ALKENESCHEMICAL PROPERTIES OF ALKENES

1. The cloud of electrons which forms the Л bond in alkene lies above and below the plane of the molecule. In this position, the Л electrons are susceptible to attack by electrophiles. 2. In an addition reaction of alkene, the Л bond of the carbon-carbon double bond breaks in order to release electrons to form two new α bonds with the reactant molecule and a saturated molecule is formed.

a) Electrophilic addition1) Hydrogenation, H2

2) Halogenations, Cl2, Br2

3) Hydrohalogenation, HCl, HBr4) Hydration, H2O

b) oxidation 5) complete combustion6) present catalyst, Ag7) cold KMnO4, H+

8) hot KMnO4, H+

9) ozonolysis, O3

c) addition polymerization

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ADDITION REACTIONS OF ALKENES (ELECTROPHILIC ADDITION)

(1) Addition Of Hydrogen (Hydrogenation)

carried out by passing hydrogen and the alkene, both in the gaseous state, over a metal catalyst(very

finely powdered nickel) at 150ºC and 5 atm pressure(If platinum catalyst is used, the process takes place under normal laboratory conditions.) An example of hydrogenation reaction of alkene is the reaction of ethene with hydrogen to produce ethane.

QUESTIONS ANSWERS(1) Write down the structure of the products, if any, of the following reactions(a)

(ethene)(b)

(c)

(2) Write down the structure of the products, if any, of the following reactions.(a)

(b)

(3) 10.8 g of an alkene required 9.6 dm3 of hydrogen gas for complete hydrogenation at room conditions. Write down the possible structural formula of the alkene. [Molar mass of the alkene = 54.0 g mol-1; 1 mol of gas occupies 24.0 dm3 at room conditions]

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(2) Addition Of Halogens (Chlorine Or Bromine) - Halogenation1. Both chlorine, Cl2, and bromine, Br2, add rapidly to the carbon-carbon double bond at room temperature without the need of any catalyst.

(a) Ethene reacts with chlorine gas at room temperature to form1,2-dichloroethane.

(b) Cyclohexene reacts rapidly with bromine to form 1,2-dibromocyclohexane

(c)When an alkene is bubbled through a solution of bromine in hexane or tetrachloromethane (organic solvent), the brown colour of bromine is rapidly decolorised (Decolorisation of bromine solution is a useful test reaction to detect a double bond)

THE MECHANISM OF ADDITION OF BROMINE TO ETHENE(Electrophilic addition- the bromine molecule, Br2, acts as an electrophile because it is attracted to the electron-rich carbon-carbon double bond. (a) When a bromine molecule approaches an ethene molecule, the pi electron cloud of ethene interacts with the approaching bromine molecule, causing a polarisation of the Br-Br bond. The double bond of ethene induces a dipole on Br2.

(b) A loose association forms between the ethene and bromine molecules. Electrons move from the double bond towards the partially positively charged bromine atom and at the same time electrons in the Br-Br bond are repelled to the partially negatively charged bromine atom . This results in the formation of two ions, a carbocation and a bromide ion, Br-. (A carbocation is a species that contains a carbon atom with only three bonds to it and bearing a positive charge).(c) The carbocation is very unstable and quickly combines with the bromide ion, Br-, to form 1,2-dibromoethane. (Iodine does not add to the carbon-carbon double bond and fluorine reacts explosively to give a mixture of products)

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QUESTION ANSWER(1) Give the equations, the conditions and the names of the products when

(a) propene reacts with bromine

(b) cydohexene reacts with bromine

(2)Would you expect a species which attacks alkenes to be an electrophile or a nudeophile? Give a reason for your answer.

(3)Name and draw structural formulae for the products of the following halogenation

(a)

(b)

(4) How would you distinguish between cyclohexene and cyclohexane using simple test-tube reactiond?

(5) Briefly outline the mechanism for the reaction between cyclohexene dan bromine in hexane at room temperature.

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1. Write an overall equation for the reaction of propene with bromine water.

Note: 2-Bromopropan-l-ol is formed and not I-bromopropan-2-01 because the addition reaction of an unsymmetrical alkene follows Markovnikoff's rule.

2. Write an overall equation for the reaction of (a) but-2-ene with bromine water

(b) but-l-ene with bromine water

(3) Reaction With Hydrogen Halides

1. Hydrogen halides readily add to the carbon-carbon double bond in the cold to give haloalkanes.

2. Ethene produces chloroethane on bubbling through concentrated hydrochloric acid at room temperature. (The rate of addition decreases in the order H-I > H-Br > H-Cl because the bond energy of the hydrogen halides increases from H-I to H-Cl. )

ADDITION OF HYDROGEN HALIDE TO UNSYMMETRICAL ALKENES

(a) When propene reacts with a hydrogen halide such as hydrogen bromide, HBr, there are two possible products. 1- Bromopropane (minor product).

REACTION WITH BROMINE WATER

1. An alkene reacts readily at room temperature with an aqueous solution of bromine (bromine water) to produce a mixture of products, bromo alcohol and dibromoalkane. 2. The bromine water would be decolorised

Water has lone pairs of electrons and can act as nucleophiles in competition with bromide ions, Br- .

3. When ethene is bubbled into bromine water, the reddish-brown bromine water is rapidly decolorised and a mixture of 2-bromoethanol and 1,2-dibromoethane is produced

4. If the bromine water is dilute, bromoalcohol would be the main product because there will be more water molecules than bromide ions present.

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2-Bromopropane(major product).(b) The addition of hydrogen halide to a carbon-carbon double bond in an unsymmetrical alkene follows the MARKOVNIKOFF'S RULE, that is, the hydrogen atom of the hydrogen halide adds to the carbon atom of the double bond with the greatest number of hydrogen atoms.

(c)An unsymmetrical carbon-carbon double bond is one whose two carbon atoms hold unequal number of hydrogen atoms.

EXAMPLES :

The carbon-carbon double bond is a site that can accept a proton from an acid and forms carbocationSTABILITY OF CARBOCATION

(a) In the reaction between propene and hydrogen bromide, the initial attack is by the acid catalyst(b) When the hydrogen ion, H+, attacks the double bond in propene, two different carbocatino can be formed.

(c) In the primary carbocation there is only one alkyl (ethyl) group pushing electrons onto the positively charged carbon atom.

The secondary carbocation is more stable than the primary carbocation. This is because in the secondary carbocation there are two alkyl groups (methyl groups) pushing electrons onto the positively charged carbon atom.

(d) As a result, the positive charge in the secondary carbocation is stabilised slightly more than that in the primary carbocation. This is because the donated electrons tend to cancel out the charge.

(e) Secondary carbocation is therefore more stable of the two possible intermediate carbocations, making it more likely to form and combine with Br- ion to form 2-bromopropane. (f)The order of stability of the carbocations is as follows:

The tertiary carbocation is the most stable because it has three electron-donating alkyl groups which tend to cancel out the positive charge more than the secondary or primary carbocation.

QUESTIONS ANSWERS(a) When but-1-ene, CH3 - CH2 - CH = CH2, reacts with hydrogen bromide, what are the

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structures of the two possible intermediate carbocations formed?

(b) Which one of those two ions is more stable?

(c) What will be the major product of the reaction between but-I-ene and hydrogen bromide?

Arrange the following alkenes in order of increasing reactivity on addition of hydrogen bromide.

QUESTIONS ANSWERS(1)Name and draw the structural formulae for the major products of these addition reactions of alkenes.(a)

(b)

(c)

(2)Suggest why when 2-methylpropene reacts with hydrogen iodide, the major product is 2 - iodo- 2 - methylpropane.

When 2-methylpropene reacts with hydrogen iodide, two different carbocations could be formed:

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The tertiary carbocation is more stable than the primary carbocation.

The tertiary carbocation has three alkyl groups pushing electrons onto the positively charge carbon atom whereas the primary carbocation has only one alkyl group. As a result, the positive charge in the tertiary carbocation is stabilised slightly more because the donated electrons tend to cancel to charge.

Hence, the tertiary carbocation is more stable and more likely to form and react with iodide ions than the primary carbocation. Thus, the major product is 2-iodo-2-methylpropane. (3) Briefly outline the reaction mechanism for the reaction between ethene and hydrogen bromide at room temperatureWhen ethene reacts with hydrogen bromide, a carbocation forms together with a bromide ion. In the second step, the bromide ion attacks the carbocation

(4) Name and draw structural formulae for the major products of these addition reactions of alkenes

(a)

(b)

(c)

REACTION WITH CONCENTRATED SULPHURIC (VI) ACID

1. An alkene undergoes addition reaction with concentrated sulphuric(VI) acid in the cold to give alkyl hydrogensulphate

2.When added to water and warmed, the alkyl hydrogensulphate is converted to an alcohol. This is a hydrolysis reaction, i.e. breaking up a compound by reacting with water.

3. When ethene is bubbled into concentrated sulphuric(VI) acid in the

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cold, ethyl hydrogensulphate is produced4. The product, ethyl hydrogensulphate, when added to water and warmed, is hydrolysed to ethanol. (The addition of concentrated sulphuric(VI) acid to unsymmetrical alkenes follows Markovnikoff's rule)

QUESTIONS ANSWERS1 (a) What is formed when propene reacts with concentrated sulphuric(VI) acid in the cold?

(b) What is the product of hydrolysis of the compound formed in (a)?

2. Draw structural formulae for the major products of these acid-catalysed hydration reactions of alkenes.

(a)

(b)

(c)

(4) Reaction With Steam (Hydration)

1. When steam and ethene are passed over a catalyst, ethanol is produced. A temperature of 330°C and a pressure of 60 atm are used in the presence of phosphoric acid catalyst.

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(a) The hydration of alkenes follows Markovnikoff's The hydration of propene:

Name and draw structural formulae for the major products of these addition reactions of alkenes(a)

(b)

OXIDATION: REACTION WITH POTASSIUM MANGANATE (VII)

(1) Reaction With Cold Dilute Potassium Manganate(Vii) Acidified With Dilute Sulphuric(Vi) Acid

1. An alkene will react with dilute potassium manganate(VII) in an acidic or alkaline solution to give a diol. The reaction with manganate(VII) ion, Mn04-, involves both addition and oxidation.

2. When ethene is added to dilute potassium manganate(VII) acidified with dilute sulphuric(VI) acid at room temperature, the purple colour of the potassium manganate(VII) is decolorised and ethene is oxidised to ethane- I,2-diol.

3. If the reaction involves cold dilute potassium manganate(VII) in alkaline conditions, the purple colour of the potassium manganate(VII) is decolorised, producing ethane-l,2-diol and a brown precipitate of manganese(IV) oxide.

(2) Reaction With Hot, Concentrated Potassium Manganate (Vii) Acidified With Dilute Sulphuric(Vi) Acid

1. If an alkene is added to hot, concentrated potassium manganate(VII) acidified with dilute sulphuric(VI) acid, the carbon-carbon double bond is oxidatively cleaved, i.e. the double bond breaks and produces

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ketones, aldehydes, carboxylic acid (from further oxidation of aldehydes formed), carbon dioxide or mixtures of these.

2. On addition of ethene to hot, concentrated acidified potassium manganate(VII), ethene is oxidatively cleaved to give methanal (an aldehyde) which is further oxidised to methanoic acid (a carboxylic acid). On further oxidation, methanoic acid will form carbon dioxide and water.

3. When 2-methylbut-2-ene is added to hot, concentrated acidified potassium manganate(VII), it is oxidised with the breaking of the carbon-carbon double bond to give ethanal (an aldehyde) and propanone (a ketone). The ethanal is further oxidised to ethanoic acid. The purple colour of the potassium manganate(VII) is decolorised.

1. An unknown alkene X with the formula CSH16 was found on oxidation with hot, acidified potassium manganate(VII) to yield propanoic acid and pentanoic acid. Name the alkene X. Realign the acids to give a better picture of the alkene: Hence, the alkene is cis- or trans-oct - 3 -ene

2. Write structures for the product(s) that would form under the conditions shown:

(a)

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(b)

(c)

(d)

(e)

QUESTIONS ANSWERS3.Write structures for the organic product(s) that would form under the conditions shown(a)

(b)

(c)

(d)

4. Draw the structural formulae for the alkenes X and Y below.(a)

(b)

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(3) Reaction With Oxygen

Complete combustion1. Alkenes burn in air with a sooty flame.Since alkenes have a higher percentage of carbon than their corresponding alkanes, they burn with a more sooty flame than alkanes. Hence alkenes are not used as fuels.

Present Catalyst, Ag

2. In the presence of finely divided silver catalyst at about 180°C, ethene reacts with oxygen to produce epoxyethane. Epoxyethane is an unstable compound because of the ring strain.(b) Epoxy thane reacts rapidly with water to produce ethane-1,2-diol. This is an industrial method to prepare ethane- l,2-diol which is used to manufacture antifreeze and polyesters

POLYMERISATION

1. Alkenes containing carbon-carbon double bonds can polymerise (addition polymerization) joining together to form long chain molecules. No atoms or molecules are eliminated in this process.

2.Poly(ethene) is the most common synthetic polymer whose monomer is ethene.

3. There are two types of poly(ethene), depending on the conditions used for preparation.

(a) (i) Low-density poly(ethene) is manufactured by heating ethene at about 200°C and a pressure of 1 200 atmosphere with traces of oxygen (initators). (ii) Low-density poly(ethene) has chain branching which makes it difficult to pack closely. It has a low melting point, 105°C, softens in boiling water and deforms easily. (iii) It is used for making plastic bags and wrappers.

(b) High-density poly(ethene) is manufactured by heating ethene at a lower temperature (60°C) and pressure with the use of catalysts, a mixture a triethyl aluminium and titanium(IV) chloride (Ziegler- Natta catalysts). (ii) It has no chain branching and can be packed closely together; hence, it is more rigid and has a higher melting point, 135°C. (iii) High-density poly(ethene) is used in making bottle crates and bottles.

IMPORTANCE OF ETHENE IN INDUSTRY

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(a) Manufacture of a number of plastics

Ethene is the raw material for the manufacture of poly(ethenel), poly( chloroethene) and polyester fibres.

(b) Manufacture of ethane-I,2-diol

Ethane-1,2-diol is prepared by reacting ethene with oxygen in the presence of a heated silver catalyst, followed by addition of water.

(c) Manufacture of ethanol

Ethanol is manufactured by hydration of ethene using phosphoric acid catalyst on silica al 300°C and 60 atmospheres.

Ethane-l,2 -diol is used as antifreeze in car radiators and for making detergents.

Ethanol is used as a solvent for varnishes, cosmetic and toilet preparations and also in the manufacture of ethanal.

IMPORTANCE OF ETHENE IN INDUSTRY

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REACTIONS TYPE OF REACTION

REAGENT(S) CONDITION(S)

1 Alkene → alkane Electrophilic addition

H2(g); Ni catalyst 150°C; 5 atm

H2(g); Pt catalyst Room temperature

2 Alkene → dihaloalkane Electrophilic addition

Cl2 in CC14 or Br2 in CC14 Room temperature

3 Alkene →haloalkane Electrophilic addition

Concentrated HCl or concentrated HBr

4 Alkene→dibromoalkane + bromoalcohol

Electrophilic addition Bromine water

Room temperature

5 Alkene→alcohol Electrophilic addition

Concentrated H2SO4 acid Coldfollowed by water Warm

6 Alkene→diol Oxidation Dilute KMnO 4/H+ ColdDilute KMnO 4/OH- Cold

7 Alkene→ ketones + aldehydes

Carboxylic acid

Oxidation and cleavage

Concentrated /KMnO 4/H+ Hot

Concentrated /KMnO 4/ OH- Hot

8 Alkene→ diol(industrial method)

Oxidation O2(g); Ag catalyst 180ºCfollowed by water Room temperature

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