Topic 6Alkenes

Alkenes (olefins) are hydrocarbons with at least one C=C double bond. Their carbons are sp²-hybridized, and the electron-rich, accessible π cloud of the double bond makes electrophilic addition their most characteristic reaction. The topic also covers their structure, their stability and the methods used to prepare them.

YouTube video: carbon chains with multiple bonds: alkenes

YouTube video: the electronic structure of double bonds

Structural Features

Nomenclature

Table of boiling and melting points of alkanes and alkenes

Physical Properties

Dipole moment of cis- and trans-2-butene

Bonding Properties

Rotation barrier of the double bond: the cis and trans isomers do not interconvert

Cis-trans isomerism: relative stability

Preparation

Elimination Reactions

Reduction of alkynes: Lindlar catalyst (cis alkene) and Na/NH₃ (trans olefin)

Reduction Reactions

Wittig Reaction

Reactivity

A C=C double bond has a π electron cloud that can donate electrons to an electrophile. Hence, the most important reaction of alkenes is electrophilic addition. Addition to alkenes is the reverse of elimination:

Addition of X–Y to a double bond: ΔH° = (ΔH°π + ΔH°X–Y) − (ΔH°C–X + ΔH°C–Y) — The enthalpy of addition of X–Y is the difference between the energy of the bonds broken (π and X–Y) and that of the bonds formed (C–X and C–Y).

Bond energies (kcal/mol) and enthalpies of addition of H₂, F₂, Cl₂, Br₂, I₂, HF, HCl, HBr, HI and H₂O to a double bond: all are exothermic — Bond energies and enthalpies of addition (kcal/mol): all additions to the double bond are exothermic.

Addition reactions are exothermic and therefore thermodynamically favourable. However, in general they do not take place spontaneously.

Therefore, if a feasible reaction pathway exists, that is, one with transition states (TS) that are not too high in energy, addition reactions take place with release of energy.