CH2CHCN Lewis structure

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CH₂CHCN (acrylonitrile) has three carbon atoms, three hydrogen atoms, and one nitrogen atom.

In the CH₂CHCN Lewis structure, there is one double bond and one single bond between the three carbon atoms. The left carbon is attached with two hydrogen atoms, the center carbon is attached with one hydrogen atom, and the right carbon makes a triple bond with one nitrogen atom. And on the nitrogen atom, there are two lone pairs.

Steps

To properly draw the CH₂CHCN Lewis structure, follow these steps:

#1 Draw a rough sketch of the structure
#2 Next, indicate lone pairs on the atoms
#3 Indicate formal charges on the atoms, if necessary
#4 Minimize formal charges by converting lone pairs of the atoms
#5 Repeat step 4 if necessary, until all charges are minimized

Let’s break down each step in more detail.

#1 Draw a rough sketch of the structure

• First, determine the total number of valence electrons

In the periodic table, carbon lies in group 14, hydrogen lies in group 1, and nitrogen lies in group 15.

Hence, carbon has four valence electrons, hydrogen has one valence electron, and nitrogen has five valence electrons.

Since CH₂CHCN has three carbon atoms, three hydrogen atoms, and one nitrogen atom, so…

Valence electrons of three carbon atoms = 4 × 3 = 12
Valence electrons of three hydrogen atoms = 1 × 3 = 3
Valence electrons of one nitrogen atom = 5 × 1 = 5

And the total valence electrons = 12 + 3 + 5 = 20

Learn how to find: Carbon valence electrons, Hydrogen valence electrons, and Nitrogen valence electrons

• Second, find the total electron pairs

We have a total of 20 valence electrons. And when we divide this value by two, we get the value of total electron pairs.

Total electron pairs = total valence electrons ÷ 2

So the total electron pairs = 20 ÷ 2 = 10

• Third, determine the central atom

Here hydrogen can not be the central atom. Because the central atom is bonded with at least two other atoms, and hydrogen has only one electron in its last shell, so it can not make more than one bond.

Now we have to choose the central atom from carbon and nitrogen. Place the least electronegative atom at the center.

Since carbon is less electronegative than nitrogen, assume that the central atom is carbon.

Here, there are three carbon atoms, so we can assume any one as the central atom.

Let’s assume that the central atom is center carbon.

Therefore, place carbons in the center and hydrogen and nitrogen on either side.

• And finally, draw the rough sketch

#2 Next, indicate lone pairs on the atoms

Here, we have a total of 10 electron pairs. And six bonds are already marked. So we have to only mark the remaining four electron pairs as lone pairs on the sketch.

Also remember that both (carbon and nitrogen) are the period 2 elements, so they can not keep more than 8 electrons in their last shell. And hydrogen is a period 1 element, so it can not keep more than 2 electrons in its last shell.

Always start to mark the lone pairs from outside atoms. Here, the outside atoms are hydrogens, nitrogen, left carbon, and right carbon. But no need to mark on hydrogen, because each hydrogen has already two electrons.

So for nitrogen, there are three lone pairs, for left carbon, there is one lone pair, and for remaining two carbons, there is zero lone pair because all four electron pairs are over.

Mark the lone pairs on the sketch as follows:

#3 Indicate formal charges on the atoms, if necessary

Use the following formula to calculate the formal charges on atoms:

Formal charge = valence electrons – nonbonding electrons – ½ bonding electrons

For left carbon atom, formal charge = 4 – 2 – ½ (6) = -1

For center carbon atom, formal charge = 4 – 0 – ½ (6) = +1

For right carbon atom, formal charge = 4 – 0 – ½ (4) = +2

For each hydrogen atom, formal charge = 1 – 0 – ½ (2) = 0

For nitrogen atom, formal charge = 5 – 6 – ½ (2) = -2

Here, both carbon and nitrogen atoms have charges, so mark them on the sketch as follows:

The above structure is not a stable Lewis structure because both carbon and nitrogen atoms have charges. Therefore, reduce the charges (as below) by converting lone pairs to bonds.

#4 Minimize formal charges by converting lone pairs of the atoms

Convert a lone pair of the left carbon atom to make a new C — N bond with the center carbon atom as follows:

In the above structure, you can see that the central atom (center carbon) doesn’t forms an octet. Hence, the octet rule is satisfied.

#5 Repeat step 4 (minimize charges again)

Since there are charges on carbon and nitrogen atoms, again convert a lone pair of the nitrogen atom to make a new C — N bond with the right carbon atom as follows:

#6 Minimize charges again

There are still charges on carbon and nitrogen atoms, so again convert a lone pair of the nitrogen atom to make a new C — N bond with the right carbon atom as follows:

In the above structure, you can see that the central atom (center carbon) forms an octet. The outside atoms (left carbon, right carbon, and nitrogen) also form an octet, and all hydrogens form a duet. Hence, the octet rule and duet rule are satisfied.

Also, the above structure is more stable than the previous structures. Therefore, this structure is the stable Lewis structure of CH₂CHCN.

Next: CH₃NCO Lewis structure

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