The pi bond prevents rotation because of the electron overlap both above and below the plane of the atoms. A single bond is analogous to two boards nailed together with one nail. A double bond is analogous to two boards nailed together with two nails.
In the first case you can twist the boards, while in the second case you cannot twist them. Geometric Isomers are compounds with different spatial arrangements of groups attached to the carbons of a double bond. In alkenes, the carbon-carbon double bond is rigidly fixed. Even though the attachment of atoms is the same, the geometry the way the atoms "see" each other is different.
A "group" can be hydrogen, alkyls, halogens, etc. Identical compounds may appear to have different arrangements as written, but closer examination by rotation or turning will result in the molecules being superimposed. If they are super impossible or if they have identical names, then the two compounds are in fact identical.
Isomers of compounds have a different arrangement of the atoms. Isomer compounds will differ from identical compounds by the arrangement of the atoms. Label them cis and trans. If no cis-trans isomers exist, write none. Learning Objectives Recognize that alkenes that can exist as cis-trans isomers.
Classify isomers as cis or trans. Draw structures for cis-trans isomers given their names. These are not isomers. Draw them. This compound meets rule 2; it has two nonidentical groups on each carbon atom H and Cl on one and H and Br on the other. It exists as both cis and trans isomers:. Concept Review Exercises What are cis-trans geometric isomers?
Classify each compound as a cis isomer, a trans isomer, or neither. Answers Cis-trans isomers are compounds that have different configurations groups permanently in different places in space because of the presence of a rigid structure in their molecule. There are no isomers for this molecule. Key Takeaway Cis-trans geometric isomerism exists when there is restricted rotation in a molecule and there are two nonidentical groups on each doubly bonded carbon atom.
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 butene as. If you write it like this, you will almost certainly miss the fact that there are geometric isomers. In other words, use the format shown in the last diagrams above. You obviously need to have restricted rotation somewhere in the molecule. Compounds containing a carbon-carbon double bond have this restricted rotation. As we have seen, other sorts of compounds may have restricted rotation as well, but we are concentrating on the case you are most likely to meet when you first come across geometric isomers.
If you have a carbon-carbon double bond, you need to think carefully about the possibility of geometric isomers. Note: This is much easier to understand if you have actually got some models to play with. If your school or college hasn't given you the opportunity to play around with molecular models in the early stages of your organic chemistry course, you might consider getting hold of a cheap set.
The models made by Molymod are both cheap and easy to use. An introductory organic set is more than adequate. Google molymod to find a supplier and more about them, or have a look at this set or this set or something similar from Amazon. Share the cost with some friends, keep it in good condition and don't lose any bits, and resell it via eBay or Amazon at the end of your course. Alternatively, get hold of some coloured Plasticene or other children's modelling clay and some used matches and make your own.
It's cheaper, but more difficult to get the bond angles right. Although we've swapped the right-hand groups around, these are still the same molecule. To get from one to the other, all you would have to do is to turn the whole model over.
You won't have geometric isomers if there are two groups the same on one end of the bond - in this case, the two pink groups on the left-hand end.
The cases we've been exploring earlier are like this:. Or you could go the whole hog and make everything different. You still get geometric isomers, but by now the words cis and trans are meaningless. This is where the more sophisticated E-Z notation comes in. It doesn't matter whether the left-hand groups are the same as the right-hand ones or not.
Note: The rest of this page looks at how geometric isomerism affects the melting and boiling points of compounds. If you are meeting geometric isomerism for the first time, you may not need this at the moment. If you need to know about E-Z notation , you could follow this link at once to the next page.
But be sure that you understand what you have already read on this page first! Alternatively, read to the bottom of this page where you will find this link repeated. The table shows the melting point and boiling point of the cis and trans isomers of 1,2-dichloroethene. There must be stronger intermolecular forces between the molecules of the cis isomers than between trans isomers.
Both of the isomers have exactly the same atoms joined up in exactly the same order. That means that the van der Waals dispersion forces between the molecules 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. Note: If you aren't sure about intermolecular forces and also about bond polarity , it is essential that you follow this link before you go on. You need to know about van der Waals dispersion forces and dipole-dipole interactions, and to follow the link on that page to another about bond polarity if you need to.
Both molecules contain polar chlorine-carbon bonds, but in the cis isomer they are both on the same side of the molecule. That means that one side of the molecule will have a slight negative charge while the other is slightly positive. The molecule is therefore polar. Because of this, there will be dipole-dipole interactions as well as dispersion forces - needing extra energy to break.
That will raise the boiling point. A similar thing happens where there are CH 3 groups attached to the carbon-carbon double bond, as in cis-butene. Alkyl groups like methyl groups tend to "push" electrons away from themselves.
0コメント