Reaction of alkenes More reactions of alkenes Objective

Reaction of alkenes

More reactions of alkenes Objective: To know some more electrophilic reactions of alkenes Success criteria: • understand the addition reactions of alkenes with: hydrogen, in the presence of a nickel catalyst, to form an alkane halogens to produce dihalogenoalkanes steam, in the presence of an acid catalyst, to produce alcohols potassium manganate(VII), in acid conditions, to oxidise the double bond and produce a diol • understand that heterolytic bond fission of a covalent bond results in the formation of ions • understand the mechanism of the electrophilic addition reactions for the reaction of alkenes with halogens including using curly arrow notation and other given binary compounds

Testing for alkenes The presence of unsaturation (a carbon– carbon double bond) can be detected using bromine water, a red/orange coloured solution of bromine. A few drops of bromine water are added to the test liquid and shaken. If a carbon–carbon double bond is present, the bromine adds across it and the solution turns colourless. 3 of 34 © Boardworks Ltd 2009

Electrophilic Addition of Br 2 to Ethene • • • The same mechanism with one small difference… Br 2 has an induced dipole. Draw out the mechanism – Connect the stages with just like an equation.

Halgoens reacting with alkenes Halogens react with alkenes to form dihalogenalkanes

• • Heterolytic Fission In heterolytic fission the bond breaks unevenly One of the bonded atoms receives both electrons from the bonded pair Two different substances can be formed E. g. A positively charged cation (X+) and negatively charged anion (Y-) X – Y X+ + Y The curly arrow shows the movement of an electron pair

Halgoens reacting with alkenes Mechanism 1) The double bond repels the electrons in Br 2 polarising Br - Br

Halgoens reacting with alkenes Mechanism 2) Heterolytic (unequal) fission of Br 2. The closer Br gives up the bonding electrons to the other Br and bonds to the C atom

Halgoens reacting with alkenes Mechanism 3) You get a positiviely charged carbocation intermediate. The Br- now zooms over……

Halgoens reacting with alkenes Mechanism 4) …. . And bonds to the other C atom, forming 1, 2 -dibromoethane

Halgoens reacting with alkenes But-1 -ene is an alkene. Alkenes contain at least one C=C double bond. 1. Describe how bromine water can be used to test for C=C double bonds. 2. Name and draw the mechanism for involved in the above test.

More reactions of alkenes Objective: To know some more electrophilic reactions of alkenes Success criteria: • understand the addition reactions of alkenes with: hydrogen, in the presence of a nickel catalyst, to form an alkane halogens to produce dihalogenoalkanes steam, in the presence of an acid catalyst, to produce alcohols potassium manganate(VII), in acid conditions, to oxidise the double bond and produce a diol • understand that heterolytic bond fission of a covalent bond results in the formation of ions • understand the mechanism of the electrophilic addition reactions for the reaction of alkenes with halogens including using curly arrow notation and other given binary compounds

• • Alkenes reacting with hydrogen Ethene will react with hydrogen gas in an addition reaction to produce ethane It needs a nickel catalyst and a temperature of 150 o. C Margarine is made by “hydrogenating” unsaturated vegetable oils By removing some double bonds you raise the melting point of the oil so that it becomes solid at room temperature

Alkenes reacting with hydrogen • Ethene will react with hydrogen gas in an addition reaction to produce ethene • It needs a nickel catalyst and a temperature of 150 o. C Ni H 2 C = CH 2 + H 2 CH 3 150 o. C

• • Alkenes reacting with steam Alkenes can be hydrated by steam at 300 o. C and a pressure of 60 – 70 atm The reaction needs a solid phosphoric (V) acid catalyst The reaction is used to manufacture ethanol from ethene It is an electrophilic addition reaction and an example of a hydration reaction H 3 PO 4 H 2 C = CH 2 (g) + H 2 O (g) CH 3 CH 2 OH (g) 300 o. C, 60 atm

Alkenes reacting with steam Mechanism Can you suggest a mechanism?

Alkenes reacting with acidified potassium manganate (VII) H 2 C = CH 2 + [O] + H 2 O CH 2 OH-CH 2 OH ethane-1, 2 -diol • If you shake an alkene with acidified potassium manganate (VII), the purple solution is decolourised (no change is observed with alkanes). • You’ve oxidised the alkene and made a diol (an alcohol with two –OH groups) • The oxidising agent is potassium manganate (VII), KMn. O 4, acidified with dilute H 2 SO 4. • This is represented [O].

Questions Write the equation, and name the organic product, for the reaction between but-2 -ene and: 1. Steam 2. Acidified potassium manganate (VII)

More reactions of alkenes Objective: To know some more electrophilic reactions of alkenes Success criteria: • understand the addition reactions of alkenes with: hydrogen, in the presence of a nickel catalyst, to form an alkane halogens to produce dihalogenoalkanes steam, in the presence of an acid catalyst, to produce alcohols potassium manganate(VII), in acid conditions, to oxidise the double bond and produce a diol • understand that heterolytic bond fission of a covalent bond results in the formation of ions • understand the mechanism of the electrophilic addition reactions for the reaction of alkenes with halogens including using curly arrow notation and other given binary compounds

Reactions of alkenes plenary What can you remember about the reactions of alkenes? Anything from the last two lessons

More reactions of alkenes Objective: To know some more electrophilic reactions of alkenes Success criteria: • understand the addition reactions of alkenes with: hydrogen, in the presence of a nickel catalyst, to form an alkane halogens to produce dihalogenoalkanes steam, in the presence of an acid catalyst, to produce alcohols potassium manganate(VII), in acid conditions, to oxidise the double bond and produce a diol • understand that heterolytic bond fission of a covalent bond results in the formation of ions • understand the mechanism of the electrophilic addition reactions for the reaction of alkenes with halogens including using curly arrow notation and other given binary compounds