Bonding in alkenes involves a double covalent bond, a centre of high…

The idea

The carbon–carbon double bond is made of two different bonds. One is a σ bond from head-on overlap of orbitals directly between the two carbons; the other is a π bond, formed by sideways overlap of p orbitals, giving electron density above and below the plane of the molecule. Those π electrons are exposed and held less tightly than σ electrons, so the double bond is a region of high electron density that attracts electrophiles, species that accept a pair of electrons. That is the whole basis of alkene chemistry. Two other consequences follow: the three groups around each double-bond carbon lie in a plane at about 120°, and rotation about the double bond is restricted, which is why E-Z stereoisomers exist.

What earns the marks

• Describing the π bond as sideways overlap of p orbitals above and below the plane.
• Stating that the high electron density attracts electrophiles.
• Defining an electrophile as an electron pair acceptor.
• Linking restricted rotation about the C=C to the existence of E-Z stereoisomers.

Common mistakes and how to put them right

• Saying the double bond attracts nucleophiles, reversing the whole of alkene chemistry.

• Describing the C=C as two identical bonds rather than one σ and one π bond.

• Claiming free rotation about the double bond, which would remove E-Z isomerism.

• Giving the bond angle around the double bond as 109.5° instead of about 120°.

Exam tips

• Whenever you write "high electron density", follow it immediately with "attracts electrophiles" to secure the link.
• If a question asks for the origin of stereoisomerism in an alkene, the phrase wanted is restricted rotation about the C=C.
• Draw the π electron density above and below the plane, not around the whole molecule.