Organic Chemistry, Form, Part 8 - Introduction to Alkenes

Introduction to Alkenes

Greetings,

This post begins a new series on Alkene Hydrocarbons.  Aliphatic Alkenes are chain-type compounds with one or more double bonds between adjacent carbon atoms.  The presence of double bonds between carbon atoms may not seem to be a significant change compared to alkanes, but in reality the effects (of double bonds) on physical and chemical properties are quite dramatic!

Properties

Alkene molecules are, literally, less flexible than alkanes because double-bonded carbon atoms are not free to rotate.  The less flexible nature of terminal alkenes (double bond on last carbon) limits intermolecular attractions which, in turn, causes terminal alkenes (double bond on last carbon atom) to boil at lower temperatures compared to similar molecular weight alkanes.  The effect reverses for non-terminal alkenes.(1)  Alkenes are more reactive than alkanes because a pair of electrons in the double bond is available for electrophilic (literally "electron loving") compounds, such as hydrogen bromide, HBr.(2)

Nomenclature

Terminal alkenes are named very similarly to comparable alkanes with the "-ene" suffux used instead of "-ane".  The nomenclature of non-terminal alkenes (applicable to C>=4 chains) is a bit more involved.  The position of the double bond can change and must be specified in the compound name; by numbering carbon atoms and including the number of the double-bonded carbon closer to the end of the chain.  Additionally, the carbon atom numbering begins at the chain-end closest to the double bonded.  Also, cis and trans isomerism occurs, requiring additional notation in the alkene compound name.  These nomenclature rules will be explained in greater detail in future posts.

That's all for now and thank you for reading.  Please enjoy the animation of a propene (aka propylene) molecule, below.  Directly clicking on the image starts and stops the animation.  There is also a nice acoustic guitar tune, which seems to be timed with the animation.

1. http://forums.studentdoctor.net/threads/boiling-point-of-alkanes-vs-alkenes-vs-alkynes.649390/
 2. http://www.chemguide.co.uk/mechanisms/eladd/whatis.html

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