VCE Chemistry: Organic Chemistry Notes

Organic Compounds: Structure, Nomenclature, and Properties

7.1 Characteristics of the Carbon Atom

Diversity of Organic Compounds

Carbon's ability to form a vast array of organic compounds is attributed to:

• Valence Electron Number: Carbon has a valency of 4, enabling it to form four bonds.
• Relative Bond Strength: Carbon forms strong, stable bonds with itself and other elements.
• Relative Stability of Carbon Bonds: C-C, C-H, and C-halogen bonds are notably stable.
• Degree of Unsaturation: Carbon can form single, double, and triple bonds, leading to diverse structures.
• Formation of Structural Isomers: The same atoms can be arranged in different ways, resulting in isomers.

Hydrocarbons

• Composed predominantly of carbon and hydrogen atoms, sometimes with oxygen and nitrogen.
• Crucial in fuels (like petrol) and industrial applications.

Homologous Series

• Series of molecules that differ by the addition of carbon atoms ( CH_2 groups).
• Exhibit similar chemical and physical properties and share the same general formula.

Carbon Bonding

• Carbon typically forms four bonds.
• Saturated hydrocarbons contain only C-C single bonds.
• Unsaturated hydrocarbons contain at least one C=C or C≡C bond.

Bond Strengths

• C-C bonds are the strongest, followed by C-H bonds and then C-halogen bonds.

7.2 & 7.3 Formulas and Structures of Organic Compounds

Representations of Organic Compounds

• Structural Formula: Shows all bonds and the entire compound.
  • Example: Butane
• Semi-structural Formula: A condensed formula representing the structure to some extent.
  • Example: Butane: CH3CH2CH2CH3
  • Note: Bonded groups like -CH_3 are represented in brackets.
• Skeletal Formula: Carbon backbone is represented as a line, with hydrogen atoms omitted.
  • Example: Butane
• Molecular Formula: Shows the distinct atoms and their numbers in the molecule.
  • Example: Butane: C4H{10}

Homologous Series Examples:

• Alkanes (including cyclohexane)
• Alkenes
• Benzene
• Haloalkanes
• Primary Amines
• Primary Amides
• Alcohols (primary, secondary, and tertiary)
• Aldehydes
• Ketones
• Carboxylic Acids
• Non-branched Esters

IUPAC Nomenclature (Systematic Naming)

• Applies to organic compounds up to C8, with no more than two functional groups for a molecule.
• Limited to non-cyclic hydrocarbons, haloalkanes, primary amines, alcohols, aldehydes, ketones, carboxylic acids, and non-branched esters.

Components of IUPAC Name

• Word Root: Indicates the number of carbon atoms in the longest continuous chain containing the functional group.
  • If two chains are identical in length, the chain with fewer substituents is chosen.
• Suffixes:
  • Primary Suffix: Indicates saturation (-ane, -ene, -yne).
  • Secondary Suffix: Indicates the highest priority functional group (-ol, -amide, -oic acid).
• Prefixes:
  • Primary Prefix: Indicates lower priority functional groups (hydroxy, amino, methyl).
    • Ordered alphabetically, not by number (e.g., 3-iodo-2-methyldecanoic acid).

Nomenclature priorities for functional groups

• Alkyl and halogen groups are always treated as secondary functional groups (prefixes).
• Alphabetical order determines priority between alkyl and halogen groups (chloro > methyl).

Alkyl: -CnH{2n+1}

Halogen: -C-F/Br/Cl-/I-

Key Notations

• "Di," "tri," and "tetra" are used for multiple identical constituents (e.g., dimethyl).
• The carbon number for each constituent should be mentioned (e.g., 3-methyl, 4,5-dimethyl).
  • Omit number if it is superfluous.
• Higher priority groups get the lower number, but the longest carbon chain must be chosen.
• Hyphens are used between numbers and letters; no space between words.
• The "e" of "ane/ene/yne" can sometimes be omitted (prop-3-enoic acid vs. methanetetramine).

Naming Esters

• Named in two parts:
  • Main chain prefix + "anoate" (at the end).
  • Secondary part prefix + "yl" (at the front).
  • Example: ethyl butanoate.

7.4 Physical Properties and Trends

Intermolecular Bonding

• Bonding between covalently bonded molecules.
• Types (in descending order of strength):

Hydrogen Bond

• Requirements:
  • Hydrogen attached to a highly electronegative atom (N, O, F).
  • Lone pair of electrons on a small, highly electronegative atom (N, O, F).

Dipole-Dipole Bond

• Occurs between molecules with different partial polarities.

Dispersion Force

• Weakest type, found in all organic compounds, most important in non-polar compounds.
• Forms from instantaneous dipoles.

Boiling Point

• Depends on the amount of uniform and strong intermolecular bonding.

• Uniformity (packing ability): Butane has a higher boiling point than methyl propane due to better packing.

• Bond Strength:

  • Higher molar mass or electron-saturated compounds have higher boiling points due to increased dispersion forces.
  • Adding -CH_3 groups increases boiling point due to increased dispersion forces.

• General Trend (functional groups only, carbon chain length constant):

Amide >> Acid ≈ Alcohol >> Ketone > Aldehyde = Amine > Ester > Ether >> Alkane > Alkene

• Relate boiling/melting point to the energy required to break bonds.
  • “Hence, considering the dense packing and higher intermolecular attraction, more energy will be required to break these intermolecular bonds giving the molecule a high boiling point/melting point.”

Viscosity

• Effective thickness or resistance to flow.
• Factors:
  • Stronger intermolecular forces increase viscosity.
  • Longer chain lengths increase the probability of tangling, increasing viscosity.

Solubility

• Assess whether molecule A can bond with molecule B (miscibility).
• If the solvent is water, determine if molecule A can bond with water.
• Smaller molecules are more easily able to bond to the solvent, improving solubility.

Checklist for Properties-Based Questions

1. All types of intermolecular attraction.
2. Polarity and electronegativity.
3. Formation of resonant structures like dimers (in carboxylic acids).
4. Side groups/effects on packing.
5. Size and bonds with solvent (for solubility).

Keywords in Organic Chemistry

1. Electronegative
2. Polar
3. Hydrogen Bonds
4. Dipole-dipole bonds
5. Instantaneous dipoles
6. Vapour
7. Viscosity
8. Packing closely/tightly
9. Boiling/melting point
10. Dimers
11. Molar mass
12. Ion-dipole forces
13. Intermolecular/intramolecular forces AND size
14. Relate bond-breaking to energy (KEY)
15. Dipole-moment
16. Interaction with water molecules for solubility

Isomers and Isomerism

Structural Isomers

• Atoms are attached to different atoms, and configurations of atoms are changing.
  • Chain isomers
  • Functional isomers
  • Positional isomers

Chain Isomers

• Switching of alkyl groups.

Functional Isomers

• Interchanging of different functional groups.
  • Ether for alcohol
  • Esters for carboxylic acid
  • Aldehyde and ketone

Positional Isomers

• Attachment of the same functional group to different carbons in the chain.

Checklist for Isomerism

1. Structural isomers
   • Chain
   • Functional
     • Ether for alcohol
     • Esters for carboxylic acid
     • Aldehyde and esters
   • Positional

• If two molecules have the same name, they are the same molecule (not isomers).