L7 - Reactions of Hydrocarbons

objectives

• investigate, write equations and construct models to represent the reactions of unsaturated hydrocarbons when added to a range of chemicals, including but not limited to:
  • hydrogen (H2)
  • halogens (X2)
  • hydrogen halides (HX)
  • water (H2O)
• investigate, write equations and construct models to represent the reactions of saturated hydrocarbons when substituted with halogens

ELECTROPHILIC ADDITION MECHANISM

Electrophillic Addition - Mechanism

Electrophilic Addition of Halogen Halides to Alkenes

Reagent:

• Hydrogen bromide... it is electrophilic as the H is slightly positive

Condition: Room temperature. NO Catalyst

Equation:

C₂H₄(g) + HBr(g) ———> C₂H₅Br(l) bromoethane

Mechanism

Electrophilic Addition of Bromine to an Alkene

Reagent:

• Bromine (Neat liquid or dissolved in tetrachloromethane, CCl₄ )

Condition:

• Room temperature. No catalyst or UV light required!

Equation:

C₂H₄(g) + Br₂(l) ——> CH₂BrCH₂Br(l) 1,2 - dibromoethane

Mechanism

Testing for Saturation using Br Water

The addition of bromine dissolved in tetrachloromethane (CCl₄) or water (known as bromine water) is used as a test for unsaturation. If the reddish-brown colour is removed from the bromine solution, the substance possesses a C=C bond.

Because the bromine adds to the alkene, it no longer exists as molecular bromine and the typical red-brown colour disappears

Testing for unsaturation using KMnO₄

• As an oxidising agent the Potassium Permanganate can react with a double bond to change colour from Purple to orange - if no double or triple bond is present it will stay Purple
• Although this could indicate unsaturation it is not enough on it's own as it also reacts with aldehydes and alcohols

Reaction of Halogens with Alkenes

• Alkenes are much more reactive than alkanes because the C=C moiety is a reactive functional group.
• A π bond, being a weaker bond, is disrupted much more easily than a σ bond.
• Thus, alkenes undergo a characteristic reaction in which the π bond is broken and replaced by two σ bonds.
• This reaction is called an addition reaction.
• The hybridization of the carbon atoms in the double bond in an alkene changes from sp2 to sp3 during an addition reaction.
• For example, halogens add to the double bond in an alkene instead of replacing hydrogen, as occurs in an alkane:

Reactions of Alkynes with Halogens

Chemically, the alkynes are similar to the alkenes. Since the C≡C functional group has two π bonds, alkynes typically react even more readily, and react with twice as much reagent in addition reactions. The reaction of acetylene with bromine is a typical example:

Electrophilic Addition of HBr to Propene

• addition of HBr to propene gives two isomeric brominated compounds

• HBr is unsymmetrical and can add in two ways

• products are not formed to the same extent

• the problem doesn't arise in ethene because it is symmetrical.

Possibility 1

Possibility 2

Markownikoff's Rule

• A Russian scientist, Markownikoff, investigated the products of the addition of hydrogen halides to alkenes.
• He found that, when two products were formed, one was formed in a larger quantity.
• His original rule was based only on this reaction.
• The modern version uses carbocation stability as a criterion for predicting the products.

In the electrophilic addition to alkenes the major product is

formed via the more stable carbocation (carbonium ion)

Carbocation Stability

• Build up of charge in one place leads to instability.
• If it can be spread around or neutralised in some way, stability is increased.
• Alkyl groups are electron releasing and can “push” electrons towards the carbocations thus reducing the charge density.

In the addition to propene, path A involves a 2° carbocation, path B a 1° carbocation.

As the 2° ion is more stable, the major product (i.e. 2-bromopropane) is formed this way.

Reaction of Water with Alkenes (Direct Hydration)

Reagent:

• steam

Conditions:

• high pressure

Catalyst:

• phosphoric acid

Product:

• alcohol

Equation:

C₂H₄(g) + H₂O(g) --> C₂H₅OH(g) ethanol

Use:

• ethanol manufacture

Comments:

• It may be surprising that water needs such vigorous conditions to react with ethene.
• It is a highly polar molecule and you would expect it to be a good electrophile.
• However, the O-H bonds are very strong so require a great deal of energy to be broken.
• This necessitates the need for a catalyst.

Reaction of Hydrogen with Alkenes (Hydrogenation)

Reagent:

• hydrogen

Conditions:

• nickel catalyst - finely divided

Product:

• alkanes

Equation:

C₂H₄(g) + H₂(g) ———> C₂H₆(g) ethane

Use:

margarine manufacture