Organic Formulas: Molecular, Structural, Semi-Structural, and Skeletal

1. Introduction to Organic Formulas

Organic chemistry is the study of carbon-containing compounds. Representing these compounds requires different types of formulas, each with varying levels of detail. The main types are molecular, structural, semi-structural (condensed), and skeletal formulas.

2. Types of Organic Formulas

2.1. Molecular Formula

• Shows the type and number of each atom in a molecule.
• Does not show the arrangement of atoms.
• Example: Butane is $C_4H_{10}$.

KEY TAKEAWAY: Molecular formulas provide the simplest representation, but lack information about bonding.

2.2. Structural Formula

• Shows all atoms and bonds in a molecule.
• Provides a complete representation of the molecular structure.
• Can be time-consuming to draw for large molecules.

• Example: Butane

  ```
  H H H H

  H - C - C - C - C - H

  H H H H
  ```

2.3. Semi-Structural (Condensed) Formula

• A simplified version of the structural formula.
• Shows atoms grouped together, indicating their connectivity.
• Easier and faster to write than structural formulas.
• Example: Butane is $CH_3CH_2CH_2CH_3$.

EXAM TIP: Pay attention to the order of atoms in semi-structural formulas to accurately represent the functional group.

2.4. Skeletal Formula (Line-Angle Formula)

• The most simplified representation.
• Carbon atoms are represented by the end of a line or the intersection of two lines.
• Hydrogen atoms attached to carbon are not shown.
• All heteroatoms (atoms other than C and H) are shown.

• Example: Butane

  /\/\

COMMON MISTAKE: Forgetting to include heteroatoms and their attached hydrogens in skeletal formulas.

3. Representing Different Organic Compounds

3.1. Alkanes

• Definition: Saturated hydrocarbons containing only single bonds.
• General Formula: $C_nH_{2n+2}$
• Examples:
  • Methane ($CH_4$): $CH_4$
  • Ethane ($C_2H_6$): $CH_3CH_3$
  • Propane ($C_3H_8$): $CH_3CH_2CH_3$
  • Butane ($C_4H_{10}$): $CH_3CH_2CH_2CH_3$ or /\/\

3.2. Alkenes

• Definition: Unsaturated hydrocarbons containing at least one carbon-carbon double bond.
• General Formula: $C_nH_{2n}$
• Examples:
  • Ethene ($C_2H_4$): $CH_2=CH_2$
  • Propene ($C_3H_6$): $CH_3CH=CH_2$
  • But-1-ene ($C_4H_8$): $CH_3CH_2CH=CH_2$ or /\=

3.3. Benzene

• Definition: Cyclic unsaturated hydrocarbon with alternating single and double bonds.
• Formula: $C_6H_6$

• Representations:

  • Structural: A six-membered ring with alternating single and double bonds.
  • Skeletal: A hexagon with a circle inside.

3.4. Haloalkanes (Alkyl Halides)

• Definition: Alkanes where one or more hydrogen atoms are replaced by halogen atoms (F, Cl, Br, I).
• Examples:
  • Chloromethane ($CH_3Cl$): $CH_3Cl$
  • Bromoethane ($C_2H_5Br$): $CH_3CH_2Br$
  • 2-Chloropropane ($C_3H_7Cl$): $CH_3CHClCH_3$ or Cl/\

3.5. Primary Amines

• Definition: Organic compounds containing an amino group ($NH_2$) bonded to a primary carbon atom.
• Examples:
  • Methanamine ($CH_3NH_2$): $CH_3NH_2$
  • Ethanamine ($C_2H_5NH_2$): $CH_3CH_2NH_2$ or /-NH2

3.6. Primary Amides

• Definition: Organic compounds containing an amide group ($CONH_2$) bonded to a primary carbon atom.
• Examples:
  • Methanamide (formamide) ($HCONH_2$): $HCONH_2$
  • Ethanamide (acetamide) ($CH_3CONH_2$): $CH_3CONH_2$ or /-C=O-NH2

3.7. Alcohols

• Definition: Organic compounds containing a hydroxyl group (-OH).
• Types:
  • Primary (1°): The carbon atom bonded to the -OH group is attached to one other carbon atom.
  • Secondary (2°): The carbon atom bonded to the -OH group is attached to two other carbon atoms.
  • Tertiary (3°): The carbon atom bonded to the -OH group is attached to three other carbon atoms.
• Examples:
  • Ethanol (1°): $CH_3CH_2OH$ or /-OH
  • Propan-2-ol (2°): $CH_3CH(OH)CH_3$
  • 2-Methylpropan-2-ol (3°): $(CH_3)_3COH$

3.8. Aldehydes

• Definition: Organic compounds containing a carbonyl group ($C=O$) at the end of the carbon chain, bonded to at least one hydrogen atom.
• General Formula: $RCHO$
• Examples:
  • Methanal (formaldehyde) ($HCHO$): $HCHO$
  • Ethanal (acetaldehyde) ($CH_3CHO$): $CH_3CHO$ or /\=O-H

3.9. Ketones

• Definition: Organic compounds containing a carbonyl group ($C=O$) bonded to two other carbon atoms.
• General Formula: $RCOR’$
• Examples:
  • Propanone (acetone) ($CH_3COCH_3$): $CH_3COCH_3$ or /\=O/\
  • Butanone ($CH_3CH_2COCH_3$): $CH_3CH_2COCH_3$

3.10. Carboxylic Acids

• Definition: Organic compounds containing a carboxyl group ($COOH$).
• General Formula: $RCOOH$
• Examples:
  • Methanoic acid (formic acid) ($HCOOH$): $HCOOH$
  • Ethanoic acid (acetic acid) ($CH_3COOH$): $CH_3COOH$ or /-C=O-OH

3.11. Esters

• Definition: Organic compounds formed by the reaction of a carboxylic acid and an alcohol. Contain the ester linkage ($COO$).
• General Formula: $RCOOR’$
• Examples:
  • Methyl ethanoate ($CH_3COOCH_3$): $CH_3COOCH_3$ or /-C=O-O/\
  • Ethyl methanoate ($HCOOCH_2CH_3$): $HCOOCH_2CH_3$

4. Summary Table

STUDY HINT: Create flashcards with the name of the compound on one side and its functional group, general formula, and example formulas on the other.

5. Cycloalkanes

• Alkanes in a ring structure.
• General formula: $C_nH_{2n}$
• Example: Cyclohexane ($C_6H_{12}$)

VCAA FOCUS: VCAA often tests the ability to convert between different types of formulas, so practice this skill extensively.