Organic Chemistry Exam 3 Study Guide: Reaction Mechanisms, Alkene Additions, and NMR Spectroscopy

Organic Reaction Mechanisms

Substitution and Elimination Reactions (SN1, SN2, E1, E2)

Organic molecules can undergo substitution and elimination reactions, which are fundamental to organic synthesis. The mechanism depends on the substrate, nucleophile/base, solvent, and reaction conditions.

• SN1 (Unimolecular Nucleophilic Substitution): Occurs in two steps: formation of a carbocation intermediate, followed by nucleophilic attack. Favored by tertiary substrates and polar protic solvents.

• SN2 (Bimolecular Nucleophilic Substitution): Occurs in a single concerted step. The nucleophile attacks the substrate directly, leading to inversion of stereochemistry. Favored by primary substrates and strong nucleophiles.

• E1 (Unimolecular Elimination): Involves carbocation formation followed by loss of a proton to form an alkene. Competes with SN1 under similar conditions.

• E2 (Bimolecular Elimination): A strong base abstracts a proton while the leaving group departs, forming an alkene in a single step. Requires anti-coplanar geometry.

• Example: Reaction of 2-bromobutane with KOH in ethanol proceeds via E2, yielding 2-butene.

Key Equations:

• SN2 rate law: $\text{Rate} = k[\text{Substrate}][\text{Nucleophile}]$

• E2 rate law: $\text{Rate} = k[\text{Substrate}][\text{Base}]$

Electrophilic Addition to Alkenes

Alkene Addition Reactions

Alkenes react with electrophiles to form addition products. The regioselectivity and stereochemistry depend on the reagents and mechanism.

• Hydrogenation: Addition of H2 across the double bond using a metal catalyst (e.g., Pd/C) yields an alkane.

• Hydrohalogenation: Addition of HX (e.g., HBr) follows Markovnikov's rule: the proton adds to the less substituted carbon.

• Hydration: Addition of H2O (acid-catalyzed or via oxymercuration-demercuration) yields alcohols.

• Halogenation: Addition of Br2 or Cl2 forms vicinal dihalides, often with anti stereochemistry.

• Hydroboration-Oxidation: Adds water across the double bond with anti-Markovnikov regioselectivity and syn stereochemistry.

• Example: 1-hexene treated with Br2/H2O yields 2-bromo-3-hexanol with anti addition.

Key Equations:

• Markovnikov's Rule: "In the addition of HX to an alkene, the hydrogen attaches to the carbon with more hydrogens."

Organic Reaction Mechanisms: Curved Arrow Notation

Drawing Mechanisms

Mechanisms illustrate the movement of electrons during chemical reactions. Curved arrows show electron flow from nucleophile to electrophile.

• Stepwise Mechanisms: Show intermediates and transition states.

• Formal Charges: Assign charges to atoms as electrons move.

• Example: Ozonolysis of cyclohexene produces adipic acid via formation and cleavage of ozonide intermediates.

NMR Spectroscopy

Interpreting 1H and 13C NMR Spectra

NMR spectroscopy is a powerful tool for determining molecular structure. The number of signals, chemical shifts, and splitting patterns provide information about the chemical environment of nuclei.

• Number of Signals: Each unique proton or carbon environment gives a separate signal.

• Chemical Shift: Indicates the electronic environment; deshielded protons appear downfield (higher ppm).

• Splitting Patterns: Determined by the number of neighboring protons (n+1 rule).

• Example: Ethanol shows three signals in 1H NMR: CH3 (triplet), CH2 (quartet), OH (singlet).

Key Equations:

• n+1 Rule: $\text{Multiplicity} = n + 1$ (where n = number of neighboring protons)

Table: NMR Signal Count for Selected Molecules

Molecule	1H NMR Signals	13C NMR Signals
CH3CH2OH	3	2
Benzene	1	1
p-Xylene	2	2
Additional info: Table entries inferred for common molecules.

Structure Elucidation Using NMR and IR

Combining NMR and IR data allows for the determination of molecular structure. IR identifies functional groups, while NMR provides connectivity and environment information.

• IR Absorptions: O-H stretch (~3350 cm-1), C=O stretch (~1738 cm-1).

• NMR Data: Chemical shifts and coupling constants help assign protons and carbons.

• Example: A molecule with formula C4H8O2, IR peaks at 3350 and 1738 cm-1, and NMR signals at 3.08, 2.51, 1.68, 1.48, 1.09 ppm is likely a hydroxy ester.

Summary Table: Key Reaction Types and Spectroscopy Features

Reaction Type	Key Features	Example
SN2	Inversion of stereochemistry, strong nucleophile	CH3Br + OH- → CH3OH
E2	Anti-coplanar elimination, strong base	2-bromobutane + KOH → 2-butene
Hydroboration-Oxidation	Anti-Markovnikov, syn addition	1-hexene + BH3/H2O2 → 1-hexanol
NMR	Signal count, chemical shift, splitting	Ethanol: 3 signals, triplet/quartet/singlet