Organic Chemistry 3rd Exam Study Guide

Alkynes

Naming of Alkynes

Alkynes are hydrocarbons containing at least one carbon-carbon triple bond. The IUPAC naming system is used to assign names based on the longest chain containing the triple bond.

• Suffix: The suffix -yne is used for alkynes.

• Numbering: Number the chain so the triple bond gets the lowest possible number.

• Examples: Ethyne (acetylene), propyne, 1-butyne, 2-butyne.

Classes of Alkynes

Alkynes are classified based on the position of the triple bond.

• Terminal Alkynes: Triple bond at the end of the chain (e.g., 1-butyne).

• Internal Alkynes: Triple bond within the chain (e.g., 2-butyne).

Acidity of Alkynes

Terminal alkynes are more acidic than alkenes and alkanes due to the sp-hybridized carbon.

• Acetylene: $pK_a \approx 25$

• Deprotonation: Terminal alkynes can be deprotonated by strong bases (e.g., sodium amide).

Reactions of Alkynes

Alkynes undergo a variety of addition and substitution reactions.

• Halogenation: Addition of halogens (Cl2, Br2) across the triple bond.

• Addition of Hydrogen Halide: Markovnikov and anti-Markovnikov addition (e.g., HBr with and without peroxide).

• Hydrogenation: Complete reduction to alkane using catalysts (e.g., Pd/C).

• Partial Reduction:

  • Lindlar Catalyst: Produces cis-alkenes.

  • Dissolving Metal Reaction: (Na/NH3) Produces trans-alkenes.

• Acid Catalyzed Hydration: Forms ketones or aldehydes via enol intermediate.

• Hydroboration-Oxidation: Using S-ia2BH, forms aldehydes from terminal alkynes.

• Conversion to Carbanion: Terminal alkynes react with sodium amide, alkyl lithium, or Grignard reagents to form acetylide ions.

• Acetylide Ion in Substitution: Acetylide ions can react with alkyl halides to form larger alkynes.

• Double Dehydrohalogenation: Preparation of alkynes from dihalides.

• Ozonolysis: Cleavage of triple bond to form carboxylic acids or ketones.

• KMnO4 Reaction: Oxidative cleavage under neutral, basic, or acidic conditions.

• Preparation of Acetylene: From calcium carbide and water.

• Acetylide Reactions: Acetylide ions react with epoxides, aldehydes, or ketones to form alcohols.

Mechanisms Involving Alkynes

• Dissolving Metal Reaction: Mechanism involves electron transfer and formation of trans-alkene.

• Enol Rearrangement: Tautomerization of enol to ketone or aldehyde.

• Addition of Hydrogen Halide: Stepwise mechanism, Markovnikov or anti-Markovnikov depending on conditions.

• Halogenation: Electrophilic addition mechanism.

• Mercury Catalyzed Hydration: Formation of enol intermediate, followed by tautomerization.

Stereochemistry

Definitions and Key Concepts

Stereochemistry deals with the spatial arrangement of atoms in molecules.

• Enantiomers: Non-superimposable mirror images.

• Diastereomers: Stereoisomers that are not mirror images.

• Meso Compounds: Achiral compounds with stereocenters and an internal plane of symmetry.

• Geometric Isomers: Cis/trans or E/Z isomers due to restricted rotation.

• Optical Isomers: Compounds that rotate plane-polarized light.

• Racemic Mixture: 1:1 mixture of enantiomers, optically inactive.

• Chiral Carbon: Carbon atom with four different substituents.

• Stereogenic Center: Atom at which the interchange of two groups produces a stereoisomer.

• Optical Activity: Ability to rotate plane-polarized light.

Recognizing Examples

• Identify enantiomers, diastereomers, meso compounds, optical isomers, racemic mixtures, and chiral carbons in molecular structures.

Properties of Enantiomers and Diastereomers

• Enantiomers: Same physical properties except optical rotation and reaction with chiral reagents.

• Diastereomers: Different physical and chemical properties.

Optical Purity Calculation

Optical purity measures the excess of one enantiomer over the other.

• Formula:

$\text{Optical purity} = \frac{\text{Observed rotation}}{\text{Rotation of pure enantiomer}} \times 100\%$

Number of Stereoisomers

The number of possible stereoisomers for a molecule with n chiral centers is:

$2^n$

Exceptions occur for meso compounds.

R/S Designation for Chiral Carbons

• Assign priorities to substituents using Cahn-Ingold-Prelog rules.

• Orient the molecule so the lowest priority group is away from you.

• If the sequence is clockwise, the configuration is R; if counterclockwise, it is S.

Nucleophilic Substitution and Elimination

Overview of Reactions

Nucleophilic substitution and elimination reactions are fundamental in organic chemistry, involving the replacement or removal of groups from a molecule.

• Nucleophilic Substitution: SN1 and SN2 mechanisms.

• Elimination: E1 and E2 mechanisms.

• Predicting Major Product: Consider mechanism, substrate, nucleophile, and conditions.

Kinetics of Reactions

• SN1: First-order kinetics; rate depends on substrate.

• SN2: Second-order kinetics; rate depends on substrate and nucleophile.

• E1: First-order kinetics; rate depends on substrate.

• E2: Second-order kinetics; rate depends on substrate and base.

Rate Law Examples:

$\text{Rate}_{\text{SN1}} = k[\text{substrate}]$

$\text{Rate}_{\text{SN2}} = k[\text{substrate}][\text{nucleophile}]$

Factors Affecting Products

• Leaving Group: Better leaving groups favor substitution/elimination.

• Class of Alkyl Halide: Primary, secondary, tertiary affect mechanism.

• Branching at Beta Carbon: Steric hindrance affects SN2/E2.

• Nucleophile Identity: Strong nucleophiles favor SN2; weak favor SN1.

Predicting Equilibrium Direction

• Consider relative stability of products and reactants.

• Use pKa values and nucleophile/base strength.

Relative Strength of Nucleophiles

• Strong nucleophiles: OH-, OR-, CN-, N3-

• Weak nucleophiles: H2O, ROH

Mechanisms of Nucleophilic Substitution and Elimination

• SN1: Two-step mechanism with carbocation intermediate.

• SN2: One-step, concerted mechanism with backside attack.

• E1: Two-step mechanism with carbocation intermediate.

• E2: One-step, concerted mechanism; requires anti-periplanar geometry.

Alkene Reactions (Review)

Students are expected to recall alkene reactions from previous exams.

• Electrophilic Addition: Addition of HX, X2, H2O, etc.

• Hydroboration-Oxidation: Anti-Markovnikov addition of water.

• Ozonolysis: Cleavage to form aldehydes and ketones.

• Other: Catalytic hydrogenation, halohydrin formation, epoxidation.

Exam Format and Weighting

Section	Weight (%)
Reactions	20
Mechanisms	20
Synthesis	20
Stereochemistry, Nucleophilic Substitution, Naming, Miscellaneous	40

All values above are +/- 10%.

Example: Predict the product of the reaction between 1-butyne and HBr in the presence of peroxide. The product will be 2-bromobutene (anti-Markovnikov addition).

Example: Assign R/S configuration to a chiral center in 2-butanol.

Example: Calculate the number of stereoisomers for 2,3-dibromobutane (should be 3 due to meso form).

Example: Predict the major product of an SN2 reaction between bromomethane and hydroxide ion.