Intramolecular Forces vs. Intermolecular Forces  Intramolecular Forces  Chemical bonds  Intermolecular Forces  Attractive forces between molecules

What are intermolecular forces?  NOT chemical bonds, less strength  Attractive forces between molecules involved in covalent bonding  Molecular level, not individual atoms  Develop solid and liquid physical properties  Types  London Dispersion  Dipole-Dipole  Ion-Dipole  Hydrogen Bonding

(London) Dispersion Forces(London) Dispersion Forces  Attractive force between dipoles  TEMPORARY charge separation in an atom (instantaneous dipole)  Sudden dipole in one atom causes “domino” effect— influences electron distribution within adjacent atoms ( induced dipoles  At SOME point in time, electron density is greater around one atom than the other  Induces temporary dipoles in adjacent molecules  Found in ALL atoms/molecules

Intermolecular Force StrengthIntermolecular Force Strength  Influenced by: 1)Polarizability 2)Molecular Shape

1. Polarizability1. Polarizability  How easily can a dipole be induced in a molecule or atom  How easily can the electron density of an atom/molecule be altered  polarizability,  strength of intermolecular forces (WHY?) **Atomic Size and Mass have an influence on polarizability

Example 1:Example 1:  Which molecule is more likely to a gas at room temperature: F 2 or Br 2 ?

2. Molecular Shape2. Molecular Shape  More contact with adjacent molecules creates more dispersion forces  Bigger/elongated molecules have this advantage  Compact molecules do not have as much surface area

Example 2:Example 2:  Which compound has the strongest intermolecular forces?

Dipole-Dipole ForcesDipole-Dipole Forces  PERMANENT charge separation (dipole)  All polar molecules  More intermolecular forces (dipole-dipole + dispersion)  Molecules arranged so the POSITIVE end of one molecule is oriented toward the NEGATIVE end of another molecule  Molecular ends with like charges repel each other

Example 3: Propane vs. acetaldehydeExample 3: Propane vs. acetaldehyde  Which has stronger intermolecular forces?

Example 4Example 4  Based on intermolecular forces, arrange the following in order from decreasing to increasing boiling point:  CBr 4  CH 3 CH 2 CH 2 CH 3  F 2  CH 3 CHO

Example 5Example 5  Examine the following 2 chemical compounds: BrCl and IBr. Which compound is a gas at room temperature? A solid?

Ion-Dipole  Attractive forces between ions and polar molecules  Results from soluble ionic compounds in polar solvent (ex. Water)  Ions split up in solvent and are surrounded by water molecules  Negative portion of water molecule surrounds POSITIVE ion  Positive portion of water molecule surrounds NEGATIVE ion

Hydrogen BondingHydrogen Bonding  Type of intermolecular force, strong dipole-dipole force  H atom is bonded to an electronegative (nonmetal) atom and is attracted to the electronegative (nonmetal) atom in a neighboring molecule  Majority of H-bonding occurs among small, very electronegative, nonmetal atoms (N, O, F)  Dotted lines represent hydrogen bonding

Hydrogen Bonding (cont.)Hydrogen Bonding (cont.)  Considered a FORCE, not a type of bond  Force holding water molecules together  Results from a negative charge on an atom and a positive charge on hydrogen  H-X format

Example: H 2 O, HFExample: H 2 O, HF

Unique Properties of Water due to Hydrogen Bonding  Freezes and expands as solid  Solid is less dense than liquid (ex. Ice floats)  High melting point  High boiling point  H-bonding strength greater than other intermolecular forces  Highest surface tension (Hg only exception)  Other compounds can easily dissolve in it  High specific heat

Biological Applications to H-BondingBiological Applications to H-Bonding  Protein Structure  Enzyme Activity  DNA structure

ALL DUE TO HYDROGEN BONDING !

Intermolecular Force StrengthIntermolecular Force Strength  All chemical compounds have dispersion forces  Other intermolecular forces are dependent on molecular structure  Generally, increasing molecular weight indicates an increase in intermolecular forces  Analyze molecular structure and types of intermolecular forces for compounds of similar molecule weight.  Similar forces, look at molecular weight

Strength of Intermolecular ForcesStrength of Intermolecular Forces

Example 6:Example 6:  The following compounds have similar molecular weights. Arrange in order from DECREASING to INCREASING boiling point.  Formaldehyde (H 2 C=O)  Methanol (CH 3 —OH)  Ethan (CH 3 CH 3 )

Example 7:Example 7:  Arrange in order from DECREASING to INCREASING boiling point.  Carbon tetrachloride (CCl 4 )  Acetone (C 3 H 6 O)  Tetrabromobutane (C 4 H 6 Br 4 )

Example 8:Example 8:  Arrange in order from INCREASING to DECREASING boiling point.  Octanoic acid—CH 3 (CH 2 ) 6 COOH  Decane—CH 3 (CH 2 ) 8 CH 3  Nonanal—CH 3 (CH 2 ) 7 CHO