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Chapter%2014%20The%20Chemistry%20of%20Alkynes

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Alkynes Also known as Acetylenes Naturally occurring alkynes are relatively rare They do not occur as a petroleum constituents ... Alkynes NMR Spectroscopy ... – PowerPoint PPT presentation

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Title: Chapter%2014%20The%20Chemistry%20of%20Alkynes


1
Chapter 14The Chemistry of Alkynes
2
Alkynes
  • Also known as Acetylenes
  • Naturally occurring alkynes are relatively rare
  • They do not occur as a petroleum constituents

Carotatoxin
14.10 Occurrence and Use of Alkynes
3
Uses of Alkynes
  • Acetylene is a chemical feedstock for many
    important compounds
  • Occur in antiviral and antifungal drugs
  • Efavirenz

14.10 Occurrence and Use of Alkynes
4
Common Nomenclature
  • Simple alkynes are named as acetylene derivatives

acetylene
  • Certain compounds are derivatives of the
    propargyl group (HC?C-CH2-)

14.1 Nomenclature of Alkynes
5
IUPAC Nomenclature
  • Follow the rules for naming alkenes except,
    change -ane to -yne

6
IUPAC Nomenclature
  • Principal groups that have priority over the
    triple bond receive preference in numbering and
    suffix
  • Recall Carboxylic acid gt anhydride gt ester gt
    acid halide amide gt nitrile gt aldehyde gt ketone
    gt alcohol gt thiol gt amine
  • Substituent groups containing a triple bond
    alkynyl groups
  • Named by replacing final -e with -yl

14.1 Nomenclature of Alkynes
7
Substitutive Nomenclature
  • If both CC and C?C are present, the bond with
    the lowest number gets precedence
  • However, if the rule is ambiguous, the double
    bond gets precedence

14.1 Nomenclature of Alkynes
8
Problems
  • Name the following compounds

9
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10
Structure and Bonding
  • Alkynes display linear geometry
  • cis-trans isomerism cannot occur in alkynes
  • Cycloalkynes smaller than cyclooctyne cannot be
    isolated under ordinary conditions

14.2 Structure and Bonding in Alkynes
11
MO Bonding Picture
  • Alkynes utilize sp hybrid orbitals
  • Electrons in an sp hybrid orbital are held closer
    to the nucleus on average

14.2 Structure and Bonding in Alkynes
12
MO Bonding Picture
14.2 Structure and Bonding in Alkynes
13
MO Bonding Picture
14.2 Structure and Bonding in Alkynes
14
Heats of Formation
  • Alkynes are less stable than isomeric dienes
  • Internal alkynes are more stable than terminal
    alkynes

14.2 Structure and Bonding in Alkynes
15
Boiling Points and Solubilities
  • Alkyne boiling points are not much different from
    those of analogous alkenes and alkanes
  • Similarly, alkynes have low densities and are
    insoluble in water

14.3 Physical Properties of Alkynes
16
IR Spectroscopy of Alkynes
  • C?C stretch 2100-2200 cm-1
  • Symmetrical alkynes will not show this stretch
  • ?C-H stretch 3300 cm-1

14.3 Physical Properties of Alkynes
17
NMR Spectroscopy of Alkynes
  • The reason for the unusual acetylenic proton
    chemical shift is similar to that described for
    vinylic protons

14.3 Physical Properties of Alkynes
18
NMR Spectroscopy of Alkynes
  • However, the effect is in the opposite direction

14.3 Physical Properties of Alkynes
19
NMR Spectroscopy of Alkynes
  • Alkynyl carbons typically appear at d 65-80
  • Propargylic carbons also display smaller chemical
    shifts (5-10 ppm lower than alkyl)

14.3 Physical Properties of Alkynes
20
Preparation of Alkynes
  • Alkynes prepared by elimination of HX from alkyl
    halides

21
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22
Addition Rxns of Alkynes
  • Similar to addition reactions for alkenes
  • Alkyne addition rxns are often slower than
    correspondig alkene additions
  • Regioselectivity follows a similar course
  • Addition of HX and X2

23
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24
Problems
  • Give the alkene products for the following
    addition rxns

25
Regioselectivity of Second Addition Rxn
26
Problems
14.4 Introduction to Addition Reactions of the
Triple Bond
27
Hydration of Alkynes
  • Addition of H2O
  • As with alkenes, alkynes can be hydrated by two
    methods
  • Mercury (II) Catalyzed Hydration
  • The product is a ketone and not an alcohol

Recall
28
Keto-Enol Tautomerism
  • Tautomers constitutional isomers that
    interconvert rapidly
  • Equilibrium lies far to the right

29
Mechanism of Mercury (II) Catalyzed Hydration
14.5 Conversion of Alkynes into Aldehydes and
Ketones
30
Mechanism of Hydration
14.5 Conversion of Alkynes into Aldehydes and
Ketones
31
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32
Problems
  1. Give the complete mechanism for the following
    reaction
  2. What is the product for the following rxn?
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