Hybrid Orbitals

Happy St. Patricks Day

fluorescein

Chicago River

1Alkenes

EThe double bond consists of a bond and a bond bond from head-on overlap of sp2 orbitals bond from side-on overlap of p orbitals

bond bondRotating a double bond requires breaking the bondNO FREE ROTATION at room temperatureContain at least one C=C double bondGeneral formula: CnH2n (like cycloalkanes)Each carbon atom in a C=C double bond is sp2 hybridizedA bond is stronger than a bond. bonds bond2Alkene Nomenclature (Naming)Parent chain = longest chain that includes the double bond(s)The double bonds have priority and must have the lowest number(s) possibleThe first C atom in the C=C bond indicates the double bonds location (or number in naming)Name, number, & alphabetize substituents as usualReplace ane ending with ene endingTwo double bonds: -diene; three double bonds: -trienePut double bond number in front of entire root name (i.e. 2-pentene indicates the double bond starts on carbon 2)7.Cyclic alkenes: number the atoms in the ring starting with the double bond

2-hexene3-butyl-2,4-hexadiene3Naming Practice

4-ethyl-3,5-dimethyl-2-heptene2,4-hexadiene3,4-dimethyl-1,3-pentadiene4-isopropyl-3,5-dimethyl-1,3,5-heptatriene4

Cis-trans isomerism in alkenesSubstituents will stay on the same or opposite sides of the double bond (no C=C bond rotation)

X 2-butene(same side) 2-butene(opposite sides)

For cis-trans isomerism, each C in the double bond must have 2 different substituents attached (i.e. a C and a H, etc.)Determining cis or trans: follow the parent chain through the double bondcis-3-methyl-2-heptenecis

trans-cis-5Cis/Trans Naming Practice

trans-4-ethyl-3,5-dimethyl-2-heptenetrans,trans-2,4-hexadiene3,4-dimethyl-1,3-pentadienetrans,trans-4-isopropyl-3,5-dimethyl-1,3,5-heptatriene

No Cis or TransBoth HsBoth CH3s

No Cis or TransBoth Hs6Important Common NamesVinyl (branch)PropyleneAllyl (branch)Ethylene

RRCH2HCi.e. polypropylene = milk jugsi.e. polyvinyl chloride = PVC pipei.e. diallyllysergamide = derivative of LSD i.e. polyethylene = plastic bags

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Arranging many double bondsCumulatedC=C double bonds all in a row: C=C=C=CConjugatedSingle and double bonds alternate: C=CC=CC=CIsolated>1 single bond between double bonds: C=CCC=CCC=CLycopene11 conjugated double bonds2 isolated double bonds

8Reactions of AlkenesAlkanes substitution reactionsR-H + A-B R-A + H-BR = residue, a generic alkyl group

Alkenes addition reactions

+ A-B Thermodynamics:Hrxn= bonds broken bonds formed= ( bond + bond) ( bond + bond)Exothermic reaction

bond is electron-rich9Alkene Addition Reactions

+ A-B ReactionHydrohalogenation(addition of H-X, X = halogen)Hydration(addition of H2O)Addition of halogensHydrogenation(addition of H2)A-BH-F, H-Cl, H-Br, H-IH-OHBr-Br, Cl-Cl, F-FH-H10Reactions at the Double Bond

bond is electron-richActs as a nucleophileAttracts electrophiles to form bondsElectrophile:electron-loverElectron deficient reagent (often + charged cations)seeks e- in reactionsNucleophile:nucleus-loverelectron-rich reagent (often charged anions)tries to donate e- to an electrophile in reactions, forming bonds11Markovnikovs RuleConsider the reactionVladimir Markovnikov

Two products are possibleExperimentally, only 2-chloropropane is formed

+1-propene1-chloropropane2-chloropropane

Markovnikovs Rule:The alkene carbon with the most H atoms gets the HHydrohalogenation (H-X), hydration (H2O) of alkenesWhy? Look at the reaction mechanism to find out...CC12Mechanism of HydrohalogenationStep 1: Alkene electrons attack H+, forming a carbocationStep 2: Cl- nucleophile forms bond with carbocation

or+carbocation

The carbocation formed in step 1 determines the final product

13Hydrohalogenation

Predict the product of the following hydrohalogenation reaction+ HBr

ORMarkovnikov ProductAnti-Markovnikov ProductRemember:Markovnikovs rule says that the H (from HBr) will bond to the alkene C with the most Hs14Remember:Markovnikovs rule says that the H (from H2O) will bond to the alkene C with the most HsHydration

Predict the product of the following hydration reaction+ H2OORMarkovnikov ProductAnti-Markovnikov Product

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Carbocation StabilityRelative carbocation stability: 3 > 2 >> 1 > methyl

>>>>231methylMost stableLeast stableWhy?Electrons can drift into the empty p orbital from C-H bonds on neighboring C atoms to help stabilizeResult:The final product comes from the most stable carbocation intermediate16

Mechanism of Hydration (R + H2O)Step 1: bond attacks electrophilic H+ catalyst forming carbocationStep 2: Nucleophilic H2O attacks carbocation, forming oxonium ionStep 3: Oxonium ion loses a proton, regenerating H+ catalyst

Most stable carbocation

+

Markovnikov productoxonium ion17

Mechanism of Halogen Addition (R + X2)Step 1: Br2 is polarized by electrons, Br+ attaches to alkeneStep 2:Br+ blocks access to one face of the alkeneBr- adds to the other face (anti addition)

bromonium ionintermediate

+trans isomer is the only product18Halogenation

+ Br2

+ Br2The Brs will add to opposite sides of a RING (anti addition) The Brs will be forced into a trans conformation ALWAYSTrans isomer

19Mechanism of Hydrogenation (R + H2)Occurs in the presence of a metal catalyst (like Pt)PtHH+PtHHStep 1: H2 adsorbs to catalyst surfaceStep 2: Both H atoms add to same face of alkene (syn addition)PtHHPt

cis isomer is the only product20HydrogenationOccurs in the presence of a metal catalyst (like Pt)

+ H2/Pt

+ H2/PtBoth Hs will add to the same side of a RING (syn addition) If branches are present, they will be forced into a cis conformationcis isomer21Hydrogenation of alkenesvegetable oils+ H2/Pt

unsaturatedsaturatedContains double bondsContains NO double bonds22Hydrohalogenation

Determine any reactant(s) that could yield the given product of the following hydrohalogenation reaction+ HBr

trans-3-methyl-2-hexene3-methyl-1-hexene

+ HBr

Major product using Ms ruleOnly possible reactant for this product23Hydrohalogenation

Determine any reactant(s) that could yield the given product of the following hydrohalogenation reaction+ HBrtrans-3-methyl-2-hexene

trans-3-methyl-3-hexeneAll three reactants could give this product

2-ethyl-1-pentene24HalogenationDetermine any reactant(s) that could yield the given product of the following halogenation reactiontrans-3,5-dimethyl-2-hepteneOnly one possible reactant in this case

+ Br225Alkene Reaction SummaryHydrohalogenation (+ HX)

Hydration (+ H2O)

Halogenation (+ X2)

Hydrogenation (H2/Pt)

Markovnikovs ruleCis/trans with rings26Polymerization of AlkenesPolymer: a large molecule made by linking together small repeat units called monomersPolymerization mechanism: radical chain reactionMonomerPolymerethene(ethylene)polyethylenepropene(propylene)polypropylene

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EAlkynesContain at least one CC triple bond with sp-hybridized C atomsTriple bond: one bond (sp orbitals), two bonds (p orbitals)Naming: triple bond indicated by yne endingReactivity: same addition reactions as alkenesUse 2 equivalents of addition reagent (i.e. + 2HCl)Use Markovnikovs rule in the same manner

ethyne(acetylene)

4-methyl-1-pentyne28Alkyne Naming Practice

3,4-dimethyl-1-pentyne2-methyl-3-hexyne3-methyl-1-pentyne2,5-dimethyl-3-hexyne

29Hydrohalogenation of Alkynes

Predict the product of the following hydrohalogenation reaction+ 2HBr?Break the reaction into two steps, adding 1 HBr each time to the multiple bond

+ HBr

+ HBrFinal Product1-pentyneMarkovnikovs rule still applies30Hydration of Alkynes

Predict the product of the following hydration reaction+ 2H2O?Break the reaction into two steps, adding H2O each time to the multiple bond

+ H2O

+ H2OFinal Product3,3-dimethyl-1-butyneMarkovnikovs rule still applies

31Halogenation of Alkynes

Predict the product of the following halogenation reaction+ 2Br2?Break the reaction into two steps, adding Br2 each time to the multiple bond

+ Br2

+ Br2Final Product3,3-dimethyl-1-butyne

32Hydrogenation of Alkynes

Predict the product of the following hydrogenation reaction+ 2H2?This reaction will simply turn the alkyne to an alkene, and then to an alkane

+ H2

+ H2Final Product3,3-dimethyl-1-butyne

33Alkyne Reaction Practice1.

2.

3.

4.

5.

+ 2HCl + 2H2O + 2Br2 + 2H2 + 2HCl

AND

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