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