Formulae, functional groups and terminology
Definition: A displayed formula shows all the atoms and all the bonds in a molecule. It provides a complete visual map of the structure.
Key Rules for Drawing:
- Carbon (C) always forms 4 bonds. If you see a carbon with fewer than 4 lines attached, it is incorrect.
- Hydrogen (H) always forms 1 bond.
- Oxygen (O) typically forms 2 bonds.
- You must draw every single bond line explicitly. Do not use condensed notation here.
Definition: A structural formula is an unambiguous description of the way the atoms in a molecule are arranged, showing how they are connected but not necessarily all bonds (unlike displayed formula).
How to recognize it:
- It groups atoms together (e.g., CH_3, OH) rather than drawing every bond.
- It shows the order of atoms clearly.
- Examples: CH_2=CH_2, CH_3CH_2OH, CH_3COOCH_3.
| Formula Type | Description |
|---|---|
| Displayed Formula | Shows every atom and every bond. (e.g., Ethene: H_2C=CH_2 drawn with lines) |
| Structural Formula | Shows arrangement/grouping but omits some bonds. (e.g., Ethene: CH_2=CH_2) |
| Molecular Formula | Shows only the number of each atom type. (e.g., Ethene: C_2H_4) |
Note on Structural Isomers: Compounds with the same molecular formula but different structural formulae are called structural isomers. They have the same atoms but arranged differently.
Example: C_4H_{10} can be:
- CH_3CH_2CH_2CH_3 (straight chain)
- CH_3CH(CH_3)CH_3 (branched chain)
The Correct Understanding: Carbon is in Group 4 and has 4 valence electrons. It must form exactly 4 covalent bonds to be stable. If you draw a structure where a carbon has 5 lines attached, or only 3, the structure is chemically impossible.
Why examiners accept this: They are checking for correct valency. Every carbon must have 4 bonds, every hydrogen 1 bond.
Correct Usage Example:
Question: Draw the displayed formula of ethene (C_2H_4).
Answer: You must draw two carbon atoms connected by a double bond (C=C), and each carbon must have two single bonds to hydrogen atoms. Do not write CH_2=CH_2; you must draw the lines.
Definition: A homologous series is a family of similar compounds with similar chemical properties due to the presence of the same functional group.
General Characteristics (Must know all 5):
- Same functional group: All members have the same reactive part.
- Same general formula: They follow a mathematical pattern (e.g., C_nH_{2n+2}).
- Differ by a –CH₂– unit: Consecutive members differ by one carbon and two hydrogens (CH_2).
- Trend in physical properties: Properties like boiling point change predictably as chain length increases.
- Similar chemical properties: They undergo the same types of reactions.
Unsaturated Compound: A compound in which one or more carbon–carbon bonds are not single bonds (i.e., they contain double or triple bonds). (e.g., Alkenes).
Note: The term 'unsaturated' specifically refers to the carbon-carbon bonding, not combustion.
| Homologous Series | General Formula | Functional Group |
|---|---|---|
| Alkanes | C_nH_{2n+2} | None (only C-C single bonds) |
| Alkenes | C_nH_{2n} | Carbon-carbon double bond (C=C) |
| Alcohols | C_nH_{2n+1}OH | Hydroxyl group (-OH) |
| Carboxylic Acids | C_nH_{2n+1}COOH | Carboxyl group (-COOH) |
Understanding Carboxylic Acid Formula:
In the general formula C_nH_{2n+1}COOH, n represents the number of carbon atoms in the alkyl chain (the tail). The COOH part contains one additional carbon atom.
- Total carbons = n + 1
- Example: If n=1, the formula is CH_3COOH (ethanoic acid), which has 2 total carbons.
The Correct Understanding:
- An alcohol has an -OH group attached to a carbon chain (e.g., R-OH).
- A carboxylic acid has the -COOH group, where the carbon is double-bonded to one oxygen and single-bonded to an -OH. The presence of the C=O bond next to the OH makes it a carboxylic acid, not an alcohol.
Also: Do not confuse 'unsaturated' (carbon-carbon double bonds) with 'incomplete combustion'. They are unrelated concepts.
Why examiners accept this: They want to see that you understand members must share the same functional group and thus have similar chemical properties. They also expect you to note they differ by CH_2.
Correct Usage Example:
Question: State two characteristics of a homologous series.
Answer: 'Members have the same general formula' AND 'Members have similar chemical properties due to the same functional group.' (Do not just say 'they are similar'; specify why).