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Explain the trend in BP of alkenes?
- increases with molecualr weight
- increased branching leads to greater volatility and lower boiling points
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Like alkanes, alkenes are relatively __. They are __ in water but __ in nonpolar solvennts. They tend to be slightly more __ than alkanes for two reasons: what are these reasons?
- nonpolar
- insoluble
- soluble
- polar
- 1) the more weakly held electrons in the pi bond are more polarizable (instantaneous dipole moment)2) the vinylic bonds tend to be slightly polar (contributing to a permanent dipole moment)
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Alkyl groups are slightly __ toward a double bond, helping to stabilize it. This donation polarizes the __, with a small partial positive charge on the allkyl group and a small negative charge on the C=C atom.
- electron donating
- vinylic bond
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In a cis-disubbed alkene, the vector sum of hte two dipole moments is directed __. In a trans-disubbed alkene, the two dipole moments __.
If an alkene is symmetrically trans disubbed, teh dipole moment is __.
- perpendicular to the double
- tend to cancel out
- zero
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Compounds with permanent dipole moments engage in __ while those without permanent dipole moments engage in __. cis-2-butene and trans-2-butene have similar __ but only cis has __. Because of its increased, intermolecular attractions, cis must be heated to a slighly higher temp to boil.
- dipole dipole attractions
- van der Waals attractions
- van der Waals attractions
- dipole dipole attractions
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__ is the elimination of a hydrogen and a halogen from an alkyl halide to form an alkene.
dehydrohaogenation
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The __ give excellent yields with bulky secondary and teritiary alkyl halides. A strong base forces __ by __. The molecule's bulkiness hinders __, and a relatively pure elimination proudct results. Tertiary halides are the best __ because they are prone to elimination and cannot under Sn2 sub.
- E2 dehydrohalogenation
- second order elimination
- abstracting a proton
- sn2
- E2 substrates
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If the substrate is probe to substitution, what can a bulky base do?
large alykl groups on a bulky base do what?
- minimize the amount of substitution
- hinder its approach to attack a carbon atom (sub), but can easily abstract a proton
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Bulky bases can also accomplish __ that don't follow __. __ often prevents a bulky base from abstracting a proton that leads to the most highly subbed alkene. In these cases, it abstracts a less hindered proton, leading to the formation of the __.
- dehydrohalogenation
- Zaitsev rule
- Steric hindrance
- hoffman product
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Hoffman vs. Zaitsev
Zaitsev product will occur on an unhindered molecule, but Hofmann will on a bulky ion
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Like Sn2, E2 is__: Different stereoisomers of the reactant give different stereoisomers of the product. The E2 is stereospecific because it __.. The products are alkenes, and different __ of starting materials commonly give different __.
- stereospecific
- normally goes through an anti and coplanar transition state
- diastereomers
- diastereomers of alkenes
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Nearly all cyclohexanes are most stable in __, which places the C-C bonds as __ and any two adjacent C atoms with __ in an __, ideally oriented for E2 reaction.
- chair conformations
- staggered
- axial bonds
- anti-coplanar conformation
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On any two adjacent carbon atoms, one has its __ pointing up and the other _ pointing down. These two bonds are trans to each other, and they are __.
- axial bond x2
- trans-diaxial
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An E2 elim can take place on a chair conformation only if what?
the proton and the leaving group can get into a trans-diaxial arrangement
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__ are converted to alkenes by __ with iodide ion in acetone. This __ is rarely an important synthetic reaction, because the most likely origin of a __ is from __.
- vicinal dibromides
- reduction
- debromination
- vicinal dibromide
- bromination of an alkene
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__ is formally a __ because a molecule of Br2 )an oxidizing agent) is removed. The reaction with iodide takes place by the E2 mechanism, with the same geonmetric constraints as the ___.
- debromination
- reduction
- E2 dehydrohalogenation
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Elimination usually takes place through an __.
anti-coplanar arrangement
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__ usually takes place in __, without a __ or base to force __. The substrate is usually a __ or __.
- First order dehydrohalogenation
- a good ionizing solvent
- strong nucleophile or base
- second order kinetics
- secondary or tertiary alkyl halide
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First order elimination requires __ to form a __, which loses a proton to a weak base. __ is generally accompanied by Sn1 sub, because the nucleophilc solvent can also attack the __ directly to form the sub product
- ionization
- carbocation
- E1 dehydrohalogenation
- carbocation
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Dehydrohalogenation mechanism of E1
- elim by the E1 mechanism
- accompanied by Sn1
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E2 debromination of a vicinal dibromide
E2 debromination takes place by a concerted, stereospecific mechanism. I- removes one Br atom and the other Br leaves as bromide ion
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E1 dehydrohalogenatioins are prone to __.
rearrangement
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