Lecture Notes on Lipids, Intermolecular Forces, Organic Molecules, Polarity, Biomolecules
Lipids: Alkanes, Alkenes, Alkynes, and Aromatics
Intermolecular Forces
- Molecules stick together (or not) due to intermolecular forces.
- Examples illustrating the importance of intermolecular forces:
- Oil and water separate because of differences in polarity and intermolecular attractions.
- Propane and ether are gases, while ethanol is a liquid due to differences in intermolecular forces (hydrogen bonding in ethanol).
- Fats are solid, and oils are liquid due to saturation/unsaturation affecting intermolecular packing.
- Phospholipids form cell membranes, and soaps form micelles due to their amphipathic nature and interactions with water.
- Proteins fold into specific shapes based on intermolecular forces and interactions between amino acids.
- Drugs bind to cell receptors through intermolecular interactions.
Two Types of Chemical Compounds
- Ionic Compounds
- Full transfer of electrons from one element to another (or polyatomic ions).
- Held together by attraction of opposite charges (+/-).
- Important ions in life: Na^+, Cl^-, HCO_3^{2-}, Fe^{3+}, Ca^{2+}, K^+.
- Covalent Compounds
- Atoms sharing pairs of electrons (molecules).
- Sometimes unequally shared, resulting in a polar bond (partial charge).
- Different types of attractions:
- London dispersion forces (weakest, always present).
- Dipole-dipole interactions (must have partial charge).
- Hydrogen bonding (sharing a hydrogen atom).
Electronegativity and Polarity
- Electronegativity: The ability of an element to attract electrons in a covalent bond.
- Bond Type and Electronegativity Difference:
- Non-polar bond: 0-0.4
- Polar bond: 0.5-1.9
- Ionic bond: 2.0 and larger
The Nature of Organic Molecules
- When carbon bonds to a more electronegative element, polar covalent bonds result.
- This creates regions of negative charge (red) and positive charge (blue).
- Polarity strongly influences physical and chemical properties.
Polarity in Organic Chemistry
- Non-polar links – Lipids (defining characteristic)
- Difference in electronegativity is 0.0-0.4
- Examples: C-H, C-Br, C-I, C-S
- Polar Links – Carbohydrates, amino acids
- Difference in electronegativity is 0.5-1.9
- Examples: C-N, C-O, C-F, C-Cl
- Polarity affects properties of organic molecules.
Hydrogen Bonding
- Strongest molecule-molecule interaction.
- Requires a polar bond with H (N-H, O-H, or F-H).
- Hydrogen bond donor (sharing the H).
- Requires an unshared pair of electrons.
- Hydrogen bond acceptor.
Impact of Polarity
- Most organic compounds are insoluble in water (lipids).
- Almost all soluble organic compounds do not conduct electricity (covalent bonds, not ionic bonds).
- Only small polar organic molecules or large molecules with many polar groups interact with water molecules and dissolve in water (carbohydrates and proteins).
Biomolecules
- Four Main Categories:
- Lipids: Defined by the property of being soluble in nonpolar solvents.
- Carbohydrates: Polyhydroxy aldehydes and ketones (contain lots of alcohol groups).
- Proteins: Built from amino acids: carboxylic acids that have an alpha-amino group.
- Nucleic acids: Heterocyclic nitrogenous bases linked by sugar phosphate esters.
- Monomer Units and Functions:
- Proteins:
- Monomer units: Amino Acids
- Functions: Structure, Enzymes, Signaling, Regulation
- Carbohydrates:
- Monomer units: Simple sugars (glucose)
- Functions: Structure, Energy storage
- Lipids:
- Monomer units: Free fatty acids, Acetyl group
- Functions: Energy storage, Cell membranes, Signals
- Nucleic Acids:
- Monomer units: Nucleotides, Bases, purines, and pyrimidines
- Functions: Information storage, Information transfer
Biomolecules (Polarity and Bonding)
- Amino Acids:
- Two polar groups
- Hydrogen bonding capable
- Carbohydrates:
- Many hydrogen bonding groups
- Lipids:
- Nucleic Acids:
