Organic Reactions: Your Guide to Making and Breaking Molecules!
Hey everyone! Welcome to one of the most fascinating topics in Chemistry: organic reactions. Think of yourself as a molecular architect or a chef. You'll learn how to take simple carbon compounds and transform them into new, exciting substances by swapping atoms, adding new ones, or even breaking molecules apart. This is the heart of how we make medicines, plastics, and even the flavourings in your favourite snacks!
Don't worry if it seems like a lot at first. We'll break down each type of reaction step-by-step with simple explanations and real-world examples. Let's get started!
Substitution Reactions: The Great Swap!
Imagine you're at a dance, and two partners decide to swap. That's exactly what a substitution reaction is! One atom or group of atoms on a molecule is replaced by another atom or group.
1. Substitution in Alkanes (Free-Radical Substitution)
Alkanes are known as saturated hydrocarbons because all their carbon atoms are bonded with the maximum number of hydrogen atoms—they're "full"! This means you can't just add more atoms to them. Instead, you have to swap one out.
• What you need: An alkane (like methane, CH₄) and a halogen (like chlorine, Cl₂).
• The special condition: This reaction is picky! It won't happen in the dark. It needs ultraviolet (UV) light to kick things off. This light provides the energy to start the reaction.
• What happens: A chlorine atom swaps with a hydrogen atom on the methane molecule.
The Equation:
Methane + Chlorine → Chloromethane + Hydrogen Chloride
$$CH_4(g) + Cl_2(g) \xrightarrow{UV\ light} CH_3Cl(g) + HCl(g)$$Did you know? This reaction can continue, swapping more hydrogens for chlorines to form dichloromethane (CH₂Cl₂), trichloromethane (CHCl₃, also known as chloroform), and eventually tetrachloromethane (CCl₄). But for your syllabus, knowing the first substitution is the key!
2. Substitution in Haloalkanes
A haloalkane is just an alkane that already has a halogen atom (like Cl, Br, I) attached. We can swap that halogen for a different group, like the hydroxyl group (-OH) to make an alcohol!
• What you need: A haloalkane (like chloroethane, CH₃CH₂Cl) and an aqueous alkali (like sodium hydroxide solution, NaOH(aq)).
• The special condition: You need to gently heat the mixture.
• What happens: The hydroxide ion (OH⁻) from the NaOH swaps with the chlorine atom on the chloroethane.
The Equation:
Chloroethane + Sodium Hydroxide → Ethanol + Sodium Chloride
$$CH_3CH_2Cl(aq) + NaOH(aq) \xrightarrow{Heat} CH_3CH_2OH(aq) + NaCl(aq)$$This is a super useful reaction for turning one type of organic compound (a haloalkane) into another (an alcohol).
Key Takeaway: Substitution
• In Alkanes: Swap H for a Halogen. Condition: UV light.
• In Haloalkanes: Swap Halogen for an -OH group. Condition: Aqueous alkali (e.g. NaOH(aq)) and heat.
Addition Reactions: Opening the Gates!
Unlike alkanes, alkenes have a carbon-carbon double bond (C=C). This double bond is like a closed gate that can swing open to let new atoms "add on". Alkenes are unsaturated because they can hold more atoms.
In an addition reaction, the double bond breaks, and atoms are added across the two carbon atoms. Nothing is removed!
1. Adding Hydrogen (Hydrogenation)
This turns an alkene into an alkane.
• What you need: An alkene (like ethene, CH₂=CH₂) and hydrogen gas (H₂).
• The special condition: A nickel (Ni) catalyst and heat (around 150°C).
$$CH_2=CH_2(g) + H_2(g) \xrightarrow{Ni\ catalyst,\ Heat} CH_3-CH_3(g)$$Ethene becomes Ethane.
Real-world link: This exact reaction is used to turn liquid vegetable oils (which are unsaturated) into solid margarine (which is more saturated)!
2. Adding a Halogen (Halogenation)
This is the classic test for unsaturation!
• What you need: An alkene (like ethene) and a halogen, usually bromine (Br₂) dissolved in an organic solvent or water (known as bromine water).
• The special condition: None needed! This happens easily at room temperature.
$$CH_2=CH_2(g) + Br_2(aq) \rightarrow CH_2Br-CH_2Br(aq)$$Ethene becomes 1,2-dibromoethane.
The Observation: When you bubble an alkene through reddish-brown bromine water, the colour disappears! This is because the bromine is used up in the addition reaction. Alkanes won't do this. This is how you can tell them apart!
3. Adding a Hydrogen Halide (Hydrohalogenation)
• What you need: An alkene and a hydrogen halide gas (like hydrogen bromide, HBr).
• The special condition: None needed.
$$CH_2=CH_2(g) + HBr(g) \rightarrow CH_3-CH_2Br(g)$$Ethene becomes bromoethane.
Key Takeaway: Addition
• What reacts? Alkenes (compounds with C=C double bonds).
• What happens? The double bond breaks open and new atoms are added.
• The big test: Alkenes decolourise reddish-brown bromine water. Alkanes do not.
Oxidation Reactions: Gaining Oxygen, Losing Hydrogen
In organic chemistry, oxidation often means gaining oxygen atoms or losing hydrogen atoms. A common oxidising agent you need to know is acidified potassium dichromate(VI) solution (K₂Cr₂O₇ / H⁺).
Memory Aid: A key observation is the colour change of the oxidising agent. Acidified dichromate(VI) solution is orange. When it successfully oxidises something, the chromium ions are reduced, and the solution turns green. So, Orange → Green means oxidation happened!
Oxidation of Alcohols
What happens when you oxidise an alcohol depends on what type it is!
1. Primary (1°) Alcohols: The carbon with the -OH group is attached to only ONE other carbon atom (e.g., ethanol).
• They can be oxidised TWICE!
• Step 1 (gentle oxidation): Oxidise to an aldehyde. To get the aldehyde, you must distil it off as it forms, otherwise it will oxidise further.
Ethanol + [O] → Ethanal + Water
• Step 2 (strong oxidation): Oxidise all the way to a carboxylic acid. This happens if you heat the mixture under reflux (so nothing escapes).
Ethanol + 2[O] → Ethanoic Acid + Water
2. Secondary (2°) Alcohols: The carbon with the -OH group is attached to TWO other carbon atoms (e.g., propan-2-ol).
• They can be oxidised ONCE to form a ketone.
Propan-2-ol + [O] → Propanone + Water
• Ketones are tough and cannot be easily oxidised further with this reagent.
3. Tertiary (3°) Alcohols: The carbon with the -OH group is attached to THREE other carbon atoms (e.g., 2-methylpropan-2-ol).
• They cannot be oxidised by acidified potassium dichromate(VI). The orange colour will remain orange.
This difference in oxidation is another great way to distinguish between the types of alcohols!
Key Takeaway: Oxidation
• Reagent: Acidified potassium dichromate(VI) (K₂Cr₂O₇ / H⁺).
• Observation: Colour change from Orange → Green.
• Primary Alcohol → Aldehyde → Carboxylic Acid
• Secondary Alcohol → Ketone
• Tertiary Alcohol → No reaction
Esterification and Hydrolysis: Making and Breaking Esters
1. Esterification: Making Fruity Smells!
Esters are compounds known for their pleasant, fruity smells. They are responsible for the natural fragrances of fruits like bananas, pineapples, and apples. We can make them in the lab!
An esterification reaction is when a carboxylic acid reacts with an alcohol to form an ester and water.
• What you need: A carboxylic acid + an alcohol.
• The special condition: A few drops of concentrated sulphuric acid (H₂SO₄) as a catalyst, and you need to heat the mixture (often in a warm water bath).
General Equation:
$$R-COOH + R'-OH \rightleftharpoons R-COO-R' + H_2O$$Carboxylic Acid + Alcohol ⇌ Ester + Water
Example: Making Ethyl Ethanoate (smells like pear drops or nail polish remover)
Ethanoic Acid + Ethanol ⇌ Ethyl Ethanoate + Water
$$CH_3COOH(l) + CH_3CH_2OH(l) \rightleftharpoons CH_3COOCH_2CH_3(l) + H_2O(l)$$Naming Esters: The name comes in two parts. The first part is from the alcohol (e.g., Ethanol → Ethyl). The second part is from the carboxylic acid (e.g., Ethanoic Acid → Ethanoate). Put them together: Ethyl ethanoate.
2. Hydrolysis: Breaking Esters Apart
Hydrolysis literally means "splitting with water" (hydro-lysis). It's the reverse of esterification. You can break an ester back down into its original carboxylic acid and alcohol.
a) Acidic Hydrolysis
• This is the exact reverse of making an ester.
• Conditions: Heat the ester with a dilute acid (like dilute H₂SO₄).
• It's a reversible reaction, so you get a mixture of reactants and products at equilibrium.
$$CH_3COOCH_2CH_3(l) + H_2O(l) \rightleftharpoons CH_3COOH(aq) + CH_3CH_2OH(aq)$$b) Alkaline Hydrolysis
• This is a more effective way to break down an ester.
• Conditions: Heat the ester with an alkali (like sodium hydroxide, NaOH).
• It's an irreversible reaction. It goes to completion.
• Instead of getting a carboxylic acid, you get its salt (a carboxylate salt).
$$CH_3COOCH_2CH_3(l) + NaOH(aq) \rightarrow CH_3COONa(aq) + CH_3CH_2OH(aq)$$Ethyl Ethanoate → Sodium Ethanoate + Ethanol
Real-world link: Alkaline hydrolysis of large, naturally occurring esters (fats and oils) is called saponification. It's how soap is made!
Key Takeaway: Esters
• Making them (Esterification): Carboxylic Acid + Alcohol. Condition: conc. H₂SO₄ catalyst, heat.
• Breaking them (Hydrolysis): Add water. Can be done with dilute acid (reversible) or dilute alkali (irreversible).
The Big Picture: Inter-conversions
The best part about these reactions is that they all connect! You can use them to build a "reaction map" to get from one type of compound to another. This is the foundation of organic synthesis.
Let's plan a synthesis route!
Challenge: How can you make ethyl ethanoate (an ester) starting from only ethene (an alkene)?
Our Plan:
1. We need an alcohol (ethanol) and a carboxylic acid (ethanoic acid) to make our ester.
2. How can we make ethanol from ethene? Through an addition reaction with steam!
Step 1: Ethene → Ethanol
$$CH_2=CH_2(g) + H_2O(g) \xrightarrow{H_3PO_4\ catalyst,\ High\ T\ \&\ P} CH_3CH_2OH(g)$$3. Great, we have our alcohol! Now, how can we make ethanoic acid? We can make it by oxidising the ethanol we just made!
Step 2: Ethanol → Ethanoic Acid
$$CH_3CH_2OH(aq) + 2[O] \xrightarrow{K_2Cr_2O_7/H^+,\ reflux} CH_3COOH(aq) + H_2O(l)$$4. Perfect! We have both ingredients. Now we just react them together in an esterification reaction.
Step 3: Ethanol + Ethanoic Acid → Ethyl Ethanoate
$$CH_3CH_2OH(l) + CH_3COOH(l) \rightleftharpoons CH_3COOCH_2CH_3(l) + H_2O(l)$$(Don't forget the conc. H₂SO₄ catalyst and heat!)
Success! By linking three different types of reactions, we've completed a multi-step synthesis. You are now officially a molecular architect!