6C.18-6C.23 - Alkenes And Addition Reactions

Recognising Alkenes And Unsaturation

Imagine two hydrocarbons with the same number of carbon atoms: propane, $\ce{C3H8}$, and propene, $\ce{C3H6}$. Propene has fewer hydrogen atoms because it contains a carbon-carbon double bond. That double bond is not just a formula detail; it is the functional group that gives alkenes their characteristic addition reactions.

Ethene is $\ce{C2H4}$, propene is $\ce{C3H6}$, and butene is $\ce{C4H8}$.

Alkenes and cycloalkenes are unsaturated hydrocarbons.

In addition reactions, atoms can add across the C=C bond without needing to replace an atom already present.

Cycloalkenes also contain a C=C bond, so they are unsaturated, but do not assume they use the same simple formula as non-cyclic mono-alkenes. Cyclohexene is $\ce{C6H10}$: the ring and the double bond each reduce the number of hydrogen atoms compared with the open-chain alkane formula $\ce{C_{n}H_{2n+2}}$. For this lesson, use $\ce{C_{n}H_{2n}}$ for ordinary one-double-bond, non-cyclic alkenes, and use the displayed or structural formula to decide whether a cycloalkene is present.

Common-error contrast

The Sigma And Pi Bonds In C=C

A carbon-carbon double bond is not two identical single bonds. It contains one sigma bond and one pi bond.

The sigma bond's electron density lies on the internuclear axis, so it holds the carbon atoms together strongly; the pi electron density is more exposed.

[DIAGRAM: asset_name: Alkene Bonding Addition Map; asset_slug: edexcel_a_level_chemistry_l032_alkene_bonding_addition_map; recommended_method: deterministic_chart; description: Side view of ethene p orbitals with sideways overlap above and below the plane, a sigma bond along the C–C axis, and an explanation of bonding changes during addition.]

The pi bond is the more exposed part of the double bond. Its electron density can attract electrophiles, so the pi bond is usually the part of the C=C bond involved first in addition reactions. When an alkene reacts by addition, the C=C double bond becomes a $\ce{C-C}$ single bond and two new sigma bonds are formed to the atoms added.

There is also a structural consequence: rotation about a C=C bond is restricted. Rotating would break the sideways overlap that makes the pi bond. That restricted rotation matters for E/Z stereoisomerism, but the main use in this lesson is simpler: the double bond is a fixed, electron-rich functional group.

Worked example: count the bonds in ethene

Ethene has the displayed structure $\ce{H2C=CH2}$.

• Each $\ce{C-H}$ bond is a sigma bond, so there are four $\ce{C-H}$ sigma bonds.
• The C=C bond contains one $\ce{C-C}$ sigma bond and one $\ce{C-C}$ pi bond.
• Total: five sigma bonds and one pi bond.

The chemical interpretation is that the pi bond is not an extra sigma bond. It is a different type of overlap, and its exposed electron density explains why alkenes react differently from alkanes.

Electrophiles And Heterolytic Fission

An electron-rich alkene can supply the pair of electrons that forms a bond to an electron-deficient atom.

The word means "electron-loving", but the key definition is not about liking electrons; it is about accepting a pair of electrons to form a new covalent bond.

Electrophiles are attracted to regions of high electron density. A positive ion such as $\ce{H+}$ is an electrophile because it can accept an electron pair. A molecule with a strongly delta-positive atom can also behave as an electrophile in a reaction, because the electron-rich pi bond can be attracted to that atom.