Alkenes: Reactions and Synthesis
CHM 247 CH.8
Alkenes: Reactions and Synthesis •
Important reactions include the addition of a halogen to give a 1,2-dihalide, addition of a hypohalous acid to give a halohydrin, addition of water to give an alcohol, addition of hydrogen to give a halohydrin, addition of a single oxygen to give a 3-membered cyclic ether called an epoxide, and addition of 2 hydroxyl groups to give a 1,2-diol.
8.1 Preparing Alkenes: A Preview of Elimination Reactions • Alkenes are readily available from simple precursors. • Preparation of alkenes dominated by elimination reactions. • Addition tends to break double bonds, where as elimination tends to form them. • Two most common elimination reactions are dehydrohalogenation – the loss of HX from an alkyl halide – and dehydration – the loss of water from an alcohol. • Dehydrohalogenation usually occurs by reaction of an alkyl halide with strong base such as potassium hydroxide.
• •
Dehydration is often carried out in the laboratory by treatment of an alcohol with a strong acid.
• 8.2 Halogenation of Alkenes: Addition of X2 • Bromine and chlorine add rapidly to alkenes to yield 1,2-dihalides, a process called halogenation. • Possible mechanism below. Electrophilic addition of Br+ to the alkene, giving a carbocation intermediate that could undergo further reaction with Br- to yield the dibromo addition product. • Stereochemistry important with these reactions though. • Anti stereochemistry: The opposite of syn. An anti addition reaction is one in which the 2 ends of the double bond are attacked from different sides. An anti elimination reaction is one in which the two groups leave from opposite sides of the molecule.
• • •
• •
In CH2Cl2 Stereochemistry observed resulted in an explanation involving a bromonium ion, R2Br+, being formed instead of a carbocation. Similarly, a chloronium ion is formed with chlorine. Bromonium ion: A species with a divalent, positively charged bromine, R2Br+.
The first bromine joins on one side to both carbons. This blocks that side from having the other bromine come in. Therefore, the second bromine has to come in from the top. The result is anti stereochemistry.
8.3 Halohydrins from Alkenes: Addition of HOX • Halohydrin: A 1,2-haloalcohol, such as that obtained on addition of HOBr to an alkene. • Electrophilic addition is the reaction of alkenes with the hypothalous acids HO-Cl or HO-Br to yield 1,2-halo alcohols, or halohydrins. • Addition done indirectly by the reaction of an alkene with either Br2 or Cl2 in the presence of water. • Br2 only reacts to form a dibromide, but if the reaction is arried out in the presence of an additional nucleophile, the intermediate bromonium ion can be intercepted by the added nucleophile and diverted to a different product.
• •
• •
In H2O The alkene reacts with Br2 to form a bromonium ion. The bromonium ion is then attacked by water. Water now has a positive charged on the molecule, and another water molecule comes in to take off an extra hydrogen, making the product plus a hydronium ion. The reaction is often carried out using NBS as a source of bromine, and water along with DMSO (dimethyl sulfoxide) as a solvent. This reaction is the same as above, just safer. Aromatic rings are more stable and less reactive causing the double bonds to be broken to be the ones not present in a ring.
8.4 Hydration of Alkenes: Addition of H2O by Oxymercuration • Water adds to alkenes to yield alcohol, a process called hydration. Takes place in water in the presence of a strong acid catalyst like H2SO4.
• • •
•
•
Requires H2SO4 or H3PO4, H2O. High temperatures also needed. Oxymercuration: A method for double-bond hydration by reaction of an alkene with aqueous mercuric acetate followed by treatment with NaBH4.
8.5 Hydration of Alkenes: Addition of H2O by Hydroboration • Hydroboration: Addition of borane (BH3) or an alkylborane to an alkene. The resultant trialkylborane products can be oxidized to yield alcohols.
• • •
• • •
• •
Hydroboration yields the non-Markovnikov product. Borane is an electrophile. It will bond to the electrons on the O in THF. When an alkene reacts with BH3 in THF solution, rapid addition to the double bond occurs 3 times and a trialkylborane, R3B, is formed.
Syn stereochemistry: The opposite of anti. A syn addition reaction is one in which the two ends of the double bond react from the same side. A syn elimination is one in which 2 groups leave from the same side of the molecule. Hydroboration has syn stereochemistry as well as anti-Markovnikov addition.
Hydroboration occurs in a single step without a carbocation intermediate. Because both C-H and C-B bonds form at the same time and from the same face of the alkene, syn stereochemistry occurs. Non-Markovnikov regiochemistry occurs because attachment of boron is favoured at the less sterically crowded carbon atom of the alkene rather than at the more crowded carbon.
8.6 Reduction of Alkenes: Hydrogenation • Hydrogenation: Addition of hydrogen to a double or triple bond to yield a saturated product. • Alkenes react with H2 in the presence of a metal catalyst such as palladium or platinum to yield the corresponding saturates alkane addition products. • Reduction: A reaction that causes an increase of electron ownership by carbon, either by bond-breaking between carbon and a more electronegative atom or by bond formation between carbon and a less electronegative atom. • Reductions increase electron density on carbon by either forming a C-H bond or breaking a C-O, C-N, or C-X bond. • Hydrogenation occurs with syn stereochemistry.
•
•
•
As a result of needing to bind to the surface, binding can often occur on one side only.
• 8.7 Oxidation of Alkenes: Epoxidation and Hydroxylation • Oxidation: A reaction that causes a decrease in electron ownership by carbon, either by bond formation between carbon and a more electronegative atoms (usually oxygen, nitrogen, or a halogen) or by bond-breaking between carbon and a less electronegative atom (usually hydrogen). • In oxidation, a decrease of electron density on carbon can be from forming C-O, C-N, or C-X bonds or by breaking C-H bonds. • Epoxide: A 3-membered-ring ether functional group.
• • •
• • • •
•
Epoxides have syn stereochemistry. Methods involve CH2Cl2 solvent OR the following:
Glycol: A diol, such as ethylene glycol, HOCH2CH2OH. Hydroxylation: Addition of 2-OH groups to a double bond. Epoxides can undergo an acid-catalyzed ring opening reaction with water to give the corresponding 1,2-dialcohol, or diol.
• • •
• •
Epoxidation followed by hydroxylation leads to a diol that has anti addition. Hydroxylation can be carried out directly without going through an epoxidation by treating an alkene with osmium tetroxide, OsO4. This reaction occurs with SYN stereochemistry.
OsO4 is expensive and very toxic, so the reaction is carried out oftenly in the presence of NMO. The initially formed osmate intermediate reacts rapidly with NMO to yield the product diol plus N-Methylmorpholine and reoxidized OsO4.
• 8.8 Oxidation of Alkenes: Cleavage to Carbonyl Compounds • Ozone is a powerful oxidizing reagent that will cleave a C=C bond in two. • Ozonide: The product initially formed by addition of ozone to a carbon-carbon double bond. Ozonides are usually treated with a reducing agent, such as zinc in acetic acid, to produce carbonyl benzenes. • Using ozone can produce ketones or aldehydes.
• •
• • •
Other agents can cut alkenes as well.
When using potassium permanganate, if hydrogens are present on the double bond, carboxylic acids are produced; if 2 hydrogens are present in one carbon, CO2 is formed. From a diol, a cleavage can occur as well.
• 8.9 Addition of Carbenes to Alkenes: Cyclopropane Synthesis • Carbene, R2C: : A neutral substance that contains a divalent carbon atom having only 6 electrons in its outer shell (R2C:). • Reaction of an alkene with a carbine yields a cyclopropane. The reaction occurs with no intermediates.
• •
Simplest ways for generating a substituted carbine is by treatment of chloroform, CHCl3, with a strong base such as KOH.
• • •
• • •
•
If dichlorocarbene is generated in the presence of an alkene, addition to the double bond occurs and a dichlorocyclopropane is formed. Stereospecific: A term indicating that only a single stereoisomer is produced in a given reaction rather than a mixture.
Only cis is formed in this reaction with the carbon. The chlorines can be in any way. Simmons-Smith reaction: The reaction of an alkene with CH2I2 and Zn-Cu to yield a cyclopropane.
Compound in g is called mCBA
3a) ozonolysis
Alkenes: Reactions and Synthesis •
Important reactions include the addition of a halogen to give a 1,2-dihalide, addition of a hypohalous acid to give a halohydrin, addition of water to give an alcohol, addition of hydrogen to give a halohydrin, addition of a single oxygen to give a 3-membered cyclic ether called an epoxide, and addition of 2 hydroxyl groups to give a 1,2-diol.
8.1 Preparing Alkenes: A Preview of Elimination Reactions • Alkenes are readily available from simple precursors. • Preparation of alkenes dominated by elimination reactions. • Addition tends to break double bonds, where as elimination tends to form them. • Two most common elimination reactions are dehydrohalogenation – the loss of HX from an alkyl halide – and dehydration – the loss of water from an alcohol. • Dehydrohalogenation usually occurs by reaction of an alkyl halide with strong base such as potassium hydroxide.
• •
Dehydration is often carried out in the laboratory by treatment of an alcohol with a strong acid.
• 8.2 Halogenation of Alkenes: Addition of X2 • Bromine and chlorine add rapidly to alkenes to yield 1,2-dihalides, a process called halogenation. • Possible mechanism below. Electrophilic addition of Br+ to the alkene, giving a carbocation intermediate that could undergo further reaction with Br- to yield the dibromo addition product. • Stereochemistry important with these reactions though. • Anti stereochemistry: The opposite of syn. An anti addition reaction is one in which the 2 ends of the double bond are attacked from different sides. An anti elimination reaction is one in which the two groups leave from opposite sides of the molecule.
• • •
• •
In CH2Cl2 Stereochemistry observed resulted in an explanation involving a bromonium ion, R2Br+, being formed instead of a carbocation. Similarly, a chloronium ion is formed with chlorine. Bromonium ion: A species with a divalent, positively charged bromine, R2Br+.
The first bromine joins on one side to both carbons. This blocks that side from having the other bromine come in. Therefore, the second bromine has to come in from the top. The result is anti stereochemistry.
8.3 Halohydrins from Alkenes: Addition of HOX • Halohydrin: A 1,2-haloalcohol, such as that obtained on addition of HOBr to an alkene. • Electrophilic addition is the reaction of alkenes with the hypothalous acids HO-Cl or HO-Br to yield 1,2-halo alcohols, or halohydrins. • Addition done indirectly by the reaction of an alkene with either Br2 or Cl2 in the presence of water. • Br2 only reacts to form a dibromide, but if the reaction is arried out in the presence of an additional nucleophile, the intermediate bromonium ion can be intercepted by the added nucleophile and diverted to a different product.
• •
• •
In H2O The alkene reacts with Br2 to form a bromonium ion. The bromonium ion is then attacked by water. Water now has a positive charged on the molecule, and another water molecule comes in to take off an extra hydrogen, making the product plus a hydronium ion. The reaction is often carried out using NBS as a source of bromine, and water along with DMSO (dimethyl sulfoxide) as a solvent. This reaction is the same as above, just safer. Aromatic rings are more stable and less reactive causing the double bonds to be broken to be the ones not present in a ring.
8.4 Hydration of Alkenes: Addition of H2O by Oxymercuration • Water adds to alkenes to yield alcohol, a process called hydration. Takes place in water in the presence of a strong acid catalyst like H2SO4.
• • •
•
•
Requires H2SO4 or H3PO4, H2O. High temperatures also needed. Oxymercuration: A method for double-bond hydration by reaction of an alkene with aqueous mercuric acetate followed by treatment with NaBH4.
8.5 Hydration of Alkenes: Addition of H2O by Hydroboration • Hydroboration: Addition of borane (BH3) or an alkylborane to an alkene. The resultant trialkylborane products can be oxidized to yield alcohols.
• • •
• • •
• •
Hydroboration yields the non-Markovnikov product. Borane is an electrophile. It will bond to the electrons on the O in THF. When an alkene reacts with BH3 in THF solution, rapid addition to the double bond occurs 3 times and a trialkylborane, R3B, is formed.
Syn stereochemistry: The opposite of anti. A syn addition reaction is one in which the two ends of the double bond react from the same side. A syn elimination is one in which 2 groups leave from the same side of the molecule. Hydroboration has syn stereochemistry as well as anti-Markovnikov addition.
Hydroboration occurs in a single step without a carbocation intermediate. Because both C-H and C-B bonds form at the same time and from the same face of the alkene, syn stereochemistry occurs. Non-Markovnikov regiochemistry occurs because attachment of boron is favoured at the less sterically crowded carbon atom of the alkene rather than at the more crowded carbon.
8.6 Reduction of Alkenes: Hydrogenation • Hydrogenation: Addition of hydrogen to a double or triple bond to yield a saturated product. • Alkenes react with H2 in the presence of a metal catalyst such as palladium or platinum to yield the corresponding saturates alkane addition products. • Reduction: A reaction that causes an increase of electron ownership by carbon, either by bond-breaking between carbon and a more electronegative atom or by bond formation between carbon and a less electronegative atom. • Reductions increase electron density on carbon by either forming a C-H bond or breaking a C-O, C-N, or C-X bond. • Hydrogenation occurs with syn stereochemistry.
•
•
•
As a result of needing to bind to the surface, binding can often occur on one side only.
• 8.7 Oxidation of Alkenes: Epoxidation and Hydroxylation • Oxidation: A reaction that causes a decrease in electron ownership by carbon, either by bond formation between carbon and a more electronegative atoms (usually oxygen, nitrogen, or a halogen) or by bond-breaking between carbon and a less electronegative atom (usually hydrogen). • In oxidation, a decrease of electron density on carbon can be from forming C-O, C-N, or C-X bonds or by breaking C-H bonds. • Epoxide: A 3-membered-ring ether functional group.
• • •
• • • •
•
Epoxides have syn stereochemistry. Methods involve CH2Cl2 solvent OR the following:
Glycol: A diol, such as ethylene glycol, HOCH2CH2OH. Hydroxylation: Addition of 2-OH groups to a double bond. Epoxides can undergo an acid-catalyzed ring opening reaction with water to give the corresponding 1,2-dialcohol, or diol.
• • •
• •
Epoxidation followed by hydroxylation leads to a diol that has anti addition. Hydroxylation can be carried out directly without going through an epoxidation by treating an alkene with osmium tetroxide, OsO4. This reaction occurs with SYN stereochemistry.
OsO4 is expensive and very toxic, so the reaction is carried out oftenly in the presence of NMO. The initially formed osmate intermediate reacts rapidly with NMO to yield the product diol plus N-Methylmorpholine and reoxidized OsO4.
• 8.8 Oxidation of Alkenes: Cleavage to Carbonyl Compounds • Ozone is a powerful oxidizing reagent that will cleave a C=C bond in two. • Ozonide: The product initially formed by addition of ozone to a carbon-carbon double bond. Ozonides are usually treated with a reducing agent, such as zinc in acetic acid, to produce carbonyl benzenes. • Using ozone can produce ketones or aldehydes.
• •
• • •
Other agents can cut alkenes as well.
When using potassium permanganate, if hydrogens are present on the double bond, carboxylic acids are produced; if 2 hydrogens are present in one carbon, CO2 is formed. From a diol, a cleavage can occur as well.
• 8.9 Addition of Carbenes to Alkenes: Cyclopropane Synthesis • Carbene, R2C: : A neutral substance that contains a divalent carbon atom having only 6 electrons in its outer shell (R2C:). • Reaction of an alkene with a carbine yields a cyclopropane. The reaction occurs with no intermediates.
• •
Simplest ways for generating a substituted carbine is by treatment of chloroform, CHCl3, with a strong base such as KOH.
• • •
• • •
•
If dichlorocarbene is generated in the presence of an alkene, addition to the double bond occurs and a dichlorocyclopropane is formed. Stereospecific: A term indicating that only a single stereoisomer is produced in a given reaction rather than a mixture.
Only cis is formed in this reaction with the carbon. The chlorines can be in any way. Simmons-Smith reaction: The reaction of an alkene with CH2I2 and Zn-Cu to yield a cyclopropane.
Compound in g is called mCBA
3a) ozonolysis
