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Chapter 6: Intermolecular Forces: Section 6.1
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Types of Intermolecular Forces
Intermolecular forces – attractive forces between molecules
Weaker than any bond within a molecule (covalent, ionic)
Arise from interaction of δ+ and δ- regions on separate molecules
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London (Dispersion) Forces
The WEAKEST intermolecular force
Attraction formed from temporary (“induced”) dipoles on molecules
Occurs momentarily in all molecules when e- become unevenly distributed over a molecule’s surface
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Important Notes on London Dispersion
- Occur in ALL molecules
- –Only significant w/nonpolar molecules because these are the ONLY intermolecular forces in which nonpolar molecules can participate
- Temporary
- –Electrons are constantly in motion. They will re-disperse around the molecule.
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Dipole-Dipole Interactions
Stronger than London dispersion
Attraction formed between permanent dipoles (polar molecules)
Do not exist between nonpolar molecules
•Polar molecules also exhibit London forces
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Dipole-Dipole Interactions
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Hydrogen Bonding
A special kind of dipole-dipole interaction
The STRONGEST intermolecular force
Occurs when a hydrogen attached to a highly electronegative atom (N, O, F) comes close to a lone pair of e- on N, O, or F on another molecule.
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Hydrogen bond donor
Hydrogen bond donor – molecule w/ a H atom covalently bonded to an O, N, or F
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Hydrogen bond acceptor
Hydrogen bond acceptor – molecule w/ a lone pair of e- on an O, N, or F
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Ion-Dipole Interactions
Technically not intermolecular because it involves ions
Stronger than H-bonding
Occurs when an ion comes close to an opposite partial charge on a molecule
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Intermolecular Forces
Play a large role in solubility of substances in one another
Affect changes of state (solid, liquid, gas)
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Section 6.2: Intermolecular Forces and Solubility
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Intermolecular Forces and Solubility
Golden Rule: Like dissolves like
–Polar substances dissolve polar substances
–Nonpolar dissolve nonpolar
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Nonpolar Compounds
Triglycerides – dietary oils (e.g., cooking oil)
–Formed via condensation rxn of three fatty acids with glycerol (esterification rxn)
–Nonpolar
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Nonpolar Compounds
In order to dissolve, molecules must interact.
Nonpolar oil (London forces) will not interact with polar water (H-Bonding).
Oils won’t dissolve in water.
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Polar Compounds
Sucrose – a carbohydrate (table sugar)
–Multiple hydroxyl groups make it polar (capable of dipole-dipole interactions)
–OH groups give it ability to H-bond
Polar sucrose dissolves in polar water.
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Ionic Compounds
Able to interact with water via ion-dipole interactions
Ionic compounds dissolve in water
Water molecules surround ions in a process called hydration.
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Soap
Salt – an ionic compound
- Soap – composed of fatty acid salts
- –Fatty acids are nonpolar (carboxylic acid group), but fatty acid salts are polar (carboxylate group).
- –Polar “head,” nonpolar “tail”
Amphipathic – compounds with polar and nonpolar parts
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Soap
- Amphipathic compounds won’t dissolve in water.
- –Tails are hydrophobic (water-fearing)
- –Heads are hydrophilic (water-loving)
–Water interacts only with hydrophilic heads, forming a micelle.
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Soap
Most stains, including dirt and grease, are nonpolar.
They are attracted to the hydrophobic tails of soap molecules.
Stains become trapped in micelles and can be washed away with water.
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Section 6.3: Intermolecular Forces and Changes of State
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Intermolecular Forces and Changes of State
- Heat is a form of energy.
- –Causes molecules to move faster.
Intermolecular forces are strongest when molecules move slowly.
As heat increases, I.F. decrease.
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Heat and Intermolecular Forces
- Solids have slow-moving molecules.
- –Strong intermolecular forces
Liquids are faster.
- Gases are fastest.
- –Weak intermolecular forces
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Changes of State
- Changes of state (“phase transitions”)
- –Solid > Liquid = melting
- –Liquid > gas = evaporating
Adding heat to a substance causes molecules to move faster, causing these changes.
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Boiling Points and Alkanes
Boiling point – the temperature at which all molecules of a substance change from a liquid to a gas
Heat disrupts intermolecular forces.
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Boiling Points and Alkanes
Octane’s B.P. is higher than pentane’s B.P.
Octane is larger, so molecules have more chances to interact.
–Stronger intermolecular forces (London Dispersion)
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Boiling Points and Alkanes
A molecule with larger surface area (like octane) has more surface contact with other molecules and more e- to disturb.
In straight-chain alkanes, the more C atoms, the stronger the attraction between molecules.
–Leads to higher boiling points
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Boiling Points and Alkanes
For alkanes with the same number of C atoms, straight-chain alkanes have higher boiling points than do branched alkanes.
–“Spaghetti-and-meatball” interactions
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The Unusual Behavior of Water
H2O has three atoms, propane has 11 (C3H8).
–Expectation: propane’s boiling point is higher
–Reality: Water’s B.P.: 100oC; propane’s: -42oC.
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The Unusual Behavior of Water
- Why?
- –Propane: London dispersion forces
- –Water: Hydrogen bonding
Stronger I.F. à more energy required to disrupt à higher boiling point
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Section 6.4: Fats, Oils, and Margarine – Solid to Liquid and Back Again
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Fat
Fat – a lipid molecule composed of three fatty acids joined to a glycerol backbone. Solid at room temperature. (AKA, triglyceride)
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Oils
Oils have the same structure, but are liquids at room temp. (AKA, triglyceride)
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Fats and Oils
•Why is fat solid and oil liquid? Intermolecular forces.
Mostly saturated hydrocarbon tails > Fats
–Saturation creates more surface area > stronger I.F.
–Only London forces > low melting point
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Fats and Oils
Fatty acid tails of oils contain more double bonds.
–Multiple double bonds à “polyunsaturated”
–Double bonds create “kinks” in fatty acid tails, making interaction more difficult à weaker I.F.
–Natural fatty acids contain only cis- double bonds
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Trans Fats
Partial hydrogenation – the saturation of some double bonds, while others are left intact
–Allows for manipulation of London forces
Butter is saturated. Margarine is partially saturated, but still has double bonds, making margarine more spreadable.
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Trans Fats
Hydrogenation is a difficult reaction to control.
Sometimes causes the formation of trans- double bonds, instead of cis-.
Trans- fats have been shown to have negative health effects (Heart disease, cancer).
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Section 6.5: Intermolecular Forces and the Cell Membrane
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Intermolecular Forces and the Cell Membrane
Cell membranes are selectively permeable.
Selective permeability – ability to allow certain substances through while denying other substances
Cell membranes are composed of phospholipids.
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Phospholipids
Phospholipids – glycerol backbone with two fatty acids and a phosphate-containing group
Phosphate group is ionic (polar); fatty acid tails are nonpolar.
–Overall phospholipid is amphipathic.
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Phospholipids
Cartoon highlights strongly polar head with two nonpolar tails
–Similar to soap, which has just one nonpolar tail
Two tails > does NOT form a micelle
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Phospholipid Bilayer
Recall: nonpolar = hydrophobic
–Outside the cell: watery. Inside the cell: aqueous.
–How does the hydrophobic portion escape?
Phospholipids form a bilayer – a double layer of phospholipids
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Other Components of the Bilayer
Phospholipids provide structure.
Proteins provide function.
–Allow molecules to move into or out of the cell.
–Can protrude through the bilayer (integral membrane proteins) or associate with one polar surface (peripheral membrane proteins)
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The Fluid Mosaic Model
Fluid Mosaic Model – the current model for how the cell membrane works
Implies that components of the bilayer are able to move freely within the bilayer
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Cholesterol
- Polar end: -OH
- Nonpolar: rest of
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- OH protrudes into the surrounding
- (aqueous) environment; rigid rings sit inside the bilayer, itself
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Cholesterol
Determines fluidity/rigidity of the cell membrane
- More cholesterol > more rigid bilayer
- –Cholesterol interacts with phospholipid tails via London forces
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