Electrophilic addition reactions of alkenes

Also making alkanes

Electrophiles

Electron pair acceptors

Neucleophile

An electron pair donor

Carbocation intermediate

An organic ion containing a positively charged carbon atom

Examples of electrophiles

Positively charge molecules, polar molecules as the delta positive atom is attracted to places with lots of electrons

What is the double bind in an alkene

Nucleophilic

Why do these reactions occur

Because the double bind in an alkene has a high electron density so is easily attacked by electrophiles

General equation for reaction using ethene

H2C=CH2 + X-Y —> CH2XCH2Y

Step one of reaction mechanism

The double bond repels the electrons in the X-Y bond polarising the molecule if it isn’t already polar foriming an induced dipole. Then two electrons from the double bond attack the slightly positive X atom creating a the new bind between on 1 atom and the X atom. This leaves carbon 2 electron deficient as carbon 1 took both electrons from the electron pair. So the carbon 2 becomes a carbocation intermediate. The X-Y bond breaks (heterolytic fission) and the electrons form the bond are taken by the Y atom to form a negative ion with a lone pair of electrons

Step 2 of reaction mechanism

The Y- ion then acts a nucleophile attacking the positively charge carbocation intermediate donating its lone pair of electrons and forming a new bond with carbon 2 forming a saturated compound

Making alkanes using alkenes

The alkenes have to react with hydrogen gas to form alkanes. These reaction require a catalyst and high temperatures. E.g H2C=CH2 + H2 —> CH3CH3

Reactions with halogens

Alkenes react with halogens to form dihaloalkanes, each carbon ends up bonded to a halogen atom. This is a type of electrophilic addition reaction

Test for unsaturation

With a pipettes add 2 cm³ of orange bromine water to a test tube then with another clean pipette add 2 cm³ of the unknown solution to the test tube. Then put a stopper in the test tube and shake and record any observations in a results table. If the bromine water goes from orange to colourless the unknown was an alkene