CHEM 250 - Ch 8 Alkenes - Reactions and Synthesis 2024 PDF

## Ch 8 - Alkenes - Reactions and Synthesis ### Addn of H₂ to an alkene (H₂, Pd/C catalyst) 1. Addn of X₂ to an alkene (Br₂, Cl₂ or I₂) 2. Halohydrins from alkenes (Br₂/H₂O or Cl₂/H₂O or NBS/DM80/420) - Variation: X₂ and ROH 3. Hydration of alkenes (H₂O, Hg(OAc)₂, THF, NaBH₄) - Variation: RO...

## Ch 8 - Alkenes - Reactions and Synthesis ### Addn of H₂ to an alkene (H₂, Pd/C catalyst) 1. Addn of X₂ to an alkene (Br₂, Cl₂ or I₂) 2. Halohydrins from alkenes (Br₂/H₂O or Cl₂/H₂O or NBS/DM80/420) - Variation: X₂ and ROH 3. Hydration of alkenes (H₂O, Hg(OAc)₂, THF, NaBH₄) - Variation: ROH/H⁺ cat. 4. Hydroboration - Oxidation of alkenes 5. Hydrogenation of alkenes (H₂, Pd/C catalyst, ethanol) 6. Epoxidation of an alkene (w/ MCPBA) or other peroxyacid 7. Epoxide formation by treating a halohydrin w/ base 8. Preparation of a trans-1,2-diol (via acid-cat. hydrolysis of epoxide) 9. Preparation of a cis-1,2-diol (w/ OsO₄) 10. Ozonolysis of alkenes 11. Oxidative cleavage of alkenes w/ KMnO₄ 12. Oxidative cleavage of 1,2-diols w/ HIOH 13. Addn of dichloro carbene to an alkene → dichlorocyclopropyl group 14. Simmons-Smith rxn-addn of a carbene → cyclopropane 15. Free radical polymerization of alkenes 16. Addn of HBr to an alkene in the presence of peroxides **It is helpful to make flashcards!** ### Most rxns in this chapter are addition rxns (some exceptions). ### The previously learned addn rxns: - **Hydrogenation (addn of H₂) ** - Syn addn of H₂ across an alkene - **Hydrohalogenation (addn of HBr, HCl, or KI/H₃PO₄)** - Note that reagents can be written over the arrow or included as a reactant - Catalysts + solvents are generally written over the arrow ### Section 8.2 - Halogenation of alkenes (addn of X₂) **Overall:** **E⊕ addn rxn** - X are "anti" (opposite) - no C⊕ intermediate/no C⊕ rearrangement possible - rxn proceeds via a halonium ion intermediate (chloronium ion or bromonium ion) **Mech:** - bromonium ion intermediate - Br attacks @ the side opposite the bridge & opens ring ### Stereochemistry when new chiral Cs are formed - rxn of cis-2-butene - rxn of trans-2-pentene **Note:** This rxn (addn of X₂ to an alkene) doesn't proceed through a C⊕ intermediate so there are no C⊕ rearrangements. ### Section 8.3 - Halohydrins from alkenes (Rxn #2) - E⊕ addn rxn - Looks like you’re adding “H-O-X” to an alkene - Conducted by reacting w/ X₂ in the presence of water - There are 2 competing nucleophiles in the 2nd step (XΘ and H₂O) but H₂O is the solvent so there’s LOTS of it (so it wins) **Ex.** - The OH becomes attached to the more subst’d C. - "Anti" stereochemistry due to halonium ion intermediate - The product is a "halohydrin" (here: bromohydrin) - Markovnikov regiochemistry **Mech:** - Br₂ is E⊕ - attacks @ more subst’d (has more δ⊖ character) at the side opposite the bridge - water deprotonates the intermed. ### Sometimes NBS (N-bromosuccinimide) is used as a source for Br₂ (it decomposes in H₂O to release Br₂). - The solvent DMSO (dimethyl sulfoxide) is also used to keep everything liquid. **Ex.** **Ex. w/ a ring (cyclohexene)** - when stereochemistry is considered, there are 2 products which are a pair of enantiomers. ### Variation of run #2 (run 2A) - This rxn also works if an alcohol is used instead of water → makes a haloether ### Section 8.4 - Hydration of alkenes (Rxn#3A &3B) - addn of H₂O to an alkene to form an alcohol - E⊕ addn- follows Markovnikov’s rule - catalyzed by an acid: either a Brønsted acid such as H₃PO₄ or H₂SO₄ or a Lewis acid such as Hg²⁺ (ex. Hg(OAc)₂- Mercury (II) acetate) **Two methods** - Catalyzed by a Brønsted acid - Oxymercuration (catalyzed by Hg (OAc)₂) - **3A. Hydration catalyzed by a Brønsted acid.** - **Overall:** - **Mechanism:** 1st remember that an acid dissociates in water to form hydronium ion. This is the E⊕. - catalyst is regenerated - **3B. Hydration by oxymercuration** - **Overall rxn:** - This rxn has 2 distinct steps, need a "1" and "2" - NaBH₄ is sodium borohydride - Source of nucleophilic hydride ion (H⊖) - OAc = acetate ion: - **How it works (not the mechanism, just a description)** - Hg is electropositive (a metal) so it complexes with the π bond of the alkene - (NO C⊕ RARs) - NaBH₄ is H⊖ source - HgOAc is an organo mercury compound - **The “OH” in the product ends up on the more subst’d C so this rxn follows Markovnikov regiochemistry.** - **3C. Variations: use an alcohol in place of water:** - acid-catalyzed addn of ROH - Product is an ether ### Section 8.5 - Hydroboration - Oxidation of alkenes (Rxn #4) - Behaves as a non-Markovnikov addn of H₂O (anti- Markovnikov). - The OH ends up on the less-substituted C. - Syn addn of H & OH (stereochem). **Overall (another 2-step rxn)** - (alkaline, aqueous peroxide) = 9-borabi cyclo [3.3.1] nonane) = C₈H₁₄B - BH₃ is a Lewis acid. It is not stable by itself so it’s dissolved in THF (with which it reacts) - H is more eneg - BH₃ is a Lewis complex (but behaves like pure BH₃). - organo borane intermediate - Syn addn of HBH₂ across a double bond. **Step I - Hydroboration** **Step II: Oxidation w/ peroxide** ### Stereochemistry of hydroboration-oxidation reactions - BH₂ attaches to less subst’d C - Syn addn of BH₂’s H (stereochemistry) - examples w/ regiochemistry/stereochemistry ### Working backward examples - What is the best way to prepare each of the following alcohols? ### Section 8.6 - Reduction of alkenes - Hydrogenation - Gen Chem: LEO GER: Gaining e⁻ is reduction - Organic: Reduction = gaining H and/or losing oxygen. **Rxn #5: Hydrogenation** - Common catalysts: (heterogeneous - not soluble) - Powdered Pd dispersed (supported) on charcoal - PtO₂, known as Adams catalyst - The catalysts work by adsorbing H and breaking the H-H sigma bond. - Metal at bottom - H adsorbs on metal surface - alkene also approaches metal surface - H’s transfer to alkene - 2 new H’s attacked syn - The rxn is sensitive to steric environment: - top side is blocked by the H & 2 CH₃’s - the top side can’t approach the catalyst - So H’s only attach @ the bottom. as major product. - each face is equally reactive on a trigonal planar alkene - So: 50:50 mixt. of a pair of enantiomers (racemic mixt.) ### Hydrogenation is also chemoselective. - Carbonyls & aromatic “rings”, nitriles aren’t reduced by H₂ w/ the Pd/C catalyst. - only the alkene is reduced, not the ketone - ester + benzene ring aren’t reduced - nitrile is not reduced ### Alkynes are reduced - *Note- not balanced we assume we have an excess of H₂ ### Section 8.7 - Oxidation of alkenes: Epoxidation & Hydroxylation - Gen Chem: LEO GER: Losing e⁻ is oxidation - Ochem: Oxidation = adding oxygen and/or losing hydrogen. ### 6. Epoxidation of an alkene w/ a peroxyacid reagent - an epoxide (aka oxirane) is a cyclic ether w/ a 3-membered ring - ex. CH₂-CH₂ “ethylene oxide” = 1,2-epoxyethane - 2 methods to synthesize an epoxide rxn - One reagent for an epoxidation rxn: a peroxyacid - RCO3H = R--O-O-H - ex. CH₃-C-O-H acetic acid is a carboxylic acid - common reagent: - overall: - syn addn - stereochemistry considerations ### Epoxidation w/ a peroxyacid (Method 1) ### 7. Epoxide formation via treating a halohydrin with base (Method 2) - Mechanism: - Can be written w/ a "1" and "2" on the arrow (2 distinct steps) - ignoring stereochemistry ### Preparation of 1,2-diols (hydroxylation) - Any diol w/ OHs on adjacent Cś is called a 1,2-diol - ex. 1,2-diol (aka glycol), vicinal drol - ethylene glycol - 1, 2-ethanediol - automotive antifreeze (18 million tons per year) ### 8. Synthesis of a trans-1,2-diol (anti" orientation of the 2 OH groups) - 1. epoxidation - 2. acid-catalyzed hydrolysis (a ring-opening rxn) **Mech:** - skip mech - BL - ring-opening (nuc. Substr) - H₂O attacks opposite the bridge ### 9. Preparation of a cis-1,2-diol w/ OsO₄ - Hydroxylation w/o going through an intermediate epoxide - OsO₄ is toxic and expensive - rxn occurs w/ syn stereochemistry via a cyclic osmate intermediate - cyclic osmate int, ring constrainsts produce cis-1,2-dimethyl-1,2-cyclopentanediol (not isolated) - (better) method - An alternative would be to use N-methylmorpholine N-oxide as a co-oxidant (“Upjohn dihydroxylation”) ### Section 8.8 - Oxidative cleavage of alkenes - Powerful oxidizing agents will break C-C double bonds to form two carbonyl compounds. (3 methods) (10, 11, 12) ### 10. Ozonolysis of alkenes - Ozone = O₃ - a molozonide - Rearranges - an ozonide-explosive! ### 11. Oxidative cleavage with potassium permanganate - Oxidation of alkenes - (any H on a sp²-C is oxidized to an “OH”) - A terminal alkene produces CO₂ as a product: - if cold, ag. KMnO4 can form an cis-diol ### 12. Oxidative cleavage of 1,2-diols with periodic acid or sodium periodate - Aktiv uses 1. NaIO₄ followed by 2. Na₂S₂O₈ (Sodrum thiosulfate) - cyclic periodate intermediate ### Section 8.9 - Addn of carbenes to alkenes- cyclopropane synthesis - 13. Rxn w/ dichlorocarbene (makes a dichlorocyclopropane ring) - Carbene = R-C-R - C is e deficient (therefore e⁻) - dichloro-Carbene - bent shape - C is sp² w/ 2e in an sp² orbital - vacant p-orbital) - But has formal charge can’t be isolated-highly reactive must be generated “in situ" (w/in the rxn) as follows: - trichloromethanide ion - dichloro carbene ### 14. . The Simmons- Smith rxn (makes a cyclopropane ring) - CH₂I₂ (Zinc-copper alloy) ### Section 8.10 - Radical additions to alkenes - Chain growth polymers - Polymer = a large molecule built up by repetitive bonding of many smaller molecules (monomers) - Biological polymers = cellulose, proteins, DNA/RNA - Industrial polymers- molar masses up to 10⁶ - LDPE - HDPE - Polystyrene - Poly(vinyl chloride) - polymers synthesized from alkenes are sometimes called polyolefins - (overall edition rxn) - overall rxn - Synthesis of polyethylene (a "chain growth" polymer) - Mechanism is radical/ uses fish hook arrows - up to a million units ### Rxn #16 - Addn of HBr to an alkene in the presence of peroxides - the non-Markovnikov product forms - Previously; - w/ peroxides: - no C⊕ are possible - mechanism is radical (fish-hook arrows) ### Section 8.11 - Biological additions- SKIP ### Section 8.12 - Rxn stereochemistry of E⊕ addn rxns - when 1 new chiral C forms - mech: - A C⊕ intermediate is trigonal planar, sp²-hybridized - If a chiral C is present before the rxn, a pair of diastereomers will form: - H₂O - H⁺ catalyst