Alkenes PDF - AS Chemistry 3.3.4

AS CHEMISTRY 3.3.4 ALKENES ALKENES OVERVIEW DESCRIPTION General CnH2n (where one C=C is present in each molecule) Formula Descr...

AS CHEMISTRY 3.3.4 ALKENES ALKENES OVERVIEW DESCRIPTION General CnH2n (where one C=C is present in each molecule) Formula Description Are unsaturated as they contain a C=C bond Are non-polar, but the C=C is an area of high negativity Polarity as it contain 2 pairs of bonding electrons Electrophiles are attracted to the C=C Reactions - Electrophilic Addition Solubility in Insoluble. Non-polar alkanes are not miscible with polar Water H2O. Increase with increased chain length / Mr. Larger molecules have more electrons involved in the Melting & induced-dipole IMF’s making them stronger, so more Boiling Points energy is required to break them. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.3.4 ALKENES STRUCTURE, BONDING & REACTIVITY Alkenes are highly reactive due to the C=C. You need to understand how this bond is formed and why it makes the molecule so reactive. The two bonds in the double bond are not identical as they are formed by the overlap of different electron orbitals. Formed from the overlap of two ’s’ orbitals directly between the two + ))))))) ((((((( + Bond 1 nuclei. This is known as a sigma (𝜎) bond and is very strong. Formed from the overlap of two ’p’ orbitals. They overlap above & below the two nuclei. ) ))) ) ))) ))) ))) Bond 2 + + This is known as a pi (∏) bond and is weaker than the sigma bond. This ))) ) ))) ))) ))) ) the the bond that breaks when alkenes react. H H Since the p orbitals overlap above and below the C atoms, the electrons create an area of high C C electron density. This attracts electrophiles. H H AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.3.4 ALKENES ELECTROPHILIC ADDITION Alkenes undergo electrophilic addition reactions. There are FIVE that you need to know. 1. + HBr at room temperature δ+ δ- Electrophile: H—Br The Hδ+ is the electrophile. Alkene to Halogenoalkane H H H H H H C C H C C H H C C H + H H H H Br Hδ+ Br - δ- Br HINTS | TIPS | HACKS Remember, curly arrows show the movement of a pair of electrons. That is why the first curly arrow comes from the double bond. The pair of electrons from the pi bond form a bond between a C atoms and the H from HBr. The intermediate is known as a carbocation (a positively charged hydrocarbon). This reaction forms a mono-substituted halogenoalkane. 1 HBr molecule reacts with 1 C=C. The ratio is 1:1. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.3.4 ALKENES 2. + Br2 at room temperature δ+ δ- Electrophile: Br—Br As the non-polar Br2 molecule approaches the area of high electron density of the C=C, a dipole is induced. Alkene to Halogenoalkane H H H H H H C C H C C H H C C H + H H H H Br Brδ+ - Br Br δ- HINTS | TIPS | HACKS This mechanism is identical to the one for HBr, except that Br2 has its dipole induced by the C=C. HBr is permanently polar. The intermediate is known as a carbocation (a positively charged hydrocarbon). This reaction forms a multi-substituted halogenoalkane (2 Br atoms bond). 1 Br2 molecule reacts with 1 C=C. The ratio is 1:1. CHEMICAL TEST FOR ALKENES This reaction serves as the chemical test for alkenes. Reagent: Bromine water Br2(aq) Colour Change: Orange to Colourless Bromine water added drop by drop to alkene. The alkene decolourises the bromine water. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.3.4 ALKENES 3. + H2SO4 at room temperature δ+ Electrophile: δ- H Alkene to Alkyl Hydrogen Sulfate O HO S O O H H H H H H C C H C C H H C C H + H δ+ H H H O Br δ- H O– O HO S O HO S O HO S O O O O ethyl hydrogen sulfate PRODUCING ALCOHOLS If the alkyl hydrogen sulfate is added to water it is converted to an alcohol, and the H2SO4 is regenerated. e.g. ethyl hydrogen + H2O Ethene + H2SO4 Ethanol + H2SO4 sulfate H2SO4 is regenerated, making it a catalyst for the reaction. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.3.4 ALKENES 4. + H2O / dilute H3PO4 Catalyst + Reflux (AKA Hydration) Electrophile: H+ H+ is provided by the acid to start the reaction. H2O then hydrates the molecule. H+ is regenerated. Alkene to Alcohol H H H H H H H H C C H C C H H C C H H C C H + H H H H +O H H OH + O H H H + H+ H 5. + H2 / Ni / Pt Catalyst + 150oC (AKA Hydrogenation) Alkene to Alkane H H H H C C + H2 H C C H H H H H You do not need to know the mechanism for hydrogenation. This is an important reaction as it is used in the food industry to turn unsaturated fats into saturated fats. Electrophilic Addition Reactions 1, 3, 4 & 5 are all subject to “Markovnikov’s Rule”. In other words, if the alkene is asymmetrical, it produces two different isomers! See next page. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.3.4 ALKENES MARKOVNIKOV’S RULE If an asymmetrical alkene is reacted with HX (e.g. HBr) by electrophilic addition, two different structural positional isomers are produced. More of one isomer is produced, known as the major product. The other isomer that you get less of is known as the minor product. e.g. propene + HBr (follow the H) H H H C C C H H δ+ H H Brδ- Primary (1o) Carbocation Secondary (2o) Carbocation Intermediate Intermediate H H H H H H OR H C C C H H C C C H + + H H H H + Br- + Br- H H H H H H H C C C H H C C C H Br H H H Br H 1-bromopropane 2-bromopropane MINOR PRODUCT MAJOR PRODUCT The relative stabilities of the carbonation intermediates dictate which is the major and which is the minor product. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.3.4 ALKENES A tertiary (3o) carbocation is more stable than a secondary (2o), which is more stable than a primary (1o). The more stable a carbocation intermediate is, the more likely the reaction will take that path. R H H R C R R C R H C R + + + 3o carbocation 2o carbocation 1o carbocation 0 H atoms bonded 1 H atom bonded 2 H atoms bonded Most Stable Least Stable How to Explain Major & Minor Products AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.3.4 ALKENES ADDITION POLYMERS Polymers (AKA plastics) are produced from organic molecules. “Poly” means “many” as polymers consist of very long chains that produced by linking “many” smaller molecules together, end to end. We can make different polymers by linking different smaller molecules together. This gives them different properties! e.g. plastic for plastic bags, appliances, toys, packaging etc. Addition polymers are produced using alkenes. e.g. poly(ethene) and poly(propene) These alkene molecules (monomers) react and link together to make long, saturated chains of repeating units. The reaction has no byproducts so it has 100% atom economy. H H H H n C C C C H H H H n n = the number of monomers used to make Monomer = ethene 1 repeating unit of the polymer. poly(ethene) So, if “n” number of monomers are used, the polymer chain contains “n” repeating units. H H H CH3 H n C C C H C C H H H H n Monomer = propene 1 repeating unit of poly(propene) AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.3.4 ALKENES You need to be able to How to Draw Monomers draw the repeating unit for & Repeating Units an alkene monomer and draw the alkene monomer for a repeating unit. PROPERTIES OF ADDITION POLYMERS Addition polymers are very unreactive. This is because they are non-polar, saturated molecules. They are made up of long chains of covalently bonded carbon atoms that are difficult to break. Neither electrophiles or nucleophiles are attracted to them. This means that they are non-biodegradable. They have weak induced-dipole (van der Waal’s) forces between the chains, which gives them low melting points. The properties of addition polymers can be changed by using substituted alkenes as monomers. e.g. P.V.C. poly(vinylchloride) AKA poly(chloroethane) H Cl H Cl n C C C C H H H H n The presence of Cl atoms on the sides of the chain means that stronger permanent-dipole forces are present between the chains. This gives it a high melting point and makes it rigid and waterproof. This makes it suitable for making window frames and guttering etc. By using a plasticiser, we can also make PVC flexible. This also makes it suitable for making flooring, electrical wiring coverings and even clothes! AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.3.5 ALKENES E / Z ISOMERISM IN ALKENES E / Z isomerism is a form of Stereoisomerism that is specific to alkenes. E / Z isomers have the same structural formula, but have groups that occupy different relative positions on space. In order for an organic molecules to show E /Z isomerism, the molecule must have the following: 1. A C=C group (alkene) 2. Two different groups bonded to each carbon atom in the C=C BOND ROTATION The pi (∏) bond in the C=C makes the bond non-rotational (doesn’t spin). e.g. but-2-ene H 3C CH3 H 3C H C C is different to C C H H H CH3 Molecular formula: C4H8 C4H8 Structural formula: CH3CHCHCH3 CH3CHCHCH3 You can see that they both have the same molecular and structural formula. However, the relative positions of the -CH3 groups are different. H 3C CH3 H 3C H C C C C H H H CH3 In the left isomer, they are on the same side of the C=C. In the right, they are on opposite sides of the C=C. Since the C=C is non-rotational, they are fixed in these relative positions. This makes the molecules different! AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.3.5 ALKENES NAMING E / Z ISOMERS The letters E and Z are used to name these isomers. In order to decide which is which, we need to follow the Cahn-Ingold Priority Rules. 1. Look at the atomic number of each atom that is directly bonded to each C in the C=C. H 3C CH3 2. The atom with the highest atomic number C C bonded to each C in the C=C takes priority. e.g. C takes priority over H H H 3. Look at where the priority groups are in relation to each other… H 3C H H 3C CH3 C C C C H CH3 H H E-but-2-ene Z-but-2-ene Priority groups are on Priority groups are on the opposite sides of the C=C same side of the C=C HINT! On ze zame zide How to Draw & Name E/Z Isomers AQA www.chemistrycoach.co.uk © scidekick ltd 2024