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by: Caroline Hurlbut

Alkenes Chem 341

Caroline Hurlbut
GPA 3.7

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Intro to alkene/alkyne structure, alkene reactivity and hydrohalogenation, and rules for naming alkenes and alkynes.
Modern Organic Chemistry I
Anna Elizabeth Allen
Class Notes
Organic Chemistry
25 ?




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This 3 page Class Notes was uploaded by Caroline Hurlbut on Sunday September 25, 2016. The Class Notes belongs to Chem 341 at Colorado State University taught by Anna Elizabeth Allen in Summer 2015. Since its upload, it has received 5 views. For similar materials see Modern Organic Chemistry I in Chemistry at Colorado State University.


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Date Created: 09/25/16
• alkenes and alkynes are unsaturated hydrocarbons b/c they have fewer than the max # of H atoms per carbon due to double/triple bonds rings also reduce # H atoms in a molecule • • degree of unsaturation increases as # pi bonds increases —single bonds (4 H per C) —double bonds (2 H per C) —triple bonds (1 H per C) • calculate degree of unsaturation or # pi bonds/rings: (2n+2) - x where n = # carbons 2 —ex. How many degrees of unsaturation are in C8H14?—>[2(8)+2] - 14 = 2 2 —>2 pi bonds or rings for every molecule with formula C8H14 • alkene reactivity —alkenes contain a reactive electron rich pi bond and participate in addition reactions with electrophiles —hydrohalogenation: addition of hydrogen halides to alkenes to form alkyl halides • mechanism - step by step process for a reaction ex. mechanism for hydrohalogenation of C2H4 into an alkyl halide • energetics of hydrohalogenation —transition state: highest point in energy for each step of a reaction—>occurs when bonds are breaking/forming —intermediate: molecule that is formed and consumed in a reaction b/c they are higher energy —activation energy: energy barrier that must be overcome for a reaction to proceed —enthalpy: energy difference between reactants and products • carbocation stability —hyperconjugation: stabilizing interaction of electrons in a sigma bond with an adjacent empty or partially filled p/pi orbital —more alkyl groups around carbocation—>more possibility for hyperconjugation—> more stable catalyzed addition to alkenes: hydration • —water is not revive enough to protonate an alkene, so a strong acid is also required —strong acid serves as a catalyst • Markovnikov’s rule - in a reaction between a strong acid and an alkene/alkyne, the H atom of the acid bonds to the C atom of the alkene/alkyne with the greatest # of H atoms to make most stable carbocation • retrosynthesis reactions - working backwards from product to find reactants • alkenes - hydrocarbon molecules with C—C double bonds • a C=C bond consists of 1 sigma and 1 pi bond • pi bonds are the result of side-on overlap of orbitals sigma bond is lowest energy bond—>electrons in that bond are most stable and less reactive • (opposite is true for pi bonds) • more reactions can be done with pi bonds bc they are more reactive and easier to break • most alkanes can rotate about their C—C bonds, but the C=C bonds of alkenes cannot rotate • stereoisomers - isomers that only differ in the way atoms or groups are oriented in space —trans isomers: groups on opposite side of pi bond —cis isomers: groups on same side of pi bond • alkynes - hydrocarbon molecules with C—C triple bonds • a C—C triple bond consists of 1 sigma bond and 2 pi bonds bond length and strength • —lengths range from longest for single bonds to shortest for triple bonds —length and strength are inversely related —% s-character A. % s-character = (# s orbitals)/(total # orbitals) x 100 B. ex. sp3: s + p + p + p = 1s/4 orbitals = 25% s-character C. ex. sp2: s + p + p = 1s/3 orbitals = 33% s-character • hybridization of carbons has large effect on C—H bonds —less hybridization—>fewer bonds—>increased acidity —lone pairs more stable when held in orbital of higher s-character b/c higher s-character holds electrons closer to nucleus • nomenclature for alkenes —find longest parent carbon chain that contains both carbons of the double bond and change ending to -ene —number carbons to give the double bond carbons (the alkene) the lowest numbers A. if alkene has same number from either side, number carbons to give substituents the lowest numbers —name substituents on parent chain —use prefixes as needed and indicate stereoisomers (trans or cis) —give location of alkene in the name and use prefix depending on how many double bonds are in the molecule A. 2 double bonds—>prefix-diene (ex. octadiene) • deciding if an alkene is E (trans) or Z (cis) —divide alkene in half at the double bond and examine groups attached to each double bonded carbon —identify highest priority group on each carbon of the double bond 1 atom of higher atomic # is priority A. B. go 1 bond out at a time down a chain until there is 1 atom of higher atomic # C. highest priority atoms are on the same side—>alkene is Z D. highest priority atoms are on different sides—>alkene is E • nomenclature for alkynes —find parent chain and change ending to -yne —number carbons to give triple bond the lowest numbers —name substituents on parent chain —use prefixes as needed (2 triple bonds—>diyne, 3 triple bonds—>triyne, etc.) molecules with both double and triple bonds get the ending -enyne, numbering carbons to • give the alkene or alkyne the lowest numbers possible


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