Organic Chemistry II: Study Guide for Exam #2 – Chirality, Amines, Carbonyl Compounds, Carboxylic Acids, and Derivatives

Chirality and Stereochemistry

Definitions and Concepts

• Chirality: A molecule is chiral if it is not superimposable on its mirror image. This property is often referred to as "handedness."

• Enantiomers: Stereoisomers that are non-superimposable mirror images of each other.

• Diastereomers: Stereoisomers that are not mirror images of each other.

Isomerism

• Constitutional Isomers: Compounds with the same molecular formula but different connectivity of atoms.

• Stereoisomers: Compounds with the same connectivity but different spatial arrangement of atoms.

• Cis-Trans Isomers: A type of stereoisomerism found in alkenes and cyclic compounds due to restricted rotation.

Stereocenters and Stereoisomer Counting

• Stereocenter: An atom, typically carbon, bonded to four different groups.

• Number of possible stereoisomers for a molecule with n stereocenters: $2^n$ (unless symmetry reduces the number).

R,S Configuration and Fischer Projections

• Assign priorities to groups attached to the stereocenter using the Cahn-Ingold-Prelog rules.

• Determine the configuration as R (rectus, right) or S (sinister, left).

• Fischer projections are two-dimensional representations of molecules; manipulation rules help determine absolute configuration and relationships between isomers.

• Example: Assign the configuration of 2-butanol ($CH_3CH(OH)CH_2CH_3$) at the chiral center.

Amines

Definitions and Classification

• Aliphatic amines: Amines with only alkyl groups attached to nitrogen.

• Heterocyclic amines: Amines where nitrogen is part of a ring.

• Primary (1°), Secondary (2°), Tertiary (3°), Quaternary (4°) Amines: Classified by the number of organic substituents on nitrogen.

Nomenclature

• Name the longest carbon chain attached to the nitrogen; replace the "-e" ending with "-amine."

• For secondary and tertiary amines, use "N-" to indicate substituents on nitrogen.

Physical Properties

• Amines can hydrogen bond (except tertiary amines), affecting boiling points and solubility.

• Boiling points: 1° > 2° > 3° (due to hydrogen bonding).

• Solubility in water decreases with increasing alkyl group size.

Reactions of Amines

• Amines act as bases, reacting with acids to form ammonium salts.

• Can act as nucleophiles in reactions forming amides (see Ch11).

• Example: Reaction of methylamine with HCl forms methylammonium chloride.

Aldehydes and Ketones

Definitions

• Aldehyde: Compound with a carbonyl group ($C=O$) bonded to at least one hydrogen.

• Ketone: Compound with a carbonyl group bonded to two carbons.

• Acetal: Carbon bonded to two -OR groups and two other atoms (often H or C).

• Hemiacetal: Carbon bonded to one -OR and one -OH group.

• Tollens’ reagent: $[Ag(NH_3)_2]^+$, used to oxidize aldehydes to carboxylic acids (silver mirror test).

Nomenclature

• Aldehydes: Replace "-e" with "-al" (e.g., ethanal).

• Ketones: Replace "-e" with "-one" (e.g., propanone).

Physical Properties

• Moderate boiling points (higher than alkanes, lower than alcohols).

• Solubility in water decreases with increasing chain length.

• Display dipole-dipole interactions.

Reactions

• Oxidation (aldehydes to carboxylic acids).

• Reduction (to alcohols).

• Addition of alcohols (formation of hemiacetals and acetals).

Keto-Enol Tautomerism

• Keto and enol forms are tautomers (isomers differing in the position of a proton and a double bond).

• General equilibrium: $\ce{R-CO-CH2-R' <=> R-C(OH)=CH-R'}$

• Example: Acetone ($CH_3COCH_3$) can tautomerize to its enol form ($CH_2=C(OH)CH_3$).

Carboxylic Acids

Definitions

• Saturated fatty acid: No double bonds in the hydrocarbon chain.

• Unsaturated fatty acid: One or more double bonds in the hydrocarbon chain.

• Hydrophilic: Water-attracting (polar).

• Hydrophobic: Water-repelling (nonpolar).

• Micelle: Spherical aggregate of amphiphilic molecules in water.

Nomenclature

• Replace "-e" with "-oic acid" (e.g., ethanoic acid).

Physical Properties

• High boiling and melting points due to strong hydrogen bonding.

• Solubility in water decreases with increasing chain length.

• More unsaturation (cis double bonds) lowers melting point.

Reactions

• Acid-base reactions (formation of carboxylate salts).

• Fischer esterification (reaction with alcohols to form esters).

Acidity

• Relative acidity depends on substituents (electron-withdrawing groups increase acidity).

• Predominant form in solution depends on pH (protonated at low pH, deprotonated at high pH).

• Carboxylate salts are more water-soluble than neutral acids.

Soaps

• Soaps are sodium or potassium salts of fatty acids.

• Amphiphilic structure allows them to form micelles, trapping nonpolar dirt/oil in the center.

• Example: Saponification of triglycerides yields glycerol and soap.

Esters, Anhydrides, and Amides

Definitions

• Saponification: Base-catalyzed hydrolysis of esters (especially triglycerides) to yield alcohol and carboxylate salt.

Nomenclature

• Esters: Name alkyl group from alcohol, then acid part with "-oate" ending (e.g., ethyl acetate).

• Amides: Replace "-oic acid" with "-amide" (e.g., ethanamide).

Physical Properties

• Esters: Moderate boiling points, pleasant odors, limited hydrogen bonding.

• Amides: High boiling points due to strong hydrogen bonding.

• Solubility in water decreases with increasing chain length.

Reactions

• Preparation of esters: Fischer esterification (acid + alcohol).

• Hydrolysis of esters: Acidic or basic conditions (saponification under basic conditions).

• Preparation of amides: Reaction of carboxylic acids (or derivatives) with amines.

• Hydrolysis of amides: Acidic or basic conditions.

• Example: Saponification of ethyl acetate with NaOH yields sodium acetate and ethanol.

Acids and Bases

Definitions

• Brønsted-Lowry acid: Proton donor.

• Brønsted-Lowry base: Proton acceptor.

• Strong acid/base: Completely ionizes in water.

• Weak acid/base: Partially ionizes in water.

Ka and pKa

• $K_a$ is the acid dissociation constant; larger $K_a$ means stronger acid.

• $pK_a = -\log_{10} K_a$; lower $pK_a$ means stronger acid.

pH

• Definition: $pH = -\log_{10} [H^+]$

• pH < 7: acidic; pH = 7: neutral; pH > 7: basic.

pH Buffers and Henderson-Hasselbalch Equation

• Buffer: Solution that resists changes in pH upon addition of acid or base; made from a weak acid and its conjugate base.

• Henderson-Hasselbalch equation: $pH = pK_a + \log_{10} \left( \frac{[A^-]}{[HA]} \right)$

• Used to calculate pH or buffer component ratios.

• Example: Calculate the ratio of acetate to acetic acid needed for a buffer at pH 5.0 ($pK_a$ of acetic acid = 4.76).

Nomenclature and Functional Groups (Review)

Prefixes for Carbon Chains

• 1: meth-, 2: eth-, 3: prop-, 4: but-, 5: pent-, 6: hex-, 7: hept-, 8: oct-, 9: non-, 10: dec-

• Multiple identical substituents: di-, tri-, tetra-, etc.

Common Alkyl Substituents

• methyl, ethyl, propyl, isopropyl, butyl, sec-butyl, isobutyl, tert-butyl

Common Monosubstituted Benzenes

Name	Structure
Phenol	Benzene ring with -OH
Anisole	Benzene ring with -OCH3
Aniline	Benzene ring with -NH2
Benzaldehyde	Benzene ring with -CHO
Benzoic acid	Benzene ring with -COOH
Toluene	Benzene ring with -CH3

Summary Table: Physical Properties and Intermolecular Forces

Compound Type	IMFs Present	Boiling Point	Water Solubility
Alkanes	London dispersion	Low	Poor
Amines (1°, 2°)	H-bonding, dipole-dipole	Moderate	Good (small amines)
Aldehydes/Ketones	Dipole-dipole	Moderate	Moderate
Carboxylic acids	H-bonding (dimers)	High	Good (small acids)
Esters	Dipole-dipole	Moderate	Moderate
Amides	H-bonding	High	Good (small amides)

Additional info: Where specific reactions are referenced but not detailed, students should refer to their course slides for mechanisms and examples. This guide covers all foundational concepts and properties required for Exam #2 as outlined in the study guide.