Introduction to Organic Chemistry: Formulae, Homologous Series, and Functional Groups

Introduction to Organic Chemistry and Formula Representations

• Prepared By: Siti Fatimah Mohammad Yaacob (SFY).

• Foundation in Science: FS1024 Chemistry I.

Types of Chemical Formulae

• General Formula

  • Definition: An algebraic formula that can describe any member of a family of compounds (homologous series).

  • Example (Alcohols): $C_nH_{2n+1}OH$

• Empirical Formula

  • Definition: The simplest whole-number ratio of atoms of each element in a compound.

  • Example (Butan-1-ol): $C_4H_{10}O$

• Molecular Formula

  • Definition: The actual number of atoms of each element in a molecule. This can be calculated from the empirical formula.

  • Example (Butan-1-ol): $C_4H_{10}O$

• Structural Formula

  • Definition: Shows how the atoms are bonded together in a molecule. It is divided into three subtypes: Expanded, Condensed, and Skeletal.

Subtypes of Structural Formulae

• Expanded Structural Formula

  • Shows how all the atoms are attached and all the bonds between them.

  • Note: This is NOT a representation of the actual shapes of the molecules.

  • Example: For Cyclobutane, it displays four carbons in a square with two Hydrogens branching off each carbon.

• Condensed Structural Formula

  • Does not show single bonds between carbon and hydrogen atoms, but double and triple bonds are explicitly shown.

  • Brackets $()$ are used for repeating $-CH_2-$ units or any substituent branching off the main chain.

  • All atoms attached to a carbon are written immediately after that carbon.

  • Example:

    • Butan-1-ol: $CH_3CH_2CH_2CH_2OH$ or $CH_3(CH_2)_3OH$

    • Isopropane variant: $CH_3CH(Br)CH_3$

• Skeletal Structural Formula

  • Shows the bonds of the carbon skeleton only, along with any other functional groups.

  • Hydrogen and carbon atoms attached to the main chain are not shown.

  • This method is considered handy for drawing large, complicated structures like cyclic hydrocarbons.

3-Dimensional Structural Formula (Wedge-Dashed Wedge-Line)

• Purpose: Describes how atoms are arranged in space. It is a 3D depiction of a molecule on a 2D surface (plane projection).

• Typical Use: Used for molecules with chiral centers.

• Notation Rules:

  • Wedged Line: Signifies bond projections towards the observer (coming forward out from the plane of the page).

  • Solid Line: Denotes a bond existing within the plane of the page.

  • Dashed Line: Indicates a bond projected away from the observer (going backward out from the plane of the page).

Homologous Series

• Definition: A series of organic compounds that have the same functional group.

• Characteristics:

  • Obey a specific general formula.

  • Possess similar chemical properties.

  • Show a gradual change in physical properties.

  • Consecutive members of a homologous series differ by a $-CH_2-$ unit.

Examples of Homologous Series General Formulae

• Alkane: $C_nH_{2n+2}$

• Alkene: $C_nH_{2n}$

• Alcohol: $C_nH_{2n+1}OH$

Physical Properties Trend: Alkanes Table

Prefix and Suffix Naming in Homologous Series

• Alkanes: Suffix "-ane". Example: Propane ($CH_3CH_2CH_3$).

• Branched Alkanes: Prefix "alkyl-" (ending in "-yl"). Example: Methylpropane ($CH_3CH(CH_3)CH_3$).

• Alkenes: Suffix "-ene". Example: Propene ($CH_3CH=CH_2$).

• Halogenoalkanes: Prefix "chloro-", "bromo-", or "iodo-". Example: Chloroethane ($CH_3CH_2Cl$).

• Alcohols: Suffix "-ol". Example: Ethanol ($CH_3CH_2OH$).

• Aldehydes: Suffix "-al". Example: Ethanal ($CH_3CHO$).

• Ketones: Suffix "-one". Example: Propanone ($CH_3COCH_3$).

• Cycloalkanes: Prefix "cyclo-" and suffix "-ane". Example: Cyclohexane ($C_6H_{12}$).

• Carboxylic acids: Suffix "-oic acid". Example: Ethanoic acid ($CH_3COOH$).

Functional Groups

• Definition: An atom or group of atoms in an organic molecule that characterizes the molecule and enables it to react in specific ways. It determines the chemical properties.

• Influences: Functional groups also impact the physical properties of the organic compound.

• Classification: Functional groups are the basis by which organic compounds are divided into different classes and homologous series, and serve as the basis for naming (nomenclature).

Classes and Their Functional Groups

• Arene: Phenyl ring.

• Alcohol: Hydroxyl group ($-OH$).

• Phenol: Hydroxyl group directly attached to an aromatic ring.

• Alkene: Carbon-carbon double bond ($C=C$).

• Alkyne: Carbon-carbon triple bond ($C\equiv C$).

• Halogenoalkane: Carbon bonded to a halogen ($X = F, Cl, Br, I$).

• Aldehyde: Carbonyl group at the end of a chain ($-CHO$).

• Ketone: Carbonyl group within a chain ($C=O$).

• Nitro: Nitro group ($-NO_2$).

• Carboxylic Acid: Carboxyl group ($-COOH$).

• Acyl Chloride: Carbonyl bonded to chlorine ($-COCl$).

• Eter: Oxygen bonded between two alkyl groups ($R-O-R'$). Example: Methoxyethane ($H_3C-O-CH_2CH_3$).

• Ester: ($-COOR$).

• Amine: Amino group ($-NH_2$ for primary).

• Amide: Carbonyl group bonded to nitrogen ($-CONH_2$).

• Nitrile: Cyano group ($-C\equiv N$).

• Amino Acid: Contains both an amine ($-NH_2$) and a carboxylic acid ($-COOH$) group.

• Anhydride: Two acyl groups joined by an oxygen atom.

Practice exercises

Practice 1: Drawing Structures

• For compound X: $CH_3CH_2CCH$

  • Substituent Handling: Brackets are needed for open-chain substituents.

• For compound Y: $CH_3CH(OH)(CH_3)CH_2CH_2Cl$

  • Requires proper placement of brackets for the hydroxyl group to signify it is a branch.

Practice 3: Aspartame Identification

• Molecule: Aspartame.

• Identified Functional Groups:

  • Amine: Located at the end of the chain ($H_2N-$).

  • Amide: The peptide bond connecting the two amino acid units ($-NH-C=O$).

  • Ester: ($-C(=O)OCH_3$).

  • Carboxylic Acid: ($-COOH$).

Classification of Carbon Atoms

• Certain functional groups behave differently based on the classification of the carbon atom they are bonded to.

• Primary ($1^\circ$): A carbon atom bonded to only one other carbon atom.

• Secondary ($2^\circ$): A carbon atom bonded to two other carbon atoms.

• Tertiary ($3^\circ$): A carbon atom bonded to three other carbon atoms.

• Quaternary ($4^\circ$): A carbon atom bonded to four other carbon atoms.

Summary of Essential Concepts

• Molecular Formula: Only shows the quantity of atoms, not their arrangement.

• Structural Formulae: (Expanded, Condensed, Skeletal, 3D) are used to visualize the connectivity and arrangement of atoms.

• Reactivity: Organic compounds react according to functional groups, which are the determining factors for the class of the compound.

• Trends: Homologous series share functional groups and demonstrate predictable trends in physical properties.

• Carbon Classification: Reactivity of functional groups can be influenced by whether the attached carbon is primary, secondary, or tertiary.