Alkenes (as exemplified by ethene)

Welcome to the World of Alkenes!

Hello there! Today, we are diving into one of the most exciting groups in organic chemistry: Alkenes. While they might just seem like molecules with double bonds, they are the reason we have everything from plastic bags to the butter-alternative on your toast. We will focus on the simplest alkene, ethene, to help you master the basics. Don't worry if organic chemistry feels like a different language right now—we'll break it down step-by-step!

Quick Review: Before we start, remember that hydrocarbons are molecules made only of carbon and hydrogen. Alkenes are a special type of hydrocarbon called unsaturated because they contain at least one carbon-carbon double bond \( (C=C) \).

1. Structure and Bonding: The "Secret Sauce" of Alkenes

The most famous alkene you need to know is ethene \( (C_2H_4) \). Its structure determines how it behaves.

The Double Bond (\( \sigma \) and \( \pi \) bonds)

In ethene, the two carbon atoms are held together by two different types of "glue":

1. The Sigma (\( \sigma \)) bond: This is a strong, single bond formed by the head-on overlap of orbitals. Think of this as a firm handshake between two carbons.

2. The Pi (\( \pi \)) bond: This is formed by the sideways overlap of p-orbitals. It creates a cloud of electron density above and below the plane of the molecule. Think of this as two people holding hands while also trying to balance a tray above their heads and one at their feet!

Shape and Angles

Because of this arrangement, ethene is trigonal planar. All the atoms lie on a flat sheet of paper. The bond angles are approximately \( 120^\circ \).

Memory Aid: Think of a Mercedes-Benz logo or a fidget spinner. The three "arms" coming off each carbon atom are spread out as far as possible, which is \( 120^\circ \).

Quick Review Box:
• General Formula: \( C_nH_{2n} \)
• Shape: Trigonal Planar
• Bond Angle: \( 120^\circ \)
• Bonding: 1 \( \sigma \) bond + 1 \( \pi \) bond

Key Takeaway: The \( \pi \) bond is weaker than the \( \sigma \) bond and is "exposed" above and below the molecule, making it a prime target for other chemicals to attack!

2. Cis-Trans Isomerism: The "Locked" Molecule

In alkanes (single bonds), the atoms can spin around freely like a wheel on an axle. However, in alkenes, the \( \pi \) bond prevents rotation. It "locks" the molecule in place.

What is Cis-Trans Isomerism?

If the two carbon atoms in the double bond are attached to different groups, we get two versions (isomers):

• Cis-isomer: The "same" groups are on the same side of the double bond.
• Trans-isomer: The "same" groups are on opposite sides (across from each other).

Analogy: Imagine two people sitting in a car. If they are both in the front seats, they are cis. If one is in the driver's seat and the other is in the back-left seat, they are trans.

Did you know? This simple difference is why "trans-fats" are bad for your health. Your body has enzymes designed to process "cis" fats (like those in olive oil), but it struggles to "unlock" the shape of trans-fats!

Key Takeaway: Cis-trans isomerism exists because the \( \pi \) bond restricts rotation around the \( C=C \) bond.

3. Chemical Reactions of Ethene

Alkenes are much more reactive than alkanes. Their main trick is the Addition Reaction. Because the \( \pi \) bond is relatively weak, it can "pop open" to let other atoms join the carbon chain.

A. Combustion (Burning)

Like all hydrocarbons, ethene burns in oxygen to produce carbon dioxide and water.

\( C_2H_4 + 3O_2 \rightarrow 2CO_2 + 2H_2O \)

Note: Alkenes tend to burn with a sootier flame than alkanes because they have a higher carbon-to-hydrogen ratio.

B. Addition of Hydrogen (Hydrogenation)

We can turn ethene (unsaturated) back into ethane (saturated) by adding hydrogen gas.

Reagents: \( H_2(g) \)
Condition: Nickel (Ni) catalyst and heat.
Equation: \( C_2H_4 + H_2 \rightarrow C_2H_6 \)

C. Addition of Bromine (Bromination)

This is the most important test you need to know for the lab!

Reagents: \( Br_2 \) in an organic solvent (like \( CCl_4 \)).
Observation: The reddish-brown color of bromine decolourises (turns colorless) immediately.
Equation: \( C_2H_4 + Br_2 \rightarrow C_2H_4Br_2 \) (1,2-dibromoethane)

Common Mistake to Avoid: Don't say the solution becomes "clear." In chemistry, "clear" just means you can see through it (like apple juice). Say it becomes colorless (like water).

Key Takeaway: Bromine decolourisation is the standard test to prove a substance is an alkene (unsaturated).

4. Polymers and the Environment

Since this chapter is part of the Polymers section, we must look at what happens when many ethene molecules join together. This is called Addition Polymerisation.

From Ethene to Poly(ethene)

Under high pressure and with a catalyst, the double bonds of thousands of ethene molecules "pop open" and link up like a long chain of people holding hands. This creates poly(ethene), commonly known as polythene (plastic).

Why are they a problem?

The resulting poly(alkenes) are made of very strong \( C-C \) and \( C-H \) single bonds. This makes them chemically inert (unreactive).
• Pros: They don't react with the food or chemicals stored inside them.
• Cons: They are non-biodegradable. Bacteria in nature don't have the "tools" to break those strong bonds down, so plastic stays in landfills for hundreds of years.

Quick Review:
• Addition Polymer: A long chain molecule formed by joining many short chain monomers (like ethene) without losing any atoms.
• Inertness: Alkenes are reactive, but Poly(alkenes) are very unreactive and difficult to biodegrade.

Key Takeaway: While ethene is reactive due to its double bond, the plastic made from it is stubbornly unreactive, leading to environmental challenges that require recycling and sustainable management.

Final Summary Checklist

• Can you describe the \( \sigma \) and \( \pi \) bonds in ethene? (Yes/No)
• Do you know why the shape is trigonal planar with \( 120^\circ \) angles? (Yes/No)
• Can you explain why cis-trans isomerism happens? (Hint: Restricted rotation!) (Yes/No)
• Can you state the reagents and observations for the test for unsaturation (\( Br_2 \))? (Yes/No)
• Can you explain why plastics (polyalkenes) are bad for the environment? (Yes/No)

Keep practicing! Organic chemistry is all about patterns. Once you see how the double bond "pops open," you'll be able to predict reactions for any alkene that comes your way!