|MadSci Network: Chemistry|
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 molecule. 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 molecule. 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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