Topic 5b: orbitals, bonding, and intermolecular forces

Page 1

Hydrogen bond forms when there's an interaction between a hydrogen atom and a more electronegative atom.

The electronegative atom acts as a hydrogen bond acceptor; this atom attracts the electron cloud around the hydrogen nucleus, resulting in the hydrogen atom having a partial positive charge.

There's an attraction between the partial positive charge on the hydrogen and the partial negative charge on the electronegative atom from its lone pair.

Multiple hydrogen bonds together can create a stronger electrostatic intermolecular interaction (example: DNA held together by hydrogen bonding between nitrogenous bases, properties of water, formation of protein structures).

Bond strength is determined by the electronegativity of the hydrogen bond acceptor.

Hydrogen bonds most commonly occur with the elements O, N, F, but can also occur with highly electronegative atoms like Cl.

Page 2

Dipole-dipole interaction: a type of intermolecular force arising from the interactions of molecules containing permanent dipoles.

Polar molecules contain dipoles as a result of large electronegativity differences.

The difference in electronegativity results in the creation of partial opposite charges; these opposite partial charges attract one another, this is a weak intermolecular interaction known as an electrostatic attraction.

This is not a bond because no electrons are shared between the two molecules.

The properties of water (high surface tension, polar solvent) arise from the dipole-dipole attractions between water molecules.

hydrogen bonds > Dipole-dipole interactions > London dispersion forces.

Molecules may contain polar bonds however there will be no overall dipole moment if the molecule is symmetrical—if the overall charges are evenly spread out. There will be no dipole- dipole interactions between molecules that have no overall polarity.

Page 3

London dispersion forces: the weakest intermolecular force arising from a temporary dipole that forms in a nonpolar molecule, which creates an induced dipole in an adjacent molecule.

Typically, nonpolar molecules have an even distribution of electrons because their atoms have similar electronegativities.

This even distribution can be affected by an adjacent molecule, which causes distribution to become temporarily uneven (formation of partial positive and negative charges), resulting in an instantaneous dipole.

Induced dipoles occur when there is a temporary uneven distribution of electrons in a nonpolar molecule, causing repulsion or attraction in an adjacent molecule.

The interactions of instantaneous and induced dipoles are known as London dispersion forces, and are considered part of the van der Waals forces.

Page 4

conjugated double bonds are more stable than nonconjugated. The electrons in conjugated double bonds have similar chemistry and behave together.

Conjugated double bonds arise from 2 adjacent p orbitals being occupied by surrounding electrons, from either a double bond or lone pair adjacent to the empty orbital. There are 2 pairs of electrons contributing to the electron density, which results in the molecule being more stable

hyperconjugation: sharing of electron density in carbocations and radicals resulting in stability. Reason for the trend of least to most stability (+ charge on the _ C)--> methyl < primary carbon << secondary < tertiary. This applies to groups with electron donating groups!!!!

Page 5

aromaticity: molecules with contiguous, cyclic, and coplanar p orbitals.

Aromatic vs antiaromatic molecules: determined using Hückel’s rule

4nπ electrons, (i.e., 4, 8, 12 electrons) tend to be antiaromatic

(4n+ 2)π electrons (i.e., 2, 6, 10 electrons) tend to be aromatic

all other molecules that do not fit the aromaticity criteria count as nonaromatic

Page 6

Electron withdrawing groups: stabilize groups with negative charges

electron donating groups: stabilize groups with positive charges

resonance and induction effects allow this phenomenon.

Resonance arises from the various resonance forms of the molecule.