Alkenes

AlkenesAlkenes 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 C H (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 2n

n=2

n=3

n=4

n=5

n=6

ethene

propene

but-1-ene but-2-ene

pent-1-ene pent-2-ene

hex-1-ene hex-2-ene hex-3-ene

*

*

* *

* cis/trans (E/Z) isomers also possible

Alkenes - ReactivityAlkenes 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.

H

C

H H

H

C𝜎

𝜋

C-C 346 kJ/molC=C 612 kJ/mol

The bond in alkenes is a region of high electron density and so is therefore prone to attack from electrophiles and oxidising agents.

Alkenes – Addition ReactionsThe characteristic type of reactions alkenes undergo are ‘addition reactions’ which involve the double bond. These often involve electrophiles.

H H

H H

H X

H X

X X

X X

H OH

H OH

Addition of Hydrogen

Addition of Hydrogen Halides (HCl, HBr, HI)

Addition of Halogens (Br2 , I2)

Addition of Water

alkane

haloalkane

di - haloalkane

alcohol

Hydrogenation reactions add two hydrogens and so an alkane is produced. These reactions require a catalyst such Ni, Pt or Pd.

H H

H HNi

H2+Ni

http://www.indiana.edu/~oso/Fat/trans.html

Alkenes – Addition of HCl, HBr and HIAlkenes react readily (room temperature) with concentrated aqueous hydrogen halides to form haloalkanes.

H

H

= Cl, Br or I

For example

HC

H

H

H+ HBrC H C

H

H

Br

C

H

H

(aq)

1-bromoethane

H Brd d+ -

HC

H

H

HC H C

H

H

H

C

H+

Br -

H C

H

H

Br

C

H

H

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:

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.

H

CH

H

H+ HBrC

H C

H

H

Br

C

H

H

(aq)

1-bromopropaneC

C

H

H

H C

H

H

Br

C

H

H

2-bromopropane

C

H

H

H

H

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

CH

H

H+ HBrC

(aq)

CH

H

H C

H

H

Br

C

H

H

C

H

H

1 2

Markownikov’s rule can be explained by considering the stability of the of the cabocations produced in each possible route.

H

CH

H

HC

CH

H

HBr(aq)

H C

H

H

Br

C

H

H

C

H

H

H C

H

HC

H

H

C

H

H

Br-

-

+

+H C

H

H

Br

C

H

H

C

H

H

H C

H

H

Br

C

H

H

C

H

H

A

B

primary carbocation

secondary carbocation

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.

1-bromopropane

2-bromopropane

CH

CCH3

CH3

CH3

2-methylbut-2-ene

HBr(aq)

C H

Br

C

H

CH3

CH3CH3

+

2-bromo-2-methylbutane

Markownikov’s rule predicts that the reaction between 2-methylbut-2-ene and HBr should produce predominantly 2-bromo-2-methylbutane, which it does.

This again can be explained by the stability of the carbocation leading to the predicted product being more stable than the alternative.

C HC

H

CH3

CH3CH3

C HC

H

CH3

CH3CH3

+ +

secondary carbocation

tertiary carbocation

This demonstrates the order of stability of carbocations:

tertiary carbocation

secondary carbocation primary carbocation> >

3[R C]

+[R CH]

2+

[RCH ]2+

Use Markovnikov’s rule to predict the major product of the following reactions…

+ HBr

+ HBr

+ HBr

+ HCl

Alkenes – Addition of Halogen (Cl or Br )2 2

Testing for Alkenes with Bromine

https://www.youtube.com/watch?v=PE1CDR1S5pk

Alkenes – Addition of Halogen (Cl or Br )2 2

X = Cl or BrX X

X X

For example

HC

H

H

H+ BrC H C

H

H

Br

C

H

2

1,2-dibromoethane

Br

ethene

Alkenes react readily with chlorine or bromine at room temperature to yield dichloro- or dibromo-alkanes.

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 𝜋 bond.

HC

H

H

HC

Cl

Cl

H CH

HH

C

Cl

+

Cl-

H C

H

H

Cl

C

H

Cld +

d -

Alkenes – Addition of H O2Alkenes react with steam under high pressures (6MPa) and high temperatures (600K) using H PO as a catalyst on an inert support to produce alcohols.

3 4

HC

H

H

H+ H OC H C

H

H

OH

C

H

H

ethene

2

H+

ethanol

CH

H

H+ H OC H C

H

OH

C

H

H

propene

2

H+

Propan-2-ol

CH3C

H

H

H

*

* The predominant product is arrived at via the most stable (in this case secondary) carbonium ion.

For example

+H

+

OH2

O

HH +OH

+H

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.

+

The predominant product of the acid-catalysed addition of water to alkenes is the one arrived via the most stable carbocation:

H+

+

+

secondary carbocation

primary carbocation

main product

Alkenes – Addition of H O and X (X= Cl or Br)2 2

If chorine or bromine, along with water are allowed to react with an alkene then a halohydrin is formed.

X , H O

= Cl or BrX

OH X

2 2

For example

Cl , H O2 2

OH

Cl

propene 1-chloropropane-2-ol

+ HX

+ HCl

Cl Cl Cl

Cl

+

-OH2

Cl

O

HH +

Cl

OH

+H

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

C C

HH

H H

C C

H H

H H

n

n

Ethene Poly(ethene) - Polythene

C C

ClH

C C

H Cl

H H

n

n

H H

Chloroethene - vinyl chloridePoly(chloroethene – PVC – Poly(vinylchloride)

n=several thousand

C C

CH3H

H H

H CH3

C C

H H

n

n

Propene Poly(propene)

Some examples