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.
- 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.
- 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
```
- 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.
- 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
| Compound |
Functional Group |
General Formula |
Example Formula |
Example Semi-Structural Formula |
Example Skeletal Formula |
| Alkane |
None |
\(C_nH_{2n+2}\) |
Butane |
\(CH_3CH_2CH_2CH_3\) |
/\/\ |
| Alkene |
C=C |
\(C_nH_{2n}\) |
But-1-ene |
\(CH_3CH_2CH=CH_2\) |
/\= |
| Benzene |
Aromatic Ring |
\(C_6H_6\) |
Benzene |
N/A |
Hexagon with circle inside |
| Haloalkane |
-X (Halogen) |
\(RX\) |
Chloroethane |
\(CH_3CH_2Cl\) |
/-Cl |
| Primary Amine |
-\(NH_2\) |
\(RNH_2\) |
Ethanamine |
\(CH_3CH_2NH_2\) |
/-\(NH_2\) |
| Primary Amide |
-\(CONH_2\) |
\(RCONH_2\) |
Ethanamide |
\(CH_3CONH_2\) |
/-C=O-NH2 |
| Alcohol |
-OH |
\(ROH\) |
Ethanol |
\(CH_3CH_2OH\) |
/-OH |
| Aldehyde |
-CHO |
\(RCHO\) |
Ethanal |
\(CH_3CHO\) |
/\=O-H |
| Ketone |
C=O |
\(RCOR'\) |
Propanone |
\(CH_3COCH_3\) |
/\=O/\ |
| Carboxylic Acid |
-COOH |
\(RCOOH\) |
Ethanoic Acid |
\(CH_3COOH\) |
/-C=O-OH |
| Ester |
-COOR’ |
\(RCOOR'\) |
Methyl Ethanoate |
\(CH_3COOCH_3\) |
/-C=O-O/\ |
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.
