Before starting Organic it is important to assess the topics from General Chemistry that will be expected to be understood. Organic is detailed and requires quick decisions to be made when solving problems. Don’t be wasting time trying to find elements on the Periodic Table and trying to remember properties. That should be in there before Organic 1 starts. The Periodic Table, which is available during exams, is an extremely useful source of data if used properly. A working knowledge of the Periodic Table, the row and column trends of electronegativity, atom size, nucleus and electronic structure, and the need to share or swap electrons to fill an octet are essential for a good start in the Organic Chemistry sequence.
Building from the ground up.
Using the Periodic Table to predict bonding.
Knowing that atoms strive to achieve a full valence shell (generally known as the Octet Rule) we may use the Periodic Table to predict how this will occur within the basic patterns of ionic (electrons transferred), covalent (electrons shared), and polar covalent (electrons shared but unequally). The first two are covered extensively in General Chemistry with the latter becoming more prevalent in Organic. For the most part, the nature of bonding between atoms is predictable from electronegativity (E.N.) values, the trends of which come from the Periodic Table, but assimilation of which can be very useful. Elements on the left (low E.N.) give electrons away to elements on the right (higher E.N.), while elements that are closer together (similar E.N.) tend to form covalent bonds. What can be confusing is when molecules contain both types of bonding, however those patterns are also predictable from electronegativity values. Let’s first consider the four examples shown below. How do we expect each of the atoms to communicate within the molecules?
Knowing that Carbon has 4 valence electrons, Nitrogen 5, Oxygen 6, etc. we know how many bonds each needs to form to gain an octet. This may be the consequence of equal (covalent) sharing of electrons with atoms of similar electronegativities, or transfer of electrons (ionic) between atoms of quite different electronegativities. There is no exact cutoff between ionic and polar covalent bonds, however we can usually get close. In these four examples, the following bonding patterns emerge from consideration of the octet rule and E.N. values. Note that there are two different formalisms used for charge; the bonds between metals (Na and Li) and their highly electronegative partners (O and N here) are ionic, while the charges in the first and fourth examples are formal charges showing that these atoms are bonded to more, or fewer, atoms than when they are neutral. These ideas are expanded upon in Organic 1, however it is essential that students know the fundamental differences between ionic and covalent bonding before starting the Organic sequence.
Since Chemistry is largely about changes in which starting materials react together to give products, it is important to know which bonds are strong and which are weak. It should make sense that it is desireable to swap weaker bonds for stronger, more stable bonds. In General Chemistry we learn about Hess’s Law in which known bond strengths from other reactions may be used to calculate what will happen in new situations. We can do the same in Organic Chemistry if we know some generalities; ionic bonds are strong as always, and within covalent bonds the strength depends on factors such as dipolar character, relative atom size, and the presence of adjacent lone pairs. While we don’t do as many of this type of calculation in Organic, we do benefit from having an appreciation of what differentiates a strong bond from a weak one. Examples are shown below; the QR code will take you to a LibreTexts page on bond strength as will clicking on the image on the left.
Knowing a strong bond from a weak bond.
Where does negative charge want to be?
Beginning early on with Acid-Base chemistry, we ask the fundamental question “where does charge want to be?” in intermediates and products. Negative charge wants to be on more highly electronegative elements such as oxygen and the halogens and bigger elements such as bromine or iodine. Carbon does not handle a negative charge well until extra groups are added to help stabilize the extra electrons. How reactive or stable a negative charge will be in a molecule depends on additional factors such as delocalization (studied in Organic 1), however the basics are covered in General Chemistry. The Periodic Table again helps us predict anion stability, which leads into the Acid-Base chemistry studied early in the first semester of Organic. A very simple example for transitioning from General Chemistry is that of NaOH reacting with HCl to give water and NaCl. The reaction is largely driven by the need to transfer the negative charge from oxygen to the more stable chloride anion. Knowing electronegativity values will help, as will understanding anion size.
