MadSci Network: Chemistry

Re: Is compound named 2-butene actually different from cis/trans 2-butene?

Date: Wed Dec 3 15:37:43 2003
Posted By: Plamen Angelov, Faculty, Organic Chemistry, University of Plovdiv
Area of science: Chemistry
ID: 1070173124.Ch

Dear Chris,

	“Straight chain” does not mean that the chain is actually a 
straight line. It rather means that there is no branching of the chain. 
The carbon chains, no matter straight or branched, are always zig-zag 
shaped except for some simple alkynes. (In propyne or 2-butyne for example 
the straight chain is really linear.) 
	Carbon chains are zig-zag shaped because this is the shape that 
ensures optimal distance between the atoms and minimal energy of the 
	Regardless of their shape, in saturated carbon chains (e.g. 
alkanes) there is no cis-trans isomerism because these chains are 
flexible – there is a constant fast rotation around the simple C-C bonds, 
which usually makes it impossible to separate the different forms of the 
So, the zig-zag shape is required but is not a sufficient prerequisite for 
the existence of cis-trans isomerism. In addition there should be 
something that fixes the molecular geometry in such a way that two 
different forms of the molecule could not easily convert into each other. 
Such a condition is ensured by the double C=C bond in alkenes. Unlike the 
simple C-C bonds, there could be no free rotation around a double C=C 
bond. That is why we could distinguish between the two forms of 2-butene.

	Finally, the answer to your question is that 2-butene can be 
either cis or trans – there is no other option. Linear geometry of the 
molecule is not an option!

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