Alkynes and Synthesis

1
 Alkyne Structure
• Alkynes contain a carbon-carbon triple bond.
• An alkyne has the general molecular formula CnH2n−2,
giving it four fewer hydrogens than the maximum possible
for the number of carbons present.
• The triple bond introduces two degrees of unsaturation.
• Terminal alkynes have the triple bond at the end of the
carbon chain so that a hydrogen atom is directly bonded to
a carbon atom of the triple bond.
• Internal alkynes have a carbon atom bonded to each
carbon atom of the triple bond.

2
 Alkyne Bonding
• Recall that the triple bond consists of 2  bonds and 1  bond.
• Each carbon is sp hybridized with a linear geometry and bond
angles of 180o.

3
 Strength of Alkyne Bonds

• Bond dissociation energies of the C − C bonds in ethylene

(one  and one  bond) and acetylene (one  and two
 bonds) can be used to estimate the strength of the
second  bond of the triple bond.

4
 Cyclic Alkynes
• Like trans cycloalkenes, cycloalkynes with small rings are
unstable.
• The carbon chain must be long enough to connect the two
ends of the triple bond without introducing too much strain.
• Cyclooctyne is the smallest isolated cycloalkyne, though it
decomposes upon standing at room temperature after a short
time.

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 Naming Alkynes
• Alkynes are named in the same general way that alkenes are
named.

• In the IUPAC system, change the –ane ending of the parent

alkane name to the suffix –yne.

• Choose the longest continuous chain that contains both

atoms of the triple bond and number the chain to give the
triple bond the lower number.

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 Naming Alkynes
• Compounds with two triple bonds are named as diynes, those
with three are named as triynes and so forth.
• Compounds with both a double and triple bond are named as
enynes.
• The chain is numbered to give the first site of unsaturation
(either C=C or CC) the lower number.

Figure 11.1

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Naming Alkynes
 Naming Alkynes

• Alkynyl groups are also possible

 Exercise
• Name the following alkynes:

A)

B)
 Physical Properties of Alkynes

• The physical properties of alkynes resemble those of

hydrocarbons of similar shape and molecular weight.

• Alkynes have low melting points and boiling points.

• Melting point and boiling point increase as the number of

carbons increases.

• Alkynes are soluble in organic solvents and insoluble in

water.

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 Acetylene
• The simplest alkyne, H−CC−H, named in the IUPAC system
as ethyne, is more often called acetylene, its common name.

• The two-carbon alkyl group derived from acetylene is called

an ethynyl group.

• Acetylene (H−CC−H) is a colorless gas that burns in oxygen

to form CO2 and H2O.
• The combustion of acetylene releases more energy per
mole of product formed than any other hydrocarbons.
• When combined with oxygen, it burns with a very hot flame
and is used in welding.

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 Oral Contraceptives
Figure 11.2

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 Oral Contraceptives
• RU-486 and levonorgestrel are two other synthetic hormones.
• RU-486 blocks the effects of progesterone, thus preventing
pregnancy.
• It is used to induce abortions within the first few weeks of
pregnancy.
• Levonorgestrel interferes with ovulation, and so it prevents
pregnancy if taken within a few days of unprotected sex.

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 Histrionicotoxin
Figure 11.3

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 Preparation of Alkynes
• Alkynes are prepared by elimination reactions.
• A strong base removes two equivalents of HX from a vicinal
or geminal dihalide to yield an alkyne through two successive
E2 elimination reactions.

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 Preparation of Alkynes from Alkenes
• Since vicinal dihalides are readily made from alkenes, one
can convert an alkene to the corresponding alkyne in a
two-step process involving:

• Halogenation of an alkene.

• Double dehydrohalogenation of the resulting vicinal

dihalide.

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 General Addition Reactions of Alkynes

• Like alkenes, alkynes undergo addition reactions because

they contain relatively weak  bonds.
• Two sequential reactions can take place: addition of one
equivalent of reagent forms an alkene, which can then add a
second equivalent of reagent to yield a product having four
new bonds.

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 Addition Reactions of Alkynes

Figure 11.5

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Hydrohalogenation—Electrophilic Addition of HX

• Alkynes undergo hydrohalogenation, i.e., the addition of

hydrogen halides, HX (X = Cl, Br, I).

• Two equivalents of HX are usually used: addition of one mole

forms a vinyl halide, which then reacts with a second mole of
HX to form a geminal dihalide.

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Hydrohalogenation—Markovnikov’s Rule

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Hydrohalogenation Mechanism

22
 Hydrohalogenation of Alkynes vs. Alkenes
• Electrophilic addition of HX to alkynes is slower than
electrophilic addition of HX to alkenes, even though alkynes are
more polarizable and have more loosely held  electrons than
alkenes.

• Markovnikov addition in step [3] places the H on the terminal

carbon to form the more substituted carbocation A, rather than
the less substituted carbocation B.

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 Halogen Stabilization of Carbocations
• Resonance stabilizes a molecule by delocalizing charge and
electron density.
• Halogens stabilize an adjacent positive charge by resonance.
• Carbocation A is stabilized by resonance.

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 Halogenation of Alkynes

• Halogens X2 (X = Cl or Br) add to alkynes just as they do to

alkenes.
• Addition of one mole of X2 forms a trans dihalide, which can
then react with a second mole of X2 to yield a tetrahalide.

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Halogenation Mechanism

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 Exercise

• Identify the product

 Hydration of Alkynes
• In the presence of strong acid or Hg2+ catalyst, the elements of
H2O add to the triple bond to form an enol initially.
• The enol is unstable and rearranges to a ketone.

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 Hydration of Internal vs. Terminal Alkynes
• Internal alkynes undergo hydration with concentrated acid to
form ketones.
• Terminal alkynes require the presence of an additional Hg2+
catalyst (usually HgSO4) to yield methyl ketones by
Markovnikov addition of water.

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 Keto-Enol Tautomerization
• Tautomers are constitutional isomers that differ in the location
of a double bond and a hydrogen atom.
• A and B are tautomers: A is the enol form and B is the keto
form of the tautomer.

• An enol tautomer has an O−H group bonded to a C=C.

• A keto tautomer has a C=O and an additional C−H bond.
• Equilibrium favors the keto form largely because the C=O is
much stronger than a C=C.
• Tautomerization, the process of converting one tautomer into
another, is catalyzed by both acid and base. 30
Tautomerization Mechanism

31
Hydration Mechanism

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 Exercise

• What products are obtained by hydration of:

 Hydroboration−Oxidation of Alkynes

• Hydroboration−oxidation is a two-step reaction sequence that

also converts an alkyne to a carbonyl compound.

• Addition of borane forms an organoborane.

• Oxidation with basic H2O2 forms an enol.
• Tautomerization of the enol forms a carbonyl compound.
• The overall result is addition of H2O to a triple bond.

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 Hydroboration−Oxidation of
Internal vs. Terminal Alkynes
• Hydroboration−oxidation of an internal alkyne forms a ketone,
just as the acid-catalyzed hydration did.
• However, hydroboration−oxidation of a terminal alkyne forms
an aldehyde.
• BH2 adds to the less substituted, terminal carbon resulting in
anti-Markovnikov addition of water.

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 Exercise

• Name an alkyne that can be used in the preparation of

the following compound by a hydroboration-oxidation
reaction
 Formation of Acetylide Ions
• Sp hybridized C − H bonds are considerably more acidic than
sp2 and sp3 hybridized C − H bonds.
• Therefore, terminal alkynes are readily deprotonated with
strong base in a Brønsted-Lowry acid-base reaction.
• The resulting ion is called the acetylide ion.

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Formation of Acetylide Ions

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 Reactions of Acetylide Ions
• Acetylide anions are strong nucleophiles and react with
unhindered alkyl halides to yield products of nucleophilic
substitution.
• The mechanism of substitution is SN2, and thus the reaction is
fastest with CH3X and 1o alkyl halides.

39
Elimination vs. Substitution with Acetylide Ions
• Steric hindrance around the leaving group causes 2° and 3°
alkyl halides to preferentially undergo elimination by an
E2 mechanism, as shown with 2-bromo-2-methylpropane.
• Thus, nucleophilic substitution with acetylide anions forms
new carbon-carbon bonds in high yield only with unhindered
CH3X and 1° alkyl halides.

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 Synthesis Using Acetylide Ions
• Carbon-carbon bond formation with acetylide anions is a
valuable reaction used in the synthesis of numerous natural
products.

Figure 11.6

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 Reactions of Acetylide Ions with Epoxides
• Acetylide anions are strong nucleophiles that open epoxide
rings by an SN2 mechanism.
• Backside attack occurs at the less substituted end of the
epoxide.

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 Synthesis Using Alkynes
• You can now begin to consider (for example) how to prepare a
five-carbon product from three smaller precursor molecules
using the reactions you have learned.

• To plan a synthesis of more than one step, we use the process

of retrosynthetic analysis—that is, working backwards from a
desired product to determine the starting materials from which
it is made. 43
 Retrosynthetic Analysis
• Retrosynthetic analysis is the method of working backwards from
a target compound to starting materials.
• To write a synthesis working backwards, an open arrow () is
used to indicate that the product is drawn on the left and the
starting material on the right.

• In designing a synthesis, reactions are often divided into two

categories:
1. Those that form new carbon-carbon bonds.
2. Those that convert one functional group into another—that is,
functional group interconversions. 44
 Example of a Retrosynthetic Synthesis
• Devise a synthesis of the following compound from starting
materials having two carbons or fewer.

• Thinking backwards . . .
[1] Form the carbonyl group by hydration of a triple bond.
[2] Form a new C-C bond using an acetylide anion and a 1°
alkyl halide (two 2-carbon structures are converted to a
4-carbon product).
[3] Prepare the acetylide anion from acetylene by treatment
with base.

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Steps to Develop a Retrosynthetic Analysis

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