hydroboration followed by hydrolysis provides an alternative approach. Reduction of the alkyne by sodium metal in liquid ammonia gives the trans-alkene. For the preparation multisubstituted alkenes, carbometalation of alkynes can give rise to a large variety of alkene derivatives. Rearrangements and related reactions Alkenes can be synthesized from other alkenes via rearrangement reactions. Besides olefin metathesis (described above), many pericyclic reactions can be used such as the ene reaction and the Cope rearrangement. In the Diels–Alder reaction, a cyclohexene derivative is prepared from a diene and a reactive or electron-deficient alkene. IUPAC Nomenclature Although the nomenclature is not followed widely, according to IUPAC, an alkene is an acyclic hydrocarbon with just one double bond between carbon atoms. Olefins comprise a larger collection of cyclic and acyclic alkenes as well as dienes and polyenes. To form the root of the IUPAC names for straight-chain alkenes, change the -an- infix of the parent to -en-. For example, CH3-CH3 is the alkane ethANe. The name of CH2=CH2 is therefore ethENe. For straight-chain alkenes with 4 or more carbon atoms, that name does not completely identify the compound. For those cases, and for branched acyclic alkenes, the following rules apply: Find the longest carbon chain in the molecule. If that chain does not contain the double bond, name the compound according to the alkane naming rules. Otherwise: Number the carbons in that chain starting from the end that is closest to the double bond. Define the location k of the double bond as being the number of its first carbon. Name the side groups (other than hydrogen) according to the appropriate rules. Define the position of each side group as the number of the chain carbon it is attached to. Write the position and name of each side group. Write the names of the alkane with the same chain, replacing the "-ane" suffix by "k-ene". The position of the double bond is often inserted before the name of the chain (e.g. "2-pentene"), rather than before the suffix ("pent-2-ene"). The positions need not be indicated if they are unique. Note that the double bond may imply a different chain numbering than that used for the corresponding alkane: C–– is "2,2-dimethyl pentane", whereas C–= is "3,3-dimethyl 1-pentene". More complex rules apply for polyenes and cycloalkenes. Cis–trans isomerism If the double bond of an acyclic mono-ene is not the first bond of the chain, the name as