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

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Presentation on theme: "ORGANIC CHEMISTRY 1 Chapter 7 – instructor version ALKENES and CYCLOALKENES Formulas & naming Bonding & molecular structure Physical & chemical properties."— 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, 7 th ed; 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) 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)

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

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):

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 butyl-3-methylhex-1-ene

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

7 [7.4] Alkene naming – cycloalkenes Formula 1: Name 1: Formula 2: Name 2: Make a note: No designation of alkene C-atoms as 1º, 2º, 3º, & 4º, as in alkanes cyclopropene Radical group naming: 1,4-cycloheptadiene methyl- methylene- 1-methylene-3-methylcyclopentane 2º C not a 2º C 4º C 3º C not a 3º C not a 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, ΔH H2 º - Terminal vs. central C=C bond 3-methyl-1-butene 2-methyl-2-butene -127 ΔH H2 º [kJ mol -1 ] methylbutane NOTE: The lower (more negative) the ΔH H2 º number the less stable the alkene.

12 ΔH H2 º of alkenes – what do they mean? ENERGYENERGY ΔHfºΔHfº H 2 (gas), C(soot) = 0 kJ mol -1 [Ch. 7, JBS 6 th ed.] -113 kJ -127 kJ [ΔH f º (2-MeBu)= -182 kJ mol -1 ] more stable less stable - Think of standard heats of formation, ΔH f º

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: You may not twist an alkene – like, for example, an ethane molecule, which twists ~ times a second at room temperature – unless you “pay” 264 kJ/mol and break the π – bond. ΔE = +264 kJ/mol 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 butane, gauche- (2E)-butene butane, anti-

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:

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

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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

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:

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, ] The experimentally determined structure of (Rhod)opsin

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 [s] a cascade of signals:

23 And – this is how you see:

24 [7.8] Macroscopic physical properties of alkenes - Lower alkenes (C2 to C5) are 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

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

26 [7.9] Preparation of alkene by elimination; E1 dehydration rxn of alcohols, recapitulation: Carbocation rearrangement (if possible): Acidification & loss of water; note Δ: β-elimination; Zaytsev regioselectivity:

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

28 [4] Preparation of alkenes by E2 reactions Example:

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:

30 Regiochemistry (where it occurs) of E2 reactions: Zaytsev, E1 (E2) Hoffmann, E2less stable more stable Make a note - similarities & differences:

31 Elimination regioselectivity: Zaytsev Hoffmann

32 E2 stereoselectivity – the definition:

33 E2 substrate stereochemistry – another view:

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

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

36 E2 practice: complete the reaction & name the product:

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


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