Reactions with alkenes Addition Reactions Hydrogenation Halogenation Hydration
- Slides: 28
Reactions with alkenes
Addition Reactions • Hydrogenation • Halogenation • Hydration • Addition of hydrogen halides • Oxidation to diols
Halogenation • Alkenes and halogens • Produce dihalogenoalkanes • E. g. ethene + bromine 1, 2 -dibromoethane • E. g. ethane + chlorine 1, 2 -dichloroethane
Hydration • Adding water • Adding H and OH to the two atoms in a C=C double bond • Heat the alkene with steam • Pass the mixture over a catalyst of phosphoric acid • Ethene + water ethanol
Addition of hydrogen halides • Adding HBr or HCl usually • Form a halogenoalkane • Ethene + hydrogen bromide bromoethane • Why do you think this doesn’t show the test for saturation?
Oxidation to diols • Both addition and oxidation • A diol is a compound containing two OH (alcohol groups) • Oxidising agent – acidified potassium manganate (V) • Potassium manganate (V) provides an oxygen atom • Water in the solution provides the other oxygen atom and two hydrogen atoms • CH 2=CH 2 + [O] + H 2 O CH 2 OH-CH 2 OH • Purple to colourless (like the bromine water saturation test)
Electrophilic Addition Reactions • Reactions of alkenes are typically electrophilic addition reactions • Electrophile • Addition • 4 e- in the double bond make it a centre of high electron density • Electrophiles are attracted to it • Can form a bond by using 2 of the 4 e- in the C=C (the 2 that are in the π bond) • The electrophile may be a positively charged ion OR have a positively charged area
Mechanism Main Steps 1. The electrophile is attracted to the double bond 2. Electrophiles are positively charged and accept a pair of efrom the double bond. 3. A positive ion (a CARBOCATION) is formed 4. A negatively charged ion forms a bond with the carbocation
1. Reaction with Hydrogen Halides HBr and Ethene H H C H H + Br - H H H C C : Br- H + H H C C Br H Bromoethane • The H-Br bond is polar H+ electrophile by breaking H-Br bond • Electrons from the weak π bond of C=C are attracted to the +H atom • The H-Br bond breaks – e- move to more electronegative Br • This produces an intermediate carbocation (C+) and a : Br- ion - • Lone pair of electrons on the : Br form a new bond with the C+ H
2. Reaction of Alkenes with Halogens: Br 2 with Ethene H H C H Br + Br - H H H C C + : Br- H Br H H H C C Br Br 1, 2 -Bromoethane Exactly parallel to addition of HBr except Br 2 dipole is induced This is the test for unsaturation!! H
Notes about Alkenes and Halogens • React very rapidly with chlorine gas or with solutions of bromine and iodine in an organic solvent • It gives you dihaloalkanes • The halogen atoms add across the double bond • Halogen molecules act as electrophiles
Why do you think the reaction happens very quickly? ? HINT: Instantaneous dipole
• At any instance, a halogen molecule is likely to have an instantaneous dipole e. g. Brδ+ – Br δ-. • The δ+ end of this dipole is attracted to the electron-rich double bond in the alkene; the bromine molecule has become an electrophile.
3. With Concentrated Sulfuric Acid At room temperature, alkenes react with concentrated sulphuric acid whose structure is: It is an exothermic reaction The electrophile is the partially positively charged H atom in the sulfuric acid molecule This allows the production of alkylhydrogensulphates
3. Conc H 2 SO 4 to Ethene H H C C H H + C C + H H C C H H Ethyl hydrogen sulphate + H 2 O H H C C H HO 3 SO H Ethyl hydrogen sulphate H SO 3 H HO 3 SO H : O- O - H H H C C HO H Ethanol H + H 2 SO 4
Reaction with water • Water also adds on across the double bond in alkenes. • The reaction is used industrially to make alcohols and is carried out with steam, at a suitable temperature and pressure, using an acid catalyst such as phosphoric acid (see this in Equilibria Topic).
Reaction with Asymmetrical Alkenes With propene: e. g. CH 2=CHCH 3(g) +H 2 SO 4(l) CH 3 CH(OSO 2 OH)CH 3(l) propylhydrogensulphate The reaction with alkenes other than ethene proceeds via the more stable carbocation. The main use of this reaction is that the product produces an alcohol on heating with water. 17
Reminder : Electrophilic Addition of HBr to Ethene H H C C + H + Br - H H H : Br- e. g. 1 But-2 -ene + HBr 2 -Bromobutane ONLY e. g. 2 Hex-3 -ene + HBr 3 -Bromohexane ONLY H C C Br H Bromoethane Symmetry of ethene SINGLE addition product Similarly for any SYMMETRICAL alkene H H
Addition to Unsymmetrical Alkenes H H C C + H Br H CH 3 H H H C H eg Propene + HBr C + H Br CH 3 Is 1 -bromopropane or 2 -bromopropane formed? H H H C C H H Br H C C Br H CH 3 BOTH, but ---- NB The major product depends on the TYPE OF INTERMEDIATE CARBOCATION corresponding to that product
Three types of carbocation PRIMARY SECONDARY H C C H + + at end of C chain TERTIARY H C C C + + at middle of C chain C C + + at junction of 2 C chains Increasing stability because MORE (1 2 3) alkyl groups “push” etowards C+, thus stabilising it Unsymmetrical alkenes form the product related to the MORE STABLE carbocation
The Positive Inductive Effect • • • Alkyl groups have a tendency to release electrons. This is known as a positive inductive effect. Sometimes we can see this effect by an arrow along the bond to show the direction of release:
Electrophilic Addition of HBr to Propene δ+ H H δBr H CH 3 C H or δ- Br H (2 -bromopropane) is the predominant product CH 3 C C H : Br- + H H H CH 3 C C : Br- H + H Primary carbocation H CH 3 C C H H Br 2 -Bromopropane or Secondary carbocation or δ+ H H CH 3 C C H Br H 1 -Bromopropane because reaction proceeds via MORE STABLE SECONDARY CARBOCATION
Task Decide which is the major product in the following reactions. You must draw out each mechanism: 1. 2. 3. 2 -methylpropene + hydrogen bromide Pent-2 -ene + water 2 -methyl-but-2 -ene + hydrochloric acid
eg 1. 2 -methylpropene + HBr ? H+ (CH 3)2 C=CH 2 + H-Br + (CH 3)2 CH-CH 2 Primary or + Br- + (CH 3)2 C-CH 3 Tertiary - more stable (CH 3)2 CBr-CH 3 NOT (CH 3)2 CH-CH 2 -Br
eg 2. Pent-2 -ene + H 2 O ? H+ CH 3 CH 2 CH=CHCH 3 + H-OH + CH 3 CH 2 -CHCH 3 Secondary or + (OH-) + CH 3 CH 2 CH-CH 2 CH 3 Secondary - EQUALLY stable CH 3 CH 2 -CH(OH)CH 3 and CH 3 CH 2 CH(OH)-CH 2 CH 3
eg 3. 2 -methyl-but-2 -ene + HCl H+ (CH 3)2 C=CHCH 3 + H-Cl + (CH 3)2 CH-CHCH 3 Secondary or + Cl- + (CH 3)2 C-CH 2 CH 3 Tertiary - more stable (CH 3)2 CCl-CH 2 CH 3 NOT (CH 3)2 CH-CHBr. CH 3
Summary Questions 1. 2. 3. 4. 5. Write the equation for the complete combustion of propene. What is the typical reaction of alkenes? What are the two possible products of the reaction between propene and hydrogen bromide? Which is the main product? Explain why this product is the more likely. What is the product of the reaction between ethane and hydrogen chloride? Which tests for a carbon-carbon double bond?
Apply your Knowledge! • Past paper Questions • 17 Marks: 17 Minutes
- Hydration vs hydrogenation
- Halogenation of alkenes
- 2 methylpropene + hbr
- Addition of halogens to alkenes
- Alkenes reactions and synthesis
- Reactions of halogenoalkanes 2 chemsheets answers
- Caron-carbon triple bonds are electrophiles.
- Hydrogenation of glyceryl trilinoleate
- Hydrogenation of oils
- What is hydrogenation
- Disiamylborane reaction with alkyne
- Free radical halogenation
- Halogenation of alkynes
- Saturated bond
- Halogenation of benzene
- Halogenation equation
- E or z configuration
- Halogenation of thiophene
- Heat of hydration
- Is acid catalyzed hydration syn or anti
- Chapter 8 nutrition and hydration
- Wrestling weight loss calculator
- Is acid catalyzed hydration syn or anti
- Pros and cons of artificial nutrition and hydration
- Nutrition and hydration chapter 15
- Hydration alkene
- Examples for chemical combination reaction
- Polar molecule
- Mercury catalyzed hydration of alkynes