1 / 37

ORGANIC CHEMISTRY 1 Chapter 7 – instructor version

ORGANIC CHEMISTRY 1 Chapter 7 – instructor version. ALKENES and CYCLOALKENES Formulas & naming Bonding & molecular structure Physical & chemical properties Preparation & use Elimination rxn : E1 and E2. Based on Organic Chemistry, by L.G. Wade, 7 th ed ; Ch. 7

zola
Download Presentation

ORGANIC CHEMISTRY 1 Chapter 7 – instructor version

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. ORGANIC CHEMISTRY 1 Chapter 7 – instructor version • ALKENES and CYCLOALKENES • Formulas & naming • Bonding & molecular structure • Physical & chemical properties • Preparation & use • Elimination rxn: E1 and E2 Based on Organic Chemistry, by L.G. Wade, 7thed; Ch. 7 Prepared by: Dr. Peter Ilich, St. John’s University Queens, New York, Spring 2012

  2. [7.1 & 7.4] Definition & naming: - An alkEne has one or more pairs of doubly bonded carbon atoms, C=C, in its skeleton formula: name: ethene (ethylene) H2C=CH2 H2C=CH2-CH3 propene butene H2C=CH2-CH2-CH3 (or 1-butene) But – watch: H3C-CH2=CH2-CH3 2-butene

  3. [7.4] Alkene naming - practice formula: name: H2C=CH2CH2CH2CH3 pentene (1-pentene) CH3CH2=CH2CH2CH3 2-pentene C atom # 2 CH3-CH2-CH2-H2C=CH2-CH2-CH3 3-heptene C atom # 3, not # 4

  4. [7.4] Naming alkenes – more practice: Name this compound: Formula, condensed: Expand it: Or draw the C-skeleton: 2, not 4 5 Label the position of double bond (smallest number, 2), identify the substituent group and label its position (5): 5-methyl-2-hexene (“old” IUPAC) (or, new IUPAC: 5-methylhex-2-ene)

  5. [7.4] The rules for naming alkenes; - Longest chain – the amended rule Name this alkene: 1st – expand it: Not the longest chain, but the longest (and most branched) chain which contains all C=C 1 2 2nd - label the C-skeleton: No! 5 1 3 6 2-butyl-3-methylhex-1-ene 9 6

  6. [7.4] Alkene naming continued: What do you name alkenes with 2 or more C=C? 1,3-pentadiene Name it: 1,3-pentadiene Name this: 1,3,7-octatriene But not: 1,2-butadiene Note: 1,3-butadiene Allene is not a diene; it is not even an alkene Make a note: Allene

  7. [7.4] Alkene naming – cycloalkenes Formula 1: Formula 2: Name 2: 1,4-cycloheptadiene cyclopropene Name 1: methylene- methyl- Radical group naming: 1-methylene-3-methylcyclopentane 3º C not a 2º C Make a note: No designation of alkene C-atoms as 1º, 2º, 3º, & 4º, as in alkanes 2º C not a 4º C not a 3º C 4º C

  8. [7.4] Alkene naming – practice: (A) (B) (C)

  9. [7.2] Ethene electronic structure & geometry:

  10. [7.2] El. structure & geo – comparison with alkanes

  11. [7.7] Heats of hydrogenation, ΔHH2º - Terminal vs. central C=C bond ΔHH2º [kJ mol-1] -113 2-methylbutane 3-methyl-1-butene -127 2-methylbutane 2-methyl-2-butene NOTE: The lower (more negative) the ΔHH2º number the less stable the alkene.

  12. ΔHH2º of alkenes – what do they mean? - Think of standard heats of formation, ΔHfº ΔHfº H2(gas), C(soot) = 0 kJ mol-1 [Ch. 7, JBS 6th ed.] E N E R G Y less stable more stable -127 kJ -113 kJ [ΔHfº (2-MeBu)= -182 kJ mol-1]

  13. [7.2] Bonding & molecular properties of alkenes Alkene substitution & stability LESS STABLE MORE STABLE MOST STABLE And – the “take-home message” is: The more substituted (or the fewer H atoms there are on) the C=C atoms the more stable the alkene.

  14. [7.5] Alkenes – 2-D structure & isomerism - The C=C part of an alkene is confined and locked into a plane: ΔE = +264 kJ/mol You may not twist an alkene – like, for example, an ethane molecule, which twists ~ 1014 times a second at room temperature – unless you “pay” 264 kJ/mol and break the π – bond. As a consequence, alkenes exist as 2-D isomers, as “E” (Germ., entgegen = oposite) and “Z” (Germ., zusamen = together) forms

  15. C,C bond properties: alkenes vs.alkanes (2Z)-butene (2E)-butene butane, gauche- butane, anti-

  16. [7.5] Alkenes – 2-D stereochemistry - The cis-/trans-notation for 2-butene, CH3CH=CHCH3 2-D formula: trans-2-butene (methyls opposite) cis-2-butene (methyls together) 2-D formula: Practice: trans-2-pentene or (2E)-pentene

  17. [7.5] Alkene 2-D stereochemistry: - The cis-/trans- notation for alkenes is being replaced by E(trans)-/Z(cis)- notation: 1st: Expand the C=C fragment and dissect it vertically 2nd: Identify and rank the groups at left; use the Sequence Rule 3rd: Do the same for the groups at right 4th: Compare the positions of the first-ranked groups: (Z)- for the same, (E)- for the opposite 5th: Name the compound: (Z)-3-chloropent-2-ene

  18. [7.5] Alkene 2-D stereo – practice Formula: Partial name, specify E/Z: 3-methyl-(3 )-hexene 5-methyl-(2 ,4 )-heptadiene 5-methyl-(1 ,3 )-cycloheptadiene (1 ,3 )-decadiene

  19. (A) Biologically important E-/Z-stereomers: Oleic acid C18:1(9c) & C18:1(9t) C18:1(9c) – cis-oleic acid, the common form of oleic acid which can be metabolized: C18:1(9t) – trans-oleic acid: industrially prepared form of oleic acid (through partial hydrogenation); cannot be metabolized and forms insoluble deposits in blood vessels:

  20. (B) Human/vertebrate visual pigment – retinal (retinimine) The retinal C11cis- to trans-isomerization is the physical-chemical basis of our ability to see light The experimentally determined structure of (Rhod)opsin [Palczewski et al., Science, 289, Issue 5480, 739-745]

  21. Impact of light on rhodopsin in our retinas induces retinal-opsin to isomerize at C11 from cis- to trans-form: Trans-retinal separates from the opsin protein and this event triggers within 10-3 [s] a cascade of signals:

  22. And – this is how you see:

  23. [7.8] Macroscopic physical properties of alkenes - Lower alkenes (C2 to C5) are gases @ R.T. - Boiling point increases with C- number • Boiling point within the same size alkenes • decreases with branching (same as alkanes) • Less dense than water (Gulf oil slick!); mixes poorly or not at all with water • More polarizable – C=C bond – than alkanes; induced dipole – induced dipole interactions: • they become slightly dipolar in electric field

  24. [7.9] Preparation of alkenes: - By elimination reactions - Examples: - E1 elimination of alkyl alcohols - E1 elimination of haloalkanes - E2 elimination of haloalkanes - Other elimination reactions

  25. [7.9] Preparation of alkene by elimination; E1 dehydration rxn of alcohols, recapitulation: Acidification & loss of water; note Δ: Carbocation rearrangement (if possible): β-elimination; Zaytsevregioselectivity:

  26. [4] Preparation of alkenes by E2 rxn: E2 elimination of haloalkanes: Substrate: usually hindered Reagent: very strong, bulky base Reaction flow: concerted Transition state: concerted reaction Stereochemistry: anti- and syn-periplanar

  27. [4] Preparation of alkenes by E2 reactions Example:

  28. E2 – continued: When nucleophile, Nu, in an SN2 reaction is also a strong Broensted base, it can lead to a concerted elimination, or the so-called E2 reaction, with the following mechanism: Reaction type: ELIMINATION, E RXN rate = krate * [substrate]*[base] =2nd order, E2

  29. Regiochemistry (where it occurs) of E2 reactions: Zaytsev, E1 (E2) more stable Hoffmann, E2 less stable Make a note - similarities & differences:

  30. Elimination regioselectivity: Zaytsev Hoffmann

  31. E2stereoselectivity – the definition:

  32. E2 substrate stereochemistry – another view:

  33. E2 – the substrate & the product stereochemistry meso-1,2-dibromo-1,2-diphenylethane (E)-1-Bromo-1,2-diphenylethene

  34. E2 – stereoselectivity (Important!): Practice: Good E2 geometry: Bad E2 geometry:

  35. E2 practice: complete the reaction & name the product:

  36. Summary of Ch. 7 – what have we learned today?

More Related