Chapter 9 INTRODUCTION TO STEREOCHEMISTRY

Chapter 9

INTRODUCTION TO STEREOCHEMISTRY

Isomers are compounds with the same molecular formula but not identical structures

Constitutional isomers are isomers which have the same molecular formula but differ in the way their atoms are connected

Constitutional Isomers

H3CCH2

H2C

CH3

butaneChemical Formula: C4H10

CH3

CHCH3H3C

2-methylpropaneChemical Formula: C4H10

Constitutional isomers are isomers which have the same molecular formula but differ in the way their atoms are connected

start by connecting the carbons in a line

determine the C skeleton of the other isomers

C C C C C CC C C C C

C

C C C C C

CC C C C

C

CC C C C

C C

Drawing Constitutional or Structural Isomers of Alkanes

Example 8.4Write all the constitutional isomers having the molecular formula C6H14

fill in the H to give each C 4 bonds

C C C C

C

CH

H

H

H H H

H

H

H

HHHH

H

C C C C

C

C

H

H

H H H

HHHH

H

H

H

H

H

C C C C C CH

H

H

H

H

H

H

H

H

H H

H

HH

C C C C

C

H

H

H H

HHHH

H

HH

C H

H

H

C C C C C C

C C C C C

C

C C C C C

C

C C C C

C

CC C C C

C C

C C C C

C

H

H

H H

HCHH

H

HH

H

HH

H


convert each to a carbon skeleton formula – each bend and the ends represent C atoms

C C C C

C

CH

H

H

H H H

H

H

H

HHHH

H

C C C C

C

C

H

H

H H H

HHHH

H

H

H

H

H

C C C C C CH

H

H

H

H

H

H

H

H

H H

H

HH

C C C C

C

H

H

H H

HHHH

H

HH

C H

H

H

C C C C

C

H

H

H H

HCHH

H

HH

H

HH

H


Stereoisomers have the same molecular formula, maintain the same connectivity, but differ in the way their atoms are arranged in space

Conformational isomers (or conformers or rotational isomers or rotamers) are stereoisomers produced by rotation about single bonds, and are often rapidly interconverting at room temperature

Conformations of Alkanes and Cycloalkanes

Conformations of Ethane

H3C CH3ethane

Staggered conformation of

ethane

Eclipsed conformation of

ethane

H

H HH

HH

H

H

H

H

HHH

H H

H

H H

H

H H

H

HH

Sawhorse Representation


Newman Projection

Newman Projection

ethane


ethane


ethane

H

H HH

HH

H

H

H

H

HHH

H H

H

H H

H

H H

H

HH



Newman Projection

Newman Projection

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Conformations of Butane

butane


butane


butane

H3C

H2C

CH2

CH3

CH3

H HCH3

HHH

H3C

H

CH3

HH

CH3

H H

CH3

HH

CH3

H H

CH3

H H


butane


butane

CH3

H HCH3

HHH

H3C

H

CH3

HH

CH3

H H

CH3

HH

CH3

H H

CH3

H H



Configurational Isomers are stereoisomers that do not readily interconvert at room temperature and can (in principle at least) be separated.

Geometric isomers are configurational isomers that differ in the spatial position around a bond with restricted rotation (e.g. a double bond):

Geometric (Cis and Trans) Isomers

Geometric (Cis and Trans) Isomers result from restriction rotation

Compounds with double bonds cis isomer – have same substituents on the same side of the double bond (= Z with more complex molecules having high priority groups on the same side)

trans isomer – have the same substituents on the opposite side of the double bond (= E with more complex molecules having high priority groups on opposite sides)

Compounds with bonds in a ring: cis isomer – have the same substituents on the same side of the ring trans isomer - have the same substituents on the opposite side of the ring

Cis-trans (Geometric) isomerism in Alkenes

C C

ClCl

H H

cis-1,2-dichloroethene

C C

ClH

Cl H

trans-1,2-dichloroethene

(E)-1,2-dichloroethene(Z)-1,2-dichloroethene


Cis-trans (Geometric) isomerism in Alkenes

C C

ClH

H Cl

1,1-dichloroethene

*If one of the two carbon atoms of the double bond has two identical substituents, there are no cis-trans isomers for that molecule

Identifying cis and trans isomers of Alkenes

Example 11.3Two isomers of 2-butene are shown below. Which is the cis isomer and which is the trans isomer

C C

H

H3C

H

CH3

C C

H

H3C

CH3

H

cis-2-butene trans-2-butene

Naming cis and trans compounds

Example 11.4Name the following geometric isomers.

trans-3,4-dichloro-3-heptene cis-3,4-dimethyl-3-octene

H3C

H2C

CH2

CC

Cl

Cl

H2C

CH3

H3CCH2

CH3C C

H2C

CH2

H2C

CH3

CH3

Identifying Geometric Isomers

Example 11.5Determine whether each of the following molecules can exist as cis-trans isomers:(1) 1-pentene(2) 3-ethyl-3-hexene(3) 3-methyl-2-pentene

1-pentene 3-ethyl-3-hexene

CH2CH3

CC

H3CH2C CH2CH3

H

C C

CH2CH2CH3H

H H

cis-3-methyl-2-pentene

C C

CH3H3C

H3CH2C H

C C

HH3C

H3CH2C CH3

trans-3-methyl-2-pentene

cis-9-octadecenoic acid

Cis and Trans Fatty Acids

O

OH

H H

O

OH

H

H

trans-9-octadecenoic acid

Cis and Trans Fatty Acids

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Cis-trans (Geometric) isomerism in Cycloalkanes

Cis-trans isomers are molecules having the same arrangement of atoms but differ in the spatial orientation of their substituents.

H

H

H

H

H

ClCl

H

HH

H H

cis-1,2-dichlorocyclohexane

H

H

H

H

H

ClH

Cl

HH

H H

trans-1,2-dichlorocyclohexane

Naming cis-trans Isomers of Substituted Cycloalkanes

Example 10.6Determine whether the following susbstituted cycloalkanes are cis or trans isomers.

trans-1,2-dimethylcyclopentane cis-1,2-dimethylcyclopentane

H

H

H

H

H CH3

HCH3 H

H

CH3

CH3

H

H

H

H

CH3CH3

HH H

H

CH3 CH3


Conformations of Cyclohexane

cyclohexaneChair

conformation of cyclohexane

Boat conformation of

cyclohexane

H

H

H

H

H

HH

H

HH

H H

A E

A

E

A

A

E

E

A

E

A

E

A

E

A

E

A

A

E

A

E

A

E

E

Chair conformation of

cyclohexane

Boat conformation of

cyclohexane

A E

A

E

A

A

E

E

A

E

A

E

A

E

A

E

A

A

E

A

E

A

E

E


Chair-Chair Interconversion


Optical isomers are configurational isomers that differ in the 3D relationship of the substituents about one or more atoms.

Diastereomers are optical isomers (stereoisomers) that are not enantiomers.

Enantiomers are optical isomers that are non-superimposable mirror images.

Chirality

Chiral objects are objects with left-handed and right-handed forms

Achiral objects - objects that have superimposable mirror images

Nonsuperimposable mirror images - a mirror image that is not the same as the image itself - chiral objects have nonsuperimposable mirror images

Assymetric Center

Chirality is not reserved just for objects - molecules can be chiral

Chiral molecules - generally molecules containing an asymmetric center

Asymmetric (chiral) center - tetrahedral atom bonded to four different groups - indicated with an asterisk (*)

Chiral Molecules with One Asymmetric Center

Molecules with one chiral center would have 2 enantiomers*Chiral molecules would have 2n enantiomers (where n is the number of chiral centers)

Chiral vs Achiral

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How to Represent Enantiomers: Perspective Formulas

Perspective Formulas - shows two bonds of the asymmetric center as lines in the plane of the paper, another bond as a solid wedge protruding forward out of the paper, and the fourth bond as a hatched wedge extending behind the paper


Fischer Projections

Fisher Projection - representation of an asymmetric center as the point of intersection of two perpendicular lines

Horizontal lines represent bonds that project out of the plane of the paper

Vertical lines represent bonds that extend back from the plane of the paper away from the viewer

Fischer Projections

Br

H

CH3H3CH2C

Br

H

CH2CH3H3C


Naming Enantiomers -R,S System

STEP 1. Rank the groups/atoms bonded to the asymmetric center in order of priority. - rank in terms of atomic mass. Higher atomic mass, higher priority. - let’s have 1-chloro-1-ethanol as an example.

1. Chlorine = 36 amu 2. Oxygen = 16 amu 3. Carbon = 12 amu4. Hydrogen = 1 amu

STEP 2. Rotate the molecule so that the lowest priority group is pointing away from thereader.


Step 3. Trace your finger around the three highest priority groups in order of the priority.

- If the circle is moving counterclockwise the CIP designation is “S”. If the circle is moving clockwise the CIP designation is “R”.

Step 4. Name the enantiomer in terms of its R or S configuration.- the name for this isomer is (S)-1-chloro-1-ethanol

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Assigning Priorities to Groups

1. Oxygen (from CH2OH) = 162. Carbon (from CH2CH3) = attached to carbon3. Carbon (from CH3) = attached to hydrogen4. Hydrogen = 1

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Chapter 9 INTRODUCTION TO STEREOCHEMISTRY

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