Simple Procedure for writing Lewis Structures of allene C3H4 -Ex. #14

A simple procedure for writing Lewis structures is given in a previous article entitled “Lewis Structures and the Octet Rule”. Relevant worked examples were given in the following articles: Examples #1, #2, #3 , #4, #5, #6,  #7,  #8, #9, #10, #11, #12 and #13.

Another example  for writing Lewis structures following the above procedure is given bellow:

Let us consider the case of allene (C₃H₄). Allenes have proven themselves to be valuable building blocks toward complex molecular targets, revealing novel applications in natural product synthesis, pharmaceutical chemistry and material science. The ongoing interest in allene chemistry results in a variety of new methodologies and pathways for the synthesis of allenes.

Step 1: Connect the atoms with single bonds

Fig. 1: Connecting the allene atoms with single bonds (step 1)

Step 2: Calculate the # of electrons in π bonds (multiple bonds) using  formula (1) in the article entitled “Lewis Structures and the Octet Rule”.:

Where n in this case is 3 (exclude the H atoms)

Where V = (4 + 2 + 4 + 4+ 2) = 16  

Therefore, P = 6n + 2 – V = 6 * 3 + 2 – 16 = 4   So there are 4 π electrons in  C₃H_{4 .}  Therefore, 2 double bonds or 1 triple bond must be added to the structure of Step 1.

  

Step 3 & 4: The Lewis structures for C₃H₄ are as follows:

Fig. 2: Lewis electron dot structure for the allene molecule C₃H₄