Welcome to the World of Organic Chemistry!
Hello future chemists! This chapter is your foundation stone for all of Organic Chemistry. Learning formulas and naming conventions might seem like memorizing rules, but think of it as learning the alphabet and grammar of a powerful new language.
Once you master these rules, you'll be able to look at a complex chemical name and instantly visualize the molecule—and predict its behaviour! Let's break down the essential terminology and the systematic way we communicate the structure of organic compounds (using C and H, and sometimes O, N, or X atoms).
1. Representing Organic Molecules: The Different Formulas
We have several ways to write down an organic molecule, and you must be able to recognize and deduce all of them (Syllabus 13.1, LO 4, 6).
1.1 Formula Types Based on Composition (The "What's in it?")
1. Molecular Formula (MF)
This shows the actual number of atoms of each element in a molecule.
Example: Ethane contains two carbon atoms and six hydrogen atoms. The MF is \(C_2H_6\).
2. Empirical Formula (EF)
This shows the simplest whole-number ratio of atoms in a compound.
Example: For Ethane (\(C_2H_6\)), the ratio is 2:6, which simplifies to 1:3. The EF is \(CH_3\).
Analogy: If the molecular formula is the *actual recipe* (2 eggs, 6 spoons flour), the empirical formula is the *simplified ratio* (1 egg, 3 spoons flour).
3. General Formula
A general algebraic expression that represents any member of a particular homologous series (group of related compounds).
Example: Alkanes have the general formula \(C_nH_{2n+2}\).
1.2 Formula Types Based on Structure (The "How is it arranged?")
4. Structural Formula
This shows the minimal detail necessary to indicate the arrangement of atoms, grouping the atoms attached to each carbon atom.
Example: Ethanol: \(CH_3CH_2OH\). Propane: \(CH_3CH_2CH_3\).
5. Displayed Formula
Shows all atoms and all bonds (including C-H and C-C bonds). This is the most detailed representation.
You must be able to draw these accurately, ensuring carbon forms exactly four bonds.
6. Skeletal Formula (The Shorthand)
This is the simplest, quickest way to draw complex molecules.
- Carbon atoms are represented by the vertices (corners) and the ends of lines.
- Hydrogen atoms attached to carbons are generally omitted (assumed to be there to complete carbon's four bonds).
- Functional groups and atoms other than C and H (like O, N, X, etc.) must be shown explicitly.
Master the conversions: The molecular formula tells you the quantity; the structural/displayed/skeletal formulas tell you the arrangement. Skeletal formulas are essential A-Level shorthand!
2. Homologous Series and Functional Groups
Organic chemistry deals with millions of compounds, but thankfully, they fall into distinct families called homologous series.
2.1 Defining the Families (Homologous Series) (13.1, LO 1a)
A homologous series is a family of organic compounds that:
- Have the same General Formula (e.g., alkenes are \(C_nH_{2n}\)).
- Share similar chemical properties because they have the same functional group.
- Show a gradual change in physical properties (like melting/boiling point) as the chain length increases.
- Each successive member differs by a \(CH_2\) unit.
What is a Hydrocarbon? (13.1, LO 1)
A compound made up of Carbon (C) and Hydrogen (H) atoms only.
Examples: Alkanes (\(C_nH_{2n+2}\)) and Alkenes (\(C_nH_{2n}\)).
2.2 The Reactive Part: Functional Groups (13.1, LO 3)
A functional group is the specific atom or group of atoms within a molecule that determines its characteristic chemical and physical properties (Syllabus 13.1, LO 3).
This is the most important concept! All compounds in the same series behave chemically almost identically because they have the same functional group.
2.3 Essential Functional Groups (Syllabus Tables: Pages 29, 30, 47)
| Homologous Series | Functional Group Name | Structural Formula | Characteristic Suffix/Prefix |
|---|---|---|---|
| Alkane (13.1, LO 2) | (None - saturated hydrocarbon) | C-C (single bonds only) | -ane |
| Alkene | C=C bond | R-C=C-R' | -ene |
| Halogenoalkane | Halogen (X) | R-X (X = F, Cl, Br, I) | Prefix: chloro-, bromo-, etc. |
| Alcohol (Hydroxy compound) | Hydroxyl | R-OH | -ol |
| Aldehyde (Carbonyl compound) | Carbonyl (\(C=O\) at end) | R-CHO | -al |
| Ketone (Carbonyl compound) | Carbonyl (\(C=O\) in middle) | R-CO-R' | -one |
| Carboxylic Acid | Carboxyl | R-COOH | -oic acid |
| Ester | Ester linkage | R-COO-R' | -oate |
| Primary Amine | Amine | R-\(NH_2\) | Prefix: amino- or Suffix: -amine |
| Nitrile | Nitrile | R-C\(\equiv\)N | -nitrile |
| Acyl Chloride (A-Level) | Acyl chloride | R-COCl | -oyl chloride |
| Amide (Primary, A-Level) | Amide | R-CON\(H_2\) | -amide |
| Amino Acid (A-Level) | Amine + Carboxyl | -\(NH_2\) and -COOH | amino- and -oic acid |
The group $C=O$ is the carbonyl group. If it is at the end of the chain, it's an aldehyde. If it is in the middle, it's a ketone.
3. Systematic Nomenclature (IUPAC Naming)
IUPAC stands for the International Union of Pure and Applied Chemistry. This is the worldwide standardized system we use to ensure every unique molecule has one unique name (and vice versa).
Crucial Scope Limitation (13.1, LO 5): At AS/A Level, you are required to systematically name simple aliphatic organic molecules (straight chains) with functional groups up to six carbon atoms (for esters and amides, $6+6$ carbons). Cyclic compounds are also included where specified (e.g., in isomerism topics).
3.1 Step 1: Find the Longest Carbon Chain (The Root)
The root name indicates the number of carbon atoms in the longest continuous chain that includes the functional group.
| Number of Carbons (n) | Root Name |
|---|---|
| 1 | Meth- |
| 2 | Eth- |
| 3 | Prop- |
| 4 | But- |
| 5 | Pent- |
| 6 | Hex- |
3.2 Step 2: Identify the Functional Group (The Suffix)
This determines the primary ending of the name (as listed in Section 2.3).
- If the functional group is an alkene (\(C=C\)), the 'an' changes to '-en' (e.g., Ethane becomes Ethene).
- If it's an alcohol, the 'e' of the alkane name is replaced by '-ol' (e.g., Propane becomes Propanol).
3.3 Step 3: Numbering the Chain (Giving Position)
The chain must be numbered so that the main functional group gets the lowest possible number.
- For molecules like alcohols or alkenes, you must include the number of the carbon where the group starts.
Example: A 3-carbon chain with the OH on the middle carbon is Propan-2-ol. - For functional groups that are always at the end (like carboxylic acids -COOH or aldehydes -CHO), the carbon of the functional group is always numbered C1, and the number is often omitted from the name.
Example: \(CH_3CH_2COOH\) is Propanoic acid (C3 chain, C1 is the carboxyl carbon).
3.4 Step 4: Adding Substituents (The Prefixes)
If there are side chains (alkyl groups R) or halogen atoms (X), these are named as prefixes.
- Identify the side chain (e.g., \(CH_3\) is methyl, \(C_2H_5\) is ethyl).
- Number the position of the side chain (e.g., $CH_3$ on C2).
- If multiple identical side chains exist, use prefixes di-, tri-, tetra-.
- List substituents in alphabetical order (ignoring di/tri prefixes).
Step-by-Step Example: Naming an Alcohol
Consider $CH_3CH(OH)CH_2CH_3$:
- Root: Longest chain is 4 carbons $\rightarrow$ Butane.
- Suffix: Functional group is -OH $\rightarrow$ -ol.
- Numbering: We must number to give the -OH the lowest number. Numbering left-to-right gives position 2 (Butan-2-ol); right-to-left gives position 3. We choose 2.
- Name: Butan-2-ol.
When you have multiple functional groups (like an alcohol and an alkene), the group with the highest priority determines the suffix (e.g., -ol is higher than -ene). You must use the IUPAC priority table provided in data booklets, but for AS/A Level, typically Carboxylic Acids (-oic acid) have the highest priority, followed by other oxygen-containing groups, then C=C.
3.5 Specific Naming Rules for Esters (R-COO-R')
Esters are named in two parts:
- The alcohol part (R') is named first as an alkyl group (e.g., methyl, ethyl). This is the part attached to the single-bonded oxygen.
- The carboxylic acid part (R-CO) is named second, changing the -oic acid suffix to -oate.
4. Saturated vs. Unsaturated Compounds
These terms are fundamental for describing hydrocarbons (13.2, LO 1b).
Saturated
An organic compound is saturated if it contains only single bonds between carbon atoms. It cannot hold any more hydrogen atoms.
Example: Alkanes (\(C_2H_6\)) and Alcohols (\(C_2H_5OH\)).
Unsaturated
An organic compound is unsaturated if it contains double or triple bonds between carbon atoms (C=C, C\(\equiv\)C, C\(\equiv\)N). These bonds can be broken to add more atoms.
Example: Alkenes (\(C_2H_4\)) and Alkynes.
The classic chemical test for unsaturation (\(C=C\) bond) is adding aqueous bromine. If the orange/brown colour of the bromine water is decolourised rapidly, the compound is unsaturated.
5. Aromatic Compounds (Arenes) (A-Level 29.1)
Once you transition fully into A-Level organic chemistry, you must be able to name compounds containing the benzene ring (the arene homologous series).
5.1 The Benzene Ring
Benzene (\(C_6H_6\)) is a six-carbon ring structure with alternating single and double bonds, resulting in a delocalised \(\pi\) system (discussed in detail in Section 29.3).
- When a benzene ring is attached to a main chain, it is often referred to as a phenyl group.
- When naming benzene derivatives, the ring is the primary component.
5.2 Naming Substituted Benzene Rings
When naming aromatic compounds, numbering is used to indicate the position of substituents on the ring, aiming for the lowest possible combination of numbers.
Required Complex Examples (29.1, LO 4):
-
3-nitrobenzoic acid:
The main functional group is the carboxylic acid (-COOH). We assign the carbon attached to -COOH as C1. The nitro group ($NO_2$) is located at C3. -
2,4,6-tribromophenol:
The main functional group is the hydroxyl group (-OH) attached to the ring (making it a phenol). We assign the carbon attached to -OH as C1. The three bromine atoms (bromo-) are located at positions 2, 4, and 6.
Never forget the numbers! Butene could be But-1-ene or But-2-ene. If you don't include the number, the structure is ambiguous, and you will lose marks! Always include the position of the functional group unless it must be C1 by definition.
Summary: Key Takeaways
- Formulas: Know the difference between molecular (\(C_2H_6\)), empirical (\(CH_3\)), structural (\(CH_3CH_2OH\)), displayed (all bonds), and skeletal (shorthand).
- Families: A homologous series has the same functional group, determining the chemical properties.
- Functional Groups: The $C=C$ in alkenes, the $-OH$ in alcohols, and the $C=O$ in carbonyls are the reactive sites.
- Nomenclature: Follow the four steps: Root (chain length) $\rightarrow$ Suffix (functional group) $\rightarrow$ Numbering (lowest number) $\rightarrow$ Prefix (substituents).
You've successfully covered the grammar of organic chemistry! Now you're ready to explore the exciting reactions these groups undergo. Keep practicing drawing and naming structures—it gets easier with every molecule!