Organic Chemistry Key Concepts & Reactions

Posted on Aug 26, 2024 in Chemistry

Organic Chemistry: Key Concepts & Reactions

Benzene & Its Derivatives

Structure & Stability of Benzene

Benzene is a cyclic, planar molecule with six carbon atoms. Each carbon atom is bonded to two other carbon atoms and one hydrogen atom via single covalent sigma bonds, resulting in a trigonal planar geometry with bond angles of 120 degrees. This leaves one unused electron on each carbon atom in a p-orbital perpendicular to the plane of the ring. These p-orbitals overlap with neighboring p-orbitals, forming delocalized π electron bonds. This creates a cloud of delocalized electrons above and below the plane of the ring, contributing to benzene’s stability.

The Kekulé structure, with alternating double bonds, does not accurately represent benzene’s properties. Benzene’s planar structure and equal bond lengths, resistance to addition reactions, and preference for substitution reactions indicate a more stable, delocalized electron system.

Reactions of Benzene

Benzene undergoes substitution reactions more readily than addition reactions, which would disrupt the stable delocalized π electron system. Some key reactions include:

  • Nitration: Refluxing benzene with concentrated nitric acid and sulfuric acid catalyst at 55°C. Longer reflux times and higher temperatures can lead to di- or trinitrobenzene formation.
  • Halogenation: Reacting benzene with chlorine gas in the presence of anhydrous aluminum chloride or ferric bromide catalyst, in the absence of UV light, at room temperature (Friedel-Crafts reaction).
  • Alkylation: Reacting benzene with an alkyl halide (e.g., CH3Cl, C2H5Cl) in the presence of anhydrous aluminum chloride catalyst at room temperature.

Chlorobenzene

Chlorobenzene has a shorter and stronger C-Cl bond, making it less susceptible to nucleophilic attack. This is due to the partial delocalization of lone pairs of electrons from the chlorine atom into the delocalized π electron system of the benzene ring. The polar C-Cl bond in chlorobenzene can undergo nucleophilic substitution with strong bases like NaOH.

Isomers

Structural Isomers

Structural isomers have the same molecular formula but differ in the sequence of bonds. They can be further classified as chain, position, or functional group isomers.

Stereoisomers

Stereoisomers have the same molecular formula and sequence of bonds but differ in the spatial arrangement of atoms. Two main types are:

  • Geometric Isomers: Occur in molecules with a C=C double bond and restricted rotation, with two different groups/atoms on either side of the double bond. They are designated as E or Z based on the priority of substituents according to the Cahn-Ingold-Prelog rules.
  • Optical Isomers: Molecules with a chiral center (a carbon atom with four different groups attached). They exist as non-superimposable mirror images called enantiomers. A 50/50 mixture of enantiomers is called a racemic mixture. Optical isomers have the same physical and chemical properties except for their effect on plane-polarized light.

Aspirin Synthesis

Aspirin (acetylsalicylic acid) is synthesized by refluxing 2-hydroxybenzoic acid (salicylic acid) with ethanoic anhydride at 90°C in the presence of concentrated phosphoric acid catalyst. This reaction yields aspirin and ethanoic acid. Ethanoic anhydride is preferred over ethanoyl chloride due to its lower cost, lower reactivity, and the production of ethanoic acid as a byproduct instead of HCl.

Alcohols & Phenols

Reactions of Alcohols

  • Halogenoalkane to Alcohol: Warming a halogenoalkane with aqueous NaOH or KOH and heating under reflux.
  • Ethene to Ethanol: Reacting ethene gas with water in the presence of concentrated phosphoric acid catalyst at 60-80 atm and 300°C.
  • Dehydration of Alcohols: Heating an alcohol with concentrated sulfuric acid at 170°C.
  • Alcohol with HBr: Reacting an alcohol with HBr generated in situ by refluxing concentrated sulfuric acid with NaBr.
  • Alcohol with Acid Chloride: Reacting an alcohol with an acid chloride to form an ester and HCl.
  • Alcohol with Carboxylic Acid: Reacting an alcohol with a carboxylic acid to form an ester and water in the presence of sulfuric acid catalyst.
  • Reduction of Aldehydes, Ketones, and Carboxylic Acids to Alcohols: Using LiAlH4 in ether under reflux at 70°C or NaBH4 in water at room temperature.
  • Oxidation of Alcohols: Heating an alcohol with potassium dichromate(VI) and dilute sulfuric acid. Distillation is used to prevent further oxidation to carboxylic acids. Secondary alcohols can be oxidized to ketones under reflux.

Uses of Ethanol

Ethanol is used as a solvent, biofuel (via fermentation), and in alcoholic beverages.

Phenol

Phenol is acidic and can donate a proton to form a phenoxide ion, which is stabilized by partial delocalization of the negative charge into the delocalized π electron system of the benzene ring. Phenol reacts with strong bases like NaOH to form salts, increasing its solubility in water in alkaline solutions. Phenol is not strong enough to liberate CO2 from Na2CO3 or NaHCO3. Phenol reacts with bromine to form 2,4,6-tribromophenol, a white precipitate with an antiseptic smell.

Aldehydes & Ketones

Oxidation of Aldehydes

  • Tollens’ Test: Reacting an aldehyde with dilute silver nitrate, dilute ammonium hydroxide, and sodium hydroxide to produce a silver mirror.
  • Fehling’s Test: Reacting an aldehyde with Fehling’s solution (copper sulfate) to produce a brick-red precipitate of copper(I) oxide.

Reactions of Aldehydes & Ketones

  • Test for Carbonyl Group: Adding 2,4-dinitrophenylhydrazine (2,4-DNPH) to an aldehyde or ketone to form a crystalline orange precipitate (a hydrazone).
  • Reaction with HCN: Reacting an aldehyde or ketone with hydrogen cyanide (generated in situ with sulfuric acid and potassium cyanide) to form a hydroxynitrile.
  • Iodoform Test: Reacting a compound containing a CH3CO- or CH3CHOH- group with iodine and sodium hydroxide to form a pale yellow precipitate of iodoform (CHI3) with an antiseptic smell.

Carboxylic Acids & Their Derivatives

Properties & Reactions of Carboxylic Acids

Carboxylic acids are soluble in water and have high boiling points and low volatility due to hydrogen bonding. They are highly acidic due to the polarization of the O-H bond by the electron-withdrawing carbonyl group. The resulting carboxylate anion is stabilized by resonance.

  • Alkyl Benzene to Benzoic Acid: Oxidizing an alkyl benzene with alkaline potassium permanganate under reflux, followed by acidification with concentrated HCl.
  • Carboxylic Acid to Acid Chloride: Reacting a carboxylic acid with phosphorus pentachloride (PCl5) at room temperature and pressure under anhydrous conditions.
  • Acid Hydrolysis of Esters: Refluxing an ester with dilute aqueous acid.
  • Alkaline Hydrolysis of Esters: Refluxing an ester with dilute aqueous alkali.
  • Acid Decarboxylation: Heating a carboxylic acid with soda lime.
  • Carboxylic Acid to Amide: Converting a carboxylic acid to an acid chloride (using PCl5) and then reacting it with ammonia.
  • Acid Chloride with Amine: Reacting an acid chloride with an amine to form a secondary amide.

Amides & Nitriles

Hydrolysis of Amides & Nitriles

Amides and nitriles can be hydrolyzed by heating with dilute aqueous acid or alkali under reflux.

Reactions of Nitriles

  • Halogenoalkane to Nitrile: Reacting a halogenoalkane with potassium cyanide in ethanol under reflux.
  • Nitrile to Amine: Reducing a nitrile with hydrogen gas in the presence of a nickel catalyst.

Amines

Reactions of Amines

  • Halogenoalkane to Aliphatic Amine: Heating a halogenoalkane with ammonia in ethanol in a sealed tube.
  • Nitrobenzene to Aromatic Amine: Reducing nitrobenzene with tin and concentrated HCl, followed by the addition of NaOH to release the free amine.
  • Amine with Acid Chloride: Reacting an amine with an acid chloride to form a secondary amide.
  • Reaction with Nitrous Acid: Reacting an amine with nitrous acid (generated in situ with HCl and NaNO2) at cold temperatures.
  • Phenylamine with Nitrous Acid: Reacting phenylamine with nitrous acid in excess below 10°C to form benzenediazonium chloride.

Basicity & Solubility of Amines

Primary amines are the strongest bases due to the availability of the lone pair on the nitrogen atom. Phenylamine is less basic due to the delocalization of the lone pair into the π electron system of the benzene ring. Methylamine is more soluble in water than phenylamine due to its smaller size and the availability of the lone pair for hydrogen bonding.

Azo Dyes

Azo dyes contain the azo group (-N=N-), which allows for extended conjugation and absorption of electromagnetic radiation in the visible spectrum, resulting in color. They are used in paints and textile dyes.

Amino Acids & Polymers

Amino Acids

Amino acids are polar and exist as zwitterions, with both positive and negative charges. They have high melting points due to ionic interactions and are amphoteric, acting as buffers.

Polypeptides & Proteins

Polypeptides are condensation polymers formed by the linkage of amino acids. Proteins have complex structures with primary, secondary, and tertiary levels of organization. They can be hydrolyzed by refluxing with concentrated HCl.

Polymer Synthesis

Polymers can be synthesized by condensation or addition reactions. Condensation polymerization involves the loss of a small molecule (e.g., water), while addition polymerization involves the addition of monomers across a double bond without any byproduct.

Laboratory Techniques

Recrystallization

Recrystallization is a technique for purifying solid compounds by dissolving them in a suitable solvent and allowing them to crystallize out as the solution cools.

Melting Point Determination

Melting point determination is used to assess the purity of a solid compound. A sharp melting point indicates purity.

Chromatography

Chromatography is a technique for separating mixtures based on the differential affinities of components for a stationary phase and a mobile phase.

Spectroscopy

  • Low-Resolution NMR: Provides information about the number and types of hydrogen environments in a molecule.
  • Mass Spectrometry: Determines the molecular weight and fragmentation pattern of a compound.
  • IR Spectroscopy: Identifies functional groups based on their characteristic absorption of infrared radiation.

Other Techniques

  • Reflux: Prevents the loss of volatile compounds during a reaction.
  • Distillation: Separates liquids based on their boiling points.
  • Steam Distillation: Isolates volatile compounds from non-volatile impurities.
  • Distillation Under Reduced Pressure: Lowers the boiling point of a liquid to prevent decomposition.
  • Separation of Immiscible Liquids: Uses a separating funnel to separate liquids that do not mix.
  • Filtration: Separates solids from liquids.

Health & Safety

Always follow proper safety procedures when working with chemicals. Use a fume hood for toxic substances, heat reactions in a water bath when possible, and work on the smallest scale necessary.