Organic Chemistry II: Functional Groups, Reactivity, Mechanisms, and Stereochemistry – Study Notes

Functional Groups and Their Identification

Common Functional Groups in Organic Chemistry

Functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. Recognizing and naming these groups is fundamental in organic chemistry.

• Hemiacetal: A functional group formed when an aldehyde reacts with an alcohol. Structure: R-CH(OH)-OR'.

• Acetal: Formed from the reaction of a hemiacetal with another alcohol. Structure: R-CH(OR')2.

• Trione: A compound containing three ketone groups (C=O).

• CyanoHydrin: Contains both a nitrile (C≡N) and a hydroxyl (OH) group on the same carbon.

• Lactam: A cyclic amide. Example: β-lactam.

• Hydrate: A compound with water molecules chemically bound to another compound or ion.

Example: Identifying the functional group in a given structure is a common exam question. For instance, a molecule with a C=O and an OH on the same carbon is a hemiacetal.

Reactivity of Carbonyl Compounds

Nucleophilic Addition to Carbonyls

Carbonyl compounds (aldehydes, ketones, esters, etc.) undergo nucleophilic addition reactions. The reactivity depends on the electronic and steric properties of the substituents.

• Electron-withdrawing groups (e.g., F, Cl) increase the electrophilicity of the carbonyl carbon, making it more reactive toward nucleophiles.

• Electron-donating groups decrease reactivity.

• Steric hindrance can reduce reactivity.

Example: Trifluoromethyl ketone is more reactive than acetone due to the strong electron-withdrawing effect of the CF3 group.

Electrophilic Aromatic Substitution (EAS) Reactivity

In EAS, the reactivity of aromatic compounds is influenced by substituents:

• Electron-donating groups (e.g., alkyl, -OCH3) activate the ring, increasing reactivity.

• Electron-withdrawing groups (e.g., -NO2, -CF3, -CN) deactivate the ring, decreasing reactivity.

Example: Toluene (methylbenzene) is more reactive than benzene; nitrobenzene is less reactive.

Resonance Structures

Drawing Resonance Structures

Resonance structures are different Lewis structures for the same molecule, showing delocalization of electrons. They are important for understanding stability and reactivity.

• Move only electrons (lone pairs or π bonds), not atoms.

• All resonance structures must be valid Lewis structures.

• The actual molecule is a hybrid of all resonance forms.

Example: The resonance forms of a benzyl carbocation show delocalization of the positive charge onto the ortho and para positions of the ring.

Named Reactions and Mechanisms

Electrophilic Aromatic Substitution (EAS) Reactions

EAS reactions introduce substituents onto aromatic rings. Common types include alkylation, sulfonation, halogenation, and oxidation.

• Friedel–Crafts Alkylation: Introduction of an alkyl group using an alkyl halide and AlCl3 as a catalyst.

• Sulfonation: Introduction of a sulfonic acid group using concentrated H2SO4 and heat.

• Halogenation: Introduction of a halogen (e.g., Cl2 with AlCl3).

• Side-chain Oxidation: Oxidation of alkyl side chains to carboxylic acids using strong oxidizers (e.g., KMnO4).

Example: Benzene + RCl/AlCl3 → Alkylbenzene (Friedel–Crafts alkylation)

Carbonyl Chemistry: Mechanisms and Transformations

Carbonyl compounds undergo a variety of reactions, including hydrolysis, reduction, and nucleophilic addition.

• Hydrolysis: Conversion of esters or amides to carboxylic acids using water and acid/base.

• Reduction: Conversion of carbonyls to alcohols or aldehydes using reducing agents (e.g., DIBAL-H).

• Mechanisms: Stepwise electron-pushing diagrams are used to show the flow of electrons during reactions.

Example: Ester hydrolysis under acidic conditions involves protonation, nucleophilic attack by water, and breakdown to carboxylic acid and alcohol.

Predicting Products of Organic Reactions

Common Transformations

Predicting the products of organic reactions requires knowledge of reagents and mechanisms.

• Radical Bromination: Br2 and light add Br to the benzylic position.

• Wittig Reaction: Converts aldehydes/ketones to alkenes using phosphonium ylides.

• Reduction of Nitriles: DIBAL-H reduces nitriles to aldehydes.

Example: Benzyl bromide + PPh3 forms a phosphonium ylide, which reacts with a carbonyl to give an alkene.

Stereochemistry and Carbohydrate Chemistry

Cyclic Forms and Stereochemistry of Sugars

Monosaccharides like deoxyribose exist in linear and multiple cyclic forms. The formation of cyclic forms creates new chiral centers, leading to different stereoisomers.

• Furanose and Pyranose Forms: Five- and six-membered rings formed by intramolecular hemiacetal formation.

• Chiral Centers: Assign R/S configuration to each chiral center.

• Stereochemistry: Wedges and dashes indicate the 3D arrangement of substituents.

Example: Deoxyribose can cyclize to form four different stereoisomers, depending on the configuration at each chiral center.

Summary Table: Key Reactions and Their Features

Reaction	Reagents	Product	Key Feature
Friedel–Crafts Alkylation	RCl, AlCl3	Alkylbenzene	Alkyl group added to aromatic ring
Sulfonation	H2SO4, heat	Benzenesulfonic acid	SO3H group added
Halogenation	Cl2, AlCl3	Chlorobenzene	Cl added to aromatic ring
Side-chain Oxidation	KMnO4, NaOH, heat	Benzoic acid	Alkyl side chain oxidized to COOH
Hydrolysis	H2O, HCl	Carboxylic acid	Breakdown of ester/amide
Wittig Reaction	Phosphonium ylide	Alkene	Converts C=O to C=C

Key Equations and Mechanisms

• General EAS Rate Law: $\text{Rate} = k[\text{Aromatic Compound}][\text{Electrophile}]$

• Hydrolysis of Ester (Acidic): $\ce{RCOOR'} + \ce{H2O} \xrightarrow{H^+} \ce{RCOOH} + \ce{R'OH}$

• Wittig Reaction: $\ce{R_2C=O} + \ce{Ph_3P=CHR'} \rightarrow \ce{R_2C=CHR'} + \ce{Ph_3P=O}$

Additional info:

• Some mechanistic steps and stereochemical assignments were inferred based on standard organic chemistry knowledge.

• Tables and equations were reconstructed for clarity and completeness.