slide1 l.
Download
Skip this Video
Loading SlideShow in 5 Seconds..
Sect 4.6: Monosubstituted cyclohexane rings PowerPoint Presentation
Download Presentation
Sect 4.6: Monosubstituted cyclohexane rings

Loading in 2 Seconds...

play fullscreen
1 / 71

Sect 4.6: Monosubstituted cyclohexane rings - PowerPoint PPT Presentation


  • 771 Views
  • Uploaded on

Sect 4.6: Monosubstituted cyclohexane rings. Methylcyclohexane conformations. Axial methyl. Equitorial methyl. Energy difference between an axial and an equitorial methyl group. E N E R G Y. 1.7 kcal/mol. 0 kcal/mol. 1,3-Diaxial interactions on the top of the ring.

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about 'Sect 4.6: Monosubstituted cyclohexane rings' - jacob


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
slide1

Sect 4.6:Monosubstituted

cyclohexane rings

slide2

Methylcyclohexane

conformations

Axial methyl

Equitorial methyl

slide3

Energy difference between an axial

and an equitorial methyl group

ENERGY

1.7 kcal/mol

0 kcal/mol

slide4

1,3-Diaxial interactions on the top of the ring

1,3-diaxial interactions

STERIC REPULSION RAISES THE ENERGY

OF THE AXIAL CONFORMATION

slide5

1,3-Diaxial interactions

1,3-diaxial interactions

CH3

H

H

H

H

CH

H

H

H

H

1,3-Diaxial interactions:

Newman projection view

slide6

Monosubstituted cyclohexanes: gauche steric interactions

GAUCHE STERIC

INTERACTIONS

gauche steric nteractions

60o

CH3

(like gauche butane)

CH3

CH2

CH3

Axial

CH2

180o

CH3

CH2

Equitorial

No gauche steric

problem when the group is equitorial

CH2

slide7

General rule

Large groups will generally prefer to occupy an

equatorial position where there is an absence of

1,3-diaxial (steric) interactions

axial

conformation

equatorial

conformation

Keep in mind, however, that the axial conformation will also be present, but in smaller amount.

slide8

Table 4.5: Conformational energy differences for substituents attached to a cyclohexane ring

Equitorial preferred

DGofor group in the axial position

Group X

kcal/mol

kJ/mol

Group

kcal/mol

kJ/mol

CH3-

1.7

7.1

Cl-

0.4

1.7

Br-

CH3CH2-

1.8

7.5

0.5

2.1

CH3-CH-

8.8

0.7

2.1

2.9

HO-

CH3

C6H5-

3.1

13

CH3

CH3-C-O-

0.7

2.9

>5

>21

CH3-C-

O

CH3

slide9

H

H

H

C

H

C

C

H

H

C

H

H

H

t -BUTYLCYCLOHEXANE

Too big a group to

go into the axial

position - must go

equatorial.

Basically “locks” the ring in a chair with the

tert -butyl group in the equatorial position.

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.

molecules viewed with chime
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!
cis trans isomerism
cis-trans isomerism
  • Different spatial arrangements
  • The arrangements cannot be converted into one another by rotation
  • cisBoth substituents on same side of plane
  • transSubstituents on opposite sides of plane
slide14

cis and trans isomers

applies to substituents on a ring or (later) double bond

Cl

Cl

Cl

Cl

cis

trans

both substituents are on

the same side of the ring

the substituents are on

opposite sides of the ring

These two compounds are geometric isomers

slide15

Naming cis /trans isomers

place designation

in front of name

Cl

Cl

cis-1,2-dichlorocyclopropane

notice

italics

Cl

trans-1,2-dichlorocyclopropane

Cl

slide16

cis

cis

no cis or trans

trans

trans

How many different dimethylcyclobutanes are there?

Constitutional

isomers

1,1-

1,2-

1,3-

cis /trans isomers (geometric)

no cis/trans here

slide17

Planar ring approximation

Notice that it is OK to use planar rings

when figuring out cis / trans isomers.

Use planar structures on tests!

You only need to use puckered rings

when you are dealing with conformations.

sect 4 8 disubstituted cyclohexanes cis trans isomerism use chair structures

Sect 4.8: disubstituted cyclohexanes: cis/trans isomerismUse chair structures!!

slide19

trans-1,2-dimethylcyclohexane

has two possible conformers

trans-1,2-dimethylcyclohexane

CHAIR-1

CHAIR-2

Methyl

above

Methyl

below

e,e

a,a

Which conformer is more stable?

The trans e,e one!

slide20

Calculating the energy difference using values from Table 4.5

trans-(a,a)-1,2-dimethylcyclohexane

3.4 kcal/mol higher

(two axial methyls)

2 x 1.7 kcal

trans-(e,e)-1,2-dimethylcyclohexane

Reference = 0 kcal/mol

D

G

= (2)(1.7) – 0 = 3.4 kcal/mol

o

Group

kcal/mol

kJ/mol

Group

kcal/mol

kJ/mol

CH

-

1.7

7.1

Cl-

0.4

1.7

3

Br-

CH

CH

-

1.8

7.5

0.5

2.1

3

2

CH

-CH-

8.8

0.7

2.1

2.9

HO-

3

CH

C

H

-

3.1

13

3

6

5

CH

3

CH

-C-O-

0.7

2.9

3

>5

>21

CH

-C-

3

O

CH

3

slide21

1,3-Diaxial interactions (steric)

on top and bottom of ring

No diaxial interactions

lots of room

Two axial-axial problems

@ 1.7 kcal/mol each

Equatorial groups are

assumed to be 0 kcal/mol

slide23

What about cis / trans isomers in disubstituted rings other than 1,2-dimethylcyclohexane?

1,1-dimethylcyclohexane: no cis/ trans isomers

1,3-dimethylcyclohexane: 4 chair structures

1,4-dimethylcyclohexane: 4 chair structures

slide24

DG = (1.7 - 0.7) = 1.0 kcal/mol

Which conformer has the higher energy?

Both are trans!

This one!

CH3

CH3

axial = 1.7 kcal/mol

equatorial = 0 kcal/mol

OH

OH

equatorial = 0 kcal/mol

axial = 0.7 kcal/mol

1.7 kcal/mol

0.7 kcal/mol

slide25

Guideline

In substituted cyclohexane rings, the best

(lowest energy) conformation will have the

largest groups in equatorial positions

whenever possible.

slide27

cis and trans ring fusions

trans-ring fusion

cis-ring fusion

bonds are cis

bonds are trans

cis-decalin: less stable

trans-decalin

slide28

other representations

trans

cis

Drawing

Conventions

top

solid wedge =

towards you

dashed wedge =

away from you

bottom

A dot implies

the hydrogen

is towards you

(on top).

slide29

Sect 4.10: read this section; no lectures

Skip this section, winter 07

slide31

Alkene geometry: planar

p bond

sp2

p bond

sp2

s bond

planar

s bond

END VIEW

SIDE VIEW

slide32

ROTATION BREAKS THE p BOND

Unlike s bonds, p bonds do not rotate.

NO!

It requires about 50-60 kcal/mole ( ~ 240 kJ/mole )

to break the p bond - this does not happen at

reasonable temperatures.

slide33

cis /trans isomers (geometric isomers)

Because there is no rotation about a

carbon-carbon bond, isomers are possible.

trans

cis

substituents on

the same side of

main chain

substituents on

opposite sides of

main chain

slide34

Compare cis / trans isomers in ring compounds to alkenes

cis

trans

cis / trans isomers are also called geometric isomers

slide35

Two identical substituents

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

no cis / trans isomers

slide37

Naming cis / trans isomers of alkenes

main chain stays

on same side of

double bond = cis

main chain crosses

to other side of

double bond = trans

cis-3-hexene

trans-3-hexene

notice that these

prefixes are in

italics

slide38

Rings with double bonds

trans double bonds are not possible until the ring

has at least eight carbon atoms

if C<8 then the

chain is too short

to join together

trans

cis

cis

C = 5

C = 8

smallest ring that

can have a trans

double bond

cis

C = 6

trans

Note that bothcis and trans exist for C8.

slide39

Be Careful !!!

The main chain determines cis / trans in the IUPAC name

cis-3-methyl-2-pentene

trans-3-methyl-2-pentene

This compound is cis

but the two methyl

groups are

….trans to each other.

This compound is trans

but the two methyl

groups are

….cis to each other.

but the terms cis and trans are also used to designate

the relative position of two groups: a new system is needed!

slide41

E/Z system of nomenclature

To avoid the confusion between what the main

chain is doing and the relationship of two similar

groups ….. the IUPAC invented the E/Z system.

cis ?

Cl

F

trans ?

I

H

This system also allows alkenes like the

one above to be classified …..

an impossibility with cis / trans.

slide42

E / Z Nomenclature

In this system the two groups attached to each carbon

are assigned a priority ( 1 or 2 ).

If priority 1 groups are both on same side of double bond:

Z isomer = zusammen = together (in German)

same

side

opposite

sides

Z

E

If priority 1 groups on opposite sides of double bond:

E isomer = entgegen = opposite (in German)

slide43

Assigning 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.

1

1

example

I > Br

F > H

2

2

Since the 1’s are on the same side, this compound is Z

(Z)-1-bromo-2-fluoro-1-iodoethene

notice use of parentheses

slide45

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.

Once you find a difference, you can stop.

path goes to

F not to H

1

1

comparison

stops here

2

2

path goes to

C not to H

This molecule has Z configuration.

slide46

Let’s give this compound a cis/trans name

and an E/Z name

C

H

C

H

F

3

2

H

C

H

C

H

2

3

trans-3-fluoromethyl-2-pentene (longest chain)

(Z)-3-fluoromethyl-2-pentene (priorities)

slide47

C

C

H

C

H

C

H

C

H

2

2

O

C

C

O

C

O

H

H

4. C=C double bond: equivalent to having two carbons.

C=O double bond: equivalent to having two oxygens.

2

C

1

slide48

1

2

1

2

(E)

slide49

2

2

1

1

(Z)

slide51

DIENES AND POLYENES

Hexadiene

trans, trans

trans, cis

E,E

E,Z

(2E,4E)-2,4-hexadiene

(2E,4Z)-2,4-hexadiene

(2Z,4Z)-2,4-hexadiene

(2Z,4E)-2,4-hexadiene

identical

cis, cis

cis, trans

Z,Z

Z,E

slide52

(E) structure

no E/Z

4

2

6

1

5

3

(E)-1,3-hexadiene

slide53

C

H

O

H

2

cis and Z are not always the same for a given ring

2

1

bonds in the ring

are cis

H

1

2

but this compound

is E

slide55

Hydrogenation of Alkenes

catalyst

+

C

C

H

H

C

C

H

H

an addition

reaction

The catalyst is Pt, PtO2, Pd, or Ni

slide57

CH3

H

H

CH3

CH3

CH3

CH3

CH3

H

H

Both hydrogen atoms add to the same side

of the double bond

not

observed

anti

addition

X

H2 / Pt

H2 / Pt

syn

addition

stereospecific

slide58

Hydrogenation is exothermic

DH = approx. -30 kcal/mol

Exothermic reaction!

-28.6

-27.6

-28.6

-30.3

slide59

Butene isomers --- Heats of hydrogenation

Higher energy

(less stable)

Lower energy

(more stable)

+H2

+H2

+H2

DH

-27.6

kcal/mol

-28.6

-30.3

CH3CH2CH2CH3

All are hydrogenated to the

same product (butane) therefore

their energies may be compared.

slide60

R

H

R

H

R

R

R

R

R

H

R

R

H

H

R

R

H

H

R

H

R

H

H

R

different positions

of the double bond

Alkene isomers

stability

1,1-

cis

1,2-

monosubstituted

trisubstituted

trans

1,2-

tetrasubstituted

less stable

disubstituted

more stable

increasing substitution

slide61

Steric repulsion is responsible for energy differences

among the disubstituted alkenes

steric

repulsion

steric

repulsion

1,1-

cis-1,2-

(Z)

trans-1,2-

(E)

slide62

Some examples of stabilities of isomers

EXAMPLE ONE

has lower energy than

(more stable)

ISOMERS

disubstituted

monosubstituted

EXAMPLE TWO

has lower energy than

(more stable)

ISOMERS

trisubstituted

disubstituted

sect 4 14 4 15 4 16

Sect 4.14, 4.15, 4.16

Bicyclic compounds and spiro compounds

slide64

Naming a bicyclic compound

bridgeheads

1 carbon

3 carbons

2 carbons

bicyclo[3.2.1]octane

total number

of carbon atoms

sizes of bridges,

largest first

number of

rings

slide65

Bicyclic ring compounds

bicyclo[1.1.1]pentane

bicyclo[1.1.0]butane

bicyclo[2.1.1]hexane

bicyclo[2.2.2]octane

bicyclo[2.2.1]heptane

bicyclo[3.1.1]heptane

bicyclo[4.4.1]undecane

slide66

trans

trans

trans

eq

cholestanol

(a close relative of cholesterol)

Rings in nature

Many examples of the trans ring fusion are found

in nature.

The cis ring fusion is not found nearly as often as trans.

NATURAL PRODUCTS : compounds that occur in living sytems,

such as plants and animals.

slide67

O

H

C

H

3

C

H

3

O

TESTOSTERONE

slide68

O

C

H

3

C

H

3

C

H

3

PROGESTERONE

O

ESTROGEN

slide69

Some bicyclic natural products

TURPENTINE

CAMPHOR

TREE

a-pinene

camphor

EUCALYPTUS

TURPENTINE

b-pinene

cineole

slide70

Spiranes

Here the smaller ring comes

first in the name.

Spiro ring junctions always

involve two rings, so bi- and

tricyclo, etc. are not needed.

The prefix “spiro” is used

instead.

spiro[2.4]heptane

slide71

Polycyclic compounds

These have been made synthetically.

basketane

adamantane

cubane

propellane

“bucky ball”

buckminsterfullerene