Basic Concepts of Organic Chemistry

Full Notes

Organic molecules can be written using different formula types and you need to be comfortable using and changing between the following:

Molecular Formula – Shows the actual number of atoms in a molecule.

ExampleHexane → C₆H₁₄

Empirical Formula – Shows the simplest whole number ratio of atoms in a compound.

Example Hexane (C₆H₁₄) → C₃H₇

General Formula – Represents a homologous series using an algebraic expression.

Example Alkanes → C_nH_2n+2

Structural Formula – Shows how atoms are arranged in a molecule without drawing bonds.

Example Hexane → CH₃CH₂CH₂CH₂CH₂CH₃

Displayed Formula – Shows all bonds and atoms.

Example Hexane

Skeletal Formula – Simplified representation where carbon atoms are at the ends and bends of lines, and hydrogens bonded to carbon atoms aren’t shown.

Example Hexane

Matt’s exam tip

Matt’s exam tip - if you find drawing skeletal formulas hard, start by drawing the displayed formula with a diagonal pattern between C-C bonds, then remove all the C-H bonds.

Homologous Series and Functional Groups

A homologous series is a family of organic compounds that:

• Have the same general formula.
• Contain the same functional group.
• Have similar chemical properties.
• Show a gradual change in physical properties (e.g., boiling points increase with chain length).

Examples of Homologous Series:

A functional group is an atom or group of atoms within a molecule that gives the molecule its reactivity and can determine its physical and chemical properties

IUPAC Naming Rules

IUPAC (International Union of Pure and Applied Chemistry) naming rules are used to help name compounds in chemistry, including:

• Chains and rings with up to six carbon atoms.
• Alkanes, alkenes, alcohols, aldehydes, ketones, carboxylic acids, esters, amines, nitriles, arenes.

Steps for Naming Organic Compounds:

1. Find the longest carbon chain
   Meth- (1C), Eth- (2C), Prop- (3C), But- (4C), Pent- (5C), Hex- (6C).
2. Identify the main functional group (determines the suffix - see table above).
3. Number the carbon chain to give the lowest possible numbers to functional groups.
4. Identify and name side chains (alkyl groups) as prefixes.
5. Use commas between numbers and hyphens between letters and numbers.

Example 2-Methylpentane (C₆H₁₄)

Longest chain = pentane, with a methyl group at carbon 2.

Organic Chemistry Terminology

• Homologous series: Compounds with same functional group and general formula, differing by CH₂ (e.g., alkanes).
• Functional group: Group responsible for reactions (e.g., –OH for alcohols).
• Alkyl group: Substituent from alkane by removing one H (e.g., CH₃–).
• Aliphatic: Straight/branched chains or non-aromatic rings.
• Alicyclic: Ring structures not aromatic (e.g., cyclopentane).
• Aromatic: Contains benzene ring/delocalised π-system.
• Saturated: Only single bonds.
• Unsaturated: One or more multiple bonds.

Predicting Formulas in Homologous Series

We can use the general formula to predict compounds in a homologous series.

Example Alkane

General formula = C_nH_2n+2. For n = 5 → C₅H₁₂.

Structural Isomerism

Structural isomers have the same molecular formula but different arrangements of atoms.

Example C₄H₁₀

Butane (CH₃CH₂CH₂CH₃) and 2-methylpropane (CH₃CH(CH₃)CH₃) are structural isomers as the have the same molecular formula (C₄H₁₀) however they have different structuress.

Bond Fission

Covalent bonds can break in two ways:

Homolytic fission

Each atom takes one electron, forming radicals.

Heterolytic fission

One atom takes both electrons, forming ions.

Radicals

Radicals are species with an unpaired electron, shown with a dot (•), formed by homolytic fission.

Radicals are highly reactive and can cause chain reactions.

Free radicals and their reactions are covered more here.

Curly Arrows and Reaction Mechanisms

Reactions in organic chemistry can be complex and occur in many steps. Curly arrow mechanisms are used to represent how each step occurs.

Curly arrows show how electrons move and start from a lone pair or bond and point to where a new bond forms.

Example Nucleophilic substitution

Curly arrow from nucleophile (Nu) to δ⁺ carbon shows bond forming. Arrow from C–X bond to halogen shows bond breaking.

Matt’s exam tip

Mechanisms help explain step-by-step how organic reactions can occur however they are only models that are predictions based on kinetic and energetic data. Always draw curly arrows neatly, include lone pairs and dipoles where relevant.

Summary

• IUPAC naming provides systematic names for organic compounds.
• Organic formulas include molecular, general, structural, displayed, and skeletal.
• Homologous series share functional groups and general formulas.
• Isomerism occurs when compounds share molecular formula but differ structurally.
• Bond fission can be homolytic or heterolytic.
• Radicals are unpaired electron species.
• Curly arrows show electron movement in mechanisms.