Organic Chemistry Exam Study Guide: Structure, Reactivity, and Synthesis

Ranking Organic Compounds: Acidity, Reactivity, and Solubility

Acidity of Alcohols

Alcohols can be ranked by their acidity, which depends on the stability of the conjugate base and the electronic environment around the hydroxyl group.

• Acidity Trend: Phenol is more acidic than aliphatic alcohols due to resonance stabilization of the phenoxide ion.

• Effect of Alkyl Groups: Increasing alkyl substitution on the carbon bearing the OH group decreases acidity due to electron-donating effects.

• Example: Phenol > 1° alcohol > 2° alcohol > 3° alcohol

Additional info: Acidity order is often: phenol > methanol > ethanol > isopropanol > tert-butanol.

Reactivity with Acidic Water

Reactivity of organic compounds with acidic water is influenced by their ability to stabilize carbocation intermediates and the presence of activating groups.

• Carbocation Stability: Tertiary carbocations are more stable than secondary or primary due to hyperconjugation and inductive effects.

• Aromatic Systems: Benzene rings with electron-donating groups react faster with acids.

• Example: Tertiary > Secondary > Primary

Water Solubility of Organic Compounds

Water solubility depends on the presence of polar functional groups and the length of the hydrocarbon chain.

• Polar Groups: Alcohols and carboxylic acids are more soluble than hydrocarbons.

• Chain Length: Increasing hydrocarbon chain length decreases solubility.

• Example: Methanol > Ethanol > Propanol > Butanol > Pentanol

Alkene Transformations and Synthesis

Alkene Functionalization

Alkenes can be converted to alcohols via different reactions, each with specific regioselectivity and stereochemistry.

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

• Oxymercuration-Demercuration: Markovnikov addition of water without carbocation rearrangement.

• Acid-Catalyzed Hydration: Markovnikov addition, possible rearrangement.

Equation:

$\ce{R_2C=CR_2 + H_2O \rightarrow R_2CH-CR_2OH}$

Alkene Hydrogenation

Alkenes can be hydrogenated to alkanes using H2 and a metal catalyst.

• Catalysts: Pd/C, Pt, Ni

• Stereochemistry: Syn addition of hydrogen

Equation:

$\ce{RCH=CH_2 + H_2 \xrightarrow{Pd/C} RCH_2CH_3}$

Organic Reaction Mechanisms

Oxidation and Reduction Reactions

Organic compounds undergo oxidation and reduction reactions, often involving changes in the oxidation state of carbon.

• Oxidation: Addition of oxygen or removal of hydrogen (e.g., alcohol to ketone).

• Reduction: Addition of hydrogen or removal of oxygen (e.g., ketone to alcohol).

• Common Reagents: PCC, KMnO4, NaBH4, LiAlH4

Equation:

$\ce{RCH_2OH \xrightarrow{PCC} RCHO}$

$\ce{RCOCH_3 \xrightarrow{NaBH_4} RCH(OH)CH_3}$

Substitution and Elimination Reactions

Alkyl halides can undergo nucleophilic substitution (SN1, SN2) or elimination (E1, E2) reactions.

• SN2: Bimolecular, backside attack, inversion of configuration.

• SN1: Unimolecular, carbocation intermediate, racemization.

• E2: Bimolecular, anti-coplanar elimination.

• E1: Unimolecular, carbocation intermediate.

Equation:

$\ce{RCH_2Br + NaCN \rightarrow RCH_2CN + NaBr}$

Organic Synthesis Roadmaps

Multi-Step Synthesis

Complex organic molecules are often synthesized via a series of reactions, each transforming functional groups in a controlled manner.

• Retrosynthetic Analysis: Breaking down target molecules into simpler precursors.

• Functional Group Interconversions: Changing one functional group to another (e.g., alcohol to aldehyde).

• Protecting Groups: Temporarily masking reactive groups during synthesis.

Nomenclature of Organic Compounds

IUPAC Naming Rules

Systematic naming of organic compounds follows IUPAC rules to ensure clarity and consistency.

• Longest Carbon Chain: Identify the longest continuous chain as the parent.

• Numbering: Number the chain to give substituents the lowest possible numbers.

• Functional Groups: Indicate the position and identity of functional groups.

• Example: 2-methylpentane, cyclohexanol

Summary Table: Common Organic Reactions

Reaction Type	Reagents	Product	Notes
Oxidation	PCC, KMnO4	Aldehyde, Ketone, Carboxylic Acid	Depends on starting alcohol
Reduction	NaBH4, LiAlH4	Alcohol	NaBH4 for aldehydes/ketones, LiAlH4 for esters/acids
Hydration	H2O, acid	Alcohol	Markovnikov or anti-Markovnikov
Substitution	NaCN, NaOH	Nitrile, Alcohol	SN1 or SN2 mechanism
Elimination	Strong base	Alkene	E1 or E2 mechanism