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25.4: Organic Reactions

Created by: CK-12

Lesson Objectives

  • Describe and distinguish between substitution reactions and addition reactions, and give examples of each.
  • Relate the concepts of oxidation and reduction to organic reactions.
  • Describe and give examples of condensation reactions, including esterification.
  • Define “polymer,” and describe how polymers are formed by an addition reaction or by a condensation reaction.

Lesson Vocabulary

  • addition polymer
  • addition reaction
  • condensation polymer
  • condensation reaction
  • esterification
  • hydration reaction
  • hydrogenation
  • monomer
  • polymer
  • saponification
  • substitution reaction

Check Your Understanding

Recalling Prior Knowledge

  • What is the difference between saturated and unsaturated hydrocarbons?
  • How are oxidation and reduction defined with respect to the elements oxygen and hydrogen?

Organic reactions require the breaking of strong covalent bonds, which takes a considerable input of energy. In order for relatively stable organic molecules to react at a reasonable rate, they often must be modified with the use of highly reactive materials or in the presence of a catalyst. In this lesson, you will learn about several general categories of organic reactions.

Substitution Reactions

A substitution reaction is a reaction in which one or more atoms in a molecule are replaced with another atom or group of atoms. Alkyl halides are formed by the substitution of a halogen atom for a hydrogen atom. When methane reacts with chlorine gas, ultraviolet light can act as a catalyst for the reaction.

\text{CH}_4(g) + \text{Cl}_2(g) \overset{\text{UV light}}{\longrightarrow} \text{CH}_3\text{Cl}(g) + \text{HCl}(g)

The reaction produces chloromethane and hydrogen chloride. When the mixture is allowed to react for longer periods of time, further substitution reactions may occur.

\text{CH}_3\text{Cl}(g) + \text{Cl}_2(g) \overset{\text{UV light}}{\longrightarrow} \text{CH}_2\text{Cl}_2(g) + \text{HCl}(g)

The product above is dichloromethane. Further substitution produces trichloromethane and tetrachloromethane, commonly called carbon tetrachloride. A mixture of products occurs in the reaction, with the relative amounts dependent upon the time that the reaction is allowed to proceed. Chlorofluorocarbons are produced by reacting chloroalkanes with HF, because the fluorine atom makes a stronger bond to the carbon atom than chlorine does.

\text{CCl}_4(g) + \text{HF}(g) \overset{\text{SbF}_5}{\longrightarrow} \text{CCl}_3\text{F}(g) + \text{HCl}(g)

The fluorine atom substitutes for a chlorine atom in the reaction.

As discussed previously, benzene is a fairly stable and unreactive molecule due to the electron delocalization around the six-membered ring. The treatment of benzene with a halogen in the presence of an iron catalyst causes the substitution of a halogen atom for a hydrogen atom. The resulting molecule is called an aryl halide.

\text{C}_6\text{H}_6(l) + \text{Br}_2(l) \overset{\text{FeBr}_3}{\longrightarrow} \text{C}_6\text{H}_5\text{Br}(l) + \text{HBr}(g)

Alkyl groups can be introduced onto a benzene ring by the reaction of benzene with an alkyl halide, using aluminum chloride as the catalyst. In the reaction below, benzene reacts with chloroethane to produce ethylbenzene.

\text{C}_6\text{H}_6(l) + \text{CH}_3\text{CH}_2\text{Cl}(g) \overset{\text{AlCl}_3}{\longrightarrow} \text{C}_6\text{H}_5\text{CH}_2\text{CH}_3(l) + \text{HCl}(g)

The reaction of an alkyl halide with an inorganic hydroxide base at elevated temperature produces an alcohol. The molecular reaction to produce methanol from iodomethane and sodium hydroxide is shown below.

\text{CH}_3\text{I}(l) + \text{NaOH}(aq) \overset{100^\circ\text{C}}{\longrightarrow} \text{CH}_3\text{OH}(l) + \text{NaI}(aq)

Addition Reactions

An addition reaction is a reaction in which an atom or molecule is added to an unsaturated molecule, making a single product. An addition reaction can often be thought of as adding a molecule across the double bond of an alkene or carbonyl or across the triple bond of an alkyne.

One type of addition reaction is called hydrogenation. Hydrogenation is a reaction that occurs when molecular hydrogen is added to an alkene to produce an alkane. The reaction is typically performed with the use of a transition metal catalyst. For example, ethene reacts with hydrogen to form ethane.

\text{CH}_2\text{=CH}_2(g) + \text{H}_2(g) \overset{\text{Pt}}{\longrightarrow} \text{CH}_3\text{CH}_3(g)

Note that the hydrogenation reaction is also a redox reaction. Ethene is reduced, because the oxidation numbers of the carbon atoms change from −2 to −3 as a result of the reaction.

Vegetable oils consist of long carbon chains with carboxyl groups on the end; these molecules are referred to as fatty acids. The carbon chains of the fatty acids in vegetable oils are unsaturated, usually containing multiple double bonds. When hydrogen gas is blown through a sample of the oil, hydrogen atoms add across the double bonds. This conversion changes the substance from a liquid oil into a solid fat. The word "hydrogenated" on a food product is an indication that oil has been converted into fat by this process. Margarine is manufactured from unsaturated vegetable oil in this way.

Addition reactions are also useful ways to introduce a new functional group into an organic molecule. Alkyl halides can be produced from an alkene by the addition of either the elemental halogen or the hydrogen halide. When the reactant is the halogen, the product is a disubstituted alkyl halide as in the addition of bromine to ethene.

\text{CH}_2\text{=CH}_2(g) + \text{Br}_2(l) \rightarrow \text{CH}_2\text{BrCH}_2\text{Br}(g)

The addition of bromine to an unknown organic compound can be used as a test for unsaturation in the compound. Bromine has a distinctive brownish-orange color, while most bromoalkanes are colorless. When bromine is slowly added to a solution of the compound, the orange color will fade if it undergoes an addition reaction to produce an alkyl halide. If the orange color remains, then the original compound was already saturated, and no reaction occurred.

A monosubstituted alkyl halide can be produced by the addition of a hydrogen halide to an alkene. Shown below is the formation of chloroethane.

\text{CH}_2\text{=CH}_2(g) \text{HCl}(g) \rightarrow \text{CH}_3\text{CH}_2\text{Cl}(g)

A hydration reaction is a reaction in which water is added to an alkene. Hydration reactions can take place when the alkene and water are heated to near 100°C in the presence of a strong acid, which acts as a catalyst. Shown below is the hydration of ethene to produce ethanol.

\text{CH}_2\text{=CH}_2(g) + \text{H}_2\text{O}(l) \rightarrow \text{CH}_3\text{CH}_2\text{OH}(g)

Under modest reaction conditions, benzene resists addition reactions, because adding across a double bond in a benzene ring would disrupt the ring of delocalized electrons, greatly destabilizing the molecule. However, under conditions of high temperature and pressure, and with an appropriate catalyst, benzene will slowly react with three molecules of hydrogen to produce cyclohexane.

\text{C}_6\text{H}_6 + 3\text{H}_2 \overset{\text{Pt}}{\longrightarrow} \text{C}_6\text{H}_{12}

Oxidation Reactions

Oxidation can be defined as the addition of oxygen to a molecule or the removal of hydrogen. Reduction is therefore the addition of hydrogen or the removal of oxygen. Several classes of organic compounds are related to one another by oxidation and reduction reactions. Alkanes, alkenes, and alkynes represent different levels of oxidation of a hydrocarbon. When an alkane is heated in the presence of an appropriate catalyst, it can be oxidized to the corresponding alkene in a reaction called a dehydrogenation reaction. Two hydrogen atoms are removed in the process. The alkene can be further oxidized to an alkyne by the removal of two more hydrogen atoms. This is also considered an oxidation according to the modern definition, because the oxidation number of each carbon atom goes from -3 to -2 to -1.

oxidation: \text{CH}_3\text{CH}_3 \overset{-\text{H}_2}{\longrightarrow} \text{CH}_2=\text{CH}_2 \overset{-\text{H}_2}{\longrightarrow} \text{CH}\equiv\text{CH}

The reactions are reversible, so an alkyne can be reduced first to an alkene and then to an alkane. These are addition reactions, as seen in the previous section.

reduction: \text{CH}\equiv\text{CH} \overset{+\text{H}_2}{\longrightarrow} \text{CH}_2\text{CH}_2 \overset{+\text{H}_2}{\longrightarrow} \text{CH}_3\text{CH}_3

The alkane is the most reduced form of a hydrocarbon, while the alkyne is the most oxidized form.

Oxidation reactions in organic chemistry often involve the addition of oxygen to a compound, which changes the functional group that is present. The following sequence shows how methane can be oxidized first to methanol, then to methanal, then to methanoic acid, and finally to carbon dioxide.

\underset{\text{methane}}{\text{CH}_4} \overset{\text{gain of oxygen}}{\longrightarrow} \underset{\text{methanol}}{\text{CH}_3\text{OH}}  \overset{\text{loss of hydrogen}}{\longrightarrow} \underset{\text{methanal}}{\text{CH}_2\text{O}} \overset{\text{gain of oxygen}}{\longrightarrow} \underset{\text{methanoic acid}}{\text{HCOOH}} \overset{\text{loss of hydrogen}}{\longrightarrow} \underset{\text{carbon dioxide}}{\text{CO}_2}

Each step in the process is either a gain of oxygen or a loss of hydrogen. Each step also releases energy, which explains why the complete combustion of alkanes to carbon dioxide is an extremely exothermic reaction.

The oxidation of an alcohol can produce either an aldehyde or a ketone. Ethanol can be oxidized in the laboratory by slight warming combined with the addition of an oxidizing agent such as the dichromate ion, which catalyzes this reaction in acidic solution. The reaction produces the aldehyde ethanal.

\text{CH}_3\text{CH}_2\text{OH} \overset{\text{Cr}_2\text{O}^{2-}_7}{\underset{\text{H}^+}{\longrightarrow}} \text{CH}_3\text{CHO}

When the alcohol to be oxidized is a secondary alcohol, the oxidation product is a ketone rather than an aldehyde. The oxidation of the simplest secondary alcohol, 2-propanol, yields propanone.

\text{CH}_3\text{CHOHCH}_3 \overset{\text{Cr}_2\text{O}^{2-}_7}{\underset{\text{H}^+}{\longrightarrow}} \text{CH}_3\text{COCH}_3

Tertiary alcohols cannot be oxidized in this way because the carbon to which the hydroxyl group is attached does not have another hydrogen atom attached to it.

When a primary alcohol is oxidized to an aldehyde in the presence of water, the reaction can be difficult to stop because the aldehyde can be further oxidized to the corresponding carboxylic acid. For example, the oxidation of ethanal produces ethanoic (acetic) acid.

\text{CH}_3\text{CHO} \overset{\text{Cr}_2\text{O}^{2-}_7}{\underset{\text{H}^+}{\longrightarrow}} \text{CH}_3\text{COOH}

Ethanol-containing beverages such as wine are susceptible to such oxidation if kept for long periods of time after having been opened and exposed to the air. Wine that has become oxidized will have an unpleasant vinegary taste due to the production of acetic acid.

Unlike aldehydes, ketones are resistant to further oxidation because the carbonyl group is in the middle of the carbon chain, so the ketone cannot be converted to a carboxylic acid.

Condensation Reactions

A condensation reaction is a reaction in which two molecules combine to form a single molecule. A small molecule, often water, is usually removed during a condensation reaction. Amino acids are important biological molecules that have an amine functional group on one end of the molecule and a carboxylic acid functional group on the other end. When two amino acids combine in a condensation reaction, a covalent bond forms between the amine nitrogen of one amino acid and the carboxyl carbon of the second amino acid. A molecule of water is then removed as a second product (Figure below).

Amino acids join together to form a molecule called a dipeptide. The –OH from the carboxyl group of one amino acid combines with a hydrogen atom from the amino group of the other amino acid to produce water (blue).

This reaction forms a molecule called a dipeptide, and the resulting carbon-nitrogen covalent bond is often called a peptide bond. When repeated numerous times, a long molecule called a protein is eventually produced.

Esterification

An esterification is a reaction in which an ester is formed from an alcohol and a carboxylic acid. Esterification is a subcategory of condensation reactions because a water molecule is produced in the reaction. The reaction can be catalyzed by a strong acid, usually sulfuric acid. When the carboxylic acid, butanoic acid, is heated with an excess of methanol and a few drops of sulfuric acid, the ester methyl butanoate is produced. Methyl butanoate has the scent of pineapples. The reaction is shown below with both molecular and structural formulas.

\text{CH}_3\text{CH}_2\text{CH}_2\text{COOH} + \text{HOCH}_3 \overset{\text{H}_2\text{SO}_4}{\longrightarrow} \text{CH}_3\text{CH}_2\text{CH}_2\text{COOCH}_3 + \text{H}_2\text{O}

Esterification reactions are reversible. When an ester is heated in the presence of a strong base such as sodium hydroxide, the ester breaks down. The products are an alcohol and the conjugate base of the carboxylic acid.

\mathrm{\underset{ethyl \ ethanoate}{CH_3COOCH_2CH_3} \ + \ NaOH \rightarrow \underset{sodium \ acetate}{CH_3COO^-Na^+} \ + \ \underset{ethanol}{CH_3CH_2OH}}

The sodium hydroxide is not acting as a catalyst; it is consumed in the reaction. Saponification describes the alkaline hydrolysis reaction of an ester. The term saponification originally described the hydrolysis of long-chain esters called fatty acid esters to produce soap molecules, which are the salts of fatty acids. One such soap molecule is sodium stearate, formed from the hydrolysis of ethyl stearate.

\mathrm{\underset{ethyl \ stearate}{C_{17}H_{35}COOC_2H_5} \ + \ NaOH \rightarrow \underset{sodium \ stearate \ (soap)}{C_{17}H_{35}COO^-Na^+} \ + \ C_2H_5OH}

Polymerization

Polymers are very different than the other kinds of organic molecules that you have seen so far. Whereas other compounds are of relatively low molar mass, polymers are giant molecules of very high molar mass. Polymers are the primary components of all sorts of plastics and related compounds. A polymer is a large molecule formed of many smaller molecules covalently bonded to one another in a repeating pattern. The small molecules that make up the polymer are called monomers. Polymers are generally formed by either addition or condensation reactions.

Addition Polymers

An addition polymer is a polymer formed by addition reactions between monomers that contain a double bond. Molecules of ethene can polymerize with each other under the right conditions to form a polymer called polyethylene.

nCH2=CH2( CH2-CH2 ) n

The letter n stands for the number of monomers that are joined in repeated fashion to make the polymer and can have a value in the hundreds or even thousands.

Polyethylene can have different properties depending on the length of the polymer chains and on how efficiently they pack together. Some common products made from different forms of polyethylene include plastic bottles, plastic bags, and harder plastic objects such as pipes.

Several other kinds of unsaturated monomers can be polymerized and find use in common household products. Polypropylene is stiffer than polyethylene and is used in plastic utensils and some other kinds of containers. Polystyrene is used in insulation and in molded items such as coffee cups. Polyvinyl chloride (PVC) is extensively used for plumbing pipes. Polyisoprene is a polymer of isoprene and is better known as rubber. It is produced naturally by rubber trees, but several variants have been developed which demonstrate improvements on the properties of natural rubber.

Condensation Polymers

A condensation polymer is a polymer formed by condensation reactions. Monomers of condensation polymers must contain two functional groups so that each monomer can link up with two other monomers. One type of a condensation polymer is called a polyamide. An amide is characterized by the functional group shown below, where the carbon of a carbonyl group is bonded to the nitrogen of an amine.

One pair of monomers that can form a polyamide is adipic acid and hexanediamine. Adipic acid is a carboxylic acid with two carboxyl groups on either end of the molecule. Hexanediamine has amino groups on either end of a six-carbon chain. When these molecules react with each other, a molecule of water is eliminated, making it a condensation reaction (Figure below).

Nylon-66 is a condensation polymer formed from the repeated reaction of adipic acid with hexanediamine.

The polymer that results from the repetition of the condensation reaction is a polyamide called nylon-66. Nylon-66 was first invented in 1935 and has been used for all sorts of products. It and other polyamides are commonly found in fibers, clothing, fishing line, carpeting, and many other products (Figure below).

This nylon spatula is a polymer made from the condensation reaction of a carboxylic acid and an amine.

Polyester is another common type of condensation polymer. Recall that esters are formed from the reaction of an alcohol with a carboxylic acid. When both the acid and alcohol have two functional groups, the ester is capable of being polymerized. One such polyester is called polyethylene terephthalate (PET), which is formed from the reaction of ethylene glycol with terephthalic acid. The structure of PET is shown below (Figure below).

PET is used in tires, photographic film, food packaging, and clothing. Polyester fabric is used in permanent-press clothing. Its resistance to wrinkling comes from cross-linking of the polymer strands.

Lesson Summary

  • Substitution reactions are reactions in which an atom or group of atoms in a molecule is replaced. Among other things, these reactions can be used to make alkyl halides from hydrocarbons.
  • In an addition reaction, a molecule is added across a double or triple bond of another organic molecule. Hydrogenation is the process of adding molecular hydrogen, while hydration involves adding water across a double bond to form an alcohol.
  • The oxidation of a hydrocarbon can occur by the loss of hydrogen, which makes it unsaturated. The oxidation of an alcohol produces either an aldehyde or a ketone. Further oxidation of an aldehyde produces a carboxylic acid.
  • In a condensation reaction, two organic molecules combine, producing a larger molecule while eliminating a molecule of water. Esterification is a type of condensation reaction.
  • Polymers are very large molecules made of repeating units called monomers. Polyethylene is a polymer formed by addition reactions. Nylon is a polymer formed by condensation reactions.

Lesson Review Questions

Reviewing Concepts

  1. Can an addition reaction occur between chlorine and propane? Explain why or why not.
  2. Why does the benzene ring tend to undergo substitution reactions rather than addition reactions?
  3. What is the product of the oxidation of a primary alcohol? A secondary alcohol?
  4. What is the requirement for the structure of the monomers that undergo condensation polymerization?

Problems

  1. Write the structural formulas and name the products of the following reactions.
    1. CH3CH=CH2 + Br2
    2. CH4 + I2
    3. CH3CH2CH2CH2Cl + KOH →
    4. CH3CH=CHCH3 + H2O →
    5. C6H6 (benzene) + Cl2
  2. Write the structure and give the name of the compound that would be formed from the oxidation of each of the following.
    1. methanol
    2. 3-pentanol
    3. propanal
  3. Write structures and name the products of the reactions below. Identify the type of reaction represented by both equations.
    1. CH3CH2OH + HCOOH →
    2. CH3OH + CH3(CH2)6COOH →
  4. For the following reaction, write the products and name them. CH3CH2CH2COOCH3 + NaOH →
  5. Consider the following reaction. What kind of reaction is this? What type of compound is the organic product and what is its name? CH3CH2CH2OH + CH3OH → CH3CH2CH2-O-CH3 + H2O
  6. Draw the structure of the repeating units in a polymer that has the following monomers.
    1. 1,2-dichloroethene
    2. propene

Further Reading / Supplemental Links

Points to Consider

Chemistry and chemical reactions are key factors in living things.

  • How can a basic understanding of organic chemistry provide a foundation for studying biological molecules?
  • What are the main categories of biomolecules?

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