Alkenes (Cambridge (CIE) A Level Chemistry): Flashcards
Exam code: 9701
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What are the three methods by which alkenes can be produced?
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What are the three methods by which alkenes can be produced?
Alkenes can be produced by:
Elimination reaction of a halogenoalkane with ethanolic NaOH.
Dehydration of an alcohol using a hot Al2O3 catalyst.
Cracking of long-chain hydrocarbons from crude oil.
True or False?
Reacting a halogenoalkane with aqueous NaOH produces an alkene.
False. Aqueous NaOH causes nucleophilic substitution to produce an alcohol. Ethanolic NaOH (NaOH dissolved in ethanol) is needed for the elimination reaction that produces an alkene.
Define dehydration reaction.
A dehydration reaction is an elimination reaction in which a water molecule is lost from an alcohol to produce an alkene, using a hot Al2O3 catalyst or concentrated acid.
In the elimination of a halogenoalkane, the small molecule lost is .......... The reagent used is .......... sodium hydroxide.
In the elimination of a halogenoalkane, the small molecule lost is hydrogen halide (HX). The reagent used is ethanolic sodium hydroxide.
True or False?
Small alkenes such as ethene are gases at room temperature and can be collected over water.
True. Small alkenes including ethene, propene and butene are gaseous at room temperature. They are collected by displacement of water in an inverted container.
What catalyst and conditions are used to dehydrate an alcohol to form an alkene?
Alcohol vapour is passed over a hot catalyst of aluminium oxide (Al2O3) powder. Concentrated acid or porous pot can also be used as alternative catalysts.
When bromoethane reacts with ethanolic NaOH, the Br- ion reacts with Na+ to form .........., and the H+ reacts with OH- to form .......... .
When bromoethane reacts with ethanolic NaOH, the Br- ion reacts with Na+ to form NaBr, and the H+ reacts with OH- to form water.
Define elimination reaction.
An elimination reaction is one in which a small molecule is removed from a larger molecule. In organic chemistry, it converts a halogenoalkane or alcohol into an alkene by losing HX or H2O respectively.
Why must oxygen be excluded during cracking of crude oil?
Oxygen would cause combustion of the hydrocarbons, producing CO2 and H2O rather than the desired smaller alkane and alkene molecules.
Define electrophilic addition.
Electrophilic addition is the addition of an electrophile across a C=C double bond in an alkene, breaking the double bond and forming two new single bonds.
What product is formed when an alkene is shaken with cold dilute acidified KMnO4?
A diol is formed. The pale purple KMnO4 solution turns colourless as the alkene is oxidised, with an OH group added to each of the two carbon atoms of the double bond.
True or False?
Hot concentrated acidified KMnO4 always produces a diol from an alkene.
False. Hot concentrated KMnO4 causes the C=C bond to completely break, producing ketones, aldehydes, carboxylic acids or CO2, depending on the substitution pattern. Diols are only formed with cold dilute KMnO4.
In addition polymerisation, the .......... double bond in many monomer molecules is broken to link them into a long-chain ...........
In addition polymerisation, the C=C double bond in many monomer molecules is broken to link them into a long-chain polymer.
Name four electrophilic addition reactions that alkenes can undergo.
Alkenes undergo electrophilic addition with:
Hydrogen (hydrogenation).
Steam (H2O (g)) to form an alcohol.
Hydrogen halide (HX) to form a halogenoalkane.
Halogen (X2) to form a dihalide.
True or False?
Oxidation of an alkene with hot concentrated KMnO4 can be used to identify the position of the double bond.
True. The products of hot concentrated KMnO4 oxidation depend on the groups attached to each carbon in the double bond, which allows the position of the C=C to be deduced from the products formed.
Define addition polymerisation.
Addition polymerisation is the reaction of many alkene monomers through their C=C double bonds to form a long-chain polymer as the only product.
When a carbon in an alkene's double bond carries two alkyl groups, hot concentrated KMnO4 oxidation produces a .......... . When it carries one H and one alkyl group, a .......... is produced.
When a carbon in an alkene's double bond carries two alkyl groups, hot concentrated KMnO4 oxidation produces a ketone. When it carries one H and one alkyl group, a carboxylic acid is produced.
What is poly(chloroethene) (PVC) and how is it made?
Poly(chloroethene) (PVC) is made by the addition polymerisation of chloroethene monomers. The C=C double bonds break and the monomers link together, forming a versatile plastic polymer.
What is the reagent used to test for unsaturation in an organic compound, and what is the observation if unsaturation is present?
Bromine water (Br2 (aq)) is used. If the compound is unsaturated, the orange/yellow solution decolourises as bromine undergoes an addition reaction across the C=C double bond.
True or False?
An alkane will decolourise bromine water when shaken with it.
False. Alkanes are saturated and do not contain a C=C double bond, so they cannot undergo addition with bromine. The bromine water remains orange/yellow in colour.
Define unsaturated compound.
An unsaturated compound is an organic molecule that contains at least one C=C double bond. It can be identified by its ability to decolourise bromine water.
When ethene is shaken with bromine water, an addition reaction produces .........., causing the bromine water to change from .......... to colourless.
When ethene is shaken with bromine water, an addition reaction produces 1,2-dibromoethane, causing the bromine water to change from orange/yellow to colourless.
Why does bromine water decolourise when added to an alkene?
Bromine undergoes an electrophilic addition reaction across the C=C double bond of the alkene, consuming the Br2 molecules. Since the coloured Br2 is used up, the solution becomes colourless.
True or False?
Bromine water is orange or yellow before it reacts with an unsaturated compound.
True. Br2 (aq) is orange or yellow in colour. Decolourisation to a colourless solution is the positive result for the test for unsaturation.
The test for unsaturation involves shaking the unknown compound with ........... A positive result is the .......... of the solution.
The test for unsaturation involves shaking the unknown compound with bromine water. A positive result is the decolourisation of the solution.
Define bromine water.
Bromine water is an aqueous solution of Br2 that is orange or yellow in colour. It is used to test for unsaturation — decolourisation indicates the presence of a C=C double bond.
What type of reaction occurs between an alkene and bromine water, and what product is formed from ethene?
An electrophilic addition reaction occurs. Bromine adds across the C=C double bond of ethene to form 1,2-dibromoethane (a dihalide).
Why are alkenes susceptible to attack by electrophiles?
The C=C double bond is an area of high electron density (four electrons). This attracts electrophiles (electron-loving species) that seek regions of electron density.
True or False?
In the electrophilic addition of HBr to an alkene, the Br atom acts as the electrophile.
False. The H atom acts as the electrophile because it carries the partial positive charge (δ+). The H–Br bond breaks heterolytically, releasing Br-.
Define carbocation.
A carbocation is a highly reactive intermediate formed during electrophilic addition when a carbon atom loses a bonding electron pair and carries a positive charge. It is quickly attacked by a nucleophile.
In the electrophilic addition of HBr to ethene, the H–Br bond breaks .........., forming a carbocation intermediate and a .......... ion.
In the electrophilic addition of HBr to ethene, the H–Br bond breaks heterolytically, forming a carbocation intermediate and a Br- ion.
Why does Br2 act as an electrophile when it approaches an alkene, even though it is nonpolar?
The high electron density of the C=C double bond repels the electron pair in the Br–Br bond towards the far Br atom. This creates an induced dipole with the near Br atom becoming δ+ and acting as an electrophile.
True or False?
The addition of Br2 to ethene produces a dibromoethane via a carbocation intermediate.
True. The near Brδ+ accepts electrons from the C=C bond, forming a carbocation intermediate. The Br- ion then attacks the carbocation to give 1,2-dibromoethane.
In HBr, the .......... atom is δ+ and the .......... atom is δ- due to differences in electronegativity.
In HBr, the H atom is δ+ and the Br atom is δ- due to differences in electronegativity.
Define heterolytic fission.
Heterolytic fission is the breaking of a covalent bond in which both electrons go to one atom, producing a cation and an anion. It occurs in electrophilic addition when the H–Br or Br–Br bond breaks.
What is the product of the electrophilic addition of HBr to ethene, and what is the intermediate?
The product is bromoethane (CH3CH2Br). The intermediate is a primary carbocation (CH3CH2+), which is then attacked by the Br- ion.
Define Markovnikov's rule.
Markovnikov's rule is the rule that in electrophilic addition of a hydrogen halide (HX) to an alkene, the halogen bonds to the most substituted carbon atom, forming the major product via the most stable carbocation intermediate.
Why is a tertiary carbocation more stable than a primary carbocation?
A tertiary carbocation has three alkyl groups attached to the positively charged carbon. Each alkyl group donates electron density towards the cation through the inductive effect, spreading the positive charge and making the ion more stable.
True or False?
In the electrophilic addition of HBr to propene, the major product is 1-bromopropane.
False. The major product is 2-bromopropane. The H+ attaches to the least substituted carbon, giving the more stable secondary carbocation, which then reacts with Br- to give 2-bromopropane.
The stability order of carbocations is: .......... > secondary > .........., because more alkyl groups donate greater electron density via the inductive effect.
The stability order of carbocations is: tertiary > secondary > primary, because more alkyl groups donate greater electron density via the inductive effect.
What is the inductive effect in the context of carbocation stability?
The inductive effect is the donation of electron density from alkyl groups towards the positively charged carbon of a carbocation. This reduces the positive charge and distributes it over more atoms, making the carbocation more energetically stable.
True or False?
Carbocations are nucleophiles because they carry a positive charge.
False. Carbocations are electrophiles — they are electron-deficient species attracted to regions of high electron density. Nucleophiles carry negative charge or lone pairs.
Markovnikov's rule predicts that when HBr adds to an unsymmetrical alkene, the H+ adds to the .......... carbon, and the Br- adds to the .......... carbon.
Markovnikov's rule predicts that when HBr adds to an unsymmetrical alkene, the H+ adds to the least substituted carbon, and the Br- adds to the most substituted carbon.
Define anti-Markovnikov addition.
Anti-Markovnikov addition is the minor pathway in electrophilic addition of HX to an unsymmetrical alkene, in which the halogen bonds to the least substituted carbon via the less stable carbocation intermediate.
In the reaction of propene with HBr, what are the two carbocation intermediates and which gives the major product?
The two intermediates are a secondary carbocation (CH3CH+CH3) and a primary carbocation (CH3CH2CH2+). The secondary carbocation is more stable and gives the major product, 2-bromopropane.
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