08 alkesynth

Bonding in Ethylene C=C bond is shorter than C—C bond a) sp2 hybrid orbitals have _______ s character than the sp3 hybrid orbitals. They are __________ to the nucleus. b) Pi overlap brings carbon atoms _______ Bonding requires two p orbitals overlapping __________© 2013 Pearson Education, Inc. Chapter 7 2

Cis and trans isomers cannot be interconverted without breaking the pi bond (264 kJ/mole). No rotation around the carbon–carbon bond is possible giving rise to Cistrans geometric isomers© 2013 Pearson Education, Inc. Chapter 7 3

Elements (degrees) of Unsaturation. Also called index of hydrogen deficiency. • Unsaturation: A structural element that decreases the number of hydrogens in the molecule by two. • Double bonds and rings are elements of unsaturation (can not distinguish with calculation Calculate the degrees of unsaturation for a compound with formula C5H8 and propose a structure • Reference (saturated compound CnH2n+2) • Compound© 2013 Pearson Education, Inc. Chapter 7 4

• For each Group VA atom increase Hs in reference by one • For each halogens decrease Hs atoms in reference by one • Group of oxygen does not change the C:H ratio, ignore oxygen in the reference formula. Calculate the IHD for a Compound with Formula C4H7Br. C6H7N. C5H4O.© 2013 Pearson Education, Inc. Chapter 7 5

EZ nomenclature: Use the Cahn–Ingold–Prelog rules to assign priorities to groups attached to each carbon in the double bond. If high-priority groups are on the same side, the name is Z (for zusammen). If high-priority groups are on opposite sides, the name is E (for entgegen). E-1-bromo-1-chloropropene© 2013 Pearson Education, Inc. Chapter 7 8

Heat of Hydrogenation can provide valuable data as to the stability of the double bond. The more substituted the double bond, the ___________its heat of hydrogenation.© 2013 Pearson Education, Inc. Chapter 7 9

Stability: cis-2-butene<iso-butene<trans-2-butene -120 kJ >-117 kJ > -116 kJ Cyclopropene is highly strained. Rings that are five-membered or larger can easily accommodate double bonds, and these cycloalkenes react much like straight-chain alkenes.© 2013 Pearson Education, Inc. Chapter 7 10

• Cis isomer is ______ stable than trans in small cycloalkenes.• Must have at least _________carbons to form a stable trans double bond.• For cyclodecene (and larger), the trans double bond is almost as stable as the cis.• All cycloalkenes are assumed to be cis unless otherwise specifically named trans.© 2013 Pearson Education, Inc. Chapter 7 11

Bredt’s Rule: A bicyclic compound cannot have a double bond at a bridgehead position unless one of the rings contains at least eight carbon atoms. Which of the following alkenes are stable?© 2013 Pearson Education, Inc. Chapter 7 12

Alkenes have relatively low boiling points which depend on surface area and polarizability (______________ forces _______ dense than water _________ on water Slightly polar:  Pi bond is polarizable, so instantaneous dipole– dipole interactions occur.  Alkyl groups are electron-donating toward the pi bond, so may have a small dipole moment. • Cisalkenes have a greater dipole moment than trans alkenes, so they will be slightly polar and have higher bp© 2013 Pearson Education, Inc. Chapter 7 14

Alkene Synthesis Overview 1. R-X ----E2 dehydrohalogenation (-HX) Alkene 2. R-X ----E1 dehydrohalogenation (-HX) Alkene 3. Vic dibromide---Dehalogenation (-X2) Alkene 4. Alcohol ---Dehydration (-H2O) Alkene© 2013 Pearson Education, Inc. Chapter 7 15

1. Dehydrohalogenationby the E2 Mechanism • Strong_________ abstracts H+ as double bond forms and X- ___________ from the adjacent carbon. • __________ and hindered ______________ alkyl halides give alkenes in good yields.Tertiary halides are the best E2 substrates because they cannot undergo __________substitution.© 2013 Pearson Education, Inc. Chapter 7 16

Bulky Bases for E2 Reactions • If the substrate is prone to substitution, a bulky base can minimize the amount of substitution. • Large alkyl groups on a bulky base hinder its approach to attack a carbon atom (substitution), yet it can easily abstract a proton (elimination).© 2013 Pearson Education, Inc. Chapter 7 17

Hofmann Product Bulky bases, such as potassium tert-butoxide, abstract the least hindered H+,giving the less substituted alkene as the major product (Hofmann product).© 2013 Pearson Education, Inc. Chapter 7 18

Show that the dehalogenation of 2,3-dibromobutane by iodide ion is stereospecific by showing that the two diastereomers of the starting material give different diastereomers of the product. Rotating meso-2,3-dibromobutane into a conformation where the bromine atoms are anti and coplanar, we find that the product will be trans-2-butene. A similar conformation of either enantiomer of the (±) diastereomer shows that the product will be cis-2-butene. (Hint: Your models will be helpful.) Meso-2,3-dibromobutane (R,R)-2,3-dibromobutane© 2013 Pearson Education, Inc. Chapter 7 20

E2 Reactions of bromocyclohexane is slow • An anti-coplanar conformation (180°) can only be achieved when both the hydrogen and the halogen occupy ___________ positions. • The chair must flip to the conformation with the axial halide in order for the elimination to take place.© 2013 Pearson Education, Inc. Chapter 7 21

Explain why the following deuterated 1-bromo-2- methylcyclohexane undergoes dehydrohalogenation by the E2 mechanism, to give only the indicated product. Two other alkenes are not observed. In an E2 elimination, the hydrogen atom and the leaving group must have a trans-diaxial relationship. In this compound, only one hydrogen atom—the deuterium—is trans to the bromine atom. When the bromine atom is axial, the adjacent deuterium is also axial, providing a trans-diaxial arrangement.© 2013 Pearson Education, Inc. Chapter 7 22

3. E1 reaction • Tertiary reacts faster than secondary alkyl halides: • Carbocationintermediate; Rearrangements are possible. • Solvolysis reactions in solvents as water or alcohols • Usually have SN1 products, too, since the solvent can attack the carbocation directly.© 2013 Pearson Education, Inc. Chapter 7 24

4. Dehydration of Alcohols• Use concentrated H2SO4 or H3PO4 and remove low-boiling alkene as it forms to shift the equilibrium and increase the yield of the reaction.• E1 mechanism.• Carbocation intermediate. Rearrangements can occur.• Reaction obeys Zaitsev’s rule.© 2013 Pearson Education, Inc. Chapter 7 25

Dehydration Mechanism: E1 Step 1:Protonation of the hydroxyl group (fast equilibrium). Step 2: Ionization to a carbocation (slow; rate limiting). Step 3: Deprotonation to give the alkene (fast).© 2013 Pearson Education, Inc. Chapter 7 26

Propose a mechanism for the sulfuric acid–catalyzed dehydration of t-butyl alcohol. The first step is protonation of the hydroxyl group, which converts it to a good leaving group. The second step is ionization of the protonated alcohol to give a carbocation. Abstraction of a proton completes the mechanism.© 2013 Pearson Education, Inc. Chapter 7 28

Other Industrial methods: a) Catalytic Cracking of Alkanes • Long-chain alkane is heated with a catalyst to produce an alkene and shorter alkane. • Complex mixtures are produced.© 2013 Pearson Education, Inc. Chapter 7 29

b) Dehydrogenation of Alkanes • Dehydrogenation is the removal of H2 from a molecule, forming an alkene (the reverse of hydrogenation). • This reaction has an unfavorable enthalpy change but a favorable entropy change.© 2013 Pearson Education, Inc. Chapter 7 30

08 alkesynth

by Los Angeles Trade Technical College on Jun 25, 2012

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