Alkene Structure and Reactivity

Markovnikov addition in alkenes explains why one product often forms in greater amount during hydrohalogenation. When an unsymmetrical alkene reacts, two haloalkane constitutional isomers are possible. In most cases, hydrogen adds to the vinylic carbon with more hydrogens, and the halide part adds to the more substituted carbon.

This preference is called regioselectivity. Regioselective reactions favor bond formation at one site over another, so one constitutional isomer becomes the major...

Alkenes are unsaturated hydrocarbons with one or more carbon-carbon double bonds. They are also called olefins, and they are broadly divided into alkenes and cycloalkenes. The general formula for an alkene is CnH2n.

The carbons in a double bond are sp2 hybridized. This gives each carbon a trigonal planar shape. The double bond has two parts: a sigma bond from overlap of hybrid orbitals and a pi bond from side-by-side overlap of unhybridized 2p orbitals. Each carbon also bonds to two hydrogen...

IUPAC rules for alkene naming show how to name carbon chains with a double bond. A simple alkene changes the alkane suffix -ane to -ene. The double bond must be part of the parent chain, which is the longest carbon chain that contains the maximum number of double bonds.

The chain is numbered so the doubly bonded carbon atoms get the lowest possible numbers. The location of the double bond is given by the number of its first carbon atom. In branched alkenes, the double bond takes priority over...

Degree of unsaturation helps count rings and double bonds in an organic molecule. It is also called the index of hydrogen deficiency, or IHD. The idea is based on comparing a compound with the acyclic alkane that has the same number of carbon atoms.

Each double bond or ring removes two hydrogen atoms compared with a saturated analog. That loss equals one degree of unsaturation. For hydrocarbons, the formula is U = (2C + 2 - H) / 2, where C is the number of carbon atoms and H is the number of...

Alkene isomers can differ in both bond position and spatial arrangement. 1-butene and 2-butene are constitutional isomers because the double bond is in a different place. 2-butene also shows stereoisomerism, which means its atoms are connected the same way but arranged differently in space.

In 2-butene, the two methyl groups can sit on the same side of the double bond or on opposite sides. When the methyl groups are on the same side, the compound is called cis-2-butene. When they are on...

Alkene stability depends on heats of hydrogenation and on the structure around the double bond. A lower heat of hydrogenation means the alkene is more stable. Three main factors help explain these differences: the number of substituents on the double-bond carbons, hyperconjugation, and double-bond stereochemistry.

Substitution across the double bond has a strong effect on stability. An alkene with two smaller substituents is more stable than its isomer with one large substituent. For example,...

Alkene addition reactions show how a carbon-carbon double bond reacts with electrophiles and changes during the process. A simple, unconjugated alkene has a double bond with high electron density, so it can act as a weak base or a nucleophile. Its filled π orbital, called the HOMO, can interact with the empty LUMO of an electrophile.