Aromatic Compounds and Reactions

Aromatic Compounds and Aromaticity

• Aromaticity follows Huckel's Rule (4n + 2 π-electrons).
• Aromatic systems exhibit resonance and delocalization.
• Electrophilic Aromatic Substitution (EAS) is a key reaction mechanism.
• Common EAS reactions include halogenation (Br₂, FeBr₃/Cl₂, AlCl₃), nitration (HNO₃, H₂SO₄), and sulfonation (SO₃, H₂SO₄).

Friedel-Crafts Alkylation

• Friedel-Crafts alkylation involves aromatic compounds reacting with alkyl halides in the presence of a Lewis acid catalyst (e.g. AlCl₃).
• This reaction introduces an alkyl group onto the aromatic ring.
• The major product is typically formed through a stepwise mechanism, resulting in a substitution reaction.

Friedel-Crafts Acylation

• In Friedel-Crafts acylation, an acyl group is introduced onto an aromatic ring.
• Acyl chlorides react with aromatic compounds in the presence of a Lewis acid catalyst(e.g. aluminum chloride).
• This results in the production of ketones.

Activating vs. Deactivating Substituents

• Substituents affect the reactivity of aromatic compounds.
• Some groups, like -OH, -NH₂, activate the ring toward further substitution reactions, directing the incoming group to ortho or para positions.
• Other groups, like -NO₂, -CN, deactivate the ring and lead to substitution at the meta position.

Reactions of Alcohols and Phenols

• Alcohols undergo nucleophilic substitution (Sn1, Sn2) reactions.
• Different types of alcohols can be oxidized. Primary alcohols to aldehyde, secondary alcohols to ketone. Tertiary alcohols cannot undergo oxidation.
• Jones reagent (CrO₃ in H₂SO₄/acetone) is an example of a strong oxidant used in oxidation reactions of alcohols.
• Phenols are stronger acids than alcohols. They also readily undergo electrophilic substitution reactions.

Carbonyl Chemistry

• Aldehydes and ketones undergo nucleophilic addition reactions with various nucleophiles. This reaction often involves a stepwise mechanism.
• NaBH₄ and LiAlH₄ are reagents for reducing aldehydes and ketones to alcohols.
• Aldehydes undergo oxidation to carboxylic acids.
• Aldol condensation and Cannizzaro reaction are specific reactions of aldehydes and ketones.
• Carboxylic acids participate in various reactions that include decarboxylation, reduction, and esterification.

Acid Chlorides, Esters, and Amides

• Acid chlorides, esters, and amides undergo nucleophilic acyl substitution reactions.
• Amides may undergo hydrolysis(conversion to carboxylic acids).
• Esters can be reduced using LiAlH₄ to yield alcohols or aldehydes.

Synthesis and Retrosynthesis

• Retrosynthetic analysis involves breaking down a complex molecule into simpler, more readily accessible reactants.
• Functional group interconversions and disconnections are essential strategies.
• Grignard reactions, Aldol condensation, and Claisen condensation are key reactions for building carbon-carbon bonds.

Organometallic Chemistry

• Organolithium and Grignard reagents are examples of organometallic reagents that act as nucleophiles.
• These reagents participate in nucleophilic additions and substitutions to carbonyl compounds.

Radical Reactions

• Radical substitution and addition reactions often involve the presence of heat or light.
• Radical reactions can lead to the formation of polymers from alkenes.

Pericyclic Reactions

• Cycloadditions (e.g., Diels-Alder) and sigmatropic rearrangements are examples of pericyclic reactions.
• These reactions involve ring closures or openings typically involving conjugated systems.
• Reactions often proceed through thermal or photochemical pathways.

Spectroscopy and Structure Determination

• NMR spectroscopy (proton and carbon-13) provides information about chemical shifts and coupling constants.
• IR spectroscopy reveals functional groups based on absorption frequencies.
• Mass spectrometry provides fragmentation patterns, aiding in structural elucidation.

Test your knowledge on aromatic compounds, aromaticity, and key reactions including Electrophilic Aromatic Substitution, Friedel-Crafts alkylation, and acylation. This quiz covers essential mechanisms and rules such as Huckel's Rule and delocalization in aromatic systems.