## lewis structure of organic compunds

(Polar molecules, Non-polar molecules, etc.)

Sarah Blake-2I
Posts: 153
Joined: Fri Aug 30, 2019 12:16 am

### lewis structure of organic compunds

Are there any tricks to drawing more complicated lewis structures? I keep drawing them incorrectly and I am not sure how to tell where the bonds go and what is connected to each atom.

Annie Chantasirivisal_4G
Posts: 114
Joined: Wed Sep 18, 2019 12:21 am

### Re: lewis structure of organic compunds

Do you have an example of a difficult compound?

I'm assuming most homework problems use easier compounds, but whenever they get challenging for me or if I get it wrong initially, I just start off again with evaluating the most electropositive element and putting it in the middle, then spread out the rest of the elements of the compound and draw one bond in between each. After that, I just count the number of electrons left and put them into each atom following the octet rule (and octet exception if leftover) and then figure out their formal charges. I would continue rearranging the bonds/adding double or triple bonds while removing the lone pairs after to reduce the magnitude of the formal charges, with the negative charge(s) at the most electronegative atom of the compound.

That's worked for me so far since the textbook answer follows formal charges, but I don't know what difficult structures/compounds you've faced.

Anisha Chandra 1K
Posts: 118
Joined: Thu Jul 11, 2019 12:17 am

### Re: lewis structure of organic compunds

You could try different orders of the atoms in the molecule and see which structure has the most favorable formal charges. For particularly long molecules, sometimes they group atoms attached to one another together. For example if you see CH3 or NH3 in a long molecular formula, you can likely assume that the hydrogens are attached to the carbon or nitrogen and not some other atom in the molecule.

Sarah Blake-2I
Posts: 153
Joined: Fri Aug 30, 2019 12:16 am

### Re: lewis structure of organic compunds

For instance, CH3CH2OH. How would I know how to draw this compound?

Annie Chantasirivisal_4G
Posts: 114
Joined: Wed Sep 18, 2019 12:21 am

### Re: lewis structure of organic compunds

Sarah-3C wrote:For instance, CH3CH2OH. How would I know how to draw this compound?

So, that compound is composed of carbon, hydrogen and oxygen atoms. Of the three, their electronegativities, from lowest to highest, are H<C<O. Usually the most electropositive element goes to the middle, but since hydrogen can only have two electrons in its shell, carbon should be our middle/central atom of the molecule (if you notice that carbon usually takes up the center role since it can form four bonds and is usually the most electropositive of any element in a compound).

Since there is more than one carbon, you'd place them in a line in the middle and then "sprinkle" the hydrogens to surround those carbons following the octet rule, followed by the oxygen at the end. The middle of your structure would look like C-C-O with hydrogens around the C atoms, so the left C would have three hydrogens and the middle C would have two hydrogens and then connect to the O on the right. O would thus have the H on its right to complete the number of hydrogens in the molecular formula (H cannot be in between C and O like C-C-H-O since it cannot form more than one bond, which is why O has to be in between such as C-C-O-H. Then, after you connect the atoms all together you notice that there are still four electrons left, which would then become two lone pairs for oxygen since it needs to complete its octet rule, and then you have the structure. The formal charges for all of the atoms are zero, so no rearrangement needed in terms of double/triple bonds.

I know our book doesn't have a lot of practice problems with longer molecular formulas, but I don't really see them in class so I don't think it will be much of a problem until ochem. I'm sure if you look up long formulas online you can find some to practice with. I hope this helps!

Tyler Angtuaco 1G
Posts: 130
Joined: Wed Sep 11, 2019 12:16 am

### Re: lewis structure of organic compunds

The rules that help me the most are the octet rule, the expanded octet trend, and formal charge. Make sure each atom satisfies the octet rule before adding any left over electrons. If there are extra electrons, create multiple bonds or add lone pairs to atoms located in period 3 or below on the periodic table (these elements can have expanded octets by filling their d-orbitals after p-orbitals are filled). Calculate the formal charge of each atom after drawing a structure to determine if it is the best arrangement. If there are not enough electrons, see if any atoms in the molecule are H, He, Li, Be, B, or Al; these elements do not require eight valence electrons.