3.7 draw displayed formulae for alkenes with up to four carbon atoms in a molecule, and name the straight-chain isomers

Below are ethene (1), propene (2) and butene (3).  In butene, it is possible to put the double bond between C atoms in two different places (structural isomers).  The one shown below has the double bond between the first and second C atoms in the chain, so it is more correctly called but-1-ene.

Only the alkene with four carbon atoms (C4H8) has isomers. The other isomers are shown below.  They are called but-2-ene (4) and methylpropene (5).  Remember, you don’t need to know how to name these structures apart from the straight chain molecules.

Tips on drawing alkene structures.

·        Remember to put only ONE double bond into the molecule.

·        Don’t automatically put 2 H atoms on every C atom.  Count up the bonds – each C atom should have EXACTLY 4 bonds (sticks) coming out of it.  A double bond obviously counts as 2 “sticks”.

·        If you are asked to draw different isomers, e.g., for C4H8, without it being specified that they have to be alkenes (i.e., with a double bond somewhere in the molecule), then you can draw a CYCLIC structure with all single bonds between 4 C atoms arranged in a square.  There will be 2 H atoms on each C atom.  (This compound is called cyclobutane.)