I. Addition Reactions of Alkenes Introduction Nuggets of Knowledge for Chapter 12 Alkenes (II) Chem 2310 An addition reaction always involves changing a double bond to a single bond and adding a new bond to each atom. The table below summarizes the addition reactions of alkenes from the previous chapter and this one that we will study. reaction what is added to the C=C what kind of molecule results addition of HX HX only alkyl halide HBr with ROOR hydration acid-catalyzed oxymercuration-reduction hydroboration-oxidation H, X (X on more subst side) H, Br (Br on less subst side) H, OH (OH on more subst side) H, OH (OH on more subst side) H, OH (OH on less subst side) X, X alcohol addition of X 2 with H 2 O X, OH (OH on more subst side) catalytic hydrogenation H, H alkane hydroxylation OH, OH vicinal diol epoxidation the same O to both sides epoxide cyclopropanation the same C to both sides cyclopropane polymerization H, C polymer vicinal dihalide halohydrin Catalytic Hydrogation of Alkenes Adding H 2 to an alkene gives an alkane. A metal catalyst is required Pt, Pd, Ni, or Rd. When precipitated onto carbon, it is written: Pd/C. The catalysts are pyrophoric and expensive, but are used in small amounts and can be recycled. Since the catalyst is a solid and the reaction is liquid, this is an example of heterogeneous catalysis. There is no regioselectivity in this reaction.
A special apparatus called a Parr shaker is used to perform this reaction. The mechanism involves formation of weak, temporary bonds between the metal catalyst and the hydrogen and carbon atoms, which helps them to become bonded to each other. No rearrangements occur no carbocation is formed. Hydroxylation of Alkenes Hydroxylation adds an OH to both sides of an alkene, forming a vicinal diol (sometimes called a glycol). Two sets of reagents can be used to perform this reaction. Potassium permanganate (KMnO 4 ) and sodium or potassium hydroxide (NaOH or KOH) is used for large scale reactions when the starting material is inexpensive and easy to obtain. Some side reactions occur; keeping the reaction cold and dilute minimizes them. This reaction can also be used as a visual test for alkenes the purple reagent turns to a brown precipitate if an alkene is present. Osmium tetroxide (OsO 4 ) and hydrogen peroxide (H 2 O 2 ) are used in small scale reactions when the starting material is expensive or difficult to obtain. OsO 4 is expensive, toxic, and volatile, but adding H 2 O 2 makes it catalytic, so only a small amount is used. Epoxidation of Alkenes Alkenes are converted to epoxides using peroxyacids. Epoxides are three-membered rings containing an oxygen; they are more reactive than other ethers, and can be synthetic targets or intermediates. Peroxyacids are carboxylic acids with a extra oxygen atom before the OH.
They can be made from carboxylic acids and hydrogen peroxide. They are abbreviated RCO 3 H. During the reaction, the peroxyacid is converted back to a carboxylic acid. This reaction selectively oxidizes alkenes, leaving other functional groups alone. Peroxyacetic acid is the simplest stable peroxy acid. It is soluble in water. Peroxybenzoic acid is useful because it is soluble in organic solvents. P-Chloroperoxybenzoic acid (MCPBA) was commonly used because it will crystallize from solution, but it is also somewhat shock sensitive. Magnesium monoperoxyphthalate (MMPP) is used because it is safer. The mechanism of this reaction is electrocyclic, and starts with attack of the pi bond of the alkene on the second oxygen of the peroxy acid. The final products are the epoxide and carboxylic acid. More carbon substitutents on the alkene make the reaction faster because they make the alkene a slightly better nucleophile (because C is more electronegative than H). Cyclopropanation of alkenes Cyclopropane rings can be formed by reacting a carbene with an alkene. There are three ways to form a carbene which can be useful in this reaction. Treating diazomethane (CH 2 N 2 ) with heat or light will cause it to decompose to N 2 gas and a carbene. However, diazomethane is toxic and can be explosive. The Simmons-Smith reagent uses iodomethane, zinc, and copper (I) chloride. Treating chloroform or bromoform with potassium tert-butoxide will first remove a hydrogen, after which a halogen will leave, creating a carbene with two halogens still attached. Addition of X 2 to alkenes Adding X 2 to an alkene gives a vicinal dihalide. One X goes to both sides of the C=C. Br 2 and Cl 2 are the most commonly used halogens. I 2 can be used, but the products easily decompose. F 2 is too reactive to use.
CH 2 Cl 2 (dichloromethane) is the most commonly used solvent it dissolves both the alkene and halogen, and doesn't react with the halogen. (It isn't always written as part of the reaction.) Alkanes like hexane won't work because although they are nonpolar, they react with halogens. This reaction works as a chemical test for alkenes: add a drop of dark red bromine to an alkene, and if it disappears then it was a positive test. In the mechanism, the alkene attacks a halogen atom, pushing the other one off, as this occurs, the lone pair on the halogen being attacked forms a bond to the other carbon of the C=C. In the second step, the halogen ion attacks one carbon in the ring, pushing the other halogen off. When water is present, a halohydrin is formed, having an OH on one carbon and a halogen on the other. The water molecule replaces the halogen ion and attacks the ring. It then loses a hydrogen atom to another water molecule, leaving the product. The OH goes to the more substituted side of the C=C, because that carbon shares more of the halogen's positive charge than the other. If both sides are equally substituted but not symmetrical, two products are formed. Polymerization of alkenes Polymers form when alkenes react together to form long chains. The reaction can be started by an acid, which forms a carbocation, or a radical initiator such as an organic peroxide. When a carbocation or radical forms, there is nothing else to react with, so it reacts with another alkene molecule. This creates a new carbocation or radical, causing the chain to continue. The carbons that were part of the C=C form the backbone of the chain. Any atoms attached to the C=C end up hanging off of the backbone. This reaction can make it difficult to store alkenes, or take their boiling point. Adding inhibitors can prevent this problem. II. Stereochemistry of Alkene Reactions Alkenes reactions are addition reactions. Therefore, both carbons involved in the reaction may become asymmetric carbons during the reaction. Two, one, or no new asymmetric carbons may be formed, depending on what substituent is added, and what substituents were already present on that carbon.
If only one new asymmetric carbon is formed, a racemic mixture of the two possible enantiomers will usually result. In order for an optically active mixture to be formed, either a chiral starting material or a chiral reagent must be used. If both carbons become asymmetric carbons, then there are three possible results. Which happens depends on the mechanism of the reaction. If syn addition occurs, both new substituents will be added to the same face of the alkene. Two products will result (unless a meso compound occurs). If anti addition occurs, both new substituents will be added to the opposite faces of the alkene. Two products will result (unless a meso compound occurs). If non-selective addition occurs, new substituents will be added to the same face and opposite faces of the alkene. Four products will result (unless meso compounds occur). If both substituents are added during the same step of the mechanism, syn addition will occur. This happens with hydroboration-oxidation, hydrogenation, hydroxylation, epoxidation, and cyclopropanation. Addition of X 2 and addition of X 2 with water both have anti addition. This is because the halide or water must attack from the opposite side as the bromonium ion. Non-selective addition occurs when there is a racemizing step, such as a carbocation or radical. This happens with addition of HX, addition of HBr with organic peroxide, acid-catalyzed hydration, and oxymercuration-reduction. III. Oxidative Cleavage of Alkenes When an alkene reacts with ozone (O 3 ), the C=C is completely broken, and both carbons gain two bond to oxygen. Note that this is NOT an addition reaction, since the C=C bonds are broken as well as the carbons gaining new bonds. If dimethyl sulfide (CH 3 SCH 3 ) is present, the products may be aldehydes or ketones; if hydrogen peroxide (H 2 O 2 ) is present, the products may be carboxylic acids or ketones. If the alkene is cyclic, it broken into a chain, with C=O's attached to both carbons that were part of the C=C. This reaction is not usually useful synthetically, since the products are smaller than the starting materials. An exception to this is dicarbonyl compounds made from cyclic alkenes. This reaction was used for determining the structure of an alkene before NMR was available. An unknown alkene was reacted with ozone, and the identity of the resulting aldehydes, ketones, or carboxylic acids were determined. They could then figure out what the original alkene had been.