Chemistry (9202) | Organic compounds – their structure and reactions

Welcome to Organic Chemistry! Understanding Carbon's Amazing World

Hello future chemists! Get ready to explore the exciting world of Organic Chemistry. This branch of science focuses almost entirely on compounds containing the element carbon. Why is carbon so special? Because it has an amazing ability to bond with itself and form massive, complicated structures—from the fuel in your car to the DNA in your body!

Don't worry if this chapter seems tricky at first. We will break down these complex molecules into simple families and predictable reactions. Let's start building!

Key Concept 1: The Magic of Carbon (Catenation)

Carbon atoms have four outer electrons, meaning they can form four strong covalent bonds. This allows carbon to link together in long chains, branched structures, or rings.

• Catenation: This is the term for carbon's ability to form bonds with other carbon atoms, creating a backbone for huge molecules. Think of it like a giant set of LEGO bricks that can connect endlessly.
• Hydrocarbons: These are the simplest organic compounds, made up of only hydrogen (H) and carbon (C) atoms.

Section 1: Families and Functional Groups

In organic chemistry, we group similar compounds into 'families' called homologous series. This makes studying them much easier!

What is a Homologous Series?

A homologous series is a group of compounds that share three key characteristics:

1. They share the same general formula (e.g., Alkanes are \(C_nH_{2n+2}\)).
2. They have the same functional group (the part of the molecule that determines its chemical properties).
3. Each member differs from the next by a single \(CH_2\) unit.

Analogy: Think of a family. All members share the same surname (functional group) and genetic characteristics (chemical properties), but they get gradually older (increasing number of carbon atoms).

Naming Organic Compounds (Nomenclature)

The name of an organic compound tells you how many carbon atoms are in its longest chain. You must memorize the prefixes for the first four:

• Meth-: 1 Carbon
• Eth-: 2 Carbons
• Prop-: 3 Carbons
• But-: 4 Carbons
• (Then Pent-, Hex-, etc.)

Memory Aid: Many Elephants Prefer Bananas!

Section 2: Alkanes – The Saturated Family

Alkanes are the simplest and least reactive type of hydrocarbon.

Structure and Formula

• Saturated: Alkanes are called saturated because all carbon bonds are single covalent bonds. The carbon atoms are "full" of hydrogen atoms and cannot hold any more.
• General Formula: \(C_nH_{2n+2}\)
• Naming: They all end in -ane (e.g., Methane, Propane).

Examples:

• Methane (\(CH_4\)) - Used in heating.
• Butane (\(C_4H_{10}\)) - Used in lighter fuel.

Reactions of Alkanes

1. Complete Combustion

Alkanes burn cleanly in a plentiful supply of oxygen, producing carbon dioxide and water. This is why they are great fuels.

\(\text{Alkane} + \text{Oxygen} \rightarrow \text{Carbon Dioxide} + \text{Water}\)

Example (Methane): \(\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O}\)

2. Incomplete Combustion

If the oxygen supply is limited, burning produces poisonous carbon monoxide (\(CO\)) and/or soot (C).

3. Substitution Reaction with Halogens

Alkanes do not react easily. However, if mixed with a halogen (like chlorine or bromine) and exposed to ultraviolet (UV) light (or high heat), one hydrogen atom can be swapped out (substituted) for a halogen atom.

\(\text{CH}_4 + \text{Cl}_2 \xrightarrow{UV\ light} \text{CH}_3\text{Cl} + \text{HCl}\)

Key Takeaway for Alkanes: They are saturated (single bonds), use the general formula \(C_nH_{2n+2}\), and their main reactions are burning (combustion) and substitution (needs UV light).

Section 3: Alkenes – The Unsaturated Family

Alkenes are much more reactive than alkanes because of their special feature: the double bond.

Structure and Formula

• Unsaturated: Alkenes are unsaturated because they contain at least one carbon-carbon double bond (\(C=C\)). This double bond means they can still accept more atoms.
• General Formula: \(C_nH_{2n}\) (Note: Alkenes must have at least 2 carbon atoms, so \(n \ge 2\)).
• Naming: They all end in -ene (e.g., Ethene, Propene).

Structural Isomers

Don't worry if this sounds complicated—it's just a different way to draw the same molecule!

• Isomers: Compounds that have the same molecular formula but different structural formulas (different arrangements of atoms).
• Example: Butane (\(C_4H_{10}\)) can exist as a straight chain (butane) or a branched chain (2-methylpropane, also called isobutane). They have the same atoms but different physical properties (like boiling point).

Reactions of Alkenes: Addition Reactions

The double bond is a highly reactive spot. In addition reactions, the double bond "breaks open" and two new atoms (or groups of atoms) attach themselves to the carbons, making the molecule saturated.

1. Hydrogenation (Adding Hydrogen)

Adding hydrogen across the double bond converts an alkene into an alkane. This process requires a catalyst (like nickel) and heat.

\(\text{Ethene} + \text{Hydrogen} \rightarrow \text{Ethane}\)

2. Halogenation (Adding Halogens) – The Test for Unsaturation

This is a very important test! Alkenes react rapidly with bromine water (a reddish-brown liquid). The bromine adds across the double bond, causing the orange/brown colour to instantly disappear (decolourise).

• Alkenes: Instantaneously decolourise bromine water. (Addition reaction)
• Alkanes: Do not react with bromine water in the dark. (No reaction)

3. Hydration (Adding Steam)

Alkenes react with steam (\(H_2O\)) at high temperatures and pressures, using a catalyst (like phosphoric acid), to produce alcohols.

\(\text{Ethene} + \text{Steam} \rightarrow \text{Ethanol}\)

Key Takeaway for Alkenes: They are unsaturated (double bonds), use the general formula \(C_nH_{2n}\), and undergo fast addition reactions, which makes them highly reactive (and easily identified by the bromine water test).

Section 4: Alcohols (Focus on Ethanol)

Alcohols belong to a homologous series containing the hydroxyl functional group (-OH). The general formula is \(C_nH_{2n+1}OH\).

Ethanol (\(C_2H_5OH\))

Ethanol is the alcohol found in alcoholic drinks, and it is widely used as a solvent and a biofuel.

Manufacturing Ethanol

There are two main methods for making ethanol, each used for different purposes:

Method A: Fermentation (The Natural Way)

• Process: Yeast (a biological catalyst) breaks down sugars (e.g., glucose, \(C_6H_{12}O_6\)) into ethanol and carbon dioxide.
• Conditions: Needs warm temperatures (\(30^{\circ}C - 40^{\circ}C\)) and anaerobic conditions (no air/oxygen).
• Uses: Used for alcoholic beverages; slower process; results in dilute solution.
• \(\text{C}_6\text{H}_{12}\text{O}_6 (aq) \xrightarrow{yeast} 2\text{C}_2\text{H}_5\text{OH} (aq) + 2\text{CO}_2 (g)\)

Method B: Hydration of Ethene (The Industrial Way)

• Process: Ethene (an alkene) reacts with steam (water) via an addition reaction.
• Conditions: High temperature, high pressure, and a catalyst (phosphoric acid).
• Uses: Used for industrial solvents and fuel; very fast, continuous process; results in a pure product.
• \(\text{C}_2\text{H}_4 (g) + \text{H}_2\text{O} (g) \xrightarrow{catalyst} \text{C}_2\text{H}_5\text{OH} (g)\)

Reactions of Ethanol

Ethanol is highly flammable and undergoes complete combustion easily, making it a viable fuel source (biofuel).

\(\text{C}_2\text{H}_5\text{OH} + 3\text{O}_2 \rightarrow 2\text{CO}_2 + 3\text{H}_2\text{O}\)

Section 5: Carboxylic Acids (Focus on Ethanoic Acid)

Carboxylic acids are organic compounds containing the carboxyl functional group (-COOH).

Ethanoic Acid (\(CH_3COOH\))

Ethanoic acid is the main component of vinegar (diluted solution).

Preparation of Ethanoic Acid (Oxidation)

Ethanoic acid can be made by oxidising ethanol. This can happen in two ways:

1. Using Chemical Oxidising Agents: Heating ethanol with an oxidising agent, such as acidified potassium manganate(VII) or acidified potassium dichromate(VI).
2. Microbial Oxidation: Leaving ethanol exposed to air where bacteria can use oxygen to oxidise it (this is how wine turns to vinegar if left uncorked).

\(\text{Ethanol} + \text{Oxygen} \xrightarrow{Oxidising\ Agent} \text{Ethanoic Acid} + \text{Water}\)

Properties of Carboxylic Acids

Carboxylic acids are weak acids.

• Weak Acid: When dissolved in water, they partially ionise (dissociate), meaning they only release a small concentration of \(H^+\) ions.
• Contrast: Strong acids (like HCl) completely ionise. Because ethanoic acid releases fewer \(H^+\) ions, it is less corrosive than strong acids at the same concentration.
• As acids, they react normally with metals, carbonates, and bases.
  Example: \(\text{Ethanoic Acid} + \text{Sodium Carbonate} \rightarrow \text{Sodium Ethanoate} + \text{Water} + \text{CO}_2\)

Section 6: Esters

Esters are another type of organic compound famous for their pleasant, fruity smells and are used in flavourings and perfumes.

Esterification

Esters are formed when a carboxylic acid reacts with an alcohol. This process is called esterification.

• Reagents: Carboxylic Acid + Alcohol
• Conditions: Requires a strong acid catalyst (usually concentrated sulfuric acid) and heating.
• Product: Ester + Water

Did you know? The smell of bananas comes partly from the ester isoamyl acetate, and the smell of apples comes from methyl butyrate.

You’ve covered the core structure and reactions of the four major organic families! Keep practicing those general formulas and reactions, and you’ll master this section!