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Alkynes
contain a C
C triple bond
Acetylene: H-C
C-H is the common name for ethyne,
used as a torch fuel
Number from the end closest to the triple bond
use branching If it's a tie between the ends to two triple bonds,
they are equal distance to the end of a chain. Double bonds take priority over triple bonds when
can still have stereochemical designators. E/Z, although any double bonds in the molecule Triple bonds are linear so there is no cis/trans or
naming rules: Alkyne nomenclature follows normal hydrocarbon Chapter 8: Alkynes: an introduction to organic synthesis
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This is useful in making an The same can be done (twice) to make an alkyne. elimination of H and Br from an alkyl bromide using KOH. Recall from 7.1 that an alkene can be prepared by
alkyne
(triple bond) from an
alkene
(double bond).
either a As we'll see later, an alkyne can be reduced selectively to
cis
or
trans
alkene, so this can be a way of
converting between
cis
and
trans
alkenes.
8.2 Preparation of alkynes: elimination of dihalides
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Alkynes contain linear (
o
bond angle),^
sp
hybridized
The triple bond is made of one carbons.
and two perpendicular
carbocation (+ is on a double-bonded C) Addition of HX to an alkyne takes place via a vinylic bond in an alkene. π bonds. Those π bonds can react very similarly to the π 8.3 Reactions of alkynes: addition of HX
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Two equivalents of HBr or HCl can add to an alkyne
have both with Markovnikov regiochemistry. H and X usually
trans
stereochemistry.
Halogens can also add twice: Addition of HX and X
2
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BH
3
adds to alkynes in a non
- Markovnikov orientation to
Terminal alkynes produce an of products. Again, unsymmetrical internal alkynes produce a mixture tautomerizes to a more stable ketone. give a vinylic borane. Oxidation makes an enol that
aldehyde
because addition
hydration. is non-Markovnikov, opposite of Hg(II)-catalyzed Hydroboration/oxidation of alkynes
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"poisoned" Pd on CaCO Reduction with a deactivated catalyst (Lindlar catalyst = reduction to an alkane just like hydrogenation of alkenes. Catalytic hydrogenation with Pd/C gives complete
3 ) occurs by a syn addition to give
a (^) cis
(^) alkene.
A
(^) trans
(^) alkene can be formed with a dissolving
metal
trans reduction. Addition of H's is stepwise so the more stable
(^) alkene is formed.
8.5 Reduction of alkynes
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Alkynes can be cleaved by KMnO
4
or O
3
to make
Ozonolysis of an unknown alkyne gives one 6 alkynes. can be used to determine the position of triple bonds in carboxylic acids. This is not so useful synthetically, but
carbon
dicarboxylic acid HOOC-(CH
2
4
-COOH and 2 equivalents
of acetic acid, CH
3
COOH. What is the structure of the
unknown alkyne? 8.6 Oxidative cleavage of alkynes
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Terminal alkynes are weak acids (p
K
a (^) ~25).
HC Which direction will the following equilibria lie?
≡ CH + OH
HC
≡ C:
2 O
HC
CH + NH
2
HC
≡ C:
3
NaNH
2 (^) is Na
NH^
2
(sodium amide)
(^) a base strong enough
an alkyne is called an to deprotonate terminal alkynes. The conjugate base of
(^) acetylide ion
(Hydrogens attached to
(^) sp
(^) carbons are more acidic than
sp 2 or^ (^) sp
3 because the^
(^) sp
(^) carbon anion is more stable)
Molecule
p K a
CH
4
CH
2 =CH
2
NH
3
HC
CH
H
2 O
8.7 Alkyne acidity: formation of acetylide anions
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Retrosynthetic analysis
(^) is the best strategy for planning
syntheses: it's working
(^) backwards
(^) - first looking at the
The retrosynthetic analysis uses a two what reactions could make that product. complex final product you need to make, and considering
bar arrow.
retrosynthetic direction are actual carbon Carbon-carbon bonds that are "broken" in the
carbon bond
Retrosynthetic direction forming reactions in the forward direction.
Forward direction: Retrosynthetic analysis
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reactions with reagents. then write the full forward sequence of synthetic Plan a retrosynthesis for the following synthesis problem, Synthesis practice
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reactions with reagents. then write the full forward sequence of syntheticPlan a retrosynthesis for the following synthesis problem, Synthesis practice
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synthetic reactions with reagents. carbons, then write the full forward sequence of starting with compounds with no more than five Plan a retrosynthesis for the following compound, Synthesis practice
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