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Organic Chemistry Reviews
Chapter 7
Cindy Boulton
November 2, 2009
Nomenclature of Alkynes
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Carbon – Carbon triple bond
Ending –yne
sp hybridized
Linear and angle = 1800
Number the bond with the carbon that has the lower
number
Terminal Alkyne:
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a triple bond at a terminal carbon
Has a acetylenic proton
Acetylenic Proton:
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Proton at the end attached to a Carbon with a triple bond
Easily pulled off with a pKa value = 25
Nomenclature of alkenes
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Carbon – Carbon double bond
Ending –ene
sp2 hybridized
All atoms are coplanar and angle = 1200
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Double bond cannot rotate
Number the bond with the carbon that has the lower
number
Cis: same groups on SAME side
Trans: same groups on OPPOSITE side
Diasteromers:
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Same molecular formula, same connectivity, not mirror images
E-Z Nomenclature
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If no 2 groups are the same, cannot use cis or trans
Identify the highest priority (highest mass) group attached
to each Carbon.
(Z): SAME side
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Zame Zide
(E): OPPOSITE side
Vinyl and Allyl Groups
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Vinyl Group
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CH2 = CH –
Allyl Group
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CH2 = CH – CH2 –
Stability of Alkenes
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Alkyl groups provide electron density to stabilize the
alkene
Hydrogen does not provide electron density
Electronics: more electron donors, more stable
Sterics: more crowding, less stable
The greater number of attached alkyl groups or the more
highly substituted the carbon atoms of the double bond,
the greater is the alkene’s stability
Stability of Alkenes cont.
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Tetrasubstituted: 4 alkyl groups attached
Trisubstiuted: 3 alkyl groups attached
Disubstitued: 2 alkyl groups attached
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On same carbon (3o Carbon)
Trans
Cis
Monosubstituted: 1 alkyl group attached
Unsubstitued: no alkyl groups attached
(In order of decreasing stability)
Stability of Cycloalkenes and Cycloalkynes
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Angle Strain
8 is the magic number
Cycloalkenes:
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Cyclopropene to Cycloheptene
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Cyclooctene
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Angle strain
Must be in cis form (not stable in trans form)
First stable cycloalkene
Tans at double bond
Cycloalkynes
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Cyclooctyne
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Can isolate at room temperature
Unstable due to angle strain
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Wants to be linear (180o) but is 145o
Synthesis of Alkenes
Dehydrohalogenation reaction (E2)
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α Carbon: Carbon with Halide/Leaving Group attached to
β Carbon: Carbon directly attached to α Carbon, has β
Hydrogens attached
E2 mechanism:
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Leaving group leaves, Nucleophile/Base attacks β Hydrogen, double
bond forms between α and β carbons
Transition step, no carbocation intermediate
Two Products:
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Zaitsev: small bases lead to more stable/substituted alkenes due to
electronics
Hoffman: big, bulky bases lead to less stable/substitued alkenes due
to sterics and crowding
SN2 Product also forms
Stereochemistry of E2 reaction
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Anti periplanar transition state
β Hydrogen needs to be oppostie the leaving group
Enantiomers will have the same E-Z nomenclature after
dehyrdohalogenation reaction
Diastereomes will have opposite E-Z nomenclature after
dehydrohalogenation reaction
Dehydration of Alcohols
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Hydroxyl group becomes protonated by an acid forming
H-O-H+ to make a good “leaving group”
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E1 mechanism:
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Acid is a catalyze
H-O-H leaves and to form Carbocation intermediate
H-O-H acts as “nucleophile” attacking β Hydrogen forming
alkene
Two Products:
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Hoffman Product: less stable/substituted with bulky base
Zaitsev Product: more stable/substituted with small base
Dehydration of Alcohols cont.
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Alkyl and hydride migration
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A hydride (H-) or alkyl group will migrate from a β Carbon to
the α Carbocation to form a more stable Carbocation
Skeletal rearrangement
Multiple products will be formed
Debromination of vic-dibromides
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Gem: halides on same carbon (twins)
Vic: halides on adjacent carbons
Reacts with Zn/H2O or 2NaI to form alkenes
Vic-dibromide must be in antiperi planar form
I- acts as nucleophile pulling off one of the Br as the other
Br leaves forming an alkene
Debromination of vic-dibromides cont.
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Why 2 NaI?
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The second I pulls the I off the I-Br complex forming I-I
With only 1 the reaction will stall
Enanitomers have same product, same E/Z nomenclature
Diastereomers have different products, different E/Z
nomenclature
A racemic mixture of vic-dibromide would have a single
product
Terminal Alkynes
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Terminal Alkynes have an acetylenic proton with a pKa =
25
Reacts with a strong base
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Forms an acetylide
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LDA
NaNH2
Carbonanion
Use acetylide as a nucleophile to attack alkyl halides to
make alkynes bigger
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Hard to add 3o RX because the elimination product will be
major
Hydrogenation of Alkenes
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Use Pt as catalyst:
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Can use Ni, Pd, Rh or others
Lowers the activation energy to speed up the reaction
H are added to the same face of the alkene
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Stereochemistry: Syn-Addition
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Z -> RS and SR
E -> RR and SS
Forms a racemic mixture of enantiomers
Regiochemistry: 1, 2 addition
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Carbons of double bond are side by side
Hydrogenation of Alkynes
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Pt as catalyst:
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Forms an alkene but can not stop so continues to form alkane
Lindlar’s Catalyst:
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H2/Pd/CaCO3
Stops as alkene
Ca prevents alkene from being hydrogenated
Stereochemistry: syn-addition
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Hydrogens added to same side
Form Z or Cis alkene
Hydrogenation of Alkynes cont.
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H2/Ni2B as Catalyst:
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Stops as alkene
B prevents alkene from being hydrogenated
Stereochemistry: syn-addition
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Hydrogens added to same side
Form Z or Cis alkene
1) Li, C2H5NH2 2) NH4Cl
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Sterochemistry: anti-addition
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Hydrogens added to opposite side
Form E or Trans alkene