions, and elimination reactions are the three main types of organic reactions. Most organic reactions can be classified as one of these three types. Addition Reactions In an addition reaction, atoms are added to a double or triple bond. ne bond of the multiple bond breaks so that two new bonds can form. To recognize an addition reaction, remember that two compounds usually react to form one major product. (Sometimes two isomers are formed.) The product has more atoms bonded to carbon atoms than the organic reactant did. A general example of addition to an alkene is given below. X Y + XY Addition reactions are common for alkenes and alkynes. Addition reactions can also occur at a bond. Some examples of addition reactions are shown on the next page. hapter 2 Reactions of rganic ompounds MR 57

2 Example 1 Br + Br ethene hydrobromic acid bromoethane Example 2 Br Br Br butyne bromine 1,2-dibromobutene You can express an addition reaction algebraically, using the equation a + b ab. ome up with similar equations for substitution and elimination reactions. Substitution Reactions In a substitution reaction, a hydrogen atom or a functional group is replaced by a different functional group. To help you recognize this type of reaction, remember that two compounds usually react to form two different products. The organic reactant(s) and the organic product(s) have the same number of atoms bonded to carbon. X + AY Y + AX Alcohols, alkyl halides, and aromatic compounds commonly undergo substitution reactions, as shown in these examples. Example I 3 2 I + ethanol hydroiodic acid iodoethane water Example 2 Br N N Br Example 3 2-bromobutane ammonia 2-butanamine hydrobromic acid N 2 + N 2 + benzene nitrous acid nitrobenzene water 58 MR Unit 1 rganic hemistry

3 Elimination Reactions In an elimination reaction, atoms are removed from a molecule to form a double bond. This type of reaction is the reverse of an addition reaction. ne reactant usually breaks up to give two products. The organic product typically has fewer atoms bonded to carbon atoms than the organic reactant did. X Y + XY Alcohols often undergo elimination reactions when they are heated in the presence of strong acids, such as sulfuric acid, 2 S 4, which acts as a catalyst. (See the first example below.) Alkyl halides also undergo elimination reactions to produce alkenes. (See the second example.) Example 1 2 S propanol propene water In the first example of an elimination reaction, the strong acid, 2 S 4, does not count as a reactant. It is not directly involved in the reaction. It is a catalyst : a compound that speeds up a reaction but is not consumed by it. Example 2 Br + Br bromoethane ethene hydrobromic acid xidation and Reduction An important type of organic reaction occurs when there is a change in the number of hydrogen or oxygen atoms that are bonded to carbon. In Unit 5, you will take a close look at oxidation-reduction reactions in terms of the transfer of electrons. As you will learn, oxidation and reduction always occur together. ne reactant is oxidized while the other reactant is reduced. In this unit, however, you will focus on the organic reactant only. Therefore, you will deal with oxidation and reduction separately, as they apply to organic compounds. In organic chemistry, oxidation and reduction are defined by the changes of the bonds to carbon atoms in the organic reactant. hapter 2 Reactions of rganic ompounds MR 59

4 xidation In organic chemistry, oxidation is defined as a reaction in which a carbon atom forms more bonds to oxygen,, or less bonds to hydrogen,. An oxidation that involves the formation of double bonds may also be classified as an elimination reaction. For example, alcohols can be oxidized to produce aldehydes and ketones. xidation occurs when an organic compound reacts with an oxidizing agent. ommon oxidizing agents include acidified potassium permanganate, KMn 4, acidified potassium dichromate, K 2 r 2 7, and ozone, 3. The symbol [] is used to symbolize an oxidizing agent, as shown below. Note that equations for the oxidation of organic compounds are often left unbalanced. The purpose of the equation is to show the changes in the organic reactant only. + [] alcohol oxidizing agent aldehyde or ketone + [] aldehyde oxidizing agent carboxylic acid To identify an oxidation, count and compare the number of and bonds in both the reactant and product. Try it for the following example. + [] ethanol ethanal Reduction In organic chemistry, reduction is defined as a reaction in which a carbon atom forms fewer bonds to oxygen,, or more bonds to hydrogen,. ften, a bond or bond is reduced to a single bond by reduction. A reduction that transforms double or bonds to single bonds may also be classified as an addition reaction. Aldehydes, ketones, and carboxylic acids can be reduced to become alcohols. Alkenes and alkynes can be reduced by the addition of 2 to become alkanes. Reduction occurs when an organic compound reacts with a reducing agent. ommon reducing agents are lithium aluminum hydride, LiAl 4, and hydrogen gas over a platinum catalyst, 2 /Pt. The symbol [] is used to symbolize a reducing agent. As is the case for oxidation, equations showing the reduction of organic compounds are often left unbalanced. 60 MR Unit 1 rganic hemistry

5 + [] aldehyde or ketone reducing agent alcohol + [] alkene reducing agent alkane To identify a reduction, count and compare the number of and bonds in both the reactant and the product. Try it for the following example [] 3 3 ther Important rganic Reactions In this chapter, you will also encounter the following classes of organic reactions: condensation reactions and hydrolysis reactions. ondensation and hydrolysis reactions are both types of substitution reactions. ondensation Reactions In a condensation reaction, two organic molecules combine to form a single organic molecule. A small molecule, usually water, is produced during the reaction. For example, a carboxylic acid and an alcohol can condense to form an ester. propanone 3 2-propanol EM FAT When you see two arrows that point in opposite directions in a chemical equation, the reaction can proceed in both directions. This type of reaction is called an equilibrium reaction. You will learn more about equilibrium reactions in Unit S carboxylic acid alcohol ester water ydrolysis Reactions In a hydrolysis reaction, water adds to a bond, splitting it in two. This reaction is the reverse of a condensation reaction. For example, water can add to an ester or amide bond. A carboxylic acid and an alcohol are produced if an ester bond is hydrolyzed, as shown in the example below. A carboxylic acid and an amine are produced if an amide bond is hydrolyzed S ester water carboxylic acid alcohol The following Sample Problems show how to identify different types of organic reactions. 3 hapter 2 Reactions of rganic ompounds MR 61

6 Sample Problem Identifying Addition, Substitution, and Elimination Reactions Problem Identify each reaction as an addition, substitution, or elimination reaction (c) 3 ( 3 ) 2 2 Br + Na 3 ( 3 ) NaBr Solution A double bond is formed. ne reactant becomes two products. The organic product has fewer atoms bonded to carbon. Thus, this reaction is an elimination reaction. A double bond becomes a single bond. Two reactants become one product. The organic product has more atoms bonded to carbon. Thus, this reaction is an addition reaction. (c) No double bond is broken or formed. Two reactants form two products. An atom in the organic reactant (Br) is replaced by a different group of atoms (). Thus, this is a substitution reaction. Sample Problem Identifying xidation and Reduction Problem Identify each reaction as an oxidation or a reduction. (The oxidizing and reducing agents are not shown.) Solution ount the number of bonds and bonds in the reactant and in the product. Reactant: 8 bonds, 2 bonds (or 1 bond) Product: 9 bonds, 1 bond The product has gained a bond and lost a bond. Thus, this is a reduction. ount the number of bonds and bonds in the reactant and in the product. Although the bonds are not shown, you know that a carbon atom forms a total of four bonds. Reactant: 11 bonds, 1 bond Product: 10 bonds, 2 bonds (or 1 bond) The product has lost a bond and gained a bond. Thus, this is an oxidation. 62 MR Unit 1 rganic hemistry

7 Practice Problems 1. Identify each reaction as an addition, substitution, or elimination reaction. Br Br l l l l (c) + l 2. Identify each reaction as an oxidation or a reduction. The oxidizing and reducing agents are not shown (c) 3. lassify each reaction in two different ways: for example, as oxidation and as an elimination reaction Identify the type of reaction hapter 2 Reactions of rganic ompounds MR 63

8 Section Summary In this section, you were introduced to some of the main types of organic reactions: addition, substitution, and elimination reactions; oxidation and reduction; and condensation and hydrolysis reactions. In the next section, you will take a close look at each type of reaction. You will find out how organic compounds, such as alcohols and carboxylic acids, can react in several different ways. Section Review 1 K/U Identify each reaction as an addition, substitution, or elimination reaction. Br Br Br (c) FeBr 3 + l 2 + l (d) Br + 2 N N Br K/U Identify each reaction as an oxidation or a reduction. (xidizing and reducing agents are not shown.) l (c) (d) (e) 3 K/U Draw a complete structural diagram for each compound that is involved in the following reaction. N-ethylpentanamide + water pentanoic acid + ethanamine K/U Is this reaction a condensation reaction or a hydrolysis reaction? Explain your reasoning. In your own words, describe each type of organic reaction. Include an example for each type. addition substitution (c) elimination The following reaction can be classified as an addition reaction. It can also be classified as a reduction. Explain why this reaction fits into both classes MR Unit 1 rganic hemistry