Organic Chemistry I - IUPAC Naming, Substitution, Elimination, and Alkene Reactions Study Guide

IUPAC Nomenclature and Structural Classification

IUPAC Naming of Alkanes and Alcohols
- Substituted Nonanes: Identifying long carbon chains and substituent priorities is essential for naming complex branched alkanes. Example: $\text{2-chloro-6-isopropyl-3,6-dimethylnonane}$ .
- Alcohol Chain Priority: The hydroxyl group ( $-OH$ ) must be on the longest continuous carbon chain and receive the lowest possible locant.
  - Example: $\text{6-methyl-3-propyl-2-heptanol}$ emphasizes the priority of the alcohol over alkyl substituents.
  - Additional names discussed: $\text{3-isobutyl-2-hexanol}$ , $\text{2-methyl-5-(1-hydroxyethyl)octane}$ , and $\text{2-methyl-5-propyl-6-heptanol}$ .
- Cyclic Compounds: Naming involves identifying the functional group and numbering the ring to give substituents the lowest numbers. Examples include $\text{3-bromo-2-methylcyclohexene}$ and $\text{6-bromo-1-methylcyclohexene}$ .
Classification of Organic Halides
- Halides are classified based on the number of carbon atoms attached to the carbon bearing the halogen.
- Tertiary ( $3^{\circ}$ ) Halides: These occur when the halogen-bearing carbon is bonded to three other carbon atoms.

Physical Properties of Organic Compounds

Water Solubility of Alcohols
- For five-carbon ( $C_5$ ) alcohols, solubility depends on the branching and the ratio of the hydrophobic alkyl part to the hydrophilic hydroxyl group. More compact, branched isomers generally exhibit higher water solubility relative to linear isomers of the same molecular weight.
Boiling Point Trends
- Boiling points are dictated by intermolecular forces (IMFs).
- Comparison of three-carbon molecules:
  1. Propane ( $CH_3CH_2CH_3$ ): Weakest (London dispersion forces only). Lowest boiling point.
  2. Dimethyl Ether ( $CH_3OCH_3$ ): Intermediate (Dipole-dipole interactions).
  3. Ethanol ( $CH_3CH_2OH$ ): Strongest (Hydrogen bonding). Highest boiling point.
- Ranking: I < III < II.

Substitution Reactions and Kinetics (SN1 and SN2)

Protonation of Alcohols
- Step 1 in many substitution/elimination reactions.
- Mechanism: The oxygen atom of the alcohol (e.g., tert-butyl alcohol) uses its lone pair to attack the proton of a strong acid like Hydrogen Bromide ( $HBr$ ), forming an oxonium ion.
Leaving Group Ability
- Leaving group ability is generally inversely related to base strength.
- Halide trend: I^- > Br^- > Cl^- > F^-.
- In a reaction between ethyl halides and sodium cyanide ( $NaCN$ ) in $DMSO$ , ethyl iodide ( $CH_3CH_2I$ ) reacts at the fastest rate due to iodide being the best leaving group.
Reaction Rates in SN1 and SN2
- SN1 Solvolysis: The rate-limiting step is the formation of a carbocation. Rates are highest for tertiary substrates and in polar protic solvents (e.g., ethanol/water).
- SN2 Substitution: The rate-limiting step involves simultaneous nucleophilic attack and leaving group departure. Substrates with the least steric hindrance (primary) react fastest.
- Nucleophile Choice: $NaCN$ is a strong nucleophile commonly used in SN2 reactions.
Rate-Limiting Steps
- For the reaction of a secondary alcohol with $HCl$ , the rate-limiting step is the dissociation of the leaving group ( $H_2O$ ) from the protonated alcohol to form a carbocation.

Carbocation Stability and Rearrangements

Stability Hierarchies
- Carbocations are stabilized by inductive effects and hyperconjugation.
- Order of Stability: \text{Tertiary } (>3^{\circ}) > \text{Secondary } (>2^{\circ}) > \text{Primary } (>1^{\circ}) > \text{Methyl}.
Thermodynamic Driving Force
- Rearrangements (such as hydride or methyl shifts) occur to transform a less stable carbocation into a more stable, lower energy carbocation (e.g., from secondary to tertiary).
Reactivity with Hydrogen Halides
- Alcohols react with $HBr$ via carbocation intermediates. The most reactive alcohols are those that form the most stable carbocations (Tertiary > Secondary > Primary).
- Regioselectivity: If a compound contains multiple types of alcohols, the most reactive (tertiary) will react first with limited equivalents of reagent (e.g., $1$ equivalent of $HCl$ ).

Alkene Structure, Isomerism, and Stability

IUPAC Naming of Alkenes
- Alkenes are named by identifying the longest chain containing the double bond.
- Examples:
  - $2,5\text{-dimethyl-2-hexene}$ .
  - $4\text{-methyl-3-propyl-3-pentene}$ .
  - $(E)\text{-3-bromo-1-fluoro-2-methylpropene}$ .
Alkene Isomerism
- Constitutional Isomers: There are five possible constitutional isomers for the formula $C_5H_{10}$ : 1-pentene, 2-pentene, 2-methyl-1-butene, 3-methyl-1-butene, and 2-methyl-2-butene.
- Stereoisomers (E-Z/Cis-Trans): Total isomers for $C_5H_{10}$ including stereoisomers reaches six.
- Requirements for E-Z Isomerism: Each carbon of the double bond must be attached to two different groups.
  - Examples: 1-chloropropene exhibits stereoisomerism, while 2-chloropropene does not.
Alkene Stability
- Stability increases with the degree of substitution (number of alkyl groups attached to the double bond).
- Measured by Heat of Combustion: The most stable alkene (most substituted) releases the least heat per mole when burned.

Elimination Reactions and Synthesis

Acid-Catalyzed Dehydration of Alcohols
- This follows an $E1$ mechanism for secondary and tertiary alcohols, proceeding through a carbocation.
- Major Product Prediction (Zaitsev's Rule): The more substituted alkene is the major product.
- Example: Dehydration of 2-methyl-2-pentanol yields 2-methyl-2-pentene as the major product.
- Ease of Dehydration: Tertiary alcohols undergo dehydration most readily due to the stability of the intermediate carbocation.
Synthesizing Bromocyclopentane
- Effective methods:
  1. $Cyclopentanol + HBr$
  2. $Cyclopentanol + PBr_3$
  3. $Cyclopentane + Br_2$ with light (radical halogenation).
- Ineffective method: $Cyclopentanol + NaBr$ (unfavorable without acid catalyst to convert $-OH$ to a good leaving group).
Dehydrohalogenation
- Reaction of alkyl halides with bases like sodium ethoxide ( $NaOCH_2CH_3$ ). Zaitsev's rule predicts the more substituted alkene will be the major isomer.

Addition Reactions to Alkenes

Hydrogenation
- Requires a metal catalyst: Palladium ( $Pd$ ), Platinum ( $Pt$ ), or Nickel ( $Ni$ ).
- Note: Sodium ( $Na$ ) is NOT used as a catalyst for alkene hydrogenation.
- Stereochemistry: Often involves syn-addition. Example: Hydrogenation of 1,4-dimethylcyclohexene results in specific cis/trans isomers.
Electrophilic Addition of HX
- Markovnikov's Rule: The proton adds to the less substituted carbon, and the halide adds to the more substituted carbon.
- Intermediate: Bromination of propene proceeds through a cyclic bromonium ion intermediate.
Hydroboration-Oxidation
- A two-step sequence:
  1. Reaction with diborane ( $B_2H_6$ ) in diglyme.
  2. Treatment with basic hydrogen peroxide ( $H_2O_2, OH^-$ ).
- Regioselectivity: Anti-Markovnikov addition of water ( $-OH$ on the less substituted carbon).
- Stereochemistry: Syn-addition of $H$ and $OH$ .
Halogenation
- Addition of $Br_2$ or $Cl_2$ in an inert solvent (e.g., $CCl_4$ ).
- Proceed via anti-addition.

Laboratory and Structural Problems

Optically Active Compounds
- If Compound X ( $C_5H_{10}O$ ) is optically active and gives an optically active hydrogenation product ( $C_5H_{12}O$ ), the chirality must be preserved during the addition of hydrogen across the double bond.
Synthesis Sequences
- Converting cyclohexanol to 1,2-epoxycyclohexane:
  1. Dehydration (e.g., $H_3PO_4, \Delta$ ) to form cyclohexene.
  2. Epoxidation using a peroxyacid (e.g., $mCPBA$ ).
Identification via Hydrogenation
- If hydrogenation of $C_7H_{14}$ (alkene B or C) yields 3-ethylpentane, the carbon skeleton of the starting material must match 3-ethylpentane exactly.

Questions & Discussion

Mechanism for Cyclic Ether Formation: Likely involves an intramolecular substitution where an internal hydroxyl group attacks a carbocation or a halide on the same molecule.
Rate of SN2 with Strong Nucleophiles: Sodium cyanide ( $NaCN$ ) reacts slowest with sterically hindered substrates (e.g., neopentyl or tertiary halides).
Product of 1-Butene and HI: Addition follows Markovnikov's rule to produce 2-iodobutane.
Hydroboration of 3-ethyl-1-pentene: Yields 3-ethyl-1-pentanol (anti-Markovnikov product).
Carbocation Rearrangement Likelihood: Carbocations like the 3rd-degree secondary cation adjacent to a quaternary center are highly likely to rearrange via a 1,2-methyl shift.