Structural Formulas
How molecules are drawn and what those drawings communicate. Covers skeletal structures, resonance notation, constitutional isomers, and the difference between open-chain and cyclic forms of the same compound.
The Kekule structure with alternating single and double bonds incorrectly implies unequal bond lengths. While historically important, it does not capture the true nature of aromatic bonding.
The delocalized circle notation accurately represents benzene's equal bond lengths and electron delocalization across all six carbons. The pi electrons are shared equally, making all C-C bonds identical at 1.40 angstroms.
Dimethyl ether (CH3OCH3) has the same molecular formula C2H6O but lacks an O-H group. It can only accept hydrogen bonds, not donate them, resulting in much weaker intermolecular interactions with water.
Ethanol (CH3CH2OH) has an O-H group that can donate and accept hydrogen bonds with water. This makes ethanol miscible with water in all proportions and gives it a much higher boiling point than dimethyl ether.
Trans-2-butene (E isomer) has methyl groups on opposite sides, creating a symmetric molecule with no net dipole. Its weaker intermolecular forces result in a lower boiling point despite having the same molecular formula.
Cis-2-butene (Z isomer) has a net dipole moment because the methyl groups are on the same side of the double bond. This gives it stronger intermolecular forces and a higher boiling point (3.7 degrees C vs 0.9 degrees C for trans).
The open-chain aldehyde form of glucose exists as less than 0.003% of total glucose in water. While it is the reactive form in many biochemical assays, it rapidly cyclizes back to the pyranose ring.
The cyclic glucopyranose form accounts for over 99% of glucose in aqueous solution. The six-membered ring is thermodynamically stable, and the intramolecular hemiacetal formation is strongly favored at equilibrium.