Post on 20-Jan-2016
transcript
Sect 4.6: Monosubstituted cyclohexane rings
Methylcyclohexane conformations
CH3
CH3
H
H
Equitorial methyl Axial methyl
1.7 kcal/mol
0 kcal/mol
EENNEERRGGYY
Energy difference between an axial and an equitorial methyl group
CH3
H
CH3
H
HHCH3
1,3-diaxial interactions
1,3-Diaxial interactions on the top of the ring
STERIC REPULSION RAISES THE ENERGY OF THE AXIAL CONFORMATION
H
HH
H
H
CHCH33 H
H
HHH
1,3-diaxial interactions
CHCH
1,3-Diaxial interactions: Newman projection view
1,3-Diaxial 1,3-Diaxial interactionsinteractions
CHCH33
CHCH22
CHCH22
gauche steric nteractionsGAUCHE STERICINTERACTIONS(like gauche butane)
CHCH33CHCH22
CHCH22
Axial
Equitorial
No gauche steric problem when the group is equitorial
Monosubstituted cyclohexanes: gauche steric interactions
CHCH33
CHCH33
60o
180o
Large groups will generally prefer to occupy an equatorial position where there is an absence of 1,3-diaxial (steric) interactions
G
Keep in mind, however, that the axial conformation will also be present, but in smaller amount.
General General rulerule
axialconformation
equatorialconformation
Go for group in the axial position
CH3-
CH3CH2-
CH3-CH-
CH3
CH3-C-
CH3
CH3
1.7
1.8
2.1
>5
7.1
7.5
8.8
>21
Group X kcal/mol kJ/mol Group kcal/mol kJ/mol
Cl-
Br-
HO-
0.4
0.5
0.7
C6H5- 3.1
CH3-C-O-
O
0.7
1.7
2.1
2.913
2.9
Table 4.5: Conformational energy differences for substituents attached to a cyclohexane ring
X
X
H
H
Equitorial preferred
CC
CC
CC
CC
HH
HH
HH
HH
HH
HH
HH
HH
HH
tt -BUTYLCYCLOHEXANE -BUTYLCYCLOHEXANE
Basically “locks” the ring in a chair with thetert -butyl group in the equatorial position.
Too big a group togo into the axialposition - must goequatorial.
The axial value for this group in Table 4-5 ( >5 Kcal/mole) indicates a minimum value because there is so little axial that it is difficult to measure any real value.
tert-Butylcyclohexane with the group axial
HUGE steric strain
Molecules viewed with Chime• Click on START, Click on PROGRAMS• Click on Netscape Communicator (4.7), then
launch Netscape Navigator• Using Google, type in the address for the Dept.
of Chemistry, WWU: http://www.chem.wwu• Select, course materials, select “WWU virtual
molecular model set”• You may need the free program, Chime, to run
this program.• Note: Internet Explorer and Netscape 7.1
won’t work!
Sect 4.7: cis and trans isomerization in cycloalkanes
cis-trans isomerism
• Different spatial arrangements• The arrangements cannot be converted into
one another by rotation• cis Both substituents on same side of plane• trans Substituents on opposite sides of plane
ciscis and and transtrans isomers isomers
cis trans
applies to substituents on a ring or (later) double bond
both substituents are on the same side of the ring
the substituents are onopposite sides of the ring
These two compounds are geometric isomers
Cl Cl Cl
Cl
Cl
Cl
Cl
Naming cis /trans isomers
cis-1,2-dichlorocyclopropane
trans-1,2-dichlorocyclopropane
noticeitalics
place designationin front of nameCl
How many different dimethylcyclobutanes are there?How many different dimethylcyclobutanes are there?
CH3
CH3
CH3
CH3
CH3
CH31,1-
1,2- 1,3-Constitutional isomers
cis /trans isomers (geometric)
CH3
CH3
no cis or transno cis or trans
CH3 CH3
CH3
CH3
ciscis
transtrans
CH3CH3
CH3
CH3
ciscis
transtrans
no cis/trans here
Notice that it is OK to use planar ringswhen figuring out cis / trans isomers.
Planar ring Planar ring approximationapproximation
You only need to use puckered ringswhen you are dealing with conformations.
CH3CH3
CH3
CH3
CH3 CH3
CH3 CH3
CH3
CH3
CH3
CH3
Use planar structures on tests!
Sect 4.8: disubstituted cyclohexanes:
cis/trans isomerism
Use chair structures!!
trans-1,2-dimethylcyclohexane
trans-trans-1,2-dimethylcyclohexane 1,2-dimethylcyclohexane has two possible conformershas two possible conformers
CHAIR-1 CHAIR-2
e,e a,a
CH3
H
H
CH3
CH3
H
CH3
H
Which conformer is more stable?The trans e,e one!
Methylbelow
Methylabove
Go = (2)(1.7) – 0 = 3.4 kcal/mol
CH3-
CH3CH2-
CH3-CH-
CH3
CH3-C-CH3
CH3
1.7
1.8
2.1
>5
7.1
7.5
8.8
>21
Group kcal/mol kJ/mol Group kcal/mol kJ/mol
Cl-Br-
HO-
0.4
0.5
0.7
C6H5- 3.1
CH3-C-O-
O
0.7
1.7
2.1
2.913
2.9
trans-(a,a)-1,2-dimethylcyclohexane
Reference = 0 kcal/mol
2 x 1.7 kcal
3.4 kcal/mol higher
trans-(e,e)-1,2-dimethylcyclohexane
(two axial methyls)
Calculating the energy difference using values from Table 4.5
CH
H
H
H
CH
HH
H
H
H
1,3-Diaxial interactions (steric) 1,3-Diaxial interactions (steric) on top and bottom of ringon top and bottom of ring
C
HC
H
HH
H
HH
H
H
H
HH
Two axial-axial problems@ 1.7 kcal/mol each
Equatorial groups are assumed to be 0 kcal/mol
No diaxial interactionsNo diaxial interactionslots of roomlots of room
What about cis-1,2-dimethylcyclohexane?
Class exercise!!
What about cis / trans isomers in disubstituted rings other than 1,2-dimethylcyclohexane?
1,3-dimethylcyclohexane: 4 chair structures1,4-dimethylcyclohexane: 4 chair structures
1,1-dimethylcyclohexane: no cis/ trans isomers
Which conformer has the higher Which conformer has the higher energy?energy?
CH3 CH3
OH OH
axial = 1.7 kcal/mol
axial = 0.7 kcal/molequatorial = 0 kcal/mol
equatorial = 0 kcal/mol
1.7 kcal/mol 0.7 kcal/mol
G = (1.7 - 0.7) = 1.0 kcal/mol
CH3
H
HO
HCH3
H
H
OH
This one!
Both are trans!
GuidelineGuideline
In substituted cyclohexane rings, the best (lowest energy) conformation will have the largest groups in equatorial positions whenever possible.
Sect 4.9: decalinskip this section, winter 07
H
H H
H
H
H
H
H
trans-ring fusion cis-ring fusion
bonds are trans
bonds are cis
ciscis and and transtrans ring fusions ring fusions
cis-decalin: less stable
trans-decalin
H
H
H
H
H
H
H
H
other representations
solid wedge = towards you
dashed wedge = away from you
A dot implies the hydrogen is towards you(on top).
trans
cisDrawing Conventions
top
bottom
Sect 4.10: read this section; no lecturesSkip this section, winter 07
Sect 4.11: cis/trans isomerism in alkenes
RR
R RC C
R
R
R
R
RRRR
Alkene geometry: planarAlkene geometry: planar
bond
bond
bond
bond
sp2
sp2
SIDE VIEWSIDE VIEW END VIEWEND VIEW
planar
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!
cis /cis / transtrans isomers (geometric isomers (geometric isomers)isomers)
Because there is no rotation about a carbon-carbon bond, isomers are possible.
ciscis transtrans
substituents onthe same side ofmain chain
substituents onopposite sides ofmain chain
C C
H
R
R
H
C C
R
H
R
H
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 isomers in ring compounds to alkenesalkenes
cis trans
cis / trans isomers are also called geometric isomers
If an alkene has two identical substituents on one ofthe 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 trans isomersisomers
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
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
CH3
H CH2CH3
CH2CH3CH3
H CH3
CH3
cis-3-methyl-2-pentene trans-3-methyl-2-pentene
Be Careful !!!Be Careful !!!
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!
Sect 4.12: E/Z nomenclature
To avoid the confusion between what the mainchain is doing and the relationship of two similargroups ….. the IUPAC invented the E/Z system.
E/Z system of nomenclatureE/Z system of nomenclature
FCl
I H
This system also allows alkenes like the one above to be classified ….. an impossibility with cis / trans.
cis ?
trans ?
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 Assigning prioritiespriorities
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)
More than one double bond: dienesMore than one double bond: dienes
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
1
2
3
4
5
6
(E)-1,3-hexadiene
no E/Z(E) structure
CH2OH
bonds in the ring are cis
12
cis and Z are not always the same for a given ring
but this compoundis E
1 2
H
Sect 4.13: Relative stabilities of alkenes:
hydrogenation
Hydrogenation of AlkenesHydrogenation of Alkenes
+C C H H C C
H H
catalyst
The catalyst is Pt, PtO2, Pd, or Ni
an addition reaction
CH3 + H2 CH3Pt
CH3 CH CH CH3Pt
H2+ CH3 CH2 CH2 CH3
CH2
PtH2+
CH3
ExamplesExamples
CH3
CH3
CH3
CH3
CH3
CH3
H
H
HH
syn addition
antiaddition
X
notobserved
Both hydrogen atoms add to the same side of the double bond
stereospecific
H2 / Pt
H2 / Pt
C C + H2 C C
H H
+ heat
Hydrogenation is exothermicHydrogenation is exothermic
H = approx. -30 kcal/mol Exothermic reaction!
-27.6 -28.6 -30.3-28.6
-30.3 -28.6 -27.6H
CH3CH2CH2CH3
+H2 +H2 +H2
kcal/mol
Butene isomers --- Heats of Butene isomers --- Heats of hydrogenationhydrogenation
All are hydrogenated to the same product (butane) thereforetheir energies may be compared.
Higher energy
Lower energy (more stable)
(less stable)
H
H
R
H
H
H
R
R
R
H
R
H
H
R
R
H
R
H
R
R
monosubstituted trisubstituted
disubstituted
tetrasubstituted
increasing substitution
R
R
R
R
stability
Alkene isomersAlkene isomers different positionsof the double bond
cis
trans
1,1-
1,2-
1,2-
less stablemore stable
H
H
R
R
R
H
R
H
H
R
R
H
stericrepulsion
stericrepulsion
1,1-
cis-1,2-
trans-1,2-
Steric repulsion is responsible for energy differencesSteric repulsion is responsible for energy differences among the disubstituted alkenesamong the disubstituted alkenes
(Z)
(E)
Some examples of stabilities of isomers
EXAMPLE ONE
has lower energy than (more stable)
EXAMPLE TWO
ISOMERS
has lower energy than (more stable)
ISOMERS
disubstituted monosubstituted
trisubstituted disubstituted
Sect 4.14, 4.15, 4.16
Bicyclic compounds and spiro compounds
bicyclo[3.2.1]octane
number ofrings
total numberof carbon atoms
sizes of bridges,largest first
3 carbons
2 carbons
1 carbon bridgeheads
Naming a bicyclic Naming a bicyclic compound compound
Bicyclic ring compoundsBicyclic ring compounds
bicyclo[2.2.1]heptane
bicyclo[1.1.0]butane
bicyclo[1.1.1]pentane
bicyclo[2.1.1]hexane
bicyclo[2.2.2]octane
bicyclo[3.1.1]heptane
bicyclo[4.4.1]undecane
Many examples of the trans ring fusion are foundin nature.
The cis ring fusion is not found nearly as often as trans.
HO
H
CH3
H
CH3
CH3H
CH3
CH3
H
H
H
cholestanol
trans
transtrans
eq
(a close relative of cholesterol)
Rings in natureRings in nature
NATURAL PRODUCTS : compounds that occur in living sytems,such as plants and animals.
CH3
CH3OH
O
TESTOSTERONE
PROGESTERONE
CH3
O
CH3
O
CH3
ESTROGEN
CH3CH3 CH3
O
CH3CH3
CH3
CH3CH3
O
CH3CH3
CH3
Some bicyclic natural productsSome bicyclic natural products
camphor -pinene
-pinenecineole
EUCALYPTUS
TURPENTINE
TURPENTINE
CAMPHORTREE
spiro[2.4]heptane
SpiranesSpiranesHere the smaller ring comes first in the name.
Spiro ring junctions alwaysinvolve two rings, so bi- and tricyclo, etc. are not needed. The prefix “spiro” is used instead.
Polycyclic compoundsPolycyclic compoundsThese have been made synthetically.
cubane
“bucky ball”
basketane adamantane
buckminsterfullerene
propellane