Naming double bond-containing substituents IUPAC: alkenyl...
Transcript of Naming double bond-containing substituents IUPAC: alkenyl...
Naming double bond-containing substituents
IUPAC: alkenyl
1-(2-butenyl)-2-methyl-1-cyclohexene
Naming double bond-containing substituents
IUPAC: alkenyl
1-(2-butenyl)-2-methyl-1-cyclohexene
Some common double bond-containing substituents:
vinyl (ethenyl)
1-methyl-4-vinyl-1-cyclohexene
Naming double bond-containing substituents
IUPAC: alkenyl
1-(2-butenyl)-2-methyl-1-cyclohexene
Some common double bond-containing substituents:
vinyl (ethenyl)
1-methyl-4-vinyl-1-cyclohexene
allyl (2-propenyl)
1-allyl-4-vinyl-1-cyclohexene
Naming double bond-containing substituents
IUPAC: alkenyl
1-(2-butenyl)-2-methyl-1-cyclohexene
Some common double bond-containing substituents:
vinyl (ethenyl)
1-methyl-4-vinyl-1-cyclohexene
allyl (2-propenyl)
1-allyl-4-vinyl-1-cyclohexene
phenyl
4,4-dimethyl-1-phenyl-1-cycloheptene
Naming double bond-containing substituents
IUPAC: alkenyl
1-(2-butenyl)-2-methyl-1-cyclohexene
Some common double bond-containing substituents:
vinyl (ethenyl)
1-methyl-4-vinyl-1-cyclohexene
allyl (2-propenyl)
1-allyl-4-vinyl-1-cyclohexene
phenyl
4,4-dimethyl-1-phenyl-1-cycloheptene
benzyl (phenylmethyl)
(R)-1-benzyl-4-methyl-4-phenyl-1-cycloheptene
Cl
When the parent chain contains multiple C=C, use a multiplicative prefix before “ene”:
(S)-6-chloro-4-methyl-2,4-heptadiene
H
H
H
H
H
H
H
H
Alenes are classified according to their degree of substitution of the C=C functional group
monosubstituted
disubstituted
terminally disubstituted
trisubstituted
tetrasubstituted
CH3 CH3
CH2 CH2
CH3 +. CH3.
:CH2 + :CH2
Csp2
F
F
B
Csp2
F
F
F
H1s
H1s
H1s
H1s
How strong is a C=C ? Is it twice the strength of a C–C?
∆HDBE, kcal/mol
83 1 σ
146 1 σ + 1 π
ˆ ∆HBDE (1 σ + 1 π) - ∆HBDE (1 σ ) = ∆HBDE (1 π)
146 - 83 = 63
cis-alkenes and trans-alkenes
Consider all the isomers of the alkene C4H8:
are these conformational isomers?
• No! They are isomers that differ by the relative positions of the substituent methylgroups in space.
• They cannot be converted into each other without breaking a bond.
• Therefore, they are diastereoisomers.
• Why does rotation about the C=C break a bond?
C
CCH3
CH3
H
H
C CH3
H
C
CH3
HC
CHCH3
H
CH3
π bond no π bond π bond reforms
These diastereoisomers are named just like disubstituted cycloalkanes:
• Identical groups on the same side of the C=C (that is, with a 0 E dihedral angle)are
cis-alkenes
• Identical groups on opposite sides of the C=C (that is, with a 180 E dihedralangle) are
trans-alkenes
O
OH
H
H
Cl
OH
CH3
Br
trans-2-butene cis-2-butene
trans-cinnamic acid ?
The E,Z system of naming diastereomeric alkenes:
• Use the Cahn-Ingold-Prelog “Priority Rules”
1. For each C of the C=C, rank the priorities of the 2 atoms attached, according toatomic number.
2. If the 2 atoms attached are identical, rank the atoms attached to those 2 atoms.Priority of the entire substituent is determined by the first point of difference.
3. If identical priority numbers are on the same side of the C=C, the alkene is a
(Z)-alkene.
4. If identical priority numbers are on opposite sides of the C=C the alkene is a
(E)-alkene.
C X
Y
Z
C X
X C
Y
Z
For comprison purposes, identical atoms must be compared in the same hybridization state:
• The “Phantom atom” Rules
1. For naming purposes, a π bonded atom is considered to replicated by a phantomσ bond to a phantom atom.
2. Rule 1 is applied to both atoms of the π bond.
3. Rules 1 and 2 apply to each π bond of multiply bonded atoms.
4. All other things being equal, phantom atoms rank below real atoms of the sameatomic number.
Cl
Cl
C4
C3 C
C
C
C
Cl
Cl
H
H
HH H
HH
1
2
CC
2
1
An example:
What is the IUPAC name of ?
1. The longest chain containing both C=C bonds is 5: pentadiene
2. Number the chain to give the C=C bonds the lowest locants: 1,3-pentadiene
3. Add substituents: 1,5-dichloro-3-ethyl-4-methyl-1,3-pentadiene
4. Add stereodesignator for the C1=C2 bond:
(1E)-1,5-dichloro-3-ethyl-4-methyl-1,3-pentadiene
5. Add stereodesignator for the C3=C4 bond:
(1E,3E)-1,5-dichloro-3-ethyl-4-methyl-1,3-pentadiene
Another example using the phantom atom rule:
Is this an E or a Z alkene?C
C
OC
O
HH
H
H H
Another example using the phantom atom rule:
Is this an E or a Z alkene?
It’s an E alkene.
1
2
1
2
Thermodynamic stabilities of isomeric alkenes:
Consider
C4H8 + 6 O2 6 4 CO2 + 4 H2O )H°rxn =
)H°rxn
kcal/mol
CO2 + H2O
The substitution - elimination continuum
what factors influence the balance between substitution and elimination for a given class of haloakanes?
Primary haloalkanes:
• SN2 for all Z-
• Increased steric hindrance in ‡ increases the proportion of E2
P "-branching in haloalkane
P strong, bulky Lewis bases
Secondary haloalkanes:
• SN2 for all Z- less basic than HO-
• E2 for all Z- more basic than HO-
• Increased steric hindrance in ‡ increases the proportion of E2
P "-branching in haloalkane
P strong, bulky Lewis bases
• SN1 or E1 when Z: is neutral and the solvent
CA of Nu: pKa
H-CN 9.2
H-OH 15.7
H-CH2CH3 50
Tertiary haloalkanes:
• E2 for all Z-
• SN1 or E1 for most Z: and the anions of strong CA
Generally, regardless of the identity of Z: or Z- :
• SN is favored at lower temperatures
• E is favored at higher temperatures