Alkenes - 3.4.% - Detailed Notes - Structure and Reactivity - Electrophilic Addition - Addition Polymers

Oct 13, 2026

Upload by acoobca

Detailed notes on AQA Chemistry A-level topic 3.3.4.1–3.3.4.3 covering alkenes as unsaturated hydrocarbons with a C=C double bond and high electron density. Explains identification using bromine water, then describes electrophilic addition across the double bond via carbocation formation, including relative carbocation stability and major product prediction. Covers addition polymers formed by breaking the double bond into repeating units, how reaction conditions affect polymer chain structure, and uses/limitations such as non-biodegradability and PVC properties from plasticisers.

LanguageEnglish

CategoryGeneral

FormatPDF

Pages5

Reading Time2 min

AccessFree to download

Alkenes

3.3.4.1 - Structure and Reactivity

3.3.4.2 - Electrophilic Addition

3.3.4.3 - Addition Polymers

AQA Chemistry A-level 3.3.4: Alkenes Detailed Notes 3.3.4.1-Structure and Reactivity Alkenes are unsaturated hydrocarbons meaning they contain a carbon-carbon double bond. This is an area of high electron density making it susceptible to attack from electrophiles (species that are attracted to ∂-areas) . It consists of a normal covalent bond and a π bond. Example: Bromine water is used to identify this double bond and other unsaturated compounds. It turns the solution from orange-brown to colourless if a double bond is present in the substance. 3.3.4.2-Electrophilic Addition Alkenes undergo electrophilic addition about the double bond. Electrophiles These are electron acceptors and are attracted to areas of high electron density. Some of the most common electrophiles are: ● HBr ● Br2 ● H2SO4 They can be used to form alkyl hydrogensulphates or halogenoalkanes from alkenes. Electrophilic Addition This is the reaction mechanism that shows how electrophiles attack the double bond in alkenes. When the double bond is broken, a carbocation forms. This is a carbon atom with only three bonds , meaning it has a positive charge. Carbocations can have varying stability , with tertiary being the most stable and primary the least. The more stable the carbocation, the more likely it is to form. Therefore in an addition reaction, multiple products can form but the major product will always be the most stable possible. Mechanism-Halogenoalkanes The π bond causes the bromine molecule to gain a temporary dipole so that electrons are transferred. Mechanism-Sulphuric Acid Alkene reacts with sulphuric acid to produce ethyl hydrogensulphate. 3.3.4.3-Addition Polymers Addition polymers are produced from alkenes where the double bond is broken to form a repeating unit. Example: The repeating unit must always be shown with extended bonds through the brackets showing that it bonds to other repeating units on both sides. Reaction Conditions The reaction conditions used in the production of these polymer chains can be altered to give the plastics produced different properties. High pressures and temperatures produce branched chain polymers with weak intermolecular forces. Whereas lower pressures and temperatures produce straight chain polymers with strong intermolecular forces. Uses of Polymers Polymers are unreactive hydrocarbon chains with multiple strong, non-polar covalent bonds. This makes them useful for manufacturing many everyday plastic products such as shopping bags (poly(ethene)) . However, the unreactive nature of the bonds in addition polymers means they are not biodegradable and cannot be broken down by species in nature. PVC Poly(chloroethene) more commonly known as PVC is an addition polymer with waterproof properties. It gains these properties by the addition of plasticisers during the reaction.

FAQs

How can bromine water identify alkenes?

Bromine water is orange-brown initially. It turns colourless if a double bond is present in the substance, indicating an alkene (and other unsaturated compounds).

What happens during electrophilic addition to an alkene?

An electrophile attacks the C=C double bond and breaks the π bond, forming a carbocation intermediate. The relative stability of the carbocation determines which product is most likely to be major.

How do reaction conditions affect addition polymer properties?

Changing pressure and temperature alters the polymer chain structure. High pressures and temperatures produce more branched chains with weaker intermolecular forces, while lower pressures and temperatures produce straighter chains with stronger intermolecular forces.

Oct 13, 2026

Upload by acoobca