For example, in alkenes, there is at least one carbon-carbon double bond present. If there are identical groups that are attached to the two carbon atoms but are located on the same side of the molecule, then they are called cis isomers.
In order to have cis isomerism, a molecule should have two identical side groups and two different side groups. The two identical side groups should be attached to the two vinylic carbon atoms carbon atoms that are in a double bond. Figure 1: The cis isomer of the 2-butene molecule. As shown in the above image, 2-butene has cis-trans isomerism. Here, the cis isomer is composed of two methyl groups attached to each vinyl carbon atom.
These two methyl groups are on the same side of the molecule. Identical groups being on the same side affects the polarity of that molecule. If there were more electronegative side groups on the same side, there is a very high polarity in that molecule. Therefore that molecule becomes a highly polar molecule. Due to this increased polarity, there are strong attraction forces between these molecules.
This results in a higher boiling point since strong attraction forces reduce the ability of molecules to leave each other. But the melting point is comparatively lower since cis isomer are not tightly packed due to the repulsion forces between molecules. Trans isomers are molecules having the same connectivity of atoms and are composed of identical side groups that can be found on the opposite sides. The E-Z system is better for naming more complicated structures but is more difficult to understand than cis-trans.
The cis-trans system of naming is still widely used - especially for the sort of simple molecules you will meet at this level. That means that irrespective of what your syllabus might say, you will have to be familiar with both systems. Get the easier one sorted out before you go on to the more sophisticated one! Isomers are molecules that have the same molecular formula, but have a different arrangement of the atoms in space.
That excludes any different arrangements which are simply due to the molecule rotating 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 as structural isomerism. Structural isomerism is not a form of stereoisomerism, and is dealt with on a separate page. Note: If you aren't sure about structural isomerism , it might be worth reading about it before you go on with this page.
In stereoisomerism, the atoms making up the isomers are joined up in the same order, but still manage to have a different spatial arrangement. Geometric isomerism is one form of stereoisomerism. These isomers occur where you have restricted rotation somewhere in a molecule. At an introductory level in organic chemistry, examples usually just involve the carbon-carbon double bond - and that's what this page will concentrate on. Think about what happens in molecules where there is un restricted rotation about carbon bonds - in other words where the carbon-carbon bonds are all single.
The next diagram shows two possible configurations of 1,2-dichloroethane. These two models represent exactly the same molecule. You can get from one to the other just by twisting around the carbon-carbon single bond.
These molecules are not isomers. If you draw a structural formula instead of using models, you have to bear in mind the possibility of this free rotation about single bonds.
You must accept that these two structures represent the same molecule:. These two molecules aren't the same. The carbon-carbon double bond won't rotate and so you would have to take the models to pieces in order to convert one structure into the other one.
That is a simple test for isomers. If you have to take a model to pieces to convert 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 a carbon-carbon double bond is that there are two links joining the carbons together. In reality, the reason is that you would have to break the pi bond.
Pi bonds are formed by the sideways overlap between p orbitals. 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 only happens if the compound is heated strongly. If you are interested in the bonding in carbon-carbon double bonds , follow this link. Be warned, though, that you might have to read several pages of background material and it could all take a long time.
It isn't necessary for understanding the rest of this page. In one, the two chlorine atoms are locked on opposite sides of the double bond. This is known as the trans isomer. In the other, the two chlorine atoms are locked on the same side of the double bond. This is know as the cis isomer. The most likely example of geometric isomerism you will meet at an introductory level is butene.
In one case, the CH 3 groups are on opposite sides of the double bond, and in the other case they are on the same side. Geometric isomers can only occur where there is restricted rotation about a bond. So far we have looked at the simplest example of this where there is a double bond between two carbon atoms, but there are other possibilities as well.
If you have a ring of carbon atoms there will also be no possibility of rotation about any of the carbon-carbon bonds. Cyclohexane is a simple example:. They are molecules having the same connectivity of atoms and are composed of identical side groups that can be found on the same side. They are molecules having the same connectivity of atoms and are composed of identical side groups that can be found on the opposite sides.
The melting point of Cis isomers is comparatively low due to the loose packing of molecules. The melting point of trans isomers is comparatively high due to tight packing of molecules.
The boiling point of Cis isomers is comparatively high due to the presence of strong intermolecular forces. The boiling point of trans isomer is comparatively low due to the absence of strong intermolecular forces.
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