alkenes e z
DESCRIPTION
asasTRANSCRIPT
Isomers Isomers
Constitutional isomersConstitutional isomers StereoisomersStereoisomers
consider the isomeric alkenes of consider the isomeric alkenes of
molecular formula Cmolecular formula C44HH88
ALKENES
07/27/12 www.themegallery.com
R
R
R
R
ROTATION BREAKS THE ROTATION BREAKS THE BOND BONDUnlike bonds, bonds do not rotate.
It requires about 50-60 kcal/mole ( ~ 240 kJ/mole )
to break the bond - this does not happen atreasonable temperatures.
NO!
2-Methylpropene2-Methylpropene1-Butene1-Butene
cis-cis-2-Butene2-Butene trans-trans-2-Butene2-Butene
CC CC
HH
HH HH
CHCH22CHCH33
HH33CC
CC CC
CHCH33
HH
HHHH
CHCH33
CC CC
HH33CC
HH
CC CC
HH
HHHH33CC
HH33CC
2-Methylpropene2-Methylpropene1-Butene1-Butene
cis-cis-2-Butene2-Butene
CC CC
HH
HH HH
CHCH22CHCH33
HH
CHCH33
CC CC
HH33CC
HH
CC CC
HH
HHHH33CC
HH33CC
Constitutional isomersConstitutional isomers
2-Methylpropene2-Methylpropene1-Butene1-Butene
trans-trans-2-Butene2-Butene
CC CC
HH
HH HH
CHCH22CHCH33
HH33CC
CC CC
CHCH33
HH
HH
CC CC
HH
HHHH33CC
HH33CC
Constitutional isomersConstitutional isomers
cis-cis-2-Butene2-Butene trans-trans-2-Butene2-Butene
HH33CC
CC CC
CHCH33
HH
HHHH
CHCH33
CC CC
HH33CC
HH
StereoisomersStereoisomers
Naming of Alkenes Number carbons in chain so that double bond carbons have
lowest possible numbers Rings have “cyclo” prefix—double bond always C#1, C#2 Multiple “C=C“ are named as “diene” “triene” “tetraene” etc… Alkenes higher priority than alkanes: even shorter chain
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Nomenclature/ Properties• Cis-Trans isomers (disubstituted)
– Compounds that have one of their carbons bonded to two identical groups cannot exist as cis-trans isomers.
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If an alkene has two identical substituents on one of
the double bond carbons, cis / trans isomers are not possible.
all of these compounds are identical
Two identical Two identical substituents substituents
no cis / trans isomers
C
H
H
C
CH3
CH2-CH3
C
H3C
CH2
C
H
HCH3
C
H
H
C
CH2-CH3
CH3
CH3
CH3
H
CH3CH2
CH3H
CH3 CH3
CH3
no cis / trans isomers
Some other compounds with no Some other compounds with no cis cis / / trans isomerstrans isomers
cis-3-hexene trans-3-hexene
Naming Naming ciscis / / transtrans isomers of isomers of alkenesalkenes
notice that theseprefixes are initalics
CH2CH3CH3CH2
H H
HCH3CH2
H CH2CH3
main chain stayson same side ofdouble bond = cis
main chain crossesto other side ofdouble bond = trans
Stability of Alkenes Cis alkenes are less stable than trans alkenes
Less stable isomer is higher in energy tetrasubstituted > trisubstituted > disubstituted > monosusbtituted
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if C<8 then thechain is too shortto join together
Rings with double bondsRings with double bondstrans double bonds are not possible until the ring has at least eight carbon atoms
CH2
CH2
CH2
CH2
cis
cis
cis
trans
smallest ring thatcan have a transdouble bond
C = 5
C = 6
C = 8
Note that both cis and trans exist for C8.
trans
C C
R
H
R
H
C C
H
R
R
H
RR
R
R
Compare Compare cis / transcis / trans isomers in ring compounds to alkenes isomers in ring compounds to alkenes
cis trans
cis / trans isomers are also called geometric isomers
Stereochemical Notation
• cis and trans are useful when substituents are identical or analogous (oleic acid has a cis double bond)
• cis and trans are ambiguous when analogies are not obvious
CC CC
CHCH33(CH(CH22))66CHCH22 CHCH22(CH(CH22))66COCO22HH
HH HH
OleicOleic acid acid
CH3
H CH2CH3
CH2CH3CH3
H CH3
CH3
cis-3-methyl-2-pentene trans-3-methyl-2-pentene
CIS OR TRANS !!!CIS OR TRANS !!!
This compound is cisbut the two methylgroups are ….trans to each other.
This compound is transbut the two methylgroups are ….cis to each other.
The main chain determines cis / trans in the IUPAC name
but the terms cis and trans are also used to designate the relative position of two groups: a new system is needed!
Sequence Rules: The E,Z Designation
Neither compound is clearly “cis” or “trans”
Substituents on C1 are different than those on C2
We need to define “similarity” in a precise way to distinguish the two stereoisomers
Cis, trans nomenclature only works for disubstituted double bonds
E/Z Nomenclature for 3 or 4 substituents
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In this system the two groups attached to each carbonare assigned a priority ( 1 or 2 ).
If priority 1 groups are both on same side of double bond:
E / Z NomenclatureE / Z Nomenclature
11
2 2
21
2 1
Z E
If priority 1 groups on opposite sides of double bond:
E isomer = entgegen = opposite (in German)
Z isomer = zusammen = together (in German)
sameside opposite
sides
Assigning prioritiesAssigning priorities
1. Look at the atoms attached to each carbon of the double bond.2. The atom of higher atomic number has higher (1) priority.
example IF
H Br
1
2
1
2
F > H I > Br
Since the 1’s are on the same side, this compound is Z
(Z)-1-bromo-2-fluoro-1-iodoethene
notice use of parentheses
Priorities in the E-Z Nomenclature system
1 1
(Z) (E)
C C C C
1
1
3. If you can’t decide using the first atoms attached, go out to the next atoms attached. If there are non-equivalent paths, always follow the path with atoms of higher atomic number.
CH2FCH3
H CH2CH3
C
H
H
F
C C
H
H
Once you find a difference, you can stop.
1
2
1
2
This molecule has Z configuration.
comparisonstops here
path goes toF not to H
path goes toC not to H
CH2FCH3
H CH2CH3
Let’s give this compound a cis/trans name and an E/Z name
trans-3-fluoromethyl-2-pentene (longest chain)
(Z)-3-fluoromethyl-2-pentene (priorities)
CH CH2
C O
H
4. C=C double bond: equivalent to having two carbons. C=O double bond: equivalent to having two oxygens.
CH CH2
C
C O
H
CO
1
2C
C C
CH2
CH2
O
CH3
H2N
Br CH2
CH2OH
1
2 1
2
(E)
C C
CH2
CH2
O
CH3
H2N
Br CH2
OH1
2 2
1
(Z)
trans, trans trans, cis
cis, cis cis, trans
DIENES AND POLYENESDIENES AND POLYENES
E,E E,Z
Z,Z Z,E
(2E,4E)-2,4-hexadiene
identical
(2E,4Z)-2,4-hexadiene
(2Z,4Z)-2,4-hexadiene (2Z,4E)-2,4-hexadiene
Hexadiene
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Nomenclature
Question. Designate each double bond as E or Z, where applicable.
OH
H
H
Br
OH
geraniol