CIE AS Chemistry Alkenes PDF

Head to www.savemyexams.com for more awesome resources CIE AS Chemistry Your notes 14.2 Alkenes Contents Producing Alkenes Reactions of Alkenes Test for Unsaturation Electrophilic Addition of Alkenes...

Head to www.savemyexams.com for more awesome resources CIE AS Chemistry Your notes 14.2 Alkenes Contents Producing Alkenes Reactions of Alkenes Test for Unsaturation Electrophilic Addition of Alkenes Markovnikov's Rule Page 1 of 24 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Producing Alkenes Your notes Production of Alkenes: Elimination, Dehydration & Cracking Alkenes can be made by a series of reactions including elimination, dehydration reactions and cracking Elimination reaction Alkenes can be produced from the elimination reaction of a halogenoalkane An elimination reaction is one in which a small molecule is lost In the case of halogenoalkanes, the small molecule that is eliminated is a hydrogen halide, HX, where X is the halogen The halogenoalkane is heated with ethanolic sodium hydroxide Making alkenes from halogenoalkanes Production of an alkene from a halogenoalkane by reacting it with ethanolic sodium hydroxide and heating it The eliminated H+ in HBr reacts with the ethanolic OH- to form water The eliminated Br- in HBr reacts with Na+ to form NaBr Overview of halogenoalkane elimination Page 2 of 24 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes The eliminated HBr reacts with ethanolic OH- and Na+ to form H2O and NaBr Note that the reaction conditions should be stated correctly as different reaction conditions will result in different types of organic reactions NaOH (ethanol): an elimination reaction occurs to form an alkene NaOH (aq): a nucleophilic substitution reaction occurs, and an alcohol is one of the products Comparing reaction conditions Page 3 of 24 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Different reaction conditions will give different products Dehydration reaction Alkenes can also be produced from the elimination reaction of alcohols in which a water molecule is lost This is also called a dehydration reaction Alcohol vapour is passed over a hot catalyst of aluminium oxide powder (Al2O3) Concentrated acid, pieces of porous pot or pumice can also be used as catalysts Making alkenes from alcchols Production of an alkene from an alcohol by using a hot aluminium oxide powder catalyst Overview of alcohol elimination Page 4 of 24 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes The formation of ethene from ethanol is an example of a dehydration reaction of alcohol The smaller alkenes (such as ethene, propene and butene) are all gases at room temperature and can be collected over water Practical set-up to form alkenes from alcohol The smaller alkenes are gases at room temperature and collected over water Page 5 of 24 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Cracking Alkenes can also be produced from the cracking of long hydrocarbon molecules in crude oil Your notes An aluminium oxide (Al2O3) catalyst and high temperatures are used to speed up this reaction. It is important to ensure that the crude oil doesn’t come into contact with oxygen as this can cause the combustion of the hydrocarbons to produce water and carbon dioxide The cracking of crude oil produces smaller alkane and alkene molecules Cracking hydrocarbons Long hydrocarbon fraction is cracked into two smaller ones The low-molecular mass alkenes are more reactive than alkanes as they have an electron-rich double bond They can therefore be used as feedstock for making new products Possible compounds formed from alkenes Page 6 of 24 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Alkenes are reactive molecules and can undergo many different types of reactions making them useful as starting compounds Page 7 of 24 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Reactions of Alkenes Your notes Reactions of Alkenes Alkenes are very useful compounds as they can undergo many types of reactions They can therefore be used as starting molecules when making new compounds Electrophilic addition Electrophilic addition is the addition of an electrophile to a double bond The C-C double bond is broken, and a new single bond is formed from each of the two carbon atoms Electrophilic addition reactions include the addition of: Hydrogen (also known as hydrogenation reaction) Steam (H2O (g)) Hydrogen halide (HX) Halogen Electrophilic addition reactions of alkenes Page 8 of 24 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes The diagram shows an overview of the different electrophilic addition reactions alkenes can undergo Oxidation Page 9 of 24 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Alkenes can also be oxidised by acidified potassium manganate(VII) (KMnO4) which is a very powerful oxidising agent Alkenes can be oxidised by both hot and cold KMnO4 which will result in different products being Your notes formed When shaken with cold dilute KMnO4 the pale purple solution turns colourless and the product is a diol When alkenes are reacted with hot concentrated KMnO4 the conditions are harsher causing the C-C double bond to completely break The O-H groups in the diol formed are further oxidised to ketones, aldehydes, carboxylic acids or carbon dioxide gas The actual products formed depend on what is bonded to the carbon atoms in the alkene Oxidising alkenes using KMnO4 Page 10 of 24 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Alkenes can be oxidised by cold dilute and hot concentrated KMnO4 to give different products The reactions of alkenes with hot concentrated KMnO4 can be used to determine the position of the double bond in larger alkenes Page 11 of 24 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Predicting the position of double bonds formed Your notes The above reactions can be used to predict where the double bond in a larger molecule is Page 12 of 24 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Worked example Your notes What are the products of the oxidation of 2-methylprop-1-ene with hot, concentrated acidified KMnO4 (aq)? Answer: The products are propanone (a ketone), carbon dioxide and water Worked example The oxidation of an alkane produces carbon dioxide gas, water and propanoic acid. Identify the alkene. Answer: The alkene is 1-butene Addition polymerisation Page 13 of 24 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Addition polymerisation is the reaction of many monomers containing at least one double C-C bond to form the long-chain polymers as the only product Monomers are small, reactive molecules that react together to make the polymer Your notes A polymer is a long-chain molecule made up of many repeating units (monomers) In an addition polymerisation reaction, the C-C double bond is broken to link together the monomers and form a polymer This is a common method of making plastics The polymerisation of ethene The polymer backbone consists of a carbon chain with monomers that contain 2 carbon atoms Page 14 of 24 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources The polymerisation of propene Your notes The polymer backbone consists of a carbon chain with monomers that contain 2 carbon atoms with the methyl group, from the propane monomer, as a side chain Other alkenes and substituted alkenes can also polymerise to make polymers with different properties E.g. poly(chloroethene), also known as PVC is the most versatile plastic used The polymerisation of chloroethene Page 15 of 24 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Poly(chloroethene) is used as plastic Page 16 of 24 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Test for Unsaturation Your notes Test for Unsaturation Halogens can be used to test if a molecule is unsaturated (i.e. contains a double bond) Br2 (aq) is an orange or yellow solution, called bromine water and this is the halogen most commonly used The unknown compound is shaken with the bromine water If the compound is unsaturated, an addition reaction will take place and the coloured solution will decolourise The unsaturation test The decolourisation of bromine water by an unsaturated compound as a result of an addition reaction Page 17 of 24 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Electrophilic Addition of Alkenes Your notes Electrophilic Addition of Alkenes The double bond in alkenes is an area of high electron density (there are four electrons found in this double bond) This makes the double bond susceptible to attack by electrophiles (electron-loving species) An electrophilic addition is the addition of an electrophile to a double bond Electrophilic addition of hydrogen bromide A molecule of hydrogen bromide (HBr) is polar as the hydrogen and bromine atoms have different electronegativities The bromine atom has a stronger pull on the electrons in the H-Br bond As a result of this, the Br atom has a partial negative and the H atom a partial positive charge Explaining the polarity of a HBr molecule Due to differences in electronegativities of the hydrogen and bromine atoms, HBr is a polar molecule In an addition reaction, the H atom acts as an electrophile and accepts a pair of electrons from the C-C bond in the alkene The H-Br bond breaks heterolytically, forming a Br- ion This results in the formation of a highly reactive carbocation intermediate which reacts with the bromide ion, Br- For example, the mechanism for the electrophilic addition of hydrogen bromide and ethene is: Electrophilic addition of HBr mechanism Page 18 of 24 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Electrophilic addition reaction of HBr and ethene to form bromoethane Electrophilic addition of bromine Bromine (Br2) is a non-polar molecule as both atoms have similar electronegativities and therefore equally share the electrons in the covalent bond However, when a bromine molecule gets closer to the double bond of an alkene, the high electron density in the double bond repels the electron pair in Br-Br away from the closest Br atom As a result of this, the closest Br atom to the double bond is slightly positive and the further Br atom is slightly negatively charged The polarity of a Br2 molecule Page 19 of 24 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Br2 is a non-polar molecule however when placed close to an area of high electron density it can get polarised Your notes In an addition reaction, the closest Br atom acts as an electrophile and accepts a pair of electrons from the C-C bond in the alkene The Br-Br bond breaks heterolytically, forming a Br- ion This results in the formation of a highly reactive carbocation intermediate which reacts with the Br- (nucleophile) Electrophilic addition of Br2 mechanism Example of an electrophilic addition reaction of Br2 and ethene to form dibromoethane Page 20 of 24 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Markovnikov's Rule Your notes Alkenes: Stability of Cations & Markovnikov's Rule Carbocations are positively charged carbon atoms with only three covalent bonds instead of four There are three types of carbocations: primary, secondary and tertiary Inductive effect The alkyl groups attached to the positively charged carbon atoms are ‘electron donating groups’ This is also known as the inductive effect of alkyl groups The inductive effect is illustrated by the use of arrowheads on the bonds to show the alkyl groups pushing electrons towards the positively charged carbon This causes the carbocation to become less positively charged As a result of this, the charge is spread around the carbocation which makes it energetically more stable This means that tertiary carbocations are the most stable as they have three electron-donating alkyl groups which energetically stabilise the carbocation Due to the positive charge on the carbon atom, carbocations are electrophiles Primary, secondary and tertiary carbocations Alkyl groups push electron density towards the carbocation making it energetically more stable; the more alkyl groups the carbocation is bonded to, the more stabilised it is Markovnikov’s rule Markovnikov’s rule predicts the outcome of electrophilic addition reactions and states that: In an electrophilic addition reaction of a hydrogen halide (HX) to an alkene, the halogen ends up bonded to the most substituted carbon atom In an electrophilic addition reaction of an interhalogen to an alkene, the most electronegative halogen ends up bonded to the most substituted carbon atom Markovnikov addition applies to electrophilic addition reactions with unsymmetrical alkanes, e.g. propene and but-1-ene Page 21 of 24 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Markovnikov addition favours the formation of the major product Anti-Markovnikov addition favours the formation of the minor product Your notes In electrophilic addition reactions, an electrophile reacts with the double bond of alkenes (as previously discussed) The mechanism for electrophilic addition reactions with unsymmetrical alkenes is slightly different, e.g. propene + hydrogen bromide Step 1 in the electrophilic addition mechanism The electrophile reacts with the electron-rich C-C double bond The electrophile can attach in two possible ways: 1. Breaking the C=C bond and attaching to the least substituted carbon This will give the most stable carbocation as an intermediate that will form the major product 2. Breaking the C=C bond and attaching to the most substituted carbon This will give the least stable carbocation as an intermediate that will form the minor product Relative stabilities of primary and secondary carbocations The major and minor carbocation intermediates formed during the reaction of propene and hydrogen bromide The nucleophile will bond to the positive carbon atom of the carbocation The more stable carbocation produces the major product The less stable carbocation produces the minor product Formation of major and minor products Page 22 of 24 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Formation of the major and minor products of the reaction of propene with hydrogen bromide The mechanism for the electrophilic addition of hydrogen bromide to propene, showing the formation of the major and minor products can be shown as: Electrophilic addition mechanism showing the formation of the major and minor products Page 23 of 24 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes The electrophilic addition reaction mechanism of HBr and propene to form 1-bromopropane and 2- bromopropane Exam Tip The stability of the carbocation intermediate is as follows: tertiary > secondary > primary When more than one carbocation can be formed, the major product of the reaction will be the one that results from the nucleophilic attack of the most stable carbocation. 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