Unformatted text preview: \g" VII. Alkyne Reactions: (read Ch.9.8 — 9.14)
Like alkenes, alkynes undergo addition reactions. ,A. a. Reduction _
I 1. Catalytic hydrogenation (complete and partial) . _
, a) complete reduction of alkynes in the presence of ﬁnely divided Pt, Pd, N1 or Rh With 2
b equivalents of hydrogen yields an alkane. Alkene is an intermediate.
«X cat. ’ , .
R_ EG_RI + 2H2 —> R—CHchZ—R I work SO T€CLC£IWJ
6/ all/KW C”
catalyst = Pt, Pd, Ni, or Rh
Heats of hydrogenation: CH3CH2CECH CH3CHECHCH3
229 kJ/mol 275 kJ/mol
Alkyl groups stabilize triple bonds in the same way that they stabilize double bonds.
*Internal triple bonds are more Stable than terminal ones. b) partial reduction Alkynes could he used to prepare alkenes if a catalyst were available that
is active enough to catalyze the hydrogenation of alkynes, but not active enough for the
hydrogenation of alkenes. H2 H2
R-CEC-R' cat R CH=CH R' X > R CHZCHZ R'
There is a catalyst that will catalyze the hydrogenation of alkynes to alkenes, but not that of
alkenes to alkanes. It is called the Lindlar catalyst and consists of palladium supported on
[d CaCO3, which has been poisoned with lead acetate and quinoline.
syn-Hydrogenation occurs; cis alkenes free of their trans isomer are formed.
‘ ﬂ,“ 1* g x
a) [email protected]—R' 2 CZC V6 l M ~— CA
Lindlar Pd R’ ‘R
5 2. Dissolving metal reduction (partial reduction to trans-alkene)
Another way to convert alkynes to alkenes is by reduction with sodium (or lithium or
potassium) in ammonia. tra’ . ! .r n s are formed.
R-CEC- +> R—CHZCHz—R'
KM) jLL l K 2 ATM% Metal (Li, Na, K) is reducing agent; H2 is not involved Mechanism: four steps Overall reaction: H\ ,R'
R—cEc—R' + 2 M + 2 NH3 pzq + 2 MNH2
/3 trans-alkene ...
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- Summer '08
- Organic chemistry