CHEM 109A
CLAS
Why is an Alkyne Less Reactive Than and Alkene?
When alkenes react with electrophiles, a carbocation is formed.
We learned that carbocations are stabilized by hyperconjugation – the more
hyperconjugation the more stable
3o > 2o > 1o > Me
When alkynes react with electrophiles, a vinylic carbocation is formed (where the +
charge is on a vinyl/sp hybridized C). Vinylic carbocations are LESS stable than
previous carbocations (where the + charge is on an sp2 hybridized C) b/c the geometry
does not allow for effective hyperconjugation.
Putting them in order of decreasing stability (pg. 266)…
The less stable the intermediate, the higher the activation energy barrier for the reaction
and therefore the slower the reaction.
Figure 6.2 from Bruice 5th Ed.
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CHEM 109A
CLAS
Why is an Alkyne Less Reactive Than and Alkene?
If we take the example reactions of ethane and ethyne/acetylene with HBr and look at the
first step…
C2H4 + HBr → H3C-C+H2 + BrBDE: 174
87
101
kcal/mol (From Table 3.2, pg146)
1o C+
ΔH = 621-101 = 160 kcal/mol
ΔG* ~ 160 kcal/mol
C2H2 + HBr → H2C-C+H + BrBDE: 231
87
~109.5
1o vinylic C+
ΔH = 318-109.5 = 208.5 kcal/mol
ΔG* ~ 208.5 kcal/mol
kcal/mol (From Table 3.2, pg146)
*ΔS for both reactions is ~ 0 (2 reactants form 2 products so no loss of “freedom” or
“disorder”)
Both reactions are endergonic, so the T.S.s are similar in energy and structure to the
products (Hammond Postulate). The product of the alkyne reaction (a 1o vinylic C+) is
less stable than the product of the alkene reaction (1o C+) so the TS for the alkyne
reaction must be less stable/higher in energy than the TS for the alkene reaction (EvansPolanyi Principle). Alkyne reaction therefore has a higher activation energy barrier (see
Fig 6.2 above) and will proceed slower than the alkene reaction.
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