Alkenes

1. Alkenes • General Formula CnH2n • Unsaturated hydrocarbons. • Each alkene contains one carbon-to-carbon double bond (two double bonds would be a DIENE, etc). • They are important feedstocks for the production of • Polymers • Antifreeze • Solvents

2. H H C C H H 120° 120° 120° 120° 120° 120° Bonding in Alkenes • Ethene is a planar (flat) molecule with bond angles of 120°

3. C-C C=C Bond Enthalpy 346 602 -1 ( kJmol ) Bond Length 0.154 0.134 ( nm) • Carbon-to-carbon double bond is shorter than a single bond. • Carbon-to-carbon double bond is stronger, but NOT twice as strong. (see Bond Enthalpy values)

4. CH3 CH3 CH3 H C C C C H H H CH3 Where as CH3 CH3 CH3 H C C C C H H H CH3 • There is not free rotation about the carbon-to-carbon double bond. i.e

5. Hybridisation in Alkenes • Reminder - In alkanes the 2s and all three 2p orbitals mix to form four sp3 orbitals. • In alkenes the 2s and only two 2p orbitals mix forming three equivalent sp2 orbitals, leaving one unhyridised p orbital. i.e.

6. Unhybridised p orbitals • The sp2 orbitals lie on a plane (@ 120° to one another) • The unhybridised p orbital is perpendicular to the plane. • sp2 orbitals can form  bonds with hydrogen and carbon atoms. i.e. Remember - to be a  bond it must lie on the axis between two atoms.

7. The remaining two parallel 2p orbitals are actually close enough to overlap. • This results in a new molecular orbital called a pi () bond (made up of only one molecular orbital, part above and part below the  bond). • The  bond pulls the carbon atoms closer together, hence a shorter bond length. •  bond has less effective overlap than  bond, hence ( bond +  bond) strength  2   bond strength.

8.  bond cannot rotate as this would reduce overlap, and eventually  bond would break (N.B. rotating a  bond does not reduce overlap)

9. Synthesis of Alkenes • Main industrial source is cracking of hydrocarbon sources such as • ethane (CH3-CH3 CH2=CH2 + H2) • natural gas liquids • naptha • gas oils

10. Laboratory preparation - Two main methods. Elimination Reactions Method 1 -Dehydration of appropriate alcohol CH3-CH2- CH=CH2 (but-1-ene) e.g. CH3-CH2 -C(OH)H-CH3 CH3-CH=CH-CH3 (but-2-ene) (Butan-2-ol) Method 2 - Reaction of monohalogenalkane withpotassium hydroxide in ethanol solution e.g. CH3-CH2 -CH2Br  CH3-CH=CH2 + HBr

11. Reactions of Alkenes • Addition reactions • Hydrogenation • Halogenation • Hydrogen halide addition (hydrohalogenation) • Acid catalysed addition of water (acid catalysed hydration)

12. Alkenes - Hydrogenation RCH=CHR + H2  RCH2- CH2R • Addition of hydrogen across the double bond, forming an alkane. • Slightly exothermic. • Slow at room temperature. • Higher temperatures and a catalyst are required. • Catalysts work by adsorbing H2 onto their surfaces.

14. Alkenes - Halogenation • Addition of Cl2, Br2 or I2 across the double bond, forming a dihalogenoalkane. • Cl2 reacts most vigorously, followed by Br2 then I2.

15. Mechanism of Halogenation Step 1 – Electrons in the double bond repel electrons in the bromine molecule, causing temporary polarisation and an electrophilic bromine atom.

16. Step 2 – The pair of electrons from the  bond ‘attacks’ the electron deficient bromine atom. The Br-Br bond breaks heterolytically (the ‘curly’ arrows show the movement of 2 electrons). This results in the formation of the ‘bromonium ion’. This step is the Rate Determining Step (RDS). bromonium ion bromide ion

17. Step 3 – The nucleophilic attack by the bromide ion. The bromide ion attacks one of the carbon atoms to give 1,2-dibromoethane. 1,2-dibromoethane • The addition of chlorine is thought to have a similar mechanism

18. Alkenes - Hydrogen Halide Addition Major product Minor product • Why is 2-bromopropane the major product? • Markovnikov’s Rule • “When a compound HX is added across a carbon-to-carbon double bond, the hydrogen becomes attached to the carbon atom of the double bond that is already bonded to the greater number of hydrogen atoms”

19. Mechanism of Hydrogen Halide Addition Step 1 – HX compounds are usually polarised. The electrophilic hydrogen atom will attack the electron rich double bond. This is the RDS

20. Step 2 – The carbocation is then attacked by the nucleophilic X- ion.

21. Markovnikov’s Rule Explained • The rule works because of the relative stability of the carbocation. • The more stable a carbocation is the more likely it is to form. • Alkyl groups can ‘donate’ electrons to adjacent atoms to stabilise the +ve charge. • The more alkyl groups surrounding the +ve carbon the more stable the ion will be.

22. e.g. The reaction of propene (CH3-CH=CH2) with a hydrogen halide. • Two carbocation intermediates are possible. • The second has more alkyl groups helping to stabilise the positive charge. • The arrows on the bonds in the diagram represent the electron donating effect of the alkyl groups.

23. Additional Notes • Hydrogen halide addition must be carried out in either the gas phase or in a non-polar solvent. • In aqueous solution an alternative mechanism is possible (acid catalysed addition of water – coming soon!).

24. Alkenes - Acid Catalysed Addition of Water • If a hydrogen halide addition is carried out in an aqueous solution a significant amount of alcohol is produced. • This is a result of the fact that hydrogen halides are strong acids. i.e. HI + H2O  H3O+ + I- • The hydrated hydrogen ion (or hydronium ion or oxonium ion) will now act as the electrophile.

25. For more complex alkenes the Markovnikov rule would apply.

26. Intermediate Type Example Reaction Summary of Intermediates Halogenation cyclic intermediate Hydrohalogenation or Acid catalysed hydration carbocation intermediate

27. Exercise • Complete the exercise on page 15 of your Unit 3(b) notes