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

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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