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Reaction Pathways (Mainly Alkenes)

Reaction Pathways (Mainly Alkenes). Alkenes. Contain at least one C=C double bond General formula: C n H 2n (like cycloalkanes). The double bond consists of a  bond and a  bond  bond from head-on overlap of s & p orbitals  bond from side-on overlap of p orbitals.  bond.  bond.

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Reaction Pathways (Mainly Alkenes)

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  1. Reaction Pathways(Mainly Alkenes)

  2. Alkenes • Contain at least one C=C double bond • General formula: CnH2n(like cycloalkanes) • The double bond consists of a  bond and a  bond •  bond from head-on overlap of s & p orbitals •  bond from side-on overlap of p orbitals  bond  bond A  bond is stronger than a  bond. • Rotating a double bond requires breaking the  bond • NO FREE ROTATION at room temperature

  3. Diagram of common organic reactions 3

  4. Reactions of Alkenes   + A-B  • Alkanes – substitution reactions • R-H + A-B  R-A + H-B • R = “residue”, a generic alkyl group • Alkenes – addition reactions  bond is electron-rich Thermodynamics: Hrxn = bonds broken – bonds formed = ( bond +  bond) – ( bond +  bond) Exothermic reaction

  5. Alkene Addition Reactions + A-B  A-B H-F, H-Cl, H-Br, H-I H-OH Br-Br, Cl-Cl, F-F H-H Reaction Hydrohalogenation (addition of H-X, X = halogen) Hydration (addition of H2O) Addition of halogens Hydrogenation (addition of H2)

  6. Reactions at the Double Bond   • Electrophile: • “electron-lover” • Electron deficient reagent (often + charged cations) • seeks e- in reactions • Nucleophile: • “nucleus-lover” • electron-rich reagent (often – charged anions) • tries to donate e- to an electrophile in reactions, forming bonds •  bond is electron-rich • Acts as a nucleophile • Attracts electrophiles to form bonds

  7. Markovnikov’s Rule • Consider the reaction C C + 1-chloropropane 2-chloropropane 1-propene • Two products are possible • Experimentally, only 2-chloropropane is formed • Markovnikov’s Rule: • The alkene carbon with the most H atoms gets the H • Hydrohalogenation (H-X), hydration (H2O) of alkenes Vladimir Markovnikov Why? Look at the reaction mechanism to find out...

  8. Carbocation Stability • Relative carbocation stability: 3° > 2° >> 1° > methyl > >> > 3° 2° 1° methyl Most stable Least stable Why? Electrons can “drift” into the empty p orbital from C-H bonds on neighboring C atoms to help stabilize Result: The final product comes from the most stable carbocation intermediate

  9. Mechanism of Hydrohalogenation Step 1: Alkene  electrons attack H+, forming a carbocation carbocation or + Step 2: Cl- nucleophile forms bond with carbocation The carbocation formed in step 1 determines the final product

  10. Hydrohalogenation Predict the product of the following hydrohalogenation reaction + HBr Anti-Markovnikov Product Remember: Markovnikov’s rule says that the H (from HBr) will bond to the alkene C with the most H’s OR Markovnikov Product

  11. Hydration Predict the product of the following hydration reaction + H2O Anti-Markovnikov Product OR Remember: Markovnikov’s rule says that the H (from H2O) will bond to the alkene C with the most H’s Markovnikov Product

  12. Mechanism of Halogen Addition (R + X2) Step 1: Br2 is polarized by  electrons, Br+ attaches to alkene + bromonium ion intermediate Step 2: Br+ blocks access to one face of the alkene Br- adds to the other face (anti addition) trans isomer is the only product

  13. Halogenation + Br2 Trans isomer + Br2 The Br’s will add to opposite sides of a RING (anti addition) The Br’s will be forced into a trans conformation ALWAYS

  14. Mechanism of Hydrogenation (R + H2) H H Pt H H Pt • Occurs in the presence of a metal catalyst (like Pt) Step 1: H2 adsorbs to catalyst surface Pt H−H + Step 2: Both H atoms add to same face of alkene (syn addition) cis isomer is the only product Pt

  15. Hydrogenation • Occurs in the presence of a metal catalyst (like Pt) + H2/Pt cisisomer + H2/Pt Both H’s will add to the same side of a RING (syn addition) If branches are present, they will be forced into a cis conformation

  16. Hydrogenation of alkenes + H2/Pt vegetable oils unsaturated saturated Contains NO double bonds Contains double bonds

  17. Alkene Reaction Summary • Hydrohalogenation (+ HX) • Hydration (+ H2O) • Halogenation (+ X2) • Hydrogenation (H2/Pt) Markovnikov’s rule Cis/trans with rings

  18. Polymerization of Alkenes • Polymer: a large molecule made by linking together small repeat units called monomers • Polymerization mechanism: radical chain reaction Monomer Polymer ethene (ethylene) polyethylene propene (propylene) polypropylene

  19. Alcohols • Contain a hydroxyl (-OH) group − + • Intermolecular forces: dipole-dipole, H-bonding • H-bonds between alcohol molecules: high boiling points • H-bonds with water: up to 4-carbon alcohols soluble in water • -OH group can act as a weak base or a weak acid + Strong acid + Strong base alkoxide alcohol oxonium ion

  20. Classes of Alcohols Primary (1°) alcohol 1-butanol OH C attached to 1 other C Secondary (2°) alcohol OH C attached to 2 other C’s 4-phenyl-2-hexanol Tertiary (3°) alcohol OH C attached to 3 other C’s 1-methylcyclohexanol

  21. Reactions of Alcohols Strong base • Reaction with strong bases • alcohol as proton donor (weak acid) • Reaction with strong acids • alcohol as proton acceptor (weak base) • Dehydration • reverse of hydration of alkenes • requires H+ catalyst • Oxidation • increase # of C-O bonds alkoxide Strong acid oxonium ion H+ - H2O alkene oxidizing agent or aldehyde ketone

  22. Oxidation of Alcohols • Oxidation: increases oxidation number • More C-O bonds (add O) or increases bond order • Fewer C-H bonds (remove H) • Needs an oxidizing agent • CrO3, Cr2O72-, MnO4- ,or PCC (pyridinium chlorochromate) PCC stops at aldehyde 1° alcohol +1 -2 -2 -2 -1 oxidizing agent oxidizing agent 0 +1 +3 +1 -2 0 0 +1 +1 +1 aldehyde carboxylic acid CrO3 (Cr6+) Cr3+

  23. Breathalyzer Tests ethanol oxidized oxidized ethanal (acetaldehyde) ethanoic acid (acetic acid) + Cr6+ + Cr3+ oxidized oxidized methanol methanal (formaldehyde) methanoic acid (formic acid)

  24. Oxidation of Alcohols 0 -2 +1 0 oxidizing agent 2° alcohol 0 +2 -2 0 0 +1 ketone propanone (acetone) 2-propanol oxidizing agent No reaction 3° alcohol

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