Welcome to the Alkenes Chapter!
Hello! This chapter is one of the most exciting parts of Organic Chemistry. Alkenes are the energetic, highly reactive cousins of the boring Alkanes. Why? Because of the double bond.
Alkenes are the crucial starting materials for thousands of useful products, including nearly all the plastics we use every day. Understanding their structure and unique reactions is key to mastering the rest of organic chemistry. Don't worry if mechanisms seem complicated at first; we will break them down step-by-step!
1. Structure, Bonding, and Reactivity (3.3.4.1)
1.1 What Defines an Alkene?
Alkenes are unsaturated hydrocarbons. The term unsaturated is key: it means they contain at least one Carbon-Carbon double bond (C=C), allowing them to add more atoms without losing existing ones.
- General Formula: For non-cyclic alkenes, the formula is \(C_nH_{2n}\).
- Nomenclature: Named using the IUPAC system, replacing the '-ane' ending of the alkane with '-ene'. The position of the double bond must be indicated (e.g., but-1-ene, but-2-ene).
1.2 The Anatomy of the Double Bond
The C=C double bond is composed of two different types of covalent bonds:
- Sigma (\(\sigma\)) Bond: Formed by the head-on overlap of orbitals. This is a strong bond and lies directly between the two carbon nuclei.
- Pi (\(\pi\)) Bond: Formed by the sideways overlap of the p-orbitals above and below the plane of the \(\sigma\) bond.
The Crucial Takeaway: Reactivity
The \(\pi\) electrons are located outside the C-C axis. This makes them exposed and less tightly held than \(\sigma\) electrons. This creates a region of high electron density right at the double bond.
- Because of this high electron density, alkenes are attacked by species that are seeking electrons. These species are called electrophiles (literally "electron lovers").
Quick Review: Bonding and Geometry
The presence of the C=C bond causes restricted rotation and affects the bond angles. Each carbon atom in the double bond has three bonding regions, leading to a trigonal planar shape and bond angles of approximately \(120^\circ\).
2. Addition Reactions of Alkenes (3.3.4.2)
The characteristic reaction of alkenes is electrophilic addition, where the double bond breaks and two new atoms or groups are added across the carbons.
2.1 Electrophilic Addition Explained
An electrophile is a species that can accept a pair of electrons (a Lewis acid). Since the double bond is a great source of electrons, it is the perfect target.
Key Reaction Example: Reaction with Bromine (\(Br_2\))
When bromine water (or dissolved bromine) is added to an alkene, the solution rapidly changes from orange-brown to colourless. This is the standard laboratory test for unsaturation.
Did you know? Even though the \(Br-Br\) bond is non-polar, as the \(Br_2\) molecule approaches the high electron density of the \(\pi\) bond, the electron cloud in \(Br_2\) is repelled, inducing a temporary dipole, making one Br atom temporarily positive (the electrophile).
The Electrophilic Addition Mechanism (General Steps)
We need to outline the mechanism for reactions with \(HBr\), \(H_2SO_4\), and \(Br_2\). All follow the same two core steps:
Step 1: Attack by the Alkene (Formation of the Carbocation)
The high electron density of the C=C double bond acts as a nucleophile (electron donor) and attacks the slightly positive or full positive charge on the electrophile (E-X). A pair of electrons from the double bond moves towards E (shown by a curly arrow starting from the bond).
The E-X bond breaks heterolytically (both electrons go to X), forming a positively charged intermediate called a carbocation (a carbon atom with only three bonds and a positive charge) and a negative ion (\(X^-\)).
Step 2: Attack by the Nucleophile
The positively charged carbocation intermediate is highly unstable and is immediately attacked by the negative ion (\(X^-\), the nucleophile).
A curly arrow shows the electron pair moving from the negative ion (nucleophile) to the positive carbon atom (carbocation). This completes the addition reaction, forming a stable product.
2.2 Specific Addition Reactions
Here are the specific addition reactions required by the syllabus:
1. Reaction with Hydrogen Bromide (HBr)
- Product: A halogenoalkane (bromoalkane).
- \(HBr\) is a permanent dipole, with H being the initial electrophile (\(H^\delta+\)).
2. Reaction with Concentrated Sulfuric Acid (\(H_2SO_4\))
- This is a two-step reaction often used to hydrate (add water to) alkenes indirectly.
- Step A (Addition): The alkene reacts with cold, concentrated \(H_2SO_4\). The \(H^+\) ion acts as the electrophile, forming an alkyl hydrogensulfate compound.
- Step B (Hydrolysis): The alkyl hydrogensulfate is then heated with water (hydrolysed) to produce an alcohol.
3. Reaction with Bromine (\(Br_2\))
- Product: A dibromoalkane (vicinal dihalide).
- Mechanism involves the induced dipole as described above.
Key Takeaway: All these addition reactions go via the same two-step mechanism: Electrophile attacks \(\pi\) bond \(\rightarrow\) Carbocation forms \(\rightarrow\) Nucleophile attacks Carbocation.
3. Carbocation Stability and Major/Minor Products
When an alkene is unsymmetrical (e.g., propene, where the two carbons in the double bond have different numbers of hydrogen atoms attached), the addition of an unsymmetrical reagent (like HBr) can lead to two possible products.
3.1 Markovnikov’s Rule (Predicting Products)
In the reaction of an unsymmetrical alkene with an unsymmetrical reagent (HX), the hydrogen atom adds to the carbon atom of the double bond that already has the greater number of hydrogen atoms.
This rule exists because the reaction proceeds through the most stable carbocation intermediate.
3.2 Carbocation Stability Explained
Carbocations are classified by how many alkyl groups are attached to the positive carbon:
- Primary (\(1^\circ\)): The positive carbon is attached to only one alkyl group.
- Secondary (\(2^\circ\)): The positive carbon is attached to two alkyl groups.
- Tertiary (\(3^\circ\)): The positive carbon is attached to three alkyl groups.
Stability Order: Tertiary (\(3^\circ\)) > Secondary (\(2^\circ\)) > Primary (\(1^\circ\))
Why this order? Alkyl groups are electron-donating. They help spread out (delocalise) the positive charge, which makes the ion more stable. The more alkyl groups, the more stable the ion.
In any addition reaction, the pathway that produces the most stable carbocation intermediate happens fastest and most often, leading to the major product.
Example: Addition of HBr to Propene (\(CH_2=CH-CH_3\))
HBr could add the H to C1 or C2:
- If H adds to C1: A Secondary Carbocation forms at C2. (More stable). This leads to 2-bromopropane (Major Product).
- If H adds to C2: A Primary Carbocation forms at C1. (Less stable). This leads to 1-bromopropane (Minor Product).
Key Takeaway: The major product always comes from the formation of the most stable (tertiary or secondary) carbocation intermediate.
4. Addition Polymers (3.3.4.3)
Alkenes are vitally important industrially because they can join together in huge numbers to form long chain molecules called addition polymers (plastics).
4.1 Formation and Structure
An addition polymer is formed when thousands of alkene monomers (the small starting units) react by opening their double bonds and linking up end-to-end.
- Monomer: The small molecule (always an alkene or substituted alkene). E.g., ethene.
- Polymer: The long chain molecule (e.g., poly(ethene)).
- Repeating Unit: The basic unit that repeats along the polymer chain. It is drawn by removing the double bond and showing extending bonds outside brackets.
Students should be able to:
- Draw the repeating unit from the monomer (remove the C=C, extend bonds).
- Draw the structure of the monomer from the repeating unit (add the C=C back).
- Apply IUPAC rules (e.g., poly(propene)).
4.2 Properties of Addition Polymers
1. Unreactivity
Addition polymers are chemically unreactive because the reactive C=C double bonds have all been converted into strong, non-polar, single C-C and C-H bonds. This stability means they are often resistant to chemical attack and non-biodegradable (a major environmental problem).
2. Intermolecular Forces
The long polymer chains are held together only by weak induced dipole-dipole forces (van der Waals/London forces).
The strength of these van der Waals forces depends on how closely the polymer chains pack together. If they pack tightly, the plastic is rigid (e.g., high-density poly(ethene)). If the packing is irregular, the plastic is softer and more flexible.
Example: Poly(chloroethene) (PVC)
PVC is naturally hard and brittle. To make it flexible (for use in tubing, insulation, etc.), substances called plasticisers are added. These small molecules get between the polymer chains, pushing them apart and weakening the van der Waals forces, making the material softer and easier to bend.
5. Epoxyethane (3.3.4.4)
Epoxyethane (also known as ethylene oxide) is a specific, highly important organic compound derived from ethene.
5.1 Production and Reactivity
Epoxyethane is produced by the partial oxidation of ethene, usually using silver as a catalyst at high temperatures.
Hazards: This is a hazardous industrial process, as epoxyethane itself is highly flammable and toxic.
High Reactivity: Epoxyethane has a three-membered ring containing two carbon atoms and one oxygen atom. This structure is severely strained (the C-C-O bond angles are forced far below the ideal tetrahedral \(109.5^\circ\)). This ring strain makes the molecule very reactive and easily opened by nucleophiles.
5.2 Reactions of Epoxyethane (Ring Opening)
The reaction mechanism involves nucleophilic attack, where the nucleophile attacks one of the carbons in the strained ring, causing the C-O bond to break and relieve the strain.
1. Reaction with Water (Hydrolysis)
Epoxyethane reacts with water (usually in the presence of an acid catalyst) to form ethane-1,2-diol (ethylene glycol).
Mechanism Outline (Nucleophilic Attack by Water):
- The epoxide ring is protonated (H+ attaches to the O atom), making the ring carbons even more susceptible to attack.
- A water molecule (\(H_2O\), the nucleophile) attacks one of the ring carbons.
- The strained C-O bond breaks, opening the ring.
- The resulting intermediate loses a proton to form the final product, ethane-1,2-diol.
Use: Ethane-1,2-diol is the main component of antifreeze, used to lower the freezing point of water in car radiators.
2. Reaction with Alcohols
Epoxyethane reacts with alcohols (e.g., ethanol) to form glycol ethers. The mechanism is similar to the reaction with water, but the alcohol molecule acts as the nucleophile.
Use: Glycol ethers are important commercial solvents and precursors for surfactants (molecules used in detergents and soaps that help oil and water mix, lowering surface tension).
5.3 Economic and Environmental Importance
The products from epoxyethane are economically vital:
- Antifreeze (Ethane-1,2-diol): Essential for vehicles in cold climates, preventing engine damage.
- Surfactants: Used extensively in cleaning products, paints, and emulsifiers.
Key Takeaway: Epoxyethane’s high reactivity is due to ring strain. This allows nucleophiles (like \(H_2O\) or alcohols) to easily break the ring open, leading to useful bifunctional products like antifreeze and surfactants.