Organic Chemistry Lectures, Unit 1

What are the two main components in determining polarity of a bond?

Atom size and electronegativity differences

Atom Size: If one is very big while one is very small but they have the same electronegativity, the big one will make it polar as most of the bond is located near the bigger atom. However, on the flipside, if there are two atoms of the same size, electronegativity has a role.

Electronegativity: If there is a significant electronegativity difference in each atom, then the more polar side will be the one with greater electronegativity as it will attract the electrons to it making electron density greater.

Describe C—H in terms of polarity

Nonpolar, both molecules are relatively small and have similar electronegativities

Describe C—I in terms of polarity

Polar, because even though the electronegativities are relatively smaller, I being in a much lower row than C makes it much bigger, so majority of the bond will be in the I section.

Why do atoms get bigger as you go down the periodic table?

1. Increased number of electron shells (n=1,n=2)

2. Shielding: the inner electrons shield the positively charged nucleus from the valence electrons which means it has not as much pull and attraction so the valence electrons are held much more loosely

What happens when an electronegative atom has a negative charge?

It is even more electronegative and polar, still relatively stable though, as electronegative atoms welcome more electrons

What happens when an electronegative atom has a positive charge?

It is very unstable as electronegative atoms are used to being the ones receiving them, but now they have to seek it out further.

In addition, it is no longer a complete octet but in the opposite direction of preference of an electronegative atom.

When drawing out molecules and their bonds, how close should they be?

As far apart as possible. Bonds are formed with electrons and negatively charged bonds will repel each other as much as possible. To maximize this, as a new bond is added, a new orientation will form to maximize this electron repulsion.

Elevator example

Definition of a bond

sharing of electrons between 2 atoms to stabilize the molecules since bonds are more stable than lone pairs

Why are bonds more stable than lone pairs/individual atoms?

This is because it lowers the overall energy of atoms to create lower energy states. This ties to Molecular Orbital Theory.

Lower energy states are more stable because of the idea of spontanaeity. When in a low energy state, there is not as much spontaneous drive to go lower, so it reaches an equilibrium almost.

Definition of an ionic bond

Most extreme case of polar covalent bonds, where they pull apart from having so much polarity. The two atoms involved are a cation and anion which are pulled together in a process called electrostatic attraction.

Resonance structures only occur in…

p orbitals. not s orbitals or hybridized sp orbitals!

sp3 is what percentage what?

25% s orbital, 75 % p orbital

Explain the connections between sigma and pi bonds to single, double, and triple bonds

Single bonds have one sigma bond

Double bonds have one sigma bond, one pi bond

Triple bonds have one sigma bond, two pi bonds

What are 5 main guidelines regarding resonance?

1. Resonance forms are imaginary, just highlights

2. Only difference between resonance structures is where the non-bonding electrons and pi bonds are placed. Electrons are in p orbitals if they take part in resonance, while sigma bonds do not take part in resonance, instead, participating in hybridization.

3. Resonance forms are not all equal contributors to the structure it averages out to

4. Resonance forms MUST follow the octet rule

5. Resonance hybrids are more stable than individual forms

What is resonance used for?

Sometimes a single Lewis Diagram is not adequate in representing a compound that as interconnected moving e-

What are the four things to look our for when you are identifying which resonance structure is major or minor (measure of significance)?

#1) As many octets as possible

#2) As many bonds as possible

#3) Negative charge is on the most electronegative atom

#4) Minimizing the amount of formal charges

What is a common pattern in resonance structures?

Minor resonance is going to be more reactive

What is considered a insignificant resonance structure?

When they have 3+ charges

What is the first thing to look for in a massive compound to identify resonance?

Polar pi bonds