Alkene Synthesis & Reactions
Post on 14-Dec-2015
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Alkenes: Synthesis & Reactions
Preparing Alkenes – Elimination Reactions
1. Dehydrohalogenation
Reaction of an alkyl halide with a strong base—HX acid is neutralized by base
2. Dehydration
Treatment of an alcohol with a strong acid—OH is neutralized by strong acid
3. Halogenation of Alkenes: Addition of X2 If halogenation reaction is carried out on a cycloalkane, only the trans isomer is formed (Anti)
If you see: Just a halogen pair Bromonium ion accounts for anti formation because the Bromine protects one side of the cycloalkene so that the other bromine can only attach from the other side
4. Halohydrins from Alkenes: Addition of HOX
Reaction of alkenes with hypohalous acids (HO-‐CL or HO-‐Br) to make 1,2-‐halo alcohols, called Halohydrins Before, we saw that the prescence of a nucleophile (Br-‐) in front of a bromonium ion makes the halogenation, but if there are two nucleophiles (halogen + water, etc.), the water nucleophile is stronger and reacts with the bromonium ion to make a bromohydrin. This other nucleophile is also added to the other side of the bromonium If you see: A halogen and another nucleophile (H2O, H2O2, etc.) NBS is a source of bromine instead of using actual bromine
5. Hydration of Alkenes: Addition of H2O by Oxymercuration Water is added to an alkene to yield an alcohol, a process called hydration. The halogen is added to the side with the least groups If you see: Water and a strong acid catalyst
6. Oxymercuration-‐Demercuration Oxymercuration involves electrophilic addition of Hg(OAc)2 in THF solvent. Intermediate then treated with NaBH4 to cause demercuration and produce an alcohol If you see: Hg(OAc)2 and THF
Very similar to halohydrin formation: Hg is at the top of the triangle. Water attaches to the other Carbon, and loses a proton (See halohydrin diagram) The OH group attaches to more highly substituted carbon and H attaches to less (Markovnikov), but Hydrogen that replaces mercury can be on either side of the molecule
7. Hydration of Alkenes: Addition of H2O by Hydroboration This is a complementary method to oxymercuration-‐demercuration that produces the non-‐Markovnikov product. Involves the addition of a B-‐H bond to an alkene to yield RBH2. Oxidation with H2O2 then yields an alcohol If you see: BH3 and H2O2
In this reaction, boron attaches to the less highly substituted carbon,
and both B and H attach on the same side of the molecule. During oxidation, the B is replaced with an OH with the same stereochemistry, and the overall molecule is syn non-‐Markovnikov.
This happens because there is no intermediate form, and the C-‐H and C-‐B bonds form at the same time and at the same face
8. Reduction of Alkenes: Hydrogenation
Alkenes react with H2 in the presence of a metal catalyst like Pd or Pt. As a result, the double bond has been hydrogenated or reduced Reduction: Creating C-‐H bonds or breaking C-‐X If you see: H2, Pt or Pd Takes place on the surface of solid catalyst particles (homogeneous solution) Usually syn – hydrogens add to the double bond from same face
Hydrogenation begins with adsorption of H2 onto the catalyst surface. Vacant orbital on the metal interacts with the filled alkene pi orbital. Hydrogen is then inserted into the double bond and saturated product diffuses away from catalyst
The catalyst usually only approaches the more accessible face of an alkene, so there is usually only a single product
9. Oxidation of Alkenes: Epoxidation and Hydroxylation
Oxidation: Form C-‐X or break C-‐H Alkenes are oxidized to give epoxides on treatment with a peroxyacid such as meta-‐ chloroperoxybenzoic acid. If you see: meta, or any other peroxyacid
Peroxyacids transfer oxygen atoms to alkene with syn stereochemistry through a one step mechanism. The oxygen farthest from the carbonyl group on the peroxyacid is transferred to the alkene. Other method: Use halohydrins and then treat with base: HX is eliminated and epoxide produced.
Epoxides then undergo ring-‐opening reaction with water (hydrolysis) to give a diol. The whole process is called hydroxylation. The product is trans
Other Method: Hydroxylation can be carried out directly without an intermediate epoxide with OsO4. The reaction is syn and creates an intermediate cyclic osmate. It is then cleaved with NaHSO3 If you see: OsO4, NaHSO3, or NMO With NaHSO3:
With NMO and catalytic OsO4:
10. Oxidation of Alkenes: Cleavage to Carbonyl Compounds
Ozone adds rapidly to C-‐C double bonds to give cyclic molozonide intermediate, which then spontaneously rearranges to form an ozonide. The ozonide is then treated with zinc to convert it to carbonyl compounds If you see: O3, Zn
If an alkene with a tetrasubstituted double bond is ozonized, two ketone fragments result; if an alkene with a trisubstituted double bond is ozonized, one ketone and one aldehyde result
Other method: Use KMnO4 to cleave; if hydrogens on double bonds, carboxylic acids are produced; if two hydrogens are present on one carbon, CO2 is formed If you see: KMnO4
Other method: Initial hydroxylation to a 1,2-‐diol, then treatment of diol with HIO4. If two OH groups are in an open chain, two carbonyl compounds result. If two OH groups are on a ring, a single, open-‐chain dicarbonyl compound is formed.
11. Addition of Carbenes to Alkenes: Cyclopropane Synthesis This reaction is an addition of a carbene to yield a cyclopropane. Carbenes are usually only intermediates. To make them: a common method is the use of chloroform, CHCl3 with a strong base If you see: R2C
If creation of the dichlorocarbene is made in the presence of an alkene, addition to the double bond occurs and a product is formed. If you are starting from a trans alkene, you get a trans product, and vice versa
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