C341/Spring 2010
Chapter 7
Chapter 7 Outline:
It takes Alkynes!!
1. Structure of Alkynes (practice nomenclature on your own)
2. Preparation of Alkynes
3. Reactions of Alkynes
A.Acidity of Alkynes
B. Alkylation of Acetylide Anions
C. Electrophilic Additions
i. Halogenation of Alkynes
ii. Hydration of Alkynes
iii. Hydroboration
D.Reductions of Alkynes
E. Oxidations of Alkynes
4. Organic Synthesis
Chapter 7 parallels chapter 6 (alkenes) in many ways, but there are
some twists. However, this will be a good review for what you learned
already.
Suggested homework problems: 7.7, 7.8, 7.10-7.12, 7.14, 7.16,- 7.18,
7.20-7.25, 7.27, 7.29-7.32.
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Chapter 7
Road Map of Alkyne Reactions (11 reactions)
X
O
O
+
OH
OH
1. O3
2. H2O
O
+ CO2
OH
X
1. O3
2. H2O
X
X
Na, NH3
2 X2
Lindlar Pd
H2
1. Strong base
2. CH3X
H2, Pd, high P
H
H2SO4,
HgSO4, H2O
O
1. BH3(THF)
2. NaOH, H2O2
2 HX
Br
2 NaNH2
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Br
O
H
C341/Spring 2010
1.
Chapter 7
Structure of Alkynes
o Internal alkynes are ca. 4.5 kcal/mol more stable than terminal alkynes.
o Triple bond must be written linearly.
o General molecular formula CnH2n-2
o The triple bond introduces two degrees of unsaturation and contains two
nucleophiles.
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2.
Chapter 7
Preparation of Alkynes by Elimination
Alkynes are prepared through an elimination reaction (opposite of addition reaction).
This mechanism will be covered in depth in chapter 9. This reaction occurs with
addiation of strong bases like:
• CH3Li
• NaNH2
• NaH
Two types of dihaloalkanes exist:
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3.
Reactions of Alkynes
A.
Deprotonation of Alkynes
Chapter 7
Alkynes can be deprotonated with addition of strong enough base.
Will NaOCH3 deprotonate an alkyne?
What bases will deprotonate an alkyne?
A common base used with alkynes is LDA = lithium diisopropylamide:
[(CH3)2CH]2N Li
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Chapter 7
B. Alkylation of Acetylide Anions
Reactions that attach an alkyl group to a molecular frame are called
“Alkylation Reactions”. The most common way alkynes are prepared are
via alkylation of alkynes.
Once the alkyne is deprotonated, it becomes a Nu- or base depending
on the RX (E+) introduced to it.
→ Acts like a Nu- when reacted with 1o and methyl carbons only
H
1. NaNH2
2. CH 3CH2I
→ Acts like a base when reacted with 2o and 3o RX
H
1. NaNH2
2.
Br
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Chapter 7
Prepare 2-pentyne from acetylene (in several steps):
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Chapter 7
C. Electrophilic Addition to Alkynes
You know both of these mechanisms from chapter 6, so you can practice doing
them twice in a row.
i.
Hydrohalgenation of Alkynes
Markovkinov’s rule still applies! (Nu adds to the most substituted carbon)
ii.
Halogenation of Alkynes
Halogens, X2 (X = Cl or Br), add to alkynes just as they do to alkenes.
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iii.
Chapter 7
Hydration – Electrophilic addition water to alkynes
o Internal alkynes undergo hydration solely with concentrated acid.
o Terminal alkynes require the presence of an additional Hg2+ catalyst (to make it
easier, we will add Hg2+ salts with all alkynes).
This reaction goes through tautomerization spontaneously once the enol is
formed.
Keto-enol tautomerism occurs quickly:
Tautomers are constitutional isomers that differ in the location of the double bond
and a hydrogen atom. These tautomers exist in equilibrium together.
Equilibrium favors the keto form due the higher bond strength of C=O > C=C.
Ka = 106
Tautomerization is the process that converts one tautomer to the other.
process is catalyzed by either acid or base.
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This
C341/Spring 2010
Chapter 7
Mechanism of hydration:
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Chapter 7
Tautomers:
Practice writing the following ketones as enols and assessing stability:
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Chapter 7
iii. Hydroboration (yes, AGAIN!)
Hydroboration takes an alkyne to an alkene (similar to chapter 6 taking an alkene to
an alkane).
Overall result is addition of water to a triple bond.
o Internal alkynes lead to ketones.
o Terminal alkynes add BH2 to the less substituted, terminal carbon. After
oxidation to the enol, tautomerization yields an aldehyde, a carbonyl
compound having a hydrogen atom bonded to the carbonyl carbon.
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Chapter 7
Overview of alkynes with boranes:
a.
b.
c.
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Chapter 7
D. Reduction of Alkynes (three methods to add H2 to a triple bond)
i. Catalytic Hydrogenation (Reduction) of Alkynes to Alkanes:
H2 gas (moderate pressures = 3 atm; ca. RT) and a catalyst (Pt, Pd or Ni)
→ it is NOT possible to stop at the alkene product
ii. Lindlar catalyst: a special catalyst that reduces an alkyne via syn-addition
“Lindlar Pd: is:
o finely powdered Pd deposited on CaCO3, lead salts and a base like quinoline.
Quinoline:
o Compared to Pd metal above, the Lindlar catalyst is deactivated or “poisoned”
iii. Chemical Reduction of an Alkyne - Alkynes may be reduced in the presence of
alkali metals in NH3(l) leading to anti-addition and a trans-alkene product.
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E.
Chapter 7
Oxidative Cleavage of Alkynes
o Alkynes undergo oxidative cleavage of the σ and both π bonds.
o Internal alkynes are oxidized to carboxylic acids (RCO2H).
o Terminal alkynes afford a carboxylic acid and CO2 from the sp hybridized
C—H bond.
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Chapter 7
Practice Addition Reactions of Alkynes
Br
Br
2 NaH
a.
b.
c.
d.
e.
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Chapter 7
Problem 7-20: Provide correct reagents for the following scheme.
a.
b.
c.
d.
e.
f.
g.
h.
i.
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Chapter 7
Problem 7-21: Provide reagents for the following scheme.
a.
b.
c.
d.
e.
f.
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Chapter 7
Retrosynthesis – putting several reactions together
This entire course will bring you to a point where you can use reactions that you
learn to actually pursue organic synthesic problems.
Planning a successful synthesis means determining where the target molecule can
be broken down into simpler molecules that join together. You will need to be able
to:
o 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.
o Break down the carbon skeleton
o Look at the functional groups present and determine what reactions you know
about those functional groups
Use open arrows to denote retrosynthetic steps:
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Chapter 7
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Chapter 7
The ketone 2-heptanone has been identified as contributing to the odor of a number
of dairy products, including condensed milk and cheddar cheese. Provide a
synthesis of 2-heptanone from acetylene and any necessary reagents.
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Chapter 7
Provide a synthesis that could bring about this reaction.
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