Chapter 6 Intermolecular forces

1. Chapter 6 Intermolecular forces Bonds (forces) Covalent Ionic Intermolecular Holds individual molecules together Holds ionic compounds together Holds groups of molecules together into liquid or solid ↑ stronger weaker ↓ Liquid water or ice Liquid chloroform CHCl3 Solid CO2 (dry ice) liquid octane or (gasoline mixture) Hydrogen bonds Dipole-Dipole attractions London Dispersion Forces

2. Intermolecular forces Hydrogen bonds Hydrogen bonds Dipole-Dipole attractions London Dispersion Forces Water (H2O) .. H – O – H ̈ .. :O Electronegativity H ~ C < N < O H H C – H bonds are nonpolar covalent O – H or N – H bonds are polar covalent To have a H-bond you need H covalently bonded to O or N

3. Intermolecular forces Dipole-Dipole attractions Hydrogen bonds Dipole-Dipole attractions London Dispersion Forces H | C ClCl Cl d+ d+ d+ d+ d+ d+ d+ d+ d+ d+ d- d- d- d- d- d- d- d- d- d- d- d- d+ d+ d- d- Chloroform, CHCl3 Is a polar molecule Each Cl is more electronegative than H d+ d+ d- d- d+ d+

4. Intermolecular forces London Dispersion Forces Hydrogen bonds Dipole-Dipole attractions London Dispersion Forces methane, CH4 Is a nonpolar molecule H | C H H H d+ d+ d+ d+ d+ d+ d+ d+ d+ d- d- d- d- d- d- d- d- d- d- d- d+ d+ However it can still form a liquid or solid at low T. This is because the electrons are constantly moving …. Sometimes they will not distribute equally throughout the molecule … This creates a temporary dipole. The temporary dipole induces neighboring molecules to also form temporary dipoles allowing them to attract just like dipole-dipole interactions only weaker. d- d- d+ d+ d- d- d+ d+

5. Can chloroform form H-bonds? (With other chloroform molecules) a) yes b) no H | C H O ― H H H | C ClCl Cl Can methanol form H-bonds? (With other methanol molecules) a) yes b) no

6. Intermolecular forces in pure liquids/solids Dominant in polar molecules with H-bonding ability Must have O-H or N-H bonds e.g. water, alcohol, amines, etc Dominant in polar molecules that form dipoles Dominant in nonpolar molecules Hydrogen bonds Dipole-Dipole attractions London Dispersion Forces What is the dominant force in alkanes? a) H-bonds b) dipole-dipole interactions c) London dispersion forces

7. 7.1 Types of Attractive Forces

8. Alkanes – nonpolar hydrocarbons - held together by dispersion forces Larger nonpolar molecules have stronger forces than smaller ones and higher melting/boiling points. } Methane CH4 Ethane C2H6 Gases at RT and 1 atm Pressure } Propane C3H8 Dispersion forces increase with size as do Melting Points & Boiling Points Larger alkanes (like paraffin wax) are solids at RT. > C20 Butane C4H10 Pentane C5H12 Hexane C6H14 Liquids at RT and 1 atm Pressure Heptane C7H16 Octane C8H18 Nonane C9H20 Decane C10H22

9. 7.2 Liquids and Solids: Attractive Forces Are Everywhere Boiling Points and Alkanes • Straight-chain alkanes with more carbons have stronger attractions between molecules. • Because these attractions must be overcome for the compound to boil, the boiling point is higher, as is the melting point.

10. 7.2 Liquids and Solids: Attractive Forces Are Everywhere Boiling Points and Alkanes • The molecules of branched alkanes have less surface contact than do the straight-chain molecules. • The more contact between two molecules, the greater the attraction of London forces between them. • Straight-chain alkanes have higher boiling points than do branched alkanes.

11. Intermolecular forces in solutions A solution is a mixture of two or more compounds in variable proportions. The compound in the higher amount is called the solvent. The other compound(s) are called solutes. Like dissolves Like Tale of two solvents ……….. water (polar) hexane (nonpolar)

12. Which is the stronger bond ….. 1 butene (C4H8) would mostly likely be souble in ….. a) CH3-CH2-OH b) water c) hexane Like dissolves Like

13. Intermolecular forces London Dispersion Forces are most like …. a) H-bonds b) dipole-dipole interactions c) ionic bonds d) covalent bonds Hydrogen bonds Dipole-Dipole attractions London Dispersion Forces H | C H H H d+ d+ d+ d+ d+ d+ d+ d+ d+ methane, CH4 Is a nonpolar molecule d- d- d- d- d- d- d- d- d- d- d- d+ d+ However it can still form a liquid or solid at low T. This is because the electrons are constantly moving …. Sometimes they will not distribute equally throughout the molecule … This creates a temporary dipole. The temporary dipole induces neighboring molecules to also form temporary dipoles allowing them to attract just like dipole-dipole interactions only weaker. d- d- d+ d+ d- d- d+ d+

14. Will it dissolve in ….. ? a benzene  • Dispersion forces • Dipole-Dipole Interactions • H-bonds • Ion-dipole interactions a CCl4 Carbontetrachloride  c Ethanol CH3 – CH2 - OH  a Vegetable oil  c glucose  d NaCl 

15. Solubility - What does it mean to dissolve? O ― H | H Glucose can H-bond to water. Instead of binding to other glucose molecules each glucose is surrounded by water. O ― H | H O ― H | H O ― H | H O ― H | H glucose

16. Solubility - What does it mean to dissolve? NaCl(s) + H2O(l) → Na+(aq) + Cl-(aq) Cl- Na+ + - + - + - + - + - + - + - + - + - + - + - + - + - + - + - + - The interaction between a hydrated ion (either + or -) is called an ion-dipole interaction. Ions dissolved in water are called electrolytes. An NaCl solution conducts electricity, but a glucose solution will not.

17. Attractive Forces and Solubility Would stearic acid dissolve in water? a) yes b) no In this case the large size of the nonpolar “tail” on the molecule would make it poorly soluble in water (to the degree that we would classify stearic acid as insoluble in water). Predicting Solubility: Amphipathic Compounds • Molecules like fatty acids that have both polar and nonpolar parts are called amphipathic (from the Greek amphimeaning “both” and pathicmeaning “condition”) compounds.

18. SOAP O _ || O - C _ Soap is prepared by the base (NaOH or lye) hydrolysis of fat. This produces a solid sample comprised of glycerol + the salt of various fatty acids. When dissolved the fatty acid salts dissociate, and the ‘soap’ ions form a micelle structure. Na+ Salt of fatty acid

19. Micelle Structure H2O out Nonpolar

20. Soap forms micelles in water. A micelle is a “soluble” structure with the (-) carboxyl groups facing the water and the nonpolar “tails” avoiding water by collecting on the inside of a “sphere”.

21. dirt & grime are nonpolar Soap Action add heat & agitate .....

22. Soap Action add heat & agitate .....

23. Soap Action cease mechanical disruption .... micelles reform spontaneously & trap dirt : dirt is ‘soluble’

24. If molecule b were added to a soapy solution where would it go? • In the aqueous part b) in the center of a micelle c) Both of the above d) none of the above Predict which is more soluble in water ….. a b

25. O -C-R O -C-R' O -C-R'' = = = CH2-O CH-O CH2-O Glycerol (glycerin) CH2-OH | CH-OH | CH2-OH glycerol A triglyceride would be best classified as … a) an aldehyde b) an alcohol c) a carboxylic acid d) an ester triglyceride Glycerol would be best classified as … a) an aldehyde b) an alcohol c) a carboxylic acid d) an ester

26. HO-C HO-C HO-C HO-C O O O O Ω3 Ω6 polyunsaturated Stearic (saturated) C18 fatty acids Oliec (monounsaturated) Linoleic Linolenic

27. O -C-R O -C-R' O -C-R'' = = = CH2-O CH-O CH2-O Triglyceride (fats or oils) esters formed between the alcohol (glycerol) and 3 fatty acids CH2-OH | CH-OH | CH2-OH glycerol O || HO — C— R fatty acid + 3x triglyceride

28. Dietary Lipids and Trans Fats • Lipids are biological molecules that are soluble in nonpolar solvents. • Animal fat is a solid at room temperature • When hydrocarbon chains (tails) of fatty acids are mostly unsaturated, the triglyceride product is an oil. These are liquid at room temperature.

29. Attractive Forces and the Cell Membrane A Look at Phospholipids • The main structural components of cell membranes are phospholipids. • Phospholipids have a glycerol backbone with fatty acids linked to it through an ester bond. • Phospholipids have only two fatty acids on their glycerol backbone. The third OH group of the glycerol is bonded to a phosphate-containing group.

30. Attractive Forces and the Cell Membrane The Cell Membrane Is a Bilayer • A cell membrane composed of phospholipids forming a double layer called a bilayer. • The polar heads are directed out into the surrounding aqueous environment and into the aqueous interior of the cell. • This arrangement leaves the nonpolar tails of both layers directed toward each other, creating a nonpolar interior region.

31. Attractive Forces and the Cell Membrane • Protein molecules can span the bilayer (integral membrane proteins) or associate with one surface (peripheral membrane proteins). • Proteins are the membrane’s functional components, allowing selected molecules to move into and out of the cell. • The exterior surface of the cell membrane also contains carbohydrates that act as cell signals. E.g. blood type • The fluid mosaic model creates “icebergs” of protein floating in a “sea” of lipids. • The membrane is fluidlike: the phospholipids move freely within their bilayer.

32. Steroids in Membranes: Cholesterol • All Steroids contain a four-memberedfused ring structure. Cholesterol is needed in animal cell membranes. Its role is to keep them from getting either too fluid or too rigid. Think of it as anti-freeze (and anti-boil) for your membrane. There is no cholesterol in plant cell membranes.