Exam 2 Study Guide: Stereochemistry, Resonance, and Acid-Base Chemistry

Chapter 5: Stereochemistry

Chirality and Chiral Centers

Stereochemistry is the study of the spatial arrangement of atoms in molecules and its effect on their chemical behavior. A chiral center (or stereocenter) is a carbon atom bonded to four different groups, resulting in non-superimposable mirror images called enantiomers.

• Chiral Molecule: A molecule that is not superimposable on its mirror image.

• Achiral Molecule: A molecule that is superimposable on its mirror image.

• Identifying Chiral Centers: Look for carbons attached to four distinct groups.

• Example: 2-butanol (CH3CH(OH)CH2CH3) has a chiral center at the second carbon.

Assigning Stereochemical Configurations (R/S and E/Z)

The R/S system is used to describe the absolute configuration at a chiral center, while the E/Z system is used for double bonds with different substituents.

• R/S Configuration: Assign priorities to the four groups attached to the chiral center using the Cahn-Ingold-Prelog rules. Orient the molecule so the lowest priority group is away from you. If the sequence 1-2-3 is clockwise, it is R; if counterclockwise, it is S.

• E/Z Configuration: For alkenes, assign priorities to the groups on each carbon of the double bond. If the higher priority groups are on the same side, it is Z (zusammen); if on opposite sides, it is E (entgegen).

• Example: (R)-2-butanol vs. (S)-2-butanol

Enantiomers and Diastereomers

Enantiomers are non-superimposable mirror images. Diastereomers are stereoisomers that are not mirror images.

• Drawing Enantiomers: Invert the configuration at all chiral centers.

• Drawing Diastereomers: Invert the configuration at one or more (but not all) chiral centers.

• Example: 2,3-dibromobutane has three stereoisomers: two enantiomers and one meso compound (see below).

Relationships Between Molecules

Understanding the relationship between two molecules is crucial in stereochemistry.

• Identical: Same connectivity and configuration.

• Constitutional Isomers: Same formula, different connectivity.

• Geometric Isomers: Differ in spatial arrangement around a double bond or ring (E/Z or cis/trans).

• Enantiomers: Non-superimposable mirror images.

• Diastereomers: Stereoisomers not related as mirror images.

• Different Formulas: Not isomers.

Meso Compounds

A meso compound is an achiral compound that contains chiral centers but has an internal plane of symmetry, making it superimposable on its mirror image.

• Identifying Meso Compounds: Look for symmetry after rotating the molecule.

• Example: Meso-tartaric acid.

Optical Activity and Enantiomeric Excess

Optical activity refers to a compound's ability to rotate plane-polarized light. Enantiomeric excess (ee) quantifies the excess of one enantiomer over the other in a mixture.

• Enantiomerically Pure: Only one enantiomer present (ee = 100%).

• Racemic Mixture: 1:1 mixture of enantiomers (ee = 0%).

• Scalemic Mixture: Any mixture with ee between 0% and 100%.

• Formula: $\text{ee} = \frac{\lvert \%\text{ of major enantiomer} - \%\text{ of minor enantiomer} \rvert}{100}$

Chapter 2: Resonance

Drawing Resonance Structures

Resonance structures are different Lewis structures for the same molecule, showing delocalization of electrons. Curved arrows indicate the movement of electron pairs.

• Curved Arrow Notation: Shows movement of electrons from a donor (lone pair or pi bond) to an acceptor (atom or bond).

• Significant Contributors: The most stable resonance structure is the major contributor. Stability increases with full octets, minimal formal charges, and negative charges on electronegative atoms.

• Example: Resonance in acetate ion (CH3COO-).

Resonance Forms and Hybrids

• Resonance Forms: Structures that differ only in the placement of electrons, not atoms.

• Resonance Hybrid: The actual structure is a hybrid of all resonance forms, with partial charges and bond orders.

• Example: Benzene is represented as a resonance hybrid of two Kekulé structures.

Partial Charges from Resonance

Resonance can result in atoms having partial negative (δ-) or partial positive (δ+) charges due to electron delocalization.

• Identifying Partial Charges: Atoms that gain electron density in some resonance forms acquire δ-, while those that lose electron density acquire δ+.

• Example: In the nitro group (NO2-), resonance delocalizes negative charge over both oxygens.

Lone Pairs: Localization and Hybridization

• Localized Lone Pairs: Not involved in resonance; reside in hybridized orbitals (sp3, sp2, etc.).

• Delocalized Lone Pairs: Participate in resonance; reside in unhybridized p orbitals.

• Hybridization: Atoms with delocalized lone pairs are often sp2 hybridized.

• Example: The lone pair on the nitrogen in an amide is delocalized into the carbonyl group.

Chapter 3: Acid-Base Chemistry

Conjugate Acids and Bases

Acid-base reactions involve the transfer of a proton (H+) from an acid to a base. The conjugate base is formed when an acid loses a proton; the conjugate acid is formed when a base gains a proton.

• Drawing Conjugates: Remove H+ from the acid to get the conjugate base; add H+ to the base to get the conjugate acid.

• Example: Acetic acid (CH3COOH) loses H+ to form acetate (CH3COO-).

Acid-Base Mechanisms

• Mechanism: Use curved arrows to show electron flow from the base to the proton, and from the acid's bond to its conjugate base.

• Example: Reaction of ammonia (NH3) with HCl.

Acid and Base Strength

The strength of an acid or base is measured by its tendency to donate or accept protons. The pKa value is commonly used to compare acid strengths.

• Lower pKa: Stronger acid.

• Higher pKa: Weaker acid.

• Benchmark pKa Values: Know common values (e.g., carboxylic acids ~5, alcohols ~16, water ~15.7, alkanes ~50).

• Comparing Bases: The conjugate base of a weaker acid is a stronger base.

Acid/Base/Solvent Selection

• Appropriate Acid/Base: Choose acids and bases with suitable strengths for the desired reaction.

• Solvent Effects: Polar protic solvents stabilize ions and can affect acid/base strength.

Equilibrium Direction

Acid-base equilibria favor the formation of the weaker acid and base (the more stable species).

• Predicting Equilibrium: Compare pKa values; equilibrium favors the side with the higher pKa (weaker acid).

• Formula: $\Delta pK_a = pK_{a,\text{product acid}} - pK_{a,\text{reactant acid}}$

• If $\Delta pK_a > 0$, equilibrium favors products.

Key Terms

• Enantiomerically Pure: Contains only one enantiomer.

• Racemic: Contains equal amounts of both enantiomers.

• Scalemic: Contains unequal amounts of enantiomers (not racemic or pure).

Summary Table: Types of Isomers

Type	Definition	Example
Constitutional Isomers	Same formula, different connectivity	Butanol vs. isobutanol
Stereoisomers	Same connectivity, different spatial arrangement	cis-2-butene vs. trans-2-butene
Enantiomers	Non-superimposable mirror images	(R)-lactic acid vs. (S)-lactic acid
Diastereomers	Not mirror images, differ at some (but not all) chiral centers	threo-2,3-dibromobutane vs. erythro-2,3-dibromobutane
Meso Compounds	Achiral, with chiral centers and internal symmetry	meso-tartaric acid

Additional info:

• For pKa values, refer to a standard pKa table for more examples.

• Practice drawing mechanisms and resonance structures using curved arrows for clarity.