ISOMERSLearning Goals: I will understand the difference

between structural isomers and stereoisomers

(diastereomers and enantiomers) and be able to

name/draw them accordingly

TYPES OF ISOMERISMTYPES OF ISOMERISM

Occurs due to the restricted rotation of C=C double bonds... two forms - CIS and TRANS

STRUCTURAL ISOMERISM

(CONSTITUTIONAL ISOMERS)

STEREOISOMERISM

(CONFORMATIONAL &

CONFIGURATIONAL

ISOMERS)

GEOMETRICAL ISOMERISM (Diastereomers)

OPTICAL ISOMERISM (Enantiomers)

Same molecular formula but different structural formulae

Occurs when molecules have a chiral centre. Get two non-superimposable mirror images.

Same molecular formula but atoms occupy different

positions in space.

caused by different arrangements of the carbon skeletonsimilar chemical propertiesslightly different physical propertiesmore branching = lower boiling point

There are two structural isomers of C4H10. One is a straight chain molecule where all the carbon atoms are in a single row. The other is a branched molecule where three carbon atoms are in a row and one carbon atom sticks out of the main chain.

BUTANEstraight chain

2-METHYLPROPANEbranched

C4H10

STRUCTURAL ISOMERSSTRUCTURAL ISOMERS

STRUCTURAL ISOMERSSTRUCTURAL ISOMERS

DIFFERENCES IN PROPERTIES

Chemical Isomers show similar chemical properties becausethe same functional group is present.

Physical Properties such as density and boiling point show trends according to the of the degree of branching

Boiling Point “straight” chain isomers have higher values than branched onesthe greater the degree of branching the lower the boiling pointbranching decreases the effectiveness of intermolecular forcesless energy has to be put in to separate the molecules

- 0.5°Cstraight chain

- 11.7°Cbranched

greater branching = lower boiling point

Number of Possible Isomers of Alkanes

Section 14.3

The more carbon atoms there are, the greater the number of possible isomers

Drawing Structural Isomers: You Try

Section 14.3

There are 5 structural alkane isomers for C6H14. Can you draw and name them all? Use molecular model kits to help.

STEREOISOMERSSTEREOISOMERS

Molecules have the SAME MOLECULAR FORMULA but the atoms are joined to each other in a DIFFERENT SPACIAL ARRANGEMENT - they occupy a different position in 3-dimensional space.

There are two types...

• GEOMETRICAL ISOMERISM (Diastereomers)

• OPTICAL ISOMERISM (Enantiomers)

GEOMETRICAL ISOMERS GEOMETRICAL ISOMERS (DIASTEREOMERS) IN ALKENES(DIASTEREOMERS) IN ALKENES

• an example of stereoisomersim• found in some, but not all, alkenes• occurs due to the RESTRICTED ROTATION OF C=C bonds• get two forms....

CISGroups/atoms are on the

SAME SIDE of the double bond

TRANSGroups/atoms are on OPPOSITE SIDES across the double bond

Isomers - have different physical properties - e.g. boiling points, density- have similar chemical properties - in most cases

DIASTEREOMERSDIASTEREOMERS

RESTRICTED ROTATION OF C=C BONDS

Single covalent bonds can easily rotate. What appears to be a different structure in an alkane is not. Due to the way structures are written out, they are the same.

ALL THESE STRUCTURES ARE THE SAME BECAUSE C-C BONDS HAVE ‘FREE’ ROTATION

Animation doesn’t work in old versions of Powerpoint

RESTRICTED ROTATION OF C=C BONDS

C=C bonds have restricted rotation so the groups on either end of the bond are ‘frozen’ in one position; it isn’t easy to flip between the two.

This produces two possibilities. The two structures cannot interchange easily so the atoms in the two molecules occupy different positions in space.

Animation doesn’t work in old versions of Powerpoint

DIASTEREOMERSDIASTEREOMERS

How to tell if it exists

Two different atoms/groups attached

Two different atoms/groups attached

Two similar atoms/groups attached

Two similar atoms/groups attached

Two similar atoms/groups attached

Two different atoms/groups attached

Two different atoms/groups attached

Two different atoms/groups attached

GEOMETRICAL ISOMERISM

GEOMETRICAL ISOMERISM

Once you get two similar atoms/groups attached to

one end of a C=C, you cannot have geometrical

isomerism

DIASTEREOMERSDIASTEREOMERS

Isomerism in butene

There are 3 structural isomers of C4H8 that are alkenes*. Of these ONLY ONE exhibits geometrical isomerism.

BUT-1-ENE 2-METHYLPROPENEtrans BUT-2-ENEcis BUT-2-ENE

* YOU CAN GET ALKANES WITH FORMULA C4H8 IF THE CARBON ATOMS ARE IN A RING

DIASTEREOMERSDIASTEREOMERS

RESTRICTED ROTATION OF CYCLOALKANES

a) Cis or Trans?b) Name it!

c) Cis or Trans?d) Name it!

DIASTEREOMERSDIASTEREOMERS

OPTICAL ISOMERS (ENANTIOMERS)OPTICAL ISOMERS (ENANTIOMERS)

Occurrence another form of stereoisomerism occurs when compounds have non-superimposable mirror images

Isomers the two different forms are known as optical isomers or enantiomers they occur when molecules have a chiral centre a chiral centre contains an asymmetric carbon atom

an asymmetric carbon has four different atoms (or groups)arranged tetrahedrally around it.

There are four different colours arranged tetrahedrally about

the carbon atom

2-chlorobutane exhibits optical isomerism because the second carbon atom has four

different atoms/groups attached

CHIRAL CENTRES

ENANTIOMERSENANTIOMERS

• two forms exist which are NON-SUPERIMPOSABLE MIRROR IMAGES of each other• non-superimposable means you you can’t stack one form exactly on top of the other

ENANTIOMERS MUST BE: non-superimposable mirror images ex. hands

What is a non-superimposable mirror image?

Animation doesn’t work in old versions of Powerpoint

ENANTIOMERSENANTIOMERS

17

Learning Check: Classify each of the following pairs as enantiomers or not enantiomers

a)CH3

ClCH3

Br

CH3

ClH3C

Br

b) CH3

ClBr

H

CH3

ClBr

H

F

H Br

F

HBrc)

APPLICATION OF ISOMERS- APPLICATION OF ISOMERS- THALIDOMIDETHALIDOMIDE

The one obvious difference between optical isomers is their response to plane polarised light. However, some naturally occurring molecules or specifically synthesised pharmaceuticals show different chemical reactivity.

The drug, THALIDOMIDE is a chiral molecule and can exist as two enantiomers. In the 1960’s it was used to treat anxiety and morning sickness in pregnant women. Tragically, many gave birth to children with deformities and missing limbs.

It turned out that only one of the enantiomers (the structure on the right) was effective and safe; its optically active counterpart was not. The major problem was that during manufacture a mixture of the isomers was produced. The drug was banned world-wide, but not after tens of thousands of babies had been affected.

19

“How would you like to live in Looking-glass House, Kitty? I wonder if they'd give you milk, there? Perhaps Looking-glass milk isn't good to drink...”

A Quote from A Quote from Lewis Carroll’s Lewis Carroll’s ‘Alice Through ‘Alice Through

the Looking the Looking Glass’…Glass’…

20

The looking-glass world Alice entered would indeed be a very different place, with mirror-image DNA and enzymes and amino acids,

meaning that - as Alice suggested to her kitten - looking-glass milk would almost certainly

not be good to drink. Its mirror-image molecules would all be the wrong way around

for her enzymes to digest

21

LEARNING CHECK: Are the following pairs of compounds consitutional isomers or stereoisomers? Name all 6!

a)

b)

c)

22

Distinguishing Isomers Flowchart

How Did We Do?Learning Goals: I will understand the difference

between structural isomers and stereoisomers

(diastereomers and enantiomers) and be able to

name/draw them accordingly