Alkenes Alkenes Alkenes are a class of hydrocarbons that contain a C=C bond. These are ‘unsaturated’ as they have not got the maximum possible amount of hydrogen atoms attached to the carbon chain. The general formula for an alkene is Cn H 2n (n>1). Apart from the simplest two alkenes (n= 2, ethene and n=3 propene) the double bond can occupy different position in a chain (structural isomers). n=2 ethene * cis/trans (E/Z) isomers also possible n=3 propene
n=4 but-1-ene but-2-ene* n=5 pent-1-ene pent-2-ene* n=6 hex-1-ene hex-2-ene* hex-3-ene* Alkenes - Reactivity Alkenes are more reactive than alkanes because the energy required to break the double bond is less than twice the energy needed to break a single bond. C-C 346 kJ/mol C=C 612 kJ/mol
휋
The bond in alkenes is a region H H of high electron density and so C C is therefore prone to attack 휎 H H from electrophiles and oxidising agents. Alkenes – Addition Reactions The characteristic type of reactions alkenes undergo are ‘addition reactions’ which involve the double bond. These often involve electrophiles. H H H H Addition of Hydrogen alkane
H X H X Addition of Hydrogen Halides (HCl, HBr, HI) haloalkane
X X X X Addition of Halogens (Br2 , I2) di - haloalkane
H OH H OH
Addition of Water alcohol Hydrogenation reactions add two hydrogens and so an alkane is produced. These reactions require a catalyst such Ni, Pt or Pd.
H H
Ni H H
Ni + H2 http://www.indiana.edu/~oso/Fat/trans.html Alkenes – Addition of HCl, HBr and HI Alkenes react readily (room temperature) with concentrated aqueous hydrogen halides to form haloalkanes.
H = Cl, Br or I
H
For example
H Br H H C C + HBr H C C (aq) H H H H H 1-bromoethane The reaction between alkenes and hydrogen halides is an example of ‘electrophilic addition’. The hydrogen halide is polar making the hydrogen atom electrophilic.
Arrow – Pushing Mechanism:
d+ d H Br - Br - H H Br H H + H C C H C C H C C H H H H H H H Have a go at the ‘arrow pushing’ mechanism for the reaction between HCl and ethene (try not to just copy the example shown earlier – but have a quick look if you get stuck). The reaction between HBr and ethene forms only one product. With asymmetrical alkenes such as propene there are two possibilities.
Br H H
H C C C H H H H H H 1-bromopropane H C C C H + HBr H H (aq) H Br H
H C C C H H H H 2-bromopropane
In this case the predominant product is 2-bromopropane, with only a small amount of 1-bromopropane in the product mixture. The predominant product of the reaction between hydrogen halides (HCl, HBr and HI) can be predicted using Markovnikov’s Rule.
When HX adds across an asymmetric double bond, the major product is the molecule in which hydrogen adds to the carbon atom in the double bond with the greater number of hydrogen atoms already attached to it.
H H H Br H H C C C H + HBr H C C C H H 1 2 H (aq) H H H Markownikov’s rule can be explained by considering the stability of the of the cabocations produced in each possible route. - Br H H Br H H + HBr H C C C (aq) H H C C C H A H H H H H H H H primary carbocation 1-bromopropane H C C C - H B Br H H H H H Br H + H C C C H H C C C H H H H H H H secondary carbocation 2-bromopropane The secondary carbocation is more stable than the primary carbocation due to having more carbons attached to the positive carbon atom, carbon atoms in alkyl groups being electron releasing. Because this is the most stable intermediate, route B, leading to 2-bromopropane is favoured. Markownikov’s rule predicts that the reaction between 2-methylbut-2- ene and HBr should produce predominantly 2-bromo-2-methylbutane, which it does. Br H H3C H C C + HBr H C C C H H (aq) 3 H3C C 3 CH H3C 3 2-methylbut-2-ene 2-bromo-2-methylbutane This again can be explained by the stability of the carbocation leading to the predicted product being more stable than the alternative. H H + + H C C C H C C 3 H3C H H C CH CH 3 3 H3C 3 tertiary carbocation secondary carbocation This demonstrates the order of stability of carbocations: tertiary carbocation > secondary carbocation > primary carbocation + + + [R3 C] [R2 CH] [RCH2 ] Use Markovnikov’s rule to predict the major product of the following reactions…
+ HBr
+ HBr
+ HCl
+ HBr Alkenes – Addition of Halogen (Cl 2 or Br2 ) Testing for Alkenes with Bromine
https://www.youtube.com/watch?v=PE1CDR1S5pk Alkenes – Addition of Halogen (Cl 2 or Br2 ) Alkenes react readily with chlorine or bromine at room temperature to yield dichloro- or dibromo-alkanes.
X X X = Cl or Br X X
For example
Br Br H H C C + Br2 H C C H H H H H ethene 1,2-dibromoethane This is a further example of electrophilic addition. Although the Cl-Cl or Br-Br bond is not polar, a dipole is ‘induced’ as it approaches the electron rich 휋 bond.
Cl d - - Cl d + Cl Cl Cl Cl H H + C C H C C H H C C H H H H H H H Alkenes – Addition of H 2O Alkenes react with steam under high pressures (6MPa) and high temperatures (600K) using H3 PO4 as a catalyst on an inert support to produce alcohols.
H OH H H H+ C C + H O H C C 2 H H H H H ethene ethanol For example H OH H + H CH3 H C C + H O H C C C H 2 H H H H H propene Propan-2-ol*
* The predominant product is arrived at via the most stable (in this case secondary) carbonium ion. The catalyst phosphoric acid supplies a proton (H+ ) which acts as an electrophile in the first step. The carbonium ion is then attacked by water (a nucleophile). The final step is the loss of a proton, which means that the acid’s role has been catalytic.
H + H OH O OH + 2 + H H + The predominant product of the acid-catalysed addition of water to alkenes is the one arrived via the most stable carbocation:
+
secondary carbocation + H main product
+ primary carbocation Alkenes – Addition of H 2O and X2 (X= Cl or Br) If chorine or bromine, along with water are allowed to react with an alkene then a halohydrin is formed.
X = Cl or Br
OH X X 2 , H2 O + HX
For example OH
Cl Cl 2 , H2 O + HCl
propene 1-chloropropane-2-ol H H OH - + 2 Cl O
Cl Cl + Cl Cl
+ H OH
Cl
The first step of the reaction is similar to the addition of halogens to alkenes. The chlorine (induced dipole) acts as an electrophile which attacks the double bond. The carbonium ion formed (the most stable), instead of being attacked by the halide, is attacked by water. The final step is the release of a proton. Alkenes – Addition Polymerisation Under suitable conditions shorter alkenes (monomers) can react with each other to form long chains (polymers). The structure (and therefore properties) are dependent upon the monomer and the conditions. Catalysts for polymerisation include TiCl and Al(C H ) . 4 2 5 3 H H H H C C n=several thousand Some examples n C C
H H H H n
Ethene Poly(ethene) - Polythene H Cl H Cl C C n C C
H H H H n
Chloroethene - vinyl chloride Poly(chloroethene – PVC – Poly(vinylchloride)
H CH3 H CH3 C C n C C
H H H H n
Propene Poly(propene)