similar: they are colorless, nonpolar, and combustible. The physical state depends on molecular mass: like the corresponding saturated hydrocarbons, the simplest alkenes (ethylene, propylene, and butene) are gases at room temperature. Linear alkenes of approximately five to sixteen carbon atoms are liquids, and higher alkenes are waxy solids. The melting point of the solids also increases with increase in molecular mass. Alkenes generally have stronger smells than their corresponding alkanes. Ethylene has a sweet and musty odor. The binding of cupric ion to the olefin in the mammalian olfactory receptor MOR244-3 is implicated in the smell of alkenes (as well as thiols). Strained alkenes, in particular, like norbornene and trans-cyclooctene are known to have strong, unpleasant odors, a fact consistent with the stronger π complexes they form with metal ions including copper. Reactions Alkenes are relatively stable compounds, but are more reactive than alkanes. Most reactions of alkenes involve additions to this pi bond, forming new single bonds. Alkenes serve as a feedstock for the petrochemical industry because they can participate in a wide variety of reactions, prominently polymerization and alkylation. Except for ethylene, alkenes have two sites of reactivity: the carbon–carbon pi-bond and the presence of allylic CH centers. The former dominates but the allylic sites are important too. Addition reactions Alkenes react in many addition reactions, which occur by opening up the double-bond. Most of these addition reactions follow the mechanism of electrophilic addition. Examples are hydrohalogenation, halogenation, halohydrin formation, oxymercuration, hydroboration, dichlorocarbene addition, Simmons–Smith reaction, catalytic hydrogenation, epoxidation, radical polymerization and hydroxylation. Hydrogenation and related hydroelementations Hydrogenation of alkenes produces the corresponding alkanes. The reaction is sometimes carried out under pressure and at elevated temperature. Metallic catalysts are almost always required. Common industrial catalysts are based on platinum, nickel, and palladium. A large scale application is the production of margarine. Aside from the addition of across the double bond, many other 's can be added. These processes are often of great commercial significance. One example is the addition of H-SiR3, i.e., hydrosilylation. This reaction is used to generate organosilicon compounds. Another reaction is hydrocyanation, the addition of across the double bond. Hydration Hydration, the addition of water across the double bond of alkenes, yields alcohols. The reaction is catalyzed by phosphoric acid or sulfuric acid. This reaction is carried out on an industrial scale to produce synthetic ethanol. CH2=CH2 + H2O -> CH3-CH2OH Alkenes can also be