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CHEM 233: CHAPTER 10: ALKENES
Alkenes |
Contains one pi bond and one sigma bond between two carbon atoms
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Terminal Alkenes
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have the double bond at the end of the carbon chain
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Internal Alkenes
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Have at least one carbon atom bonded to each end of the double bond.
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Cycloalkenes
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Contain a double bond in a ring
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C-C Pi Bond
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- 267 KJ/mol
- the pi bond is much weaker than the sigma bond of a C-C double bond, making it much more easily broken. As a result, alkenes undergo many reactions that alkanes do not.
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Restricted Rotation
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The rotation around the C-C double bond is restricted. Rotation can occur only if the pi bond breaks and then reforms, a process that is unfavorable.
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Stereoisomerism
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Whenever two groups on each end of a C=C are different from each other, two diastereomers are possible. Cis- and trans- are diastereomers.
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Stability |
Trans alkenes are generally more stable than cis alkenes. The stability of an alkene increases as the number of R groups on the C=C increases.
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Addition Reactions
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Every reaction of alkenes involves addition: the pi bond is always broken.
Because alkenes are electron rich, simple alkenes do NOT react with nucleophiles or bases, reagents that are themselves electron rich. Alkenes react with electrophiles.
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Syn Addition
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Takes place when both X and Y are added from the same side.
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Anti Addition
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Takes place when X and Y are added from opposite sides
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Hydrohalogenation |
The addition of hydrogen halides HX (X= Cl, Br, I) to alkenes to form alkyl halides.
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Addition of HX
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Two bonds are broken in this reaction - the weak pi bond of the alkene and the HX bond - and two new sigma bonds are formed - one to H and one to X.
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Electrophilic Additions
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The electrophilic (H) end of HX is attracted to the electron-rich double bond.
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Addition Reactions
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Exothermic
-Occur in two steps: 1. addition of H to form carbocation and 2. nucleophilic attack of X
- Consists of two Lewis acid-base reactions
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Markovnikov's Rule
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In the addition of HX to an unsymmetrical alkene, the H atom bonds to the less substituted carbon atom - that is, the carbon that has more H atoms to begin with.
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Carbocation Stability
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The less stable, the more energy it requires for a transition state
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Intermediate Carbocation
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The more stable, more substituted carbocation is formed by addition of the electrophile to the less substituted carbon.
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1,2-CH3 Shift
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Moving the methyl to carbon with more H's on it, so that the carbocation can be the most stable (secondary or tertiary)
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Stereochemical Principles
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Trigonal planar atoms react with reagents from two directions with equal probability and achiral starting materials yield achiral or racemic products.
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Hydrohalogenation Reactions
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Begin with achiral reactant and form an achiral product.
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Because a product has a stereogenic center at a newly formed sp3 hybridized carbons,
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and equal amount of two entiomers - a racemic mixture must form!
Two products of equal amount!
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Hydrohalogenation |
occurs with syn and anti addition of HX
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Stereospecific Reaction
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A reaction is stereospecific when each of two specific stereoisomers of a starting material yields a particular stereoisomer of a product.
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Halogenation Mechanism
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Addition of X+ forms an unstable bridged halonium ion in the rate-determining step. Nucleophilic attack of X- occurs from the back side to form trans products. The overall result is anti addition of X2 across the double bond.
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Bromohydrins |
Br2 and H2O forms bromohydrins, but can also be formed from N-bromosuccinimide (NBS) in aqueous DMSO [(CH3)2S=O].
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Halohydrins |
Preferred product has the electrophilic X+ bonded to the less substituted carbon atom (more H's); the nucleophile (H2O) bonds to the more substituted carbon. Anti addition occurs.
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Hydroboration-Oxidation
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Hydroboration-oxidation is a two step reaction sequence that converts an alkene to an alcohol. Addition of borane (BH3) to an alkene, forming an alkylborane. Oxidation converts the C-B bond of the alkylborane to a C-O bond.
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