Chem Chapter 6 Final Review

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

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

• 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.

Stronger than London dispersion

Attraction formed between permanent dipoles (polar molecules)

Do not exist between nonpolar molecules

•Polar molecules also exhibit London forces

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.

Hydrogen bond donor – molecule w/ a H atom covalently bonded to an O, N, or F

Hydrogen bond acceptor – molecule w/ a lone pair of e- on an O, N, or F

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

Play a large role in solubility of substances in one another

Affect changes of state (solid, liquid, gas)

Golden Rule: Like dissolves like

–Polar substances dissolve polar substances

–Nonpolar dissolve nonpolar

Triglycerides – dietary oils (e.g., cooking oil)

–Formed via condensation rxn of three fatty acids with glycerol (esterification rxn)

–Nonpolar

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.

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.

Able to interact with water via ion-dipole interactions

Ionic compounds dissolve in water

Water molecules surround ions in a process called hydration.

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

• 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.

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.

• 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.

• Solids have slow-moving molecules.
• –Strong intermolecular forces

Liquids are faster.

• Gases are fastest.
• –Weak intermolecular forces

• Changes of state (“phase transitions”)
• –Solid > Liquid = melting
• –Liquid > gas = evaporating

Adding heat to a substance causes molecules to move faster, causing these changes.

Boiling point – the temperature at which all molecules of a substance change from a liquid to a gas

Heat disrupts intermolecular forces.

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)

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

For alkanes with the same number of C atoms, straight-chain alkanes have higher boiling points than do branched alkanes.

–“Spaghetti-and-meatball” interactions

H2O has three atoms, propane has 11 (C3H8).

–Expectation: propane’s boiling point is higher

–Reality: Water’s B.P.: 100oC; propane’s: -42oC.

• Why?
• –Propane: London dispersion forces
• –Water: Hydrogen bonding

Stronger I.F. à more energy required to disrupt à higher boiling point

Fat – a lipid molecule composed of three fatty acids joined to a glycerol backbone. Solid at room temperature. (AKA, triglyceride)

Oils have the same structure, but are liquids at room temp. (AKA, triglyceride)

•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

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

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.

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).

Cell membranes are selectively permeable.

Selective permeability – ability to allow certain substances through while denying other substances

Cell membranes are composed of 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.

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

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

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)

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

• Polar end: -OH
• Nonpolar: rest of
• molecule

• OH protrudes into the surrounding
• (aqueous) environment; rigid rings sit inside the bilayer, itself

Determines fluidity/rigidity of the cell membrane

• More cholesterol > more rigid bilayer
• –Cholesterol interacts with phospholipid tails via London forces