Drawing the Missing Organic Structures

If you’re having trouble drawing the missing organic structures, this blog post is for you. Learn some tips and tricks to help you get the hang of it.

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Introduction

In this guide, we will learn how to draw the missing organic structures. We will focus on the functional groups and resonance structures. The functional groups are the key features in any molecule, and the resonance structures are important for understanding the delocalized electrons in a molecule. We will also learn how to use the Lewis structure to predict the shapes of molecules.

The Method

The difficult part of drawing an organic structure is not that the student does not know where to put the atoms, but that the student does not know how to determine which atoms are connected to which. The key, then, is to find a method that will reliably produce the correct answer.

Step 1: Sketch the Overall Shape of the Molecule

The first step in drawing an organic molecule is to sketch the overall shape of the molecule. This can be done by starting with a simple sketch of the chain of atoms that make up the molecule, and then adding in any other atoms or groups of atoms that are attached to the chain. When sketching the molecule, it is important to keep in mind the types of bonds that are present in the molecule (single, double, or triple bonds), as well as the relative sizes of the different atoms involved.

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Step 2: Find the Carbon Skeleton

The second step is to identify and draw the carbon skeleton for the molecule. The carbon skeleton is the basis for naming organic molecules and is formed by stringing together single atoms of carbon in straight or branched chains, or in ring structures.

Single atoms of hydrogen are then added to the carbon skeleton to complete the structure. Although multiple bonds (double or triple) between atoms of carbon are commonly drawn in organic structures, it is usually more accurate to treat these bonds as a single bond when counting the number of carbons in the skeleton. Doing so leads to more consistent results when naming molecules.

Step 3: Add in the Hydrogens

In step 3, you will add in the hydrogens. Hydrogens are the lightest elements and are found on the outside of the molecule. They are not drawn in step 1 because they do not determine the shape of the molecule.

To add hydrogens, simply draw them in on the outside of the molecule where there is an empty space. Add as many as needed to fill all the empty spaces. Be sure to add them in evenly so that the molecule does not become unbalanced.

Step 4: Fill in the Lone Pairs

In many molecules, one or more of the atoms has more electrons around it than are needed to form single bonds with its neighbors. These electrons are called lone pairs (or nonbonding pairs). They are represented as dots in Lewis structures. For example, the Lewis structure for water (H2O) shows two lone pairs of electrons on the oxygen atom:

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O
: :
H−−−H
: :
O
The two lone pairs take up more space than a single bond, so they force the bond angles in water to be closer to 104° than the ideal 109.5°.

You can place lone pairs on any of the atoms in a molecule, but they are not always Spread out evenly. In some cases, you may need to put two lone pairs on one atom and none on another in order to satisfy the octet rule and minimize formal charges. For example, consider ammonia (NH3). It has one nitrogen atom bonded to three hydrogen atoms. The nitrogen still needs one more electron to complete its octet, so it would be reasonable to place a lone pair on it:

N–H−H−H
But this structure violates the rule that says you should minimize formal charges whenever possible. In this case, it’s better to put two lone pairs on the nitrogen and one on each hydrogen:

N
: : H—N—H Formal charge = 0
: : | | Formal charge = +1
H H H Formal charge = −1

There is another reason why you might want to put two lone pairs on one atom and none on another. When two atoms have a lot of electrons around them (because they have multiple bonds or because they have lone pairs), they repel each other more than atoms with fewer electrons around them do. So it’s often necessary to put Lone Pairs on different atoms in order to minimize repulsions. For example, consider carbon dioxide (CO2). It has a double bond between its carbon and oxygen atoms:

C=O The correct Lewis structure for carbon dioxide puts both Lone Pairs on the oxygen atom:
O This is because oxygen is more electronegative than carbon, so it will attract the Lone Pairs more strongly.

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C=O O=C Placing both Lone Pairs on the same atom would give rise to too many repulsions:

O

Conclusion

Now that you know the basic steps for drawing the missing organic structures, you can practice on your own. Start with simple molecules and then try more complex ones. With a little practice, you’ll be able to draw any organic molecule!

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