What is the difference between structural isomers and stereoisomers of alkenes? Can you use an example of butene to explain the difference?
Also, why are there no stereoisomers of propene and ethene? Thank youuu
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- Thread Starter
- 29-10-2017 14:58
- 29-10-2017 15:17
Structural isomers are compounds with the same molecular formula but different structural formulae. For example, butene has 3 structural isomers: but-1-ene: CH3CH2CH=CH2, but-2-ene: CH3CH=CHCH3 and 2-methylpropene: (CH3)2C=CH2. If you were to draw these out, you'd see they all have the same molecular formula but the atoms are connected differently.
Stereoisomers are compounds with the same structural formula but a different spatial arrangement. But-2-ene has the stereoisomers of trans but-2-ene and cis but-2-ene. In the trans isomer, the CH3 groups are on different sides of the C=C double bond, but in the cis isomer, the CH3 groups are on the same side of the C=C bond.
For stereoisomerism to exist, there must be restriction of rotation around the double bond and there must be two non-identical groups on each double bonded C atom. In propene, you have one CH3 group attached to the C=C bond, and 3 H atoms. This does not comply with the rule that there must be two non-identical groups. Similarly, in ethene, all atoms attached to the carbon are H atoms.
- 29-10-2017 15:17
Ok so firstly I’ll explain structural isomers, butene has the formula C4H8 and has one double bond, this double bond can be at any point in the butene to produce either But-1-end or But-2-ene.
C4H8 could also be the formula of cyclobutane which again is a structural isomer of butene, or even 1,1 -dimethyl ethene.
Stereo isomer is to do with E/z isomerism, if you’re not sure what this is I’d suggest googling it because it’s quite lengthy to explain without diagrams but you can either have e but-2-ene or z but-2-ene
There are no stereoisomers of propene or ethene because no matter where the double bond is located, there are Two hydrogen atoms on (at least) one of the carbons that makes up the double bonds. You can’t have sterioisomerism if there are two of the same group attached on the same carbon.
Hope this helps