©Prentice Hall, 2001Chapter 21 Foundations of Individual Behavior.
© Prentice Hall 2001Chapter 51 General Formulas The General Formula for an acyclic alkyne is C n H...
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Transcript of © Prentice Hall 2001Chapter 51 General Formulas The General Formula for an acyclic alkyne is C n H...
© Prentice Hall 2001 Chapter 5 1
General Formulas
The General Formula for an acyclic alkyne is CnH2n-2
The Simplest Alkyne is HCCH, which has the common name Acetylene
Used as a fuel used in cutting torches
© Prentice Hall 2001 Chapter 5 2
Alkynes If the triple bond is at the end of the
chain, the alkyne is known as a terminal alkyne
© Prentice Hall 2001 Chapter 5 3
Alkynes If the triple bond is not at the end of the
chain, the alkyne is known as an internal alkyne
© Prentice Hall 2001 Chapter 5 4
IUPAC Nomenclature of Alkynes
1. Find the longest chain containing the triple bond and change the corresponding “-ane” ending to “-yne”
2. The chain is numbered in direction that gives the triple bond, the lower number
3. If the same number for the triple bond is obtained in numbering from both directions, the number for the substituent nearest the chain end is minimized
© Prentice Hall 2001 Chapter 5 5
Alkyne Structure
© Prentice Hall 2001 Chapter 5 6
Alkyne Structure
© Prentice Hall 2001 Chapter 5 7
Reactivity Considerations In the first step of HCl addition to an
alkyne, a bond breaks because electrons are attracted to the electrophilic hydrogen atom
© Prentice Hall 2001 Chapter 5 8
Reactivity Considerations In the second step, the positively
charged carbocation intermediate reacts rapidly with the negatively charged chloride ion
© Prentice Hall 2001 Chapter 5 9
Reactivity Considerations The product is an alkene which can
undergo a second electrophilic addition reaction
© Prentice Hall 2001 Chapter 5 10
Reactivity Considerations
© Prentice Hall 2001 Chapter 5 11
Reactivity Considerations The instability of the transition state for the
alkyne reaction can be understood from the Hammond postulate
The transition state will resemble the structure of the intermediate, which is a vinyl carbocation
The vinyl carbocation has a positive charge on an sp carbon which is more electronegative than an sp2 carbon
© Prentice Hall 2001 Chapter 5 12
Relative Stabilities of Carbocations
© Prentice Hall 2001 Chapter 5 13
Hydrogen Halide Addition
The addition of a hydrogen halide to an alkyne follows Markovnikov’s rule because a secondary vinylic cation is more stable than a primary vinylic cation
CH3CH2C CH
H
CH3CH2C CH2
Br
CH3CH2C CH2
Br
© Prentice Hall 2001 Chapter 5 14
Hydrogen Halide Addition When excess hydrogen halide is present,
a second equivalent is added Markovnikov’s Rule also is followed for the
second addition, forming a geminal dihalide
CH3CH2C CH2
Br
HBr
CH3CH2CCH3
Br
Br