Substitution Reactions of Alkanes

1. Substitution Reactions of Alkanes AS Chemistry

2. Lesson Objectives Substitution Reactions of Alkanes describe the substitution of alkanes using ultraviolet radiation, by Cl2 and by Br2, to form halogenoalkanes; define the term radical as a species with an unpaired electron Explain what homolytic and heterlytic fission are describe the mechanism of radical substitution in alkanes in terms of initiation, propagation and termination reactions explain the limitations of radical substitution in synthesis, arising from further substitution with formation of a mixture of products;

3. Substitution Reactions • Write a definition of a substitution reaction. A substitution reaction is one in which an atom or group or atoms is replaced with a different atom or group of atoms.

4. Mechanisms • In chemistry we often need to know the mechanism of a reaction. • This will show us a reaction pathway, how we actually get from the reactants to the products. • The reaction pathway will usually show us intermediate chemical species, stepping stones in the reaction.

5. Mechanisms • The activation energy for a reaction is how much energy is used to form the intermediate species. • If we can find a different reaction mechanism (e.g. a catalyst) we could change the activation energy. • Catalysts are often used in chemical reactions and will take part in the reaction mechanism. • Remember, catalysts are left unchanged at the end of a reaction.

6. Reactions • Last lesson you carried out this reaction: C6H10 + Br2 C6H10Br2 • Write out a word equation. cyclohexene + bromine  1,2-dibromocyclohexene • Now draw the displayed formulae of the species involved. • How did you know that C6H10

7. Reactions • How did you know that C6H10was an alkene? • It removed the bromine. • Alkanes can also react with halogens but only in the presence of UV light. • Write an equation for Bromine reacting with methane.

8. Reactions • Write an equation for Chlorine reacting with methane. CH4 + Br2 CH3Br + HBr • How do I know that those are the products? • I know the mechanism of the reaction.

9. What are halogenoalkanes? Halogenoalkanes are similar to alkanes but with one or more of the hydrogen atoms replaced by a halogen. trichloromethane Halogenoalkanes can contain more than one type of halogen. For example, CFCs(chlorofluorocarbons) contain both chlorine and fluorine atoms. chloro-pentafluoroethane Some halogenoalkanes are useful themselves, but many are valuable intermediates in the production of other molecules.

10. Naming halogenoalkanes halogen prefix no. halogen atoms prefix fluorine one chlorine two bromine three iodine four five A prefix is added to the name of the alkane depending on what halogens are attached. fluoro- – chloro- di- bromo- tri- iodo- tetra- penta- Another prefix is used to indicate how many atoms of each halogen is present. Numbers are used, where necessary, to indicate to which carbon atom(s) each halogen is attached.

11. Bond Fission Breaking a covalent bond is called bond fission. A single covalent bond is a shared pair of electrons and it can break in two ways.

12. 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

13. Homolytic Fission • In homolytic fission the bond breaks evenly and each bonding atom receives one electron from the bonded pair • Two electrically uncharged radicals are formed • Radicals are particles that have an unpaired electron • We use a dot to represent this X – Y  X· + Y· Because of the unpaired electron radicals are very reactive

14. Forming Halogenoalkanes • Halogens react with alkanes in photochemical reactions • These reactions are started by light – this reaction needs UV light • A hydrogen atom is substituted by chlorine or bromine • This is radical substitution UV E.g. CH4 + Br2  CH3Br + HBr

15. Free-radical Subsitution • The initiation step • Radicals are produced • Bromine can be split into two free radicals by UV light (photodissociation): Br-Br(g) Br•(g) + Br•(g) • Homolytic fission occurs and bromine radicals are produced

16. Free-radical Subsitution • The Propagation Steps • A free radical will react with other species leaving more free radicals as there will always be an electron short: Br•(g) + H-CH3(g) H-Br(g) + CH3•(g) CH3•(g) + Br-Br(g) CH3-Br(g) +Br•(g) • The first reaction uses up a bromine free radical, but the second creates a new one. • This is therefore a chain reaction.

17. Free-radical Subsitution • The Termination Steps • If a free radical reacts with another free radical the chain reaction ends: Br•(g) + Br•(g) Br-Br(g) CH3•(g) + CH3•(g)  CH3-CH3(g) • The free radicals are “mopped up” by reacting together to form stable molecules • Other termination steps are possible too.

18. Free-radical Subsitution In the reaction a Br is substituted for an H on the molecule by the attack of a free radical. This is therefore called: A free radical substitution reaction.

19. Free-radical Subsitution • These reactions are often considered unsuitable for organic synthesis. • This is because: • A mixture of products is formed. • Bromine could react with more bromine radicals forming… • Bromomethane (CH3Br) • Dibromomethane (CH2Br2) • Tribromomethane (CHBr3) • Tetrabromomethane (CBr4)

20. Tetrabromomethane (CBr4) is formed in the last possible substitution reaction • There are no more hydrogen atoms attached to the carbon atom so the reaction has to stop • To reduce the chance of the by products forming you can have an excess of methane • This means there is a greater chance of a bromine radical colliding with a methane molecule and not a bromine molecule • Another problem with a radical substitution is that it can take place at any point along the carbon chain. • So a mixture of structural isomers can be formed

21. Questions • What is a radical and how does it form? • Ethane reacts with bromine to produce bromoethane: CH3CH3 +Br2 CH3CH2Br + HBrWrite equations to describe the radical substitution mechanism involved.

22. Questions • Chlorine reacts with propane in UV light.a) Write an equation to show the initiation stepb) One of the products of the reaction will be 1-chloropropane. Show the two propagation steps of this reaction • In the chlorination of ethane, one of the products is a molecule with four carbon atoms.Explain using equations how this molecule can be formed.

23. Questions • Describe two limitations of radical substitution reactions in the synthesis of organic molecules (2 marks) Further substitution is possible, for example to produce dichloromethane, trichloromethane and tetrachloromethane, rather than the desired product, chloromethane. A mixture of products form, both by further substitution and through termination steps.

24. Lesson Objectives Substitution Reactions of Alkanes describe the substitution of alkanes using ultraviolet radiation, by Cl2 and by Br2, to form halogenoalkanes; define the term radical as a species with an unpaired electron Explain what homolytic and heterlytic fission are describe the mechanism of radical substitution in alkanes in terms of initiation, propagation and termination reactions explain the limitations of radical substitution in synthesis, arising from further substitution with formation of a mixture of products;