Title: Reactions of Alkynes' Introduction to Multistep Synthesis Chapter 6
1Reactions of Alkynes. Introduction to Multistep
SynthesisChapter 6
2Contents of Chapter 6
- General Formulas of Alkynes and Nomenclature
- Multifunctional Group Nomenclature
- Structure and Properties of Alkynes
- Reactivity Considerations
- Addition of Hydrogen Halides and Halogens
- Other Additions
- Hydrogenation of Alkynes
- Acidity of Hydrogens Bonded to sp Carbons
- Introduction to Multistep Synthesis
3Alkynes
- If the triple bond is at the end of the chain,
the alkyne is known as a terminal alkyne - Terminal alkynes must be treated with special
reagents (disiamylborane or HgSO4 with H2O/H2SO4
4Alkynes
- If the triple bond is not at the end of the
chain, the alkyne is known as an internal alkyne - Internal alkynes can be treated with normal
alkene reagents (BH3 and H2SO4/H2O)
5Alkynes
- Common names of alkynes are based on
substitution of the simplest alkyne, acetylene
6IUPAC Nomenclature of Alkynes
- Find the longest chain containing the triple bond
and change the corresponding -ane ending to
-yne - The chain is numbered in direction that gives the
triple bond, the lower number - 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
7IUPAC Nomenclature of Dienes
- Find the longest chain containing both double
bonds
1
2
3
4
5
3-butyl-1,4-pentadiene
8IUPAC Nomenclature of Dienes
- Use corresponding alkane name but replace the
ne ending with diene
3-butyl-1,4-pentadiene
pentane changed to pentadiene
9IUPAC Nomenclature of Dienes
- Number in the direction that gives the lowest
number to a double bond
1,5-heptadiene not 2,6-heptadiene
10IUPAC Nomenclature of Dienes
- List substituents in alphabetical order
5-ethyl-2-methyl-2,4-heptadiene
11IUPAC Nomenclature of Dienes
- Place numbers indicating the double bond
positions either in front of the parent compound
or in the middle of the name immediately before
the diene suffix
5-ethyl-2-methyl-2,4-heptadiene or
5-ethyl-2-methyl-hepta-2,4-diene
12IUPAC Nomenclature of Dienes
13Configurational Isomers of Dienes
14IUPAC Multifunctional Compound Nomenclature
15IUPAC Multifunctional Compound Nomenclature
- The longest chain has to go past the
highest-priority functional group - High prio group has lowest possible number
- If not highest priority NH2 is amino and OH is
hydroxy substituent - General form is n-substit-n-alken-n-yn-n-groupsuff
ix
16Reactivity Considerations
- The hydrohalogenation product is an alkene which
can undergo a second electrophilic addition
reaction - First halogen follows alkene hydrohalogenation
regioselectivity rules - Second halogen goes on same carbon as first
halogen
17Relative Stabilities of Carbocations
- Vinyl cations are one level less stable than
alkyl cations
18Hydrogen Halide Addition in the presence of
Peroxide
- Hydrogen peroxide has same effect on hydrogen
bromide addition to an alkyne as to an alkene
(reversed regioselectivity).
19Halogen Addition to Alkynes
- Halogens add to alkynes as well as to alkenes
- Excess halogen leads to the addition of a second
equivalent
20Addition of Water to Alkynes
- Water adds to alkynes in the presence of acid to
yield an enol
21Addition of Water to Alkynes
- However the initially formed enol reacts further
to produce a ketone
Such isomers, differing only in the placement of
a hydrogen atom, are called tautomers
22Mercuric-Ion-Catalyzed Addition of Water to
Alkynes
- Terminal alkynes need catalytic help to undergo
hydration. - Internal alkynes dont need the catalyst.
23HydroborationOxidation
- Hydroboration of an internal alkyne leads to a
ketone
24HydroborationOxidation
- For terminal alkynes, a bulkier borane is needed
to prevent addition of a second equivalent of
borane to the resultant double bond
25Addition of Water to an Alkyne
26Addition of Hydrogen to an Alkyne
27Acidity of a Hydrogen Bonded to an sp Carbon
- The conjugate bases have the following relative
base strength because the stronger the acid, the
weaker the conjugate base
28Syntheses Using Acetylide Ions
- Alkylation reactions work best with primary
alkyl halides and methyl halides
29Introduction to Multistep Synthesis
- The thought process is known as retrosynthetic
analysis and is indicated by using open arrows
30Introduction to Multistep Synthesis -
Considerations
- Alkene products made from alkynes
- Carbonyl products made from alkynes
- Alkane products made from alkenes
- Know reactions pgs 259-260 thoroughly
- Understand issues involved in proper choice of
reagents thoroughly
31Introduction to Multistep Synthesis Reagent
Issues
- Regioselectivity of HX alkene/alkyne rxn
- Regioselectivity of 2 HX alkyne rxn
- Regioselectivity of HBr/peroxide rxn
- Regioselectivity of oxidative hydroboration
- Stereospecificity of halogenation rxn
- Regio/stereo/specificity of X2/ROH or X2/water
rxn - Stereospecificity of H2/Lindlar reduction
- Stereoselectivity of Na/NH3 reduction
- H2 with Pd/C, Pt/C, or Ni does complete reduction
- Terminal alkynes need disiamylborane or HgSO4
- Carbocation rearrangements prevented with
Hg(OAc)2 - RX Na--R rxn works best with primary halide
32Introduction to Multistep Synthesis Practice
- Use retrosynthetic analysis to make these