An introduction to A Level organic chemistry

🔹 Formulas, Functional Groups, and the Naming of Organic Compounds

🔸 Functional Groups and Physical/Chemical Properties

• Functional Group: A specific group of atoms within a molecule responsible for its characteristic chemical reactions.
• The physical and chemical properties of an organic molecule are largely dictated by the functional group it contains.
• Examples of functional groups (see page 47 of the syllabus):
  • Alcohols (-OH)
  • Carboxylic acids (-COOH)
  • Amines (-NH₂)
  • Aldehydes (-CHO)
  • Ketones (C=O)
  • Esters (-COOR)
  • Amides (-CONH₂)
  • Nitriles (-CN)
  • Alkenes (C=C)
  • Halogenoalkanes (–X)
  • Arenes (benzene rings)

🔸 Organic Formulas

• General Formula: Represents a whole class of compounds.
  Example: Alkanes – CₙH₂ₙ₊₂
• Structural Formula: Shows the arrangement of atoms without displaying all bonds.
  Example: Butanol – CH₃CH₂CH₂CH₂OH
• Displayed Formula: Shows all atoms and bonds explicitly.
• Skeletal Formula: Shows only carbon skeleton and functional groups, omitting H on C.

🔸 Systematic Nomenclature

• Follows IUPAC rules:
  • Identify the longest carbon chain (parent chain).
  • Number the chain such that the functional group or substituent gets the lowest possible number.
  • Use prefixes/suffixes to indicate functional groups and positions.
• Aliphatic molecules with up to six carbon atoms should be named.
  • Includes cyclic compounds (e.g., cyclopentane, cyclohexanol).
  • For esters and amides: straight chain up to 6 + 6 carbons (e.g., methyl hexanoate).
  • Nitriles: straight chains only.
• Aromatic compounds:
  • One benzene ring + simple substituents.
  • Examples:
    • 3-nitrobenzoic acid
    • 2,4,6-tribromophenol
  • Use correct position numbers and names of substituents.

🔹 Characteristic Organic Reactions

🔸 Reaction Mechanisms

• Electrophilic Substitution:
  • Typical of aromatic compounds (e.g., benzene).
  • An electrophile (electron-pair acceptor) replaces a hydrogen atom in the benzene ring.
  • Examples: nitration, halogenation of benzene.
• Addition–Elimination:
  • Involves two steps: an addition followed by an elimination.
  • Common in reactions between acid derivatives (e.g., acyl chlorides) and nucleophiles like amines or alcohols.

🔹 Shapes of Aromatic Organic Molecules; σ and π Bonds

🔸 Structure of Benzene

• Benzene is a planar, cyclic molecule with six carbon atoms.
• Each carbon is sp² hybridised, forming:
  • Three sigma (σ) bonds – two with adjacent carbon atoms and one with a hydrogen.
  • One unhybridised p orbital per carbon atom, overlapping to form a delocalised π system above and below the ring.
• Delocalisation provides stability and results in equal bond lengths between all carbon atoms.

🔹 Isomerism: Optical

🔸 Chirality and Enantiomers

• Chiral carbon: A carbon atom bonded to four different groups.
• Enantiomers: Non-superimposable mirror images of each other.
• Have:
  • Identical physical properties (melting point, boiling point, solubility).
  • Identical chemical properties (except with chiral reagents).
  • Different effects on plane-polarised light:
    • One rotates light clockwise (dextrorotatory, +).
    • Other rotates light anticlockwise (laevorotatory, –).
• A racemic mixture (racemate) contains equal amounts of both enantiomers → optically inactive due to cancellation.

🔸 Relevance in Drug Synthesis

• Biological systems are often chiral, so enantiomers can have different effects.
• One enantiomer might be therapeutic, while the other could be inactive or harmful.
• Drug manufacturing requires:
  • Separation of racemic mixtures into pure enantiomers.
  • Use of chiral catalysts to produce only one enantiomer.
• Note: Some molecules have more than one chiral centre, but diastereoisomers or meso compounds are not required.

🔚 Summary of Key Terms

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