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25.3: Organic Reactions

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Lesson Objectives

  • Be able to write reactions for:
    • Hydrogenation of an alkene or alkyne.
    • Halogenation of an alkene.
    • Dehydrohalogenation of an alkene.
    • Dehydration of an alcohol.
    • Oxidation of an alcohol.
    • Ester synthesis and hydrolysis.
    • Amide synthesis and hydrolysis.
    • Addition reactions leading to the formation of polymers.
    • Condensation reactions leading to the formation of polymers.
  • Be able to list some common polymers and describe their uses.

Lesson Vocabulary

  • dehydration: A type of elimination reaction that involves the loss of a water molecule.
  • dehydrohalogenation: A reaction between an alkyl halide and an alcoholic alkali resulting in an alkene.
  • halogenation: A reaction in which one or more of the hydrogen atoms in an organic compound is replaced by a halogen.
  • hydrogenation: A reaction in which a hydrogen atom is added to a molecule, yielding a saturated compound when added to an organic molecule.
  • hydrolysis: A type of decomposition reaction in which water is used to break chemical bonds, often the formation of an acid or a base from a salt and water.
  • monomer: A chemical structure that represents building blocks used to construct more complex compounds.
  • polymer: A chemical structure built from a number of combined monomers.

Check Your Understanding

Recalling Prior Knowledge

  • Which parts of an organic molecule are considered functional groups, and why?

Introduction

With over twenty million organic compounds to keep track of, we need some way to organize information other than simply memorizing reactions for each substance. The presence of functional groups allows us to group together similar modes of chemical reactivity. Instead of learning the reactions that can be performed with a given compound, we learn the reactions that can be performed on a given functional group. Although there are always special cases, specific sets of reaction conditions tend to cause the same changes to a certain functional group regardless of what the rest of the reactant molecule looks like.

Although there are thousands of different types of organic reactions, many of the more complex ones can be broken down and understood in terms of simpler processes, such as addition, elimination, and substitution reactions. We will look at a small sampling of the most common processes that occur during organic reactions. In each case, we will focus on how one functional group is changed into another under a certain set of conditions.

Additions Across a Double Bond

There are two types of double bonds we will look at in terms of reactions, the C=C double bond and the C=O double bond. Because one of these bonds is polarized and the other is not, they differ quite a bit in terms of the conditions under which they are reactive. However, the overall change from reactant to product has the same form in either case; the double bond becomes a single bond, and a new atom or group of atoms is attached to each of the two atoms originally involved in the double bond.

Addition of Hydrogen

Hydrogenation is a very common addition reaction that involves the net addition of H2 across a double bond. Although the actual reaction mechanism for such a process can be quite complicated, the overall transformation is relatively simple. Each atom involved in the double bond is attached to a new hydrogen atom, and the double bond becomes a single bond.

For alkenes and alkynes (unpolarized multiple bonds), this process is generally accomplished by mixing the starting material and H2 gas in the presence of a metal catalyst (often Pt or Pd, although some cheaper metals can also be effective under certain conditions). An alkene can combine with one equivalent of H2 gas to form an unfunctionalized alkane. Similarly, an alkyne can combine with two equivalents of H2 (one for each pi bond) to make an alkane.

C=O bonds can also undergo a net addition of H2, but the reactants are often quite different. In fact, many conditions for adding hydrogen to an alkene will not affect a C=O bond at all, and vice versa. Complementary reactivity patterns like this allow chemists to selectively change one portion of a molecule without altering the rest.

In our chapter on Oxidation-Reduction Reactions, we learned that one definition of reduction is the addition of hydrogen atoms. The addition of H2 across a double bond is an example of this type of reduction reaction.

Addition of Halogens

Halogenation is analogous to hydrogenation, except that instead of adding a hydrogen atom to each side of the double bond, we add a halogen atom instead. In Figure below, we see that adding molecular bromine to an alkene results in the formation of a dibrominated product. Unlike hydrogenation, no catalyst is required for this reaction to occur. Again, using these same conditions on a C=O double bond will not produce the analogous addition product due to the different reactivity of these two functional groups.

Halogenation of an alkene.

Formation of Double Bonds

Formation of C=C bonds

Dehydrohalogenation of an alkane.

Dehydration of an alcohol.

Formation of C=O bonds

Oxidation of a primary alcohol.

Recall that one definition of oxidation is the loss of hydrogen atoms. Oxidation of a primary alcohol will first lead to the formation of an aldehyde. This process is the net removal of H2 (one from oxygen and one from carbon), leaving behind a double bond. In other words, this oxidation is the opposite of the hydrogenation (reduction) reaction that we looked at previously. If water is present, or if we use a strong enough oxidizing agent, the aldehyde will be further oxidized to the carboxylic acid. This fits with the idea that some oxidation reactions involve the literal addition of oxygen atoms to the molecule.

A secondary alcohol forms a ketone when oxidized. The secondary alcohol cannot be further oxidized to produce a carboxylic acid. Tertiary alcohols cannot be oxidized in this way, because the carbon atom bonded to the OH group is not also bonded to any hydrogens.

Oxidation of a secondary alcohol.

Formation and Hydrolysis of Carboxylic Acid Derivatives

Esters

In the presence of a strong acid catalyst, such as sulfuric acid, an alcohol and a carboxylic acid can combine to form an ester and water.

Ester formation from a carboxylic acid and an alcohol.

This reaction is reversible. The reverse reaction, in which water adds to an ester in order to form an acid and an alcohol, is referred to as hydrolysis of the ester. Because the reverse reaction happens so readily, we need to manipulate this equilibrium in order to achieve a high yield of the ester product. For example, if we increase the amount of one of the starting materials, the reaction will be shifted in the direction of ester formation according to Le Chatelier's Principle. Usually, the less expensive starting material would be used in excess.

Conversely, we can also remove one of the products (generally water) in order to drive the reaction forward. If the reaction is run above the boiling point of water, the H2O product will leave the system as a gas, shifting the equilibrium to the right. Chemical drying agents also provide ways to remove water from active participation in the equilibrium.

Amides

A similar reaction can occur between a carboxylic acid and an amine:

Amide synthesis.

We can see from the diagram that the –OH of the carboxyl group and a hydrogen from the amine form water as a byproduct. The nitrogen will attach to the carboxyl carbon in the same way the oxygen atom of the alcohol did in the ester synthesis. The reaction shown above uses a primary amine, which has two hydrogen atoms that could potentially be removed to form the final product. The same process can also occur with a secondary amine, which still has one hydrogen attached to the nitrogen. Tertiary amines will not form amides, because the new bond to the carbonyl carbon must replace an existing N-H bond.

The reverse reaction (amide hydrolysis) is very useful in the study of protein structure. Proteins are long chains of amino acids (each amino acid contains an amine group and a carboxyl group, both attached to a central carbon atom). The amino acids are linked together by amide bonds to form the long protein chain. One of the techniques for looking at protein structure is to break those amide linkages so we can learn the identity of the amino acids in the chain. We do this through a hydrolysis reaction:

Hydrolysis of an amide linkage.

Usually, an acid such as HCl is used for the hydrolysis. The amino acids can then be separated and identified. More complex reaction conditions allow the amino acids to be broken off one by one, allowing the amino acid sequence of an unknown protein to be determined. The three-dimensional structures and functions of large protein molecules are ultimately determined by the sequence of amino acids from which they are constructed.

Organic Polymers

What Are Polymers?

Polymers are a pervasive part of modern life. It is very likely that at least some part of your clothing is made of nylon, rayon, or polyester. The milk or juice that you have for breakfast often comes in a polyethylene container. If you don’t have breakfast at home, you might get coffee from a fast food establishment, where your order is delivered to you in a Styrofoam® container. While getting out of your car, you bang the door on another vehicle. One reason you may not have dented the door is the fact that it could be made out of a polymeric plastic material that resists deformation more than simple metallic structures. Elsewhere on your car, both synthetic and natural rubber are probably present in the tires. All these materials are examples of organic polymers.

Polymers are long-chain organic molecules made up of many smaller subunits (called monomers) that are connected together in a repeating pattern. Some polymers are constructed by living organisms, such as starch, cellulose, proteins, and DNA. These natural polymers are covered in the following chapter on Biochemistry. In this section, we will focus on synthetic polymers that are made in the laboratory.

Important Synthetic Polymers

One widely used polymer is polystyrene:

Polystyrene structure.

This polymer is used for many purposes. We find it in Styrofoam® packaging materials, plastic cutlery, CD “jewel cases”, laboratory ware, and a wide variety of other applications. Polystyrene can take on many forms, depending on how it is manufactured. It is inexpensive, easy to work with, and fairly durable.

Polyethylene can come in two forms. The low-density form is used for squeeze bottles, insulation for wiring, and flexible pipes. The high-density form is employed in bottles, pipes, and plastic bags.

Repeating unit of polyethylene.

Polyethylene has the simplest structure of all the polymers; it is essentially just a very long linear alkane. Its repeating unit is shown above.

Polytetrafluoroethylene (Teflon®) is very chemically inert and non-wettable – water does not stick to it. This polymer has found broad applications in cookware because materials do not stick to it. Since it does not react with other materials, Teflon storage containers are used to package highly reactive or corrosive chemicals.

Repeating unit of polytetrafluoroethylene.

All of these polymers (and many more) are referred to as homopolymers, because they are composed of a single repeating unit. There is another class of polymers called copolymers. This group contains two different repeating units in its structure. Kevlar®, used in body armor, is one such copolymer:

Structure of Kevlar®.

One of the repeating units is a substituted benzene ring with attached amine groups, while the other repeating unit contains carbonyl groups connected to the benzene ring. In addition to the strong covalent bonds that link these monomers together, the polymeric chains interact with one another via hydrogen bonds, further increasing the strength of the material.

Synthesis of Polymers

We will now look at two of the reactions that are commonly used to create synthetic polymers. One way to form polymers is through addition reactions. If each monomer contains an alkene or alkyne, various methods can be used to connect each side of the multiple bond with one of the other monomers, resulting in a long chain.

Steps in addition reaction.

One way to form an addition polymer is through a radical chain reaction. The steps in this process are as follows:

  1. A free radical initiator attacks the carbon-carbon double bond. The initiator can be something like hydrogen peroxide, which can be split with light or heat to form two radical species, each of which contains a reactive, unpaired electron (H-O-O-H → 2 H-O). This free radical attacks a carbon-carbon double bond. The initiator binds to one of the carbon atoms using its unpaired electron and one of the pi electrons from the double bond. The other pi electron forms a new free radical on the other carbon atom.
  2. The new free radical adds to another alkene, generating yet another unpaired electron and continuing the process of chain growth.
  3. Termination occurs whenever two free radicals meet. The two unpaired electrons form a covalent bond, ending the chain reaction.

The other general category of polymer synthesis reactions are the condensation reactions. In these situations, a reaction occurs between two different functional groups with the expulsion of water. For example, polyesters and polyamides can be formed via the condensation reactions discussed earlier in this lesson.

A condensation reaction.

Another condensation reaction can be seen in the synthesis of Kevlar®:

Synthesis of Kevlar®.

In this case, we see the formation of amide bonds between amines and acyl halides (which can be thought of as "activated" carboxylic acids). Note that each molecule must have two functional groups in order for this type of polymerization to occur.

Lesson Summary

  • Alkenes and alkynes can be reduced with hydrogen gas to form alkanes.
  • Alkenes can react with halogens to form dihalides.
  • Alkenes can react with water to form alcohols.
  • Alcohols can lose water to form alkenes.
  • Alcohols can be oxidized to aldehydes or ketones.
  • Aldehydes can be oxidized to carboxylic acids.
  • Carboxylic acids can form esters with alcohols.
  • Carboxylic acids can form amides with amines.
  • Polymers are long-chain materials with useful properties.

Lesson Review Questions

  1. Name the product (class of compound) formed in the following reactions:
    1. alcohol + carboxylic acid
    2. primary alcohol + oxidizing agent
    3. ketone + oxidizing agent
    4. alkene + hydrogen + metal catalyst
  2. What class of compound(s) would be used to form the following product:
    1. amide
    2. ketone
    3. halogenated alkane
    4. alcohol
  3. Draw the structure of the product formed in the following reactions:
    1. CH3CH2CH=CHCH3 + Cl2
    2. CH3CH2CHOHCH3+ oxidizing agent

Further Reading/Supplementary Links

Points to Consider

  • What do reactions in living cells have in common with these organic reactions? How are they similar and how are they different?

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Date Created:

Sep 09, 2013

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Sep 05, 2014
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CK.SCI.ENG.SE.1.Chemistry-Basic.25.3

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