Introduction to Organic Chemistry!
Hello students and welcome to Organic Chemistry 1! First, let’s begin with some of the basics you will need to succeed in this course. When drawing a molecule, we use lines to indicate bonds and dots to indicate electrons. This is also known as the Lewis Structure:
We also abbreviate atoms using their symbol from the periodic table:
Fluorine (F), Chlorine (Cl), Bromine (Br), and Iodine (I)
Valence Electrons and Bonds
Molecules bond with one another by exchanging electrons, thereby forming a covalent bond. Each atom has a specific number of valence electrons, or a number of electrons that they “start with”:
C has 4 valence electrons, which allows it to make 4 bonds while maintaining a neutral charge. N has 5 valence electrons, which allows it to make 3 bonds and have 1 lone electron pair while maintaining a neutral charge.
O has 6 valence electrons, which allows it to make 2 bonds and have 2 lone electron pairs while maintaining a neutral charge.
F, Cl, Br, I have 7 valence electrons, which allows them to make 1 bond and have 3 lone electron pairs while maintaining a neutral charge.
H has 1 valence electron, but because it only has one s orbital, it only has a maximum of 2 electrons it can hold which allows it to make 1 bond.
When an atom is by itself, it only has its valence electrons:
But when an atom bonds with another atom, they each donate one electron per bond and each atom wants to complete its electron octet (or have access to 8 electrons). Hydrogen can only have 2 valence electrons; the exception to the octet rule:
In the example above, C has 4 bonds and since each bond contains 2 electrons, C has a full octet since it has access to 8 electrons. Within each bond, 1 electron belongs to the donor and 1 belongs to the acceptor. In this case, C still has 4 electrons that belong to itself and 1 electron that is donated from each H and 1 from F. F has a full octet since it has 3 electron pairs (with 2 electrons each) and 1 bond that contributes 2 electrons; one that belongs to F and one that is shared with C.
Let’s take a look at 2 examples where the octet is complete, but the atom does not have the right amount of electrons in its valence shell:
In this example, the C and O are single bonded but O has only made 1 bond. When counting how many total electrons O has, we count 8: 3 lone pair electron sets (6 electrons) and 1 bond with C (2 shared electrons). However, when we count how many electrons are in O valence shell, we get 7: 3 lone pair electron sets (6 electrons) and 1 electron from the bond with C. O needs 6 electrons in its valence shell to remain neutral and since there is a 7th electron, we draw a negative charge next to it (remember: electrons are negatively charged so having an extra electron adds a negative charge, being short an electron gives a positive charge).
Let’s look at another example:
In this example, the N on the far right has 3 bonds with the N in the middle (6 electrons) and also has 1 lone pair electron set (2 electrons) which completes its octet. When counting the valence electrons to the far right N, we count 5: 2 electrons from the lone pair and 3 electrons from the bonds (1 electron from each bond belongs to N). This gives the far right N a neutral charge. When looking at the middle N, we see that there are no lone pairs but that N has 4 bonds (3 to the far right N and 1 to the C). This means that N has a complete octet (has access to 8 electrons) but when counting the valence electrons, there are only 4 electrons that the middle N can call its own. N needs to have 5 electrons in its valence shell to maintain a neutral charge. Since this N only has 4, it has a positive charge since it is short one electron (and electrons have negative charges, so being short one electron yields a positive charge).