Chapter 5: Alkenes and elimination reactions
Alkenes are unsaturated hydrocarbons. Formula is CnH2n (if one C=C).
Recall that the alkene carbons are sp2 trigonal planar. They are more e­ rich than alkanes, and react readily with electrophiles (chapter 6).
We will focus on the nomenclature, stability, and the preparation of alkenes in this chapter.
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Alkene nomenclature:
1. Number the longest chain across the alkene, giving the alkene the lowest number possible.
• OH will get a lower number before alkene or alkyne
• Then alkene
• alkyl halide or alkyl are lower priority for numbering chain carbons.
2. Name chain and branches as with alkanes, but precede the name with the number of the alkene carbon, and end with the suffix ­ene.
Number left to right so alkene carbon starts on the lower number (3 instead of 4)
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Geometric Isomers or cis and trans alkenes
An alkene has geometric isomers if
• There is restricted roatation about C=C
• AND
Does the following molecule exhibit geometric isomerism?
Naming geometric isomers.
We name geometric isomers either
cis (Z) zusammen together
trans (E) entgegen opposite
Use Cahn­Ingold­Prelog rules of nomenclature
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Cycloalkenes
By default carbon 1 must start on an alkene (unless there is an OH). C­2 must be the 2nd C of the alkene. Note: cycloalkenes have cis double bonds in rings less than 8 carbons.
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Preparing alkenes via elimination
Two mechanisms, E1 and E2
E1 competes with SN1 E2 competes with SN2
E2: • Concerted mechanism
• One step, 2nd order
• Rate = k[base][alkyl halide]
• best with non­nucleophilic bases
• the H that is removed must be anti periplanar to the leaving group.
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E1
• Competes with Sn1
• Carbocation intermediate (rearrangements)
• Rate = k[alkyl halide]
• often ROH and H2SO4 and heat
• Neutral Base and heat
• Zaitzev's rule predicts thermodynamically stable product
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