ORGANIC CHEMISTRY 1 Chapter 7 – instructor version ALKENES and CYCLOALKENES Formulas & naming Bonding & molecular structure Physical & chemical properties.

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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 Prepared by: Dr. Peter Ilich, St. Johns University Queens, New York, Spring 2012 Slide 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) propene butene 2-butene H 2 C=CH 2 H 2 C=CH 2 -CH 3 H 2 C=CH 2 -CH 2 -CH 3 But watch: H 3 C-CH 2 =CH 2 -CH 3 (or 1-butene) Slide 3 [7.4] Alkene naming - practice formula:name: pentene (1-pentene) 2-pentene 3-heptene H 2 C=CH 2 CH 2 CH 2 CH 3 CH 3 -CH 2 -CH 2 -H 2 C=CH 2 -CH 2 -CH 3 CH 3 CH 2 =CH 2 CH 2 CH 3 C atom # 3, not # 4 C atom # 2 Slide 4 [7.4] Naming alkenes more practice: Formula, condensed: Name this compound: 5-methyl-2-hexene (old IUPAC) (or, new IUPAC: 5-methylhex-2-ene) 2, not 4 5 Expand it:Or draw the C-skeleton: Label the position of double bond (smallest number, 2), identify the substituent group and label its position (5): Slide 5 [7.4] The rules for naming alkenes; - Longest chain the amended rule Name this alkene: 1 st expand it: 2 nd - label the C-skeleton: Not the longest chain, but the longest (and most branched) chain which contains all C=C 1 5 6 9 1 2 3 6 2-butyl-3-methylhex-1-ene Slide 6 [7.4] Alkene naming continued: What do you name alkenes with 2 or more C=C? 1,3-pentadieneName it: Name this: 1,3,7-octatriene Note: 1,3-butadiene But not: 1,2-butadiene Make a note: Allene Allene is not a diene; it is not even an alkene 1,3-pentadiene Slide 7 [7.4] Alkene naming cycloalkenes Formula 1: Name 1: Formula 2: Name 2: Make a note: No designation of alkene C-atoms as 1, 2, 3, & 4, as in alkanes cyclopropene Radical group naming: 1,4-cycloheptadiene methyl- methylene- 1-methylene-3-methylcyclopentane 2 C not a 2 C 4 C 3 C not a 3 C not a 4 C Slide 8 [7.4] Alkene naming practice: (A) (B) (C) Slide 9 [7.2] Ethene electronic structure & geometry: Slide 10 [7.2] El. structure & geo comparison with alkanes Slide 11 [7.7] Heats of hydrogenation, H H2 - Terminal vs. central C=C bond 3-methyl-1-butene 2-methyl-2-butene -127 H H2 [kJ mol -1 ] -113 2-methylbutane NOTE: The lower (more negative) the H H2 number the less stable the alkene. Slide 12 H H2 of alkenes what do they mean? ENERGYENERGY HfHf H 2 (gas), C(soot) = 0 kJ mol -1 [Ch. 7, JBS 6 th ed.] -113 kJ -127 kJ [H f (2-MeBu)= -182 kJ mol -1 ] more stable less stable - Think of standard heats of formation, H f Slide 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. Slide 14 [7.5] Alkenes 2-D structure & isomerism - The C=C part of an alkene is confined and locked into a plane: You may not twist an alkene like, for example, an ethane molecule, which twists ~ 10 14 times a second at room temperature unless you pay 264 kJ/mol and break the bond. E = +264 kJ/mol As a consequence, alkenes exist as 2-D isomers, as E (Germ., entgegen = oposite) and Z (Germ., zusamen = together) forms Slide 15 C,C bond properties: alkenes vs. alkanes (2Z)-butene butane, gauche- (2E)-butene butane, anti- Slide 16 [7.5] Alkenes 2-D stereochemistry - The cis-/trans-notation for 2-butene, CH 3 CH=CHCH 3 cis-2-butene (methyls together) trans-2-pentene or (2E)-pentene trans-2-butene (methyls opposite) 2-D formula: Practice: Slide 17 [7.5] Alkene 2-D stereochemistry: - The cis-/trans- notation for alkenes is being replaced by E(trans)-/Z(cis)- notation: 2 nd : Identify and rank the groups at left; use the Sequence Rule 3 rd : Do the same for the groups at right 5 th : Name the compound: 1 st : Expand the C=C fragment and dissect it vertically 4 th : Compare the positions of the first-ranked groups: (Z)- for the same, (E)- for the opposite (Z)-3-chloropent-2-ene Slide 18 Slide 19 [7.5] Alkene 2-D stereo practice Formula: Partial name, specify E/Z: 5-methyl-(2,4 )-heptadiene 3-methyl-(3 )-hexene (1,3 )-decadiene 5-methyl-(1,3 )-cycloheptadiene Slide 20 (A) Biologically important E-/Z-stereomers: Oleic acid C18:1(9c) & C18:1(9t) 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: C18:1(9c) cis-oleic acid, the common form of oleic acid which can be metabolized: Slide 21 (B) Human/vertebrate visual pigment retinal (retinimine) The retinal C11 cis- to trans-isomerization is the physical-chemical basis of our ability to see light [ Palczewski et al., Science, 289, Issue 5480, 739-745] The experimentally determined structure of (Rhod)opsin Slide 22 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: Slide 23 And this is how you see: Slide 24 [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 Slide 25 [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 Slide 26 [7.9] Preparation of alkene by elimination; E1 dehydration rxn of alcohols, recapitulation: Carbocation rearrangement (if possible): Acidification & loss of water; note : -elimination; Zaytsev regioselectivity: Slide 27 [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 Slide 28 [4] Preparation of alkenes by E2 reactions Example: Slide 29 When nucleophile, Nu, in an S N 2 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 = k rate * [substrate]*[base] =2 nd order, E2 E2 continued: Slide 30 Regiochemistry (where it occurs) of E2 reactions: Zaytsev, E1 (E2) Hoffmann, E2less stable more stable Make a note - similarities & differences: Slide 31 Elimination regioselectivity: Zaytsev Hoffmann Slide 32 E2 stereoselectivity the definition: Slide 33 E2 substrate stereochemistry another view: Slide 34 E2 the substrate & the product stereochemistry meso-1,2-dibromo-1,2-diphenylethane (E)-1-Bromo-1,2-diphenylethene Slide 35 E2 stereoselectivity (Important!): Practice: Good E2 geometry: Bad E2 geometry: Slide 36 E2 practice: complete the reaction & name the product: Slide 37 Summary of Ch. 7 what have we learned today?