Welcome to the World of Organic Reactions!

In this chapter, we are going to explore how certain "families" of carbon-based molecules behave. We’ve already met alkanes, but now we are moving on to the more "exciting" relatives: alkenes, alcohols, and carboxylic acids.

Organic chemistry is simply the study of carbon compounds. It’s the chemistry of life, fuels, plastics, and even the "fizz" in your vinegar! Don't worry if some of the names sound like a foreign language at first—by the end of these notes, you’ll be speaking "Chemist" fluently.

1. Alkenes: The "Unsaturated" Hydrocarbons

Alkenes are a homologous series (a family) of hydrocarbons. The big difference between them and alkanes is that alkenes contain a double carbon-carbon bond (\(C=C\)).

Structure and Formula

  • Functional Group: The functional group is the part of the molecule that determines how it reacts. For alkenes, it is the \(C=C\) double bond.
  • Unsaturated: Because they have a double bond, they have two fewer hydrogen atoms than the equivalent alkane. We call them unsaturated because they have "space" to bond with more atoms if that double bond opens up.
  • General Formula: \(C_nH_{2n}\)

The First Four Alkenes

You need to know the names and structures of these four:

  1. Ethene: \(C_2H_4\)
  2. Propene: \(C_3H_6\)
  3. Butene: \(C_4H_8\)
  4. Pentene: \(C_5H_{10}\)

Analogy: Think of a double bond like a "double handshake" between two carbon atoms. It’s very strong, but one of those "hands" can be let go to grab onto something new!

Quick Review: Alkenes vs Alkanes

Alkanes = Single bonds only (\(C-C\)), Saturated (Full).
Alkenes = Double bond (\(C=C\)), Unsaturated (Not full).

Key Takeaway: Alkenes are defined by their \(C=C\) double bond and follow the formula \(C_nH_{2n}\).


2. Reactions of Alkenes

Alkenes are much more reactive than alkanes because of that double bond.

Combustion (Burning)

Alkenes react with oxygen just like other hydrocarbons. However, they tend to undergo incomplete combustion in the air. This results in a smoky yellow flame because there isn't enough oxygen to burn all the carbon completely.

Addition Reactions

This is where the magic happens! The double bond "opens up" to become a single bond, and new atoms join the carbon atoms. This is called an addition reaction.

  1. With Hydrogen (Hydrogenation): Adding hydrogen (\(H_2\)) to an alkene turns it back into an alkane.
    Example: \(Ethene + Hydrogen \rightarrow Ethane\)
  2. With Steam (Hydration): When alkenes react with steam (\(H_2O\)) in the presence of a catalyst, they produce alcohols.
    Example: \(Ethene + Water (steam) \rightarrow Ethanol\)
  3. With Halogens: Alkenes react quickly with chlorine, bromine, or iodine. The halogen atoms add across the double bond.
The Bromine Water Test (Important!)

This is how we tell the difference between an alkane and an alkene:

  • Add orange bromine water to the liquid.
  • If it stays orange, it’s an alkane (no reaction).
  • If it turns colourless, it’s an alkene (the bromine added to the double bond).

Did you know? This reaction is used in the food industry to measure how "unsaturated" vegetable oils are!

Key Takeaway: Alkenes react via addition—the double bond breaks and "grabs" new atoms like hydrogen, water, or halogens.


3. Alcohols

Alcohols are another family of organic compounds. They are not just for drinks; they are vital solvents and fuels!

Structure and Properties

  • Functional Group: The -OH group (hydroxyl group).
  • First Four: Methanol (\(CH_3OH\)), Ethanol (\(C_2H_5OH\)), Propanol (\(C_3H_7OH\)), and Butanol (\(C_4H_9OH\)).
  • Solubility: They dissolve in water to form a neutral solution (pH 7).

How do Alcohols React?

  1. With Sodium: They fizz and produce hydrogen gas.
  2. Combustion: They burn in air to produce carbon dioxide and water. They burn very cleanly, which is why ethanol is used as a biofuel.
  3. Oxidation: If you react an alcohol with an oxidising agent, it turns into a carboxylic acid. (This is what happens when wine is left open and turns into vinegar!)

Making Ethanol: Fermentation

Ethanol is made by fermentation of sugar solutions using yeast.

The perfect "recipe" for fermentation:

  • Sugars dissolved in water.
  • Yeast (the catalyst).
  • Warm temperature (about \(37^\circ C\)).
  • Anaerobic conditions (no oxygen).

Memory Aid: "Warm, Wet, and No Air!" If it’s too cold, yeast sleeps. If it’s too hot, the yeast enzymes denature (break).

Key Takeaway: Alcohols have the -OH group, can be oxidised to acids, and ethanol is made through the fermentation of sugar by yeast.


4. Carboxylic Acids

These are the "vinegar" family. They have the functional group -COOH.

Key Facts

  • The First Four: Methanoic acid, Ethanoic acid (vinegar), Propanoic acid, and Butanoic acid.
  • Reaction with Carbonates: Like any acid, they react with carbonates to produce carbon dioxide (fizzing), a salt, and water.
  • Weak Acids: (Higher Tier) They are called weak acids because they do not ionise completely in water. This means they have a higher pH than a strong acid (like HCl) of the same concentration.
Common Mistake to Avoid

Don't confuse the -OH in an alcohol with the \(OH^-\) (hydroxide ion) found in alkalis. Alcohols are not alkaline; they are neutral!

Key Takeaway: Carboxylic acids contain -COOH and behave like weak acids, reacting with carbonates to produce CO2 gas.


Summary Checklist

Quick Review Box:

  • Can you name the first four alkenes, alcohols, and acids?
  • Do you know the test for an alkene (Bromine water)?
  • Can you describe the conditions for fermentation?
  • Do you recognize the functional groups: \(C=C\) (alkene), -OH (alcohol), and -COOH (acid)?

Don't worry if this seems like a lot of names. Just remember: the functional group is the "personality" of the molecule—it tells you exactly how it's going to behave!