From TB ( 1 )Alkenes & Electrophilic addition reaction

What is Alkenes ?

→ General Formula Cn H2n + 2

→ At least one C=C double covalent bond

→ Unsaturated : they can make more bonds with an extra atoms in addition reactions

→ Reactive + high electron density : there’s two pairs of electrons in the C = C bonds

With cyclic Alkene : there’s 2 few H atom than an open chain, in a C = C can make one less bond with hydrogen.

Electrophilic Addition Reaction

→ The double bond in an Alkene opens up and atoms are added to the carbon atoms

→ the double bond has plenty of electrons, and is easily attacked by electrophilies

( electron lone pair acceptors ) < short of electron, so attracted to ares with high electrons.

Example 1 : Ethene with an electrophile X-Y

CH2 CH2 + X-Y → CH2 X CH2 Y

• The double C=C repels the electron in X-Y, which polarises the X-Y bond

Carbocation Intermediate :

The 2 electron from the C = C attacks the s+ X atom → to form new bond between carbon 1 and the X atom.

The X—Y bond breaks and the electron from the bond are taken by the Y atom to form a negative ion with a lone pair of electron.

Carbon 2 is left with a positive charge , since the double bond is broke carbon 1 and took the electron to form a bond with X atom.

The Y- ion acts as a nucleophile, attacking the positively charge carbocation, donating its lone pair of electron → forming a new bond with carbon .

The X-Y molecule has been added to the Alkene across the double bond to form a saturated compound. Alkane *

Example 2 : Mechanism with ethene with water → H2 SO4 —> Ethanol

The C =C bond attacks a hydrogen s- atom on the H2 SO4 molecule → new bond is form between one of the carbon and hydrogen, and the electron from the O -H bond are taken by the oxygen atom ( lone pair )

The 2nd carbon is let with a positive charge : it has lost the electron from the double bond.