Electrophilic Addition Reactions (DP IB Chemistry): Revision Note

Electrophiles

What is an electrophile?

• An electrophile is a species that forms a covalent bond when reacted with a nucleophile by accepting electrons

• Electrophiles are electron-deficient and typically have a full or partial positive charge

Examples of neutral and charged electrophiles

• Electrophiles can be either neutral or positively charged:

  • Neutral electrophiles:

    • HX (hydrogen halides)

    • X₂ (halogens)

    • H₂O

    • RX (halogenoalkanes)

  • Positively charged electrophiles (cations):

    • H⁺

    • NO₂⁺ (nitronium or nitryl ions)

    • NO⁺ (nitrosonium or nitrosyl ion)

    • R⁺ (carbocations)

Electrophilic addition reactions

• Electrophilic addition is the reaction of an electrophile with a carbon–carbon double bond, C=C

• The C=C double bond in alkenes is an area of high electron density

  • This makes it attractive to electrophiles

• The C=C bond breaks forming:

  • A single C-C bond

  • A new bond from each carbon to the electrophile

• Electrophilic addition reactions include the addition of:

  • Steam, H₂O (g), to form alcohols

  • Hydrogen halides, HX , to form halogenoalkanes

  • Halogens, X₂, to form dihalogenoalkanes

Why does the C=C bond react with electrophiles?

• Alkenes are unsaturated molecules that contain a C=C bond

• The atoms around the C=C double bond adopt a planar arrangement with bond angle of 120^o

Diagram to show the planar arrangement of the C=C bond

• The C=C bond consists of one σ-bond and a weaker π-bond

• The π-bond can be broken and replaced by stronger σ-bonds with incoming atoms or groups

• This allows alkenes to undergo addition reactions, which are not possible for saturated alkanes

• As a result, alkenes are more reactive than alkanes

Diagram to show the general equation for addition reactions across the C=C

Addition of water (hydration)

• Hydration occurs when alkenes are treated with:

  • Steam at 300 ^oC

  • A pressure of 60 atmospheres 

  • An acid catalyst such as sulfuric acid (H₂SO₄) or phosphoric acid (H₃PO₄) catalyst

• Water is added across the double bond converting the alkene into an alcohol:

ethene ethanol

CH₂CH₂  CH₃CH₂OH

• The reaction processes via an intermediate in which H⁺ and HSO₄^-  ions are added across the double bond

  • This intermediate is quickly hydrolysed by water, regenerating the sulfuric acid catalyst

• This reaction is used industrially to produce large quantities of ethanol, a common solvent and fuel.

• It is faster and more efficient than ethanol production by fermentation.

Addition of halogens (halogenation)

• Alkenes react with halogens in an electrophilic addition reaction known as halogenation

• The π-bond in the C=C double bond breaks and is replaced by two new C–X bonds, forming a dihalogenoalkane

• This reaction occurs readily at room temperature

• Halogens like Br₂ can be used to test if a molecule is unsaturated (i.e. contains a C=C double bond)

• Bromine water is an orange/yellow solution of Br₂ (aq)

• When an unknown compound is shaken with bromine water:

  • If a C=C bond is present, an addition reaction occurs

    • The coloured solution becomes decolourised

  • If no C=C bond is present, there is no reaction

    • The coloured solution remains orange/yellow

Diagram to show the colour change that occurs when testing for unsaturation

Addition of hydrogen halides (hydrohalogenation)

• Alkenes react with hydrogen halides (e.g. HCl, HBr) to form halogenoalkanes

• This is an electrophilic addition reaction that occurs rapidly at room temperature

• All hydrogen halides react with alkenes in this way

• The reaction is fastest in the order HI > HBr > HCl

  • This is because weaker H–X bonds break more easily