Organic Chemistry: Functional Groups, Reactivity, and Reaction Mechanisms – Study Guide

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Functional Groups and Reactivity

Identification of Functional Groups

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

Carboxylic Acid: Contains a carbonyl (C=O) and hydroxyl (OH) group attached to the same carbon. Example: Acetic acid.
Ester: Contains a carbonyl group adjacent to an ether linkage (C=O–O–R). Example: Ethyl acetate.
Amide: Contains a carbonyl group bonded to a nitrogen atom. Example: Acetamide.
Alcohol: Contains a hydroxyl (OH) group attached to a saturated carbon. Example: Ethanol.
Alkene: Contains a carbon-carbon double bond (C=C). Example: Ethene.
Nitrile: Contains a carbon triple-bonded to a nitrogen (C≡N). Example: Acetonitrile.

Reactivity with Nucleophiles

Nucleophiles are species that donate an electron pair to form a chemical bond. The reactivity of compounds toward nucleophiles depends on the electrophilicity of the carbonyl carbon and the stability of the leaving group.

Acid Chlorides & Anhydrides: Highly reactive due to good leaving groups (Cl-, RCOO-).
Esters & Amides: Less reactive; amides are the least reactive due to resonance stabilization.
Reactivity Order: Acid chloride > Anhydride > Ester > Amide
Example: Acyl chlorides react rapidly with water to form carboxylic acids.

Reactivity in Electrophilic Aromatic Substitution (EAS)

Electrophilic aromatic substitution is a reaction in which an atom, usually hydrogen, attached to an aromatic system is replaced by an electrophile. The reactivity of aromatic compounds in EAS depends on the substituents present on the ring.

Activating Groups: Electron-donating groups (e.g., -OH, -OCH3, -NH2) increase reactivity.
Deactivating Groups: Electron-withdrawing groups (e.g., -NO2, -COOH, -SO3H) decrease reactivity.
Halogens: Deactivate the ring but are ortho/para-directing due to lone pair resonance.
Example: Toluene (methylbenzene) is more reactive than nitrobenzene in EAS.

Resonance Structures

Resonance structures are different Lewis structures for the same molecule that show the delocalization of electrons. Resonance increases stability by delocalizing charge.

Rules: Only electrons move, not atoms; all resonance structures must be valid Lewis structures.
Example: The acetate ion (CH3COO-) has two resonance forms with the negative charge on either oxygen.

Organic Reactions and Mechanisms

Electrophilic Aromatic Substitution (EAS) Reactions

Common EAS reactions include nitration, halogenation, Friedel–Crafts alkylation/acylation, and sulfonation. These reactions introduce substituents onto aromatic rings.

Nitration: Introduction of a nitro group (-NO2) using HNO3/H2SO4.
Halogenation: Introduction of halogens (Br, Cl) using Br2/FeBr3 or Cl2/FeCl3.
Friedel–Crafts Acylation: Introduction of an acyl group using RCOCl/AlCl3.
Reduction of Nitro Groups: Nitrobenzene can be reduced to aniline using Zn(Hg)/HCl (Clemmensen reduction).

Carbonyl Chemistry: Nucleophilic Addition and Substitution

Carbonyl compounds (aldehydes, ketones, carboxylic acids, esters, amides) undergo nucleophilic addition or substitution reactions depending on their structure.

Nucleophilic Addition: Aldehydes and ketones react with nucleophiles (e.g., HCN, Grignard reagents) to form addition products.
Nucleophilic Acyl Substitution: Carboxylic acid derivatives (esters, amides, acid chlorides) undergo substitution with nucleophiles.
Example: Reaction of an aldehyde with HCN forms a cyanohydrin.

Mechanisms and Reaction Pathways

Understanding the stepwise mechanism of a reaction is crucial for predicting products and rationalizing reactivity.

Arrow Pushing: Curved arrows show the movement of electron pairs during bond formation and breaking.
Intermediates: Carbocations, carbanions, and radicals may form as intermediates.
Example: The mechanism of esterification involves nucleophilic attack, proton transfers, and loss of water.

Summary Table: Reactivity of Carbonyl Compounds

Compound Type	General Structure	Relative Reactivity (Nucleophilic Attack)	Example
Acid Chloride	RCOCl	Most Reactive	Acetyl chloride
Anhydride	RCOOCOR'	High	Acetic anhydride
Ester	RCOOR'	Moderate	Ethyl acetate
Amide	RCONH2	Low	Acetamide

Key Equations and Concepts

General Nucleophilic Addition to Carbonyl:
Electrophilic Aromatic Substitution (EAS):
Friedel–Crafts Acylation:
Reduction of Nitrobenzene:

Additional info:

Some content, such as specific resonance structures and detailed mechanisms, would require drawing structures, which are not possible in this text format. However, the principles and steps are described above.
Questions in the file cover identification of functional groups, predicting reactivity, drawing resonance structures, naming and drawing products of reactions, and providing mechanisms—core skills in college-level organic chemistry.