Classification and Nomenclature of Organic Compounds

I. 1. What is Organic Chemistry and Why is Nomenclature Important?

Organic Chemistry Definition: Study of carbon-containing compounds, their structures, properties, reactions, and synthesis.

Importance: Foundational to life, medicine, materials, and various industries.

Scale: Millions of organic compounds exist due to carbon's unique bonding.

Carbon Bonding: Forms stable covalent bonds with itself and other elements (H, O, N, halogens, P). Can be single, double, or triple.

Examples of Organic Chemistry in Daily Life: Burning a match (cellulose combustion), food (carbohydrates), clothes (polymers), medicines.

Biological Significance: Carbohydrates, lipids, proteins, nucleic acids are organic molecules. Crucial for biochemistry and medicine (drug development).

Materials Science: Polymers (plastics, fibers, rubber) are organic. Properties depend on structure.

Industrial Applications: Agrochemicals, dyes, detergents, specialized chemicals, petrochemicals.

Nomenclature Importance: Enables clear communication about organic compound identity and structure.

1.1 The Importance of Systematic Nomenclature

Problem with Trivial Names: Ambiguous, inconsistent, don't convey structural information (e.g., formic acid, urea).

Systematic Nomenclature (IUPAC): Standardized, logical system based on structure. Universal language for chemists.

Key Aspects of Importance:

1.2 The Language of Chemistry: Communication and Standardization

Nomenclature as a Language: Facilitates clear communication and collaboration among chemists globally.

Transcends Boundaries: Enables international collaboration despite language differences.

Standardization: Consistent rules for deriving names. Allows deducing structure from name and vice versa.

Community and Shared Understanding: Common language for discussing structures, reactions, and properties.

Digital Realm: Essential for chemical databases, modeling software, and electronic notebooks.

1.3 Historical Context: Early Naming Systems vs. Modern IUPAC

Early Naming (Trivial Names): Based on source, properties, discoverer (e.g., acetic acid, citric acid, morphine).

Challenges of Trivial Names: Difficult to memorize, no structural information, inconsistencies, hindered progress.

Development of Structural Theory (Mid-19th Century): Emphasized the need for structure-based naming.

Geneva Nomenclature (1892): First major attempt at systematic naming. Introduced key principles (longest chain, suffixes).

Formation of IUPAC (1919): Goal to establish a unified international system.

IUPAC Nomenclature (1930 onwards): Regularly updated rules for naming diverse organic structures.

Shift to IUPAC: Significant advancement, providing a common language for chemists, aiding communication and knowledge dissemination.

1.4 Basic Concepts: Atoms, Bonds, Functional Groups

Carbon Atom: Central to organic chemistry. Tetravalent (forms 4 bonds).

Common Elements: H, O, N, S, P, halogens.

Chemical Bonds: Forces holding atoms together. Predominantly covalent (sharing electrons).

• Single Bond: Sharing 1 electron pair.
• Double Bond: Sharing 2 electron pairs.
• Triple Bond: Sharing 3 electron pairs.

Electronegativity: Ability of an atom to attract electrons. Leads to polar covalent bonds.

Formal Charge and Resonance: Important for representing electron structure.

Functional Group: Specific group of atoms responsible for characteristic reactions. Key to nomenclature.

Common Functional Groups (Examples):

• Alkanes: C-C single bonds (unreactive).
• Alkenes: C=C double bond (more reactive).
• Alkynes: C≡C triple bond (highly reactive).
• Alcohols: -OH group.
• Ethers: R-O-R' group.
• Aldehydes: C=O bonded to at least one H.
• Ketones: C=O bonded to two alkyl/aryl groups.
• Carboxylic Acids: -COOH group.
• Esters: -COOR group.
• Amines: Nitrogen bonded to alkyl/aryl groups.
• Amides: C=O bonded to nitrogen.
• Halides: Halogen bonded to carbon.

Importance of Functional Groups: Determine reactivity and are incorporated into IUPAC names.

2. Fundamental Principles of Organic Structure

3D Arrangement: Understanding the spatial arrangement of atoms is crucial.

2.1 Carbon's Unique Bonding Properties

Tetravalency: Carbon forms four covalent bonds. Due to sp³ hybridization (tetrahedral, 109.5°).

Hybridization: Can also undergo sp² (planar, 120°, double bonds) and sp (linear, 180°, triple bonds) hybridization.

Catenation: Ability to form stable bonds with other carbon atoms (chains, branches, rings).

Bonding with Other Nonmetals: Forms strong bonds with H, O, N, S, P, halogens. Bond type and polarity influence properties.

2.2 Drawing and Interpreting Organic Structures (Lewis Structures, Line-Angle/Skeletal Formulas, Condensed Formulas)

Lewis Structures (Dot Structures): Show all valence electrons (bonding and non-bonding). Useful for visualizing electrons and connectivity. Cumbersome for large molecules.

Example: Methane, Water.

Line-Angle/Skeletal Formulas (Zig-Zag Structures): Simplified representation. Carbons at line ends/intersections, hydrogens implied. Heteroatoms shown. Emphasizes carbon skeleton. Quick to draw.

Example: Butane, Ethanol.

Condensed Formulas: Textual representation, grouping atoms. Branches in parentheses. Double/triple bonds sometimes shown. Saves space. Can be ambiguous.

Example: Butane (CH₃CH₂CH₂CH₃), Ethanol (CH₃CH₂OH).

2.3 Representing 3D Structures: Wedge and Dash Notation, Perspective Drawings

Wedge and Dash Notation: Shows 3D arrangement around an atom (typically tetrahedral carbon).

• Solid Lines: In plane.
• Solid Wedges: Out of plane (towards viewer).
• Dashed Wedges: Into plane (away from viewer).

Importance: Illustrating chirality and distinguishing stereoisomers.

Example: A carbon atom bonded to four different groups.

Perspective Drawings: Visualize 3D arrangement, focusing on conformation around a bond.

• Newman Projections: View along a C-C bond axis. Front carbon as a circle center, back carbon implied (bonds from circle edge). Shows conformational relationships (staggered/eclipsed).
• Sawhorse Projections: View along C-C bond at an angle. Both carbons shown. Clearer view of spatial relationships.

Usefulness: Analyzing conformational isomers and steric interactions.

2.4 Bond Types and Polarity

Bond Types (Covalent):

• Single Bonds (σ): One electron pair shared (head-on overlap). Free rotation.
• Double Bonds (σ and π): Two electron pairs shared (head-on and sideways overlap). Restricted rotation. Stronger and shorter.
• Triple Bonds (σ and 2π): Three electron pairs shared. Restricted rotation. Strongest and shortest.

Bond Polarity: Unequal electron sharing due to electronegativity differences.

• Nonpolar Covalent: Equal sharing (similar electronegativity, e.g., C-C, C-H).
• Polar Covalent: Unequal sharing (different electronegativity, e.g., C-O, C-N, C-X). Partial charges (δ⁺, δ^-).

Molecular Polarity: Overall polarity depends on bond polarities and molecular geometry.

Importance: Predicting reactivity, explaining physical properties, understanding molecular interactions.

2.5 Isomers: Structural (Constitutional) Isomers, Stereoisomers

Isomers Definition: Same molecular formula, different arrangement of atoms.

Structural (Constitutional) Isomers: Different connectivity of atoms.

• Types: Different carbon skeletons, different positions of functional groups, different functional groups.
• Example: Butane isomers, propanol isomers, dimethyl ether/ethanol.
• Properties: Different physical and potentially chemical properties.

Stereoisomers: Same connectivity, different spatial arrangement.

• Enantiomers: Non-superimposable mirror images. Chiral molecules (at least one stereocenter - carbon with 4 different groups). Identical physical properties (except optical rotation). Different interactions with chiral molecules.
• Diastereomers: Not mirror images. Different physical and chemical properties. Arise with multiple stereocenters where not all configurations are opposite.
  • Cis-Trans Isomers (Geometric Isomers): Due to restricted rotation (double bonds, rings). Cis: higher priority substituents on the same side. Trans: on opposite sides.

Significance of Isomerism: Different properties and biological activities (e.g., drug enantiomers).

Classification of Organic Compounds

3. Hydrocarbons: The Foundation of Organic Chemistry

Key Definitions:

• Hydrocarbons: Organic compounds containing only carbon and hydrogen atoms.
• Saturated Hydrocarbons: Hydrocarbons with only single carbon-carbon bonds (C-C).
• Unsaturated Hydrocarbons: Hydrocarbons with at least one carbon-carbon double (C=C) or triple (C≡C) bond.
• Aliphatic Hydrocarbons: Linear or branched chain hydrocarbons and cyclic hydrocarbons (excluding aromatic).
• Aromatic Hydrocarbons: Hydrocarbons containing a benzene ring (a stable six-membered ring with alternating single and double bonds).

3.1 Alkanes: Nomenclature, Properties, and Reactions

Definition: Alkanes = saturated hydrocarbons with C-C single bonds.

Example: Methane (CH₄), ethane (C₂H₆).

Nomenclature:

• Mnemonics: "Longest Chain, Lowest Numbers, Alphabetical Order."
• Key Rules:
  • Identify the longest continuous carbon chain.
  • Number carbons to give substituents the lowest numbers.
  • Name substituents as alkyl groups (e.g., methyl, ethyl).
  • List substituents alphabetically.
  • Use di-, tri-, tetra- for multiple identical substituents.
• Quick Example: Name CH₃-CH(CH₃)-CH₂-CH₃ (Answer: 2-methylbutane)

Properties:

• Table: Properties of the First Five Alkanes

  Alkane	Molecular Formula	Boiling Point (°C)	State at Room Temp
  Methane	CH₄	-162	Gas
  Ethane	C₂H₆	-89	Gas
  Propane	C₃H₈	-42	Gas
  Butane	C₄H₁₀	-0.5	Gas
  Pentane	C₅H₁₂	36	Liquid

• General Trends:

  • Increasing boiling point and melting point with increasing molecular weight (due to stronger London Dispersion Forces).
  • Insoluble in water (nonpolar).
  • Less dense than water.

  Reactions:

• Key Reactions:

  • Combustion: React with oxygen to produce carbon dioxide and water (releases energy). Example: $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$
  • Halogenation (Radical Substitution): Reaction with halogens (Cl₂, Br₂) in the presence of UV light or heat, replacing a hydrogen atom with a halogen. Example: $CH_4 + Cl_2 \rightarrow CH_3Cl + HCl$

3.2 Cycloalkanes: Nomenclature and Ring Systems

Definition: Cycloalkanes = saturated hydrocarbons with carbon atoms joined in a ring.

Example: Cyclopropane (C₃H₆), cyclohexane (C₆H₁₂).

Nomenclature:

• Key Rules:
  • Add the prefix "cyclo-" to the parent alkane name.
  • If there's only one substituent, no number is needed.
  • If there are multiple substituents, number the ring to give the substituents the lowest possible numbers.
  • List substituents alphabetically.

• Quick Example: Name the structure with a cyclopentane ring and a methyl group attached. (Answer: methylcyclopentane)

Ring Systems:

• Small Rings (Cyclopropane, Cyclobutane): Significant ring strain due to bond angle deviation from the ideal 109.5°. More reactive.
• Common Rings (Cyclopentane, Cyclohexane): Adopt non-planar conformations (e.g., chair conformation for cyclohexane) to minimize strain. More stable.

3.3 Alkenes: Nomenclature, Cis-Trans Isomerism, and Properties

Definition: Alkenes = unsaturated hydrocarbons containing at least one carbon-carbon double bond (C=C).

Example: Ethene (C₂H₄), propene (C₃H₆).

Nomenclature:

• Key Rules:
  • Identify the longest continuous carbon chain containing the double bond.
  • Number the carbons so the double bond gets the lowest possible number.
  • Change the "-ane" ending of the parent alkane to "-ene".
  • Indicate the position of the double bond with a number before the parent name.
  • Name substituents as before.

• Quick Example: Name $CH_3-CH=CH-CH_3$ (Answer: but-2-ene)

Cis-Trans Isomerism (Geometric Isomerism):

• Requirement: Restricted rotation around the double bond and two different groups on each carbon of the double bond.
• Cis Isomer: Substituents on the same side of the double bond.
• Trans Isomer: Substituents on opposite sides of the double bond.

!(cis-but-2-ene and trans-but-2-ene)[https://www.vedantu.com/question-sets/2b92d53e-d0bd-4cf4-afc7-5122c035d3435308516844318259558.png]

• Quick Example: Identify if $CH_3-CH=CH-CH_2CH_3$ can have cis-trans isomers (Yes).

Properties:

• Physical Properties: Similar to alkanes with similar carbon numbers, but slightly lower boiling points due to less efficient packing.
• Reactivity: More reactive than alkanes due to the presence of the pi bond.
• Key Reactions:
  • Addition Reactions: The pi bond breaks, and new atoms or groups are added to the carbon atoms.
    • Hydrogenation: Addition of H₂ (requires a catalyst, e.g., Pt, Pd, Ni). Example: $CH_2=CH_2 + H_2 \rightarrow CH_3CH_3$
    • Halogenation: Addition of Cl₂ or Br₂. Example: $CH_2=CH_2 + Br_2 \rightarrow CH_2BrCH_2Br$
    • Hydrohalogenation: Addition of HX (HCl, HBr, HI). Follows Markovnikov's rule (hydrogen adds to the carbon with more hydrogens). Example: $CH_3CH=CH_2 + HBr \rightarrow CH_3CHBrCH_3$(major product)
    • Hydration: Addition of water (requires acid catalyst). Follows Markovnikov's rule. Example: $CH_3CH=CH_2 + H_2O \rightarrow CH_3CH(OH)CH_3$ (major product)
• Mnemonics (Markovnikov's Rule): "The rich get richer" (hydrogen adds to the carbon with more hydrogens).

3.4 Alkynes: Nomenclature and Properties

Definition: Alkynes = unsaturated hydrocarbons containing at least one carbon-carbon triple bond (C≡C).

Example: Ethyne (acetylene, C₂H₂), propyne (CH₃C≡CH).

Nomenclature:

• Key Rules:
  • Similar to alkene nomenclature, but change the "-ane" ending to "-yne".
  • Number the carbons to give the triple bond the lowest possible number.

• Quick Example: Name $CH_3-C\equiv C-CH_3$ (Answer: but-2-yne)

Properties:

• Physical Properties: Similar to alkanes and alkenes of comparable size, but slightly higher boiling points than corresponding alkenes due to stronger intermolecular forces.

• Reactivity: Highly reactive due to the presence of two pi bonds.

• Key Reactions:

  • Addition Reactions: Similar to alkenes, but can add two moles of reagent due to two pi bonds.
    • Hydrogenation: Can be controlled to form alkenes (using Lindlar's catalyst) or alkanes (using excess H₂ and standard catalysts).
    • Halogenation: Can add one or two moles of halogen.
    • Hydrohalogenation: Follows Markovnikov's rule for the first addition.
    • Hydration: Addition of water in the presence of HgSO₄ and H₂SO₄ forms an enol, which tautomerizes to a ketone. Example: $HC\equiv CH + H_2O \rightarrow [CH_2=CHOH] \rightarrow CH_3CHO$ (acetaldehyde)

• Table: Comparing Reactivity (Qualitative)

  Hydrocarbon	Reactivity
  Alkanes	Low
  Alkenes	Medium
  Alkynes	High

3.5 Aromatic Hydrocarbons: Benzene, Substituted Benzenes, and Polycyclic Aromatic Hydrocarbons

Definition: Aromatic Hydrocarbons = hydrocarbons containing a benzene ring, a six-membered ring with alternating single and double bonds (exhibiting resonance).

Example: Benzene (C₆H₆), toluene (methylbenzene, C₇H₈).

Benzene (C₆H₆):

• Structure: Cyclic, planar molecule with six carbon atoms and alternating single and double bonds. Exhibits resonance, meaning the electrons are delocalized, making all C-C bonds equivalent in length and strength.
• Stability: Highly stable due to resonance. Undergoes substitution reactions rather than addition reactions like alkenes.

Substituted Benzenes:

• Nomenclature (Monosubstituted): Name the substituent followed by "benzene". Example: chlorobenzene, nitrobenzene.
• Nomenclature (Disubstituted): Use prefixes ortho- (1,2), meta- (1,3), and para- (1,4) to indicate the relative positions of the two substituents. Example: o-dichlorobenzene, m-nitrotoluene, p-bromophenol.
• Nomenclature (Polysubstituted): Number the ring to give substituents the lowest possible numbers.
• Electrophilic Aromatic Substitution: The characteristic reaction of benzene. An electrophile (electron-seeking species) replaces a hydrogen atom on the benzene ring.
  • Key Reactions:
    • Halogenation: Reaction with Cl₂ or Br₂ in the presence of a Lewis acid catalyst (e.g., FeCl₃, FeBr₃).
    • Nitration: Reaction with concentrated nitric acid and sulfuric acid.
    • Sulfonation: Reaction with fuming sulfuric acid (SO₃ in H₂SO₄).
    • Friedel-Crafts Alkylation: Reaction with an alkyl halide in the presence of a Lewis acid catalyst (e.g., AlCl₃).
    • Friedel-Crafts Acylation: Reaction with an acyl halide or anhydride in the presence of a Lewis acid catalyst.
• Directing Effects of Substituents: Substituents already on the benzene ring influence where the next substituent will attach.
  • Activating and ortho, para-directing groups: Donate electron density to the ring (e.g., -OH, -NH₂, -R).
  • Deactivating and meta-directing groups: Withdraw electron density from the ring (e.g., -NO₂, -COOH, -CHO).
  • Halogens: Deactivating but ortho, para-directing.

Polycyclic Aromatic Hydrocarbons (PAHs):

• Definition: Aromatic hydrocarbons containing two or more fused benzene rings.
• Examples: Naphthalene, anthracene, phenanthrene.
• Significance: Some PAHs are carcinogenic (cancer-causing).

4. Functional Groups: The Heart of Reactivity

4.1 Definition of a Functional Group

• Specific group of atoms within a molecule that is responsible for the characteristic chemical reactions of that molecule.
• The reactive center of an organic molecule.
• Dictates the molecule's chemical behavior.

4.2 Classification by Functional Group

4.2.1 Halides (Alkyl and Aryl Halides)

• Definition: A halogen atom (F, Cl, Br, I) bonded to an alkyl (alkyl halide) or aryl (aryl halide) group.
• Example: CH₃Cl (chloromethane), C₆H₅Br (bromobenzene).
• Key Characteristic/Reactivity: Undergo nucleophilic substitution and elimination reactions. Aryl halides are less reactive towards SN reactions.

4.2.2 Alcohols, Phenols, and Ethers

4.2.3 Aldehydes and Ketones

4.2.4 Carboxylic Acids and Their Derivatives (Esters, Amides, Acid Anhydrides, Acyl Halides)

4.2.5 Amines

4.2.6 Nitriles and Isocyanides

4.2.7 Thiols, Sulfides, Disulfides and Epoxides

4.2.9 Other Less Common but Important Functional Groups

• Nitro Compounds (-NO₂): Electron-withdrawing, influence reactivity of aromatic rings.
• Azides (-N₃): Can be explosive, used in synthesis.
• Phosphines (R₃P): Act as ligands in organometallic chemistry.

4.3 Polarity and Reactivity of Functional Groups

• Polarity: Arises from differences in electronegativity between atoms in the functional group. Creates partial positive and negative charges.
• Reactivity: Polar functional groups are often more reactive.
• Electrophiles: Electron-deficient species attracted to negative charges (e.g., carbonyl carbon).
• Nucleophiles: Electron-rich species attracted to positive charges (e.g., oxygen in alcohols, nitrogen in amines).
• The presence and type of functional group largely determine the chemical reactions a molecule will undergo.

5. Heterocyclic Compounds

5.1 Introduction to Heterocycles

• Definition: Cyclic compounds where at least one atom in the ring is not carbon (a heteroatom).
• Common Heteroatoms: Nitrogen (N), Oxygen (O), Sulfur (S).
• Prevalence: Extremely common in natural products, pharmaceuticals, and materials.
• Influence of Heteroatom: Alters electron distribution, reactivity, and properties compared to carbocyclic rings.

5.2 Nomenclature of Common Heterocyclic Rings

• Trivial Names: Many common heterocycles have historical, non-systematic names.
  • 3-membered: Oxirane (epoxide), Aziridine, Thiirane.
  • 5-membered: Furan, Thiophene, Pyrrole, Imidazole, Thiazole.
  • 6-membered: Pyridine, Pyridazine, Pyrimidine, Pyrazine.
  • Fused Rings: Indole (Benzene + Pyrrole), Quinoline (Benzene + Pyridine).
• Systematic Nomenclature: IUPAC rules exist, but trivial names are often preferred for simpler rings. Involves prefixes indicating heteroatoms (oxa-, thia-, aza-) and suffixes indicating ring size and saturation.
  • Example: Oxolane (tetrahydrofuran).

5.3 Classification Based on Ring Size and Type of Heteroatom

• Ring Size:
  • 3-membered rings: Highly strained, reactive (e.g., epoxides, aziridines).
  • 4-membered rings: Also strained, less so than 3-membered (e.g., azetidines).
  • 5-membered rings: Common, relatively stable, often aromatic or can exhibit aromatic character.
  • 6-membered rings: Very common, stable, often aromatic (e.g., pyridine).
  • Larger rings: Generally less common.
• Type of Heteroatom:
  • Nitrogen-containing: Pyrrole (aromatic), Pyridine (aromatic, basic), Imidazole (aromatic, amphoteric), etc.
  • Oxygen-containing: Furan (aromatic), Tetrahydrofuran (common solvent), Epoxides (reactive).
  • Sulfur-containing: Thiophene (aromatic), Thiazole (important in biochemistry).
  • Multiple Heteroatoms: Imidazole (2 N), Thiazole (N and S), Purine (4 N, fused rings), Pyrimidine (2 N).

5.4 Important Heterocycles in Nature and Pharmaceuticals

• Nucleic Acid Bases: Purines (Adenine, Guanine) and Pyrimidines (Cytosine, Thymine, Uracil) are fundamental to DNA and RNA.
• Amino Acids: Many contain heterocyclic side chains (e.g., Tryptophan, Histidine, Proline).
• Carbohydrates: Some cyclic forms involve oxygen as a ring member (pyranose and furanose rings).
• Vitamins: Many vitamins have heterocyclic structures (e.g., Vitamin B1 (Thiamine), Vitamin B6 (Pyridoxine)).
• Alkaloids: Diverse group of naturally occurring nitrogen-containing heterocycles with significant biological activity (e.g., Morphine, Caffeine, Nicotine).
• Pharmaceuticals: A vast number of drugs contain heterocyclic rings. These rings often provide key structural features for binding to biological targets.

• Natural Pigments: Chlorophyll (porphyrin ring), Heme (porphyrin ring).
• Materials Science: Heterocycles are used as building blocks for conductive polymers and dyes.

Nomenclature of Organic Compounds

6. Basic IUPAC Rules for Naming Alkanes and Alkyl Groups

6.1 Identifying the Parent Chain

The parent chain is the longest continuous chain of carbon atoms.

If there are two chains of the same length, choose the one with the greater number of substituents.

6.2 Numbering the Parent Chain

Number the carbon atoms in the parent chain starting from the end that gives the lowest numbers to the substituents at the first point of difference.

For example, if one numbering gives substituents at positions 2 and 4, and another gives 3 and 5, the 2,4 numbering is correct.

6.3 Naming Substituents (Alkyl Groups, Halogens)

Alkyl groups are named by changing the "-ane" ending of the alkane to "-yl". For example, methane ($CH_4$) becomes methyl ($-CH_3$).

Common alkyl groups include: methyl ($-CH_3$), ethyl ($-CH_2CH_3$), propyl ($-CH_2CH_2CH_3$), isopropyl ($-CH(CH_3)_2$), butyl ($-CH_2CH_2CH_2CH_3$), sec-butyl ($-CH(CH_3)CH_2CH_3$), isobutyl ($-CH_2CH(CH_3)_2$), tert-butyl ($-C(CH_3)_3$).

Halogens are named as prefixes: fluoro-, chloro-, bromo-, iodo-.

6.4 Alphabetical Order and Multiple Substituents

List the names of the substituents in alphabetical order, ignoring prefixes like "di-", "tri-", "sec-", and "tert-". "Iso-" is considered part of the alkyl group name for alphabetization.

Use prefixes like "di-", "tri-", "tetra-" to indicate multiple identical substituents. Each substituent gets its own number.

Separate numbers with commas and numbers from names with hyphens.

Example: 2-chloro-3,3-dimethylbutane

6.5 Complex Substituents and Their Nomenclature

For complex substituents, number the carbon atoms of the substituent starting from the point of attachment to the parent chain (this carbon gets number 1).

Enclose the name of the complex substituent in parentheses and treat it as a single unit for alphabetization.

Example: 4-(1-methylethyl)heptane

7. Nomenclature of Alkenes, Alkynes, and Cycloalkanes

7.1 Naming Alkenes: Position of Double Bonds

Identify the longest continuous carbon chain containing the double bond.

Number the carbon chain so that the double bond gets the lowest possible number. The number indicates the position of the first carbon of the double bond.

Change the "-ane" ending of the parent alkane to "-ene".

Example: $CH_3CH=CHCH_3$ is but-2-ene.

If there are multiple double bonds, use "-adiene", "-atriene", etc., and indicate the position of each double bond. Example: buta-1,3-diene.

7.2 Naming Alkynes: Position of Triple Bonds

Identify the longest continuous carbon chain containing the triple bond.

Number the carbon chain so that the triple bond gets the lowest possible number. The number indicates the position of the first carbon of the triple bond.

Change the "-ane" ending of the parent alkane to "-yne".

Example: $CH_3C \equiv CCH_3$ is but-2-yne.

If there are multiple triple bonds, use "-adiyne", "-atriyne", etc., and indicate the position of each triple bond.

If both double and triple bonds are present, number the chain to give the lower number to the first multiple bond encountered. "-en" comes before "-yne" in the name. Example: pent-1-en-4-yne.

7.3 Naming Cycloalkanes: Ring as Parent Chain

For unsubstituted cycloalkanes, simply add the prefix "cyclo-" to the name of the alkane with the same number of carbons. Example: cyclopropane ($C_3H_6$).

For substituted cycloalkanes, the ring is the parent chain unless there is an alkyl chain with more carbon atoms than the ring.

Number the carbon atoms of the ring starting at the point of attachment of a substituent. Number in a direction that gives the lowest numbers to the other substituents.

If there is only one substituent, no number is needed for its position.

7.4 Naming Cycloalkenes and Cycloalkynes

For cycloalkenes, the double bond carbons are always numbered 1 and 2. Number in the direction that gives the lowest numbers to other substituents.

The position of the double bond is usually not explicitly stated unless there is ambiguity (more than one double bond). Example: cyclohexene.

For cycloalkynes, the triple bond carbons are always numbered 1 and 2. Number in the direction that gives the lowest numbers to other substituents.

Cycloalkynes with small rings are unstable due to ring strain.

8. Nomenclature of Compounds with Functional Groups

8.1 Identifying the Principal Functional Group

When naming organic compounds containing functional groups, the first crucial step is to identify the principal functional group. This group dictates the suffix of the IUPAC name and takes precedence over other functional groups present in the molecule. The principal functional group is determined based on a priority order, which we will discuss later.

A functional group is an atom or a group of atoms within a molecule that is responsible for the characteristic chemical reactions of that molecule. Common examples include alcohols ($-OH$), aldehydes ($-CHO$), ketones ($R-CO-R'$), carboxylic acids ($-COOH$), amines ($-NH_2$), and nitriles ($-CN$). Recognizing these groups is fundamental to applying the correct nomenclature rules.

To identify the principal functional group, carefully examine the molecule's structure. Look for heteroatoms (atoms other than carbon and hydrogen) and characteristic bonding patterns like double or triple bonds to heteroatoms. For instance, a carbon atom double-bonded to an oxygen atom ($C=O$) is indicative of either an aldehyde or a ketone, depending on its location within the carbon chain. Similarly, a carbon atom single-bonded to an oxygen atom that is also bonded to a hydrogen atom ($-OH$) signifies an alcohol.

If multiple functional groups are present, their relative priority determines which one becomes the principal functional group. The remaining functional groups are treated as substituents and are named using appropriate prefixes.

For example, in a molecule containing both a carboxylic acid group ($-COOH$) and a hydroxyl group ($-OH$), the carboxylic acid group is the principal functional group due to its higher priority. The hydroxyl group will then be named as a hydroxy substituent.

8.2 Applying Suffixes and Prefixes

Once the principal functional group is identified, the next step is to determine the appropriate suffix to add to the parent alkane name. The parent alkane is the longest continuous carbon chain containing the principal functional group. The suffix replaces the "-ane" ending of the corresponding alkane.

Different functional groups have specific suffixes. For example:

• Alcohols use the suffix "-ol". Example: $CH_3CH_2OH$ is ethanol.
• Aldehydes use the suffix "-al". Example: $CH_3CHO$ is ethanal.
• Ketones use the suffix "-one". Example: $CH_3COCH_3$ is propanone.
• Carboxylic acids use the suffix "-oic acid". Example: $CH_3COOH$ is ethanoic acid.
• Amines use the suffix "-amine". Example: $CH_3NH_2$ is methanamine.
• Nitriles use the suffix "-nitrile". Example: $CH_3CN$ is ethanenitrile.

The position of the principal functional group is indicated by a number placed just before the suffix, if necessary. Numbering of the parent chain starts from the end that gives the lowest number to the carbon atom of the principal functional group (or the carbon directly attached to it, depending on the group).

Functional groups that are not the principal group are treated as substituents and are named using prefixes. Common prefixes include:

• Hydroxy ($-OH$)
• Oxo ($=O$) for ketones and aldehydes when not the principal group
• Amino ($-NH_2$)
• Cyano ($-CN$)
• Alkoxy ($-OR$) for ethers

For example, if a molecule contains a ketone group as the principal functional group and an alcohol group, the alcohol group will be named as a hydroxy substituent. The name will include the position of the hydroxy group and the ketone group.

8.3 Ranking Order of Functional Groups

The priority order of functional groups determines which group is named using a suffix and which are named using prefixes. A simplified ranking order, from highest to lowest priority, is as follows (this is not exhaustive, but covers common functional groups):

1. Carboxylic acids ($-COOH$)
2. Acid anhydrides ($-CO-O-CO-$)
3. Esters ($-COOR$)
4. Acid halides ($-COX$)
5. Amides ($-CONH_2$)
6. Nitriles ($-CN$)
7. Aldehydes ($-CHO$)
8. Ketones ($R-CO-R'$)
9. Alcohols ($-OH$)
10. Thiols ($-SH$)
11. Amines ($-NH_2$)
12. Imines ($C=NH$)
13. Ethers ($-OR$)
14. Sulfides ($-SR$)
15. Alkenes ($C=C$) and Alkynes ($C \equiv C$) (treated similarly for numbering priority but generally named as suffixes if the principal group doesn't contain them)
16. Alkanes ($-CH_3$, $-CH_2CH_3$, etc.) (parent chain)
17. Halogens ($-F$, $-Cl$, $-Br$, $-I$) as substituents
18. Nitro groups ($-NO_2$) as substituents

When multiple functional groups are present, identify the one highest on this list. This will be the principal functional group and will determine the suffix of the name. All other functional groups will be named as prefixes according to their prefix names.

8.4 Examples with Multiple Functional Groups

Let's look at some examples to illustrate the naming of compounds with multiple functional groups:

Example 1: $HOCH_2CH_2COOH$

This molecule contains both a carboxylic acid group ($-COOH$) and a hydroxyl group ($-OH$). According to the priority order, the carboxylic acid is the principal functional group. The parent chain is three carbons long (prop-). The suffix will be "-oic acid". The hydroxyl group is a substituent on carbon 3. The name is 3-hydroxypropanoic acid.

Example 2: $CH_3COCH_2CH_2CHO$

This molecule contains both a ketone group ($C=O$) and an aldehyde group ($-CHO$). The aldehyde group has higher priority. The parent chain is four carbons long (but-). The suffix will be "-al". The ketone group is on carbon 3 and is named as an oxo substituent. The name is 3-oxobutanal.

Example 3: $CH_3CH=CHCH_2OH$

This molecule contains an alkene ($C=C$) and an alcohol ($-OH$). The alcohol has higher priority. The parent chain is four carbons long (but-). The suffix will be "-ol". The double bond is between carbons 2 and 3. The hydroxyl group is on carbon 1. The name is but-2-en-1-ol.

Example 4: $NH_2CH_2CH_2COOH$

This molecule contains an amine group ($-NH_2$) and a carboxylic acid group ($-COOH$). The carboxylic acid has higher priority. The parent chain is three carbons long (prop-). The suffix will be "-oic acid". The amine group is on carbon 3 and is named as an amino substituent. The name is 3-aminopropanoic acid.

These examples demonstrate how to apply the rules for naming compounds with multiple functional groups by identifying the principal group, applying the correct suffix, and naming other groups as prefixes based on their priority.

9. Nomenclature of Aromatic Compounds

Aromatic compounds are a class of cyclic, planar molecules with a delocalized pi electron system, most commonly based on the structure of benzene. Benzene ($C_6H_6$) is a six-membered ring with alternating single and double bonds, although the actual structure involves resonance where the electrons are shared equally among all carbon-carbon bonds. To understand the structure of benzene, you can search for "Benzene" on Wikipedia and view its structural diagrams.

9.1 Naming Substituted Benzenes: Ortho, Meta, Para

When a single substituent is attached to a benzene ring, the compound is named by stating the name of the substituent followed by "benzene". For example, if a chlorine atom is attached to benzene, the compound is called chlorobenzene. If a methyl group ($CH_3$) is attached, it's called methylbenzene (also known as toluene).

For disubstituted benzenes, where two substituents are attached to the benzene ring, we use the prefixes ortho-, meta-, and para- to indicate the relative positions of the two substituents.

• ortho- (o-) indicates that the two substituents are on adjacent carbon atoms (1,2-disubstituted).
• meta- (m-) indicates that the two substituents are separated by one carbon atom (1,3-disubstituted).
• para- (p-) indicates that the two substituents are on opposite carbon atoms (1,4-disubstituted).

Examples:

• If two methyl groups are attached to adjacent carbons, the compound is o-dimethylbenzene or 1,2-dimethylbenzene.
• If a chlorine and a nitro group ($NO_2$) are attached with one carbon in between, the compound is m-chloronitrobenzene or 1-chloro-3-nitrobenzene.
• If a bromine and an ethyl group ($CH_2CH_3$) are attached on opposite sides, the compound is p-bromoethylbenzene or 1-bromo-4-ethylbenzene.

When the two substituents are different, they are listed alphabetically followed by "benzene". The prefix (o-, m-, p-) indicates their relative positions. For example, if a bromine and a chlorine are in the meta position, the name is m-bromochlorobenzene.

Some substituted benzenes have common names that are accepted by IUPAC. When one of these common names is used as the parent name for a disubstituted benzene, the carbon atom bearing the group that gives the compound its special name is considered position 1. For example:

• If a hydroxyl group ($OH$) and a methyl group are on adjacent carbons, the compound can be named o-methylphenol. Here, phenol is the parent name (benzene with an $OH$ group).
• If an amino group ($NH_2$) and a chlorine are in the para position, the compound can be named p-chloroaniline. Here, aniline is the parent name (benzene with an $NH_2$ group).
• If a carboxylic acid group ($COOH$) and a nitro group are in the meta position, the compound can be named m-nitrobenzoic acid. Here, benzoic acid is the parent name (benzene with a $COOH$ group).

9.2 Naming More Complex Aromatic Compounds

For benzenes with more than two substituents, the ortho-, meta-, para- system is no longer used. Instead, the carbon atoms of the benzene ring are numbered to indicate the positions of the substituents. The numbering starts at one of the substituted carbon atoms and proceeds around the ring in a way that gives the lowest set of numbers to the substituents.

The substituents are listed alphabetically with their corresponding numbers. If one of the substituents gives the molecule a common name (like toluene, phenol, aniline, or benzoic acid), that substituent is considered to be at position 1.

Examples:

• A benzene ring with a methyl group at position 1, a chlorine at position 2, and a nitro group at position 4 is named 2-chloro-4-nitrotoluene. Toluene is the parent name, so the methyl group is at position 1.
• A benzene ring with a bromine at position 1, a chlorine at position 2, and a nitro group at position 6 is named 1-bromo-2-chloro-6-nitrobenzene. Here, since none of the substituents dictate a common name, we number to give the lowest set of numbers.
• A benzene ring with hydroxyl group at position 1, a methyl group at position 3, and a bromine at position 5 is named 5-bromo-3-methylphenol. Phenol is the parent name.

If there are identical substituents, prefixes like di-, tri-, etc., are used, and each substituent gets its own number. For example, a benzene ring with three nitro groups at positions 1, 3, and 5 is named 1,3,5-trinitrobenzene.

9.3 Naming Polycyclic Aromatic Hydrocarbons

Polycyclic aromatic hydrocarbons (PAHs) are aromatic compounds consisting of fused benzene rings. They have their own unique naming systems.

• Naphthalene ($C_{10}H_8$) consists of two fused benzene rings. The carbon atoms are numbered as shown below. When naming substituted naphthalenes, the numbering starts at one of the carbons adjacent to the fusion and proceeds around the periphery.
  
• Anthracene ($C_{14}H_{10}$) consists of three benzene rings fused linearly. The numbering is specific.
  
• Phenanthrene ($C_{14}H_{10}$) also consists of three fused benzene rings, but they are fused in an angular manner, which results in a different structure and numbering system compared to anthracene.
  

When naming substituted PAHs, the substituents are numbered according to the established numbering system for the parent PAH. For example, a naphthalene molecule with a methyl group at position 1 is called 1-methylnaphthalene.

More complex PAHs with more than three rings also exist and have their own systematic or trivial names, often based on their structure or origin. Examples include pyrene, benzopyrene, and coronene.

About the author

Written by Noah Kleij, PhD

Noah Kleij holds a Doctorate in Organic and General Chemistry from the prestigious University of Manchester, United Kingdom. With a deep passion for chemical sciences, Noah has contributed significantly to advancing knowledge in both organic synthesis and general chemistry principles. Their research encompasses cutting-edge methodologies and innovative problem-solving approaches.

In addition to their academic achievements, Noah is an accomplished author and educator, committed to sharing complex chemical concepts in accessible and engaging ways. Their work not only bridges theoretical and practical chemistry but also inspires the next generation of chemists to explore the field's transformative potential.