An electrophile is an electron pair acceptor. The name means 'electron-loving': electrophiles are attracted to regions of high electron density. They are either positive ions or molecules with a δ+ atom.The C=C bond is a region of high electron density. Its four electrons are concentrated between the two carbons, and the two π electrons sit exposed above and below the plane of the molecule. This makes alkenes open to attack by electrophiles, and explains why they react far more readily than alkanes, which have no π bond.
Common electrophiles in alkene chemistry:
• H+ ions from acids.
• Hydrogen halides such as H–Br. Br is more electronegative than H, so the bond is polar: Hδ+–Brδ–. The δ+ hydrogen is the electrophile.
• Halogens such as Br2. Br–Br is non-polar, but as it approaches a C=C the high electron density repels the electrons in the Br–Br bond. A dipole is induced, and the nearer Br atom becomes δ+.
• The δ+ hydrogen in H2SO4.Compare a nucleophile, which is an electron pair donor (it has a lone pair), such as OH– or NH3.In mechanisms, a curly arrow shows the movement of a pair of electrons. It starts at the electron pair (a bond or lone pair) and points to the atom that accepts it. When an alkene reacts with an electrophile, the first curly arrow always starts at the C=C bond and points to the electrophile.
Common electrophiles in alkene chemistry:• H+ ions from acids.
• Hydrogen halides such as H–Br. Br is more electronegative than H, so the bond is polar: Hδ+–Brδ–. The δ+ hydrogen is the electrophile.
• Halogens such as Br2. Br–Br is non-polar, but as it approaches a C=C the high electron density repels the electrons in the Br–Br bond. A dipole is induced, and the nearer Br atom becomes δ+.
• The δ+ hydrogen in H2SO4.Compare a nucleophile, which is an electron pair donor (it has a lone pair), such as OH– or NH3.In mechanisms, a curly arrow shows the movement of a pair of electrons. It starts at the electron pair (a bond or lone pair) and points to the atom that accepts it. When an alkene reacts with an electrophile, the first curly arrow always starts at the C=C bond and points to the electrophile.