7/31/2019 Stereo Isomerism

http://slidepdf.com/reader/full/stereo-isomerism 1/4

STEREOISOMERISM

Chemistry PU2A SFMS 2012 Page 1

STEREOISOMERISM - GEOMETRIC ISOMERISM 

Geometric isomerism (also known as cis-transisomerism or E-Z isomerism) is a form of 

stereoisomerism.

What is stereoisomerism? 

What are isomers? 

Isomers are molecules that have the same molecular formula, but have a different arrangement of theatoms in space. That excludes any differentarrangements which are simply due to the moleculerotating as a whole, or rotating about particular bonds.

Where the atoms making up the various isomers are

 joined up in a different order, this is known asstructural isomerism. Structural isomerism is not aform of stereoisomerism.

What are stereoisomers? 

In stereoisomerism, the atoms making up the isomersare joined up in the same order, but still manage tohave a different spatial arrangement. Geometricisomerism is one form of stereoisomerism.

Geometric (cis / trans) isomerism 

How geometric isomers arise? 

These isomers occur where you have restrictedrotation somewhere in a molecule.

Think about what happens in molecules where thereis un restricted rotation about carbon bonds - in other words where the carbon-carbon bonds are all single.The next diagram shows two possible configurationsof 1,2-dichloroethane.

 

These two models represent exactly the samemolecule. You can get from one to the other just bytwisting around the carbon-carbon single bond. Thesemolecules are not isomers .

If you draw a structural formula instead of usingmodels, you have to bear in mind the possibility of thisfree rotation about single bonds. You must accept thatthese two structures represent the same molecule:

But what happens if you have a carbon-carbon doublebond - as in 1,2-dichloroethene?

These two molecules aren't the same. The carbon-

carbon double bond won't rotate and so you wouldhave to take the models to pieces in order to convertone structure into the other one. That is a simple testfor isomers. If you have to take a model to pieces toconvert it into another one, then you've got isomers. If you merely have to twist it a bit, then you haven't!

Note: In the model, the reason that you can't rotate acarbon-carbon double bond is that there are two links

 joining the carbons together. In reality, the reason isthat you would have to break the pi bond. Pi bondsare formed by the sideways overlap between porbitals. If you tried to rotate the carbon-carbon bond,the p orbitals won't line up any more and so the pi

bond is disrupted. This costs energy and onlyhappens if the compound is heated strongly.

Drawing structural formulae for the last pair of modelsgives two possible isomers.

In one, the two chlorine atoms are locked on oppositesides of the double bond. This is known as the trans  

7/31/2019 Stereo Isomerism

http://slidepdf.com/reader/full/stereo-isomerism 2/4

STEREOISOMERISM

Chemistry PU2A SFMS 2012 Page 2

isomer. (trans : from latin meaning "across" - as intransatlantic).

In the other, the two chlorine atoms are locked on the

same side of the double bond. This is known as thecis isomer. (cis : from latin meaning "on this side")

The most likely example of geometric isomerism youwill meet at an introductory level is but-2-ene. In one

case, the CH3 groups are on opposite sides of thedouble bond, and in the other case they are on thesame side.

The importance of drawing geometric isomersproperly 

It's very easy to miss geometric isomers in exams if you take short-cuts in drawing the structural formulae.For example, it is very tempting to draw but-2-ene as

CH3CH=CHCH3 

If you write it like this, you will almost certainly missthe fact that there are geometric isomers. If there iseven the slightest hint in a question that isomersmight be involved, always draw compoundscontaining carbon-carbon double bonds showing thecorrect bond angles (120°) around the carbon atomsat the ends of the bond. In other words, use the

format shown in the last diagrams above.

How to recognise the possibility of geometricisomerism 

You obviously need to have restricted rotationsomewhere in the molecule. Compounds containing a

carbon-carbon double bond have this restrictedrotation. (Other sorts of compounds may haverestricted rotation as well, but we are concentrating onthe case you are most likely to meet when you first

come across geometric isomers.) If you have acarbon-carbon double bond, you need to thinkcarefully about the possibility of geometric isomers.

What needs to be attached to the carbon-carbon double bond?  

Think about this case:

 Although we've swapped the right-hand groupsaround, these are still the same molecule. To get fromone to the other, all you would have to do is to turnthe whole model over.

You won't have geometric isomers if there are twogroups the same on one end of the bond - in thiscase, the two pink groups on the left-hand end.

So . . . there must be two different groups on the left-hand carbon and two different groups on the right-hand one. The cases we've been exploring earlier arelike this:

But you could make things even more different andstill have geometric isomers:

7/31/2019 Stereo Isomerism

http://slidepdf.com/reader/full/stereo-isomerism 3/4

STEREOISOMERISM

Chemistry PU2A SFMS 2012 Page 3

Here, the blue and green groups are either on thesame side of the bond or the opposite side.

Or you could go the whole hog and make everythingdifferent. You still get geometric isomers, but by nowthe words cis and trans are meaningless. This iswhere the more sophisticated E-Z notation comes in.

 

Summary  

To get geometric isomers you must have:

  restricted rotation (often involving a carbon-carbon double bond for introductorypurposes);

  two different groups on the left-hand end of the bond and two different groups on theright-hand end. It doesn't matter whether theleft-hand groups are the same as the right-hand ones or not.

The effect of geometric isomerism on physicalproperties 

The table shows the melting point and boiling point of the cis and trans isomers of 1,2-dichloroethene.

isomer melting point (°C) boiling point (°C)

cis -80 60 

trans -50 48

In each case, the higher melting or boiling point isshown in red.

You will notice that:

  the trans isomer has the higher meltingpoint;

  the cis isomer has the higher boiling point.

This is common. You can see the same effect with thecis and trans isomers of but-2-ene:

isomermelting point

(°C)boiling point

(°C)

cis-but-2-ene -139 4 

trans-but-2-ene

-106 1

Why is the boiling point of the cis isomers higher?  

There must be stronger intermolecular forcesbetween the molecules of the cis isomers thanbetween trans isomers.

Taking 1,2-dichloroethene as an example:

Both of the isomers have exactly the same atoms joined up in exactly the same order. That means thatthe van der Waals dispersion forces between themolecules will be identical in both cases.

The difference between the two is that the cis isomer is a polar molecule whereas the trans isomer is non-polar.

Both molecules contain polar chlorine-carbon bonds,but in the cis isomer they are both on the same sideof the molecule. That means that one side of the

molecule will have a slight negative charge while theother is slightly positive. The molecule is thereforepolar.

7/31/2019 Stereo Isomerism

http://slidepdf.com/reader/full/stereo-isomerism 4/4

STEREOISOMERISM

Chemistry PU2A SFMS 2012 Page 4

Because of this, there will be dipole-dipoleinteractions as well as dispersion forces - needingextra energy to break. That will raise the boiling point.

 A similar thing happens where there are CH3 groupsattached to the carbon-carbon double bond, as in cis- but-2-ene .

 Alkyl groups like methyl groups tend to "push"electrons away from themselves. You again get apolar molecule, although with a reversed polarity from

the first example.

Note: The term "electron pushing" is only to help

remember what happens. The alkyl group doesn'tliterally "push" the electrons away - the other end of the bond attracts them more strongly. The arrows withthe cross on (representing the more positive end of the bond) are a conventional way of showing thiselectron pushing effect. 

By contrast, although there will still be polar bonds inthe trans isomers, overall the molecules are non-polar.

The slight charge on the top of the molecule (asdrawn) is exactly balanced by an equivalent chargeon the bottom. The slight charge on the left of the

molecule is exactly balanced by the same charge onthe right.

This lack of overall polarity means that the only

intermolecular attractions these molecules experienceare van der Waals dispersion forces. Less energy isneeded to separate them, and so their boiling pointsare lower.

Why is the melting point of the cis isomers lower?  

You might have thought that the same argumentwould lead to a higher melting point for cis isomers aswell, but there is another important factor operating.

In order for the intermolecular forces to work well, themolecules must be able to pack together efficiently inthe solid.

Trans isomers pack better than cis isomers. The "U"shape of the cis isomer doesn't pack as well as thestraighter shape of the trans isomer.

The poorer packing in the cis isomers means that theintermolecular forces aren't as effective as theyshould be and so less energy is needed to melt themolecule - a lower melting point.