ch11_Lecture MRA - SEM II 2324 (2).pdfh j j r

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Organic Chemistry, Fifth Edition
Chapter 11
Alkynes
Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.
Mohd Ridhwan ADAM
Universiti Sains Malaysia

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• 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.
Alkyne Structure

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• 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.
Alkyne Bonding

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• 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.
Strength of Alkyne Bonds

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• 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.
Cyclic Alkynes

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• 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.
Naming Alkynes

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• 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
Naming Alkynes

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• 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.
Physical Properties of Alkynes

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• 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.
Acetylene

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• 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.
Preparation of Alkynes

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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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• 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.
General Addition Reactions of Alkynes

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• The red electron-rich region is located between the two
carbon atoms forming the triple bond.
• This forms a cylinder of electron density around the center of
the molecule.
Electrostatic Potential of Acetylene
Figure 11.4

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Figure 11.5
Addition Reactions of Alkynes

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• 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.
Terminal Alkynes – Reaction as an Acid

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• Acetylide ions formed by deprotonating terminal alkynes are
strong nucleophiles.
• They can react with a variety of electrophiles.
Terminal Alkynes – Reaction as an Acid

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• 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.
• Alkynes undergo hydrohalogenation, the addition of hydrogen
halides, HX (X = Cl, Br, I).
Addition of Hydrogen Halides

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Hydrohalogenation—Markovnikov’s Rule

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Hydrohalogenation Mechanism

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• 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.
Hydrohalogenation of Alkynes vs. Alkenes

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

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• 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.
Halogenation of Alkynes

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• 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.
Hydration of Alkynes

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• 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.
Hydration of Internal vs. Terminal Alkynes

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• 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.
Keto-Enol Tautomerization

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• 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.
Hydroboration−Oxidation of Alkynes

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• 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.
Hydroboration−Oxidation of
Internal vs. Terminal Alkynes

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• Terminal alkynes are readily converted to acetylide ions with
strong bases such as NaNH2 and NaH.
• These anions are strong nucleophiles, capable of reacting with
electrophiles such as alkyl halides and epoxides.
Reactions of Acetylide Ions

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• Acetylide anions are strong nucleophiles and react with
unhindered alkyl halides to yield products of nucleophilic
substitution.
Reactions of Acetylide Ions with Alkyl
Halides

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• The mechanism of substitution is SN2, and thus the reaction is
fastest with CH3X and 1o alkyl halides.
• Nucleophilic substitution with acetylide ions forms new
carbon-carbon bonds.
Reactions of Acetylide Ions with Alkyl
Halides

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• 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.
Elimination vs. Substitution with Acetylide Ions

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• Carbon-carbon bond formation with acetylide anions is a
valuable reaction used in the synthesis of numerous natural
products.
Figure 11.6
Synthesis Using Acetylide Ions

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

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• 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.
Retrosynthetic Analysis

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• Devise a synthesis of the following compound from starting
materials having two carbons or fewer.
Example of a Retrosynthetic Synthesis
• 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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Example of a Retrosynthetic Synthesis
• Three steps are needed to complete the synthesis.
• Formation of the acetylide
• SN2 reaction with an alkyl halide
• Hydration of the alkyne

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