C=C double bonds can be reduced using hydrogen gas and a metal catalyst (usually nickel or palladium) to produce fully saturated alkanes. This process is called hydrogenation and it is exothermic (that is, energy is released) since a thermodynamically more stable product, an alkane, is produced. When cis-cyclooctene is hydrogenated to cyclooctane, 96 kJ mol−1 of energy is released. Cyclooctatetraene releases 410 kJ mol−1 on hydrogenation. This value is approximately four times one double bond’s worth, as we might expect. However, whereas the heat of hydrogenation for cyclohexene is 120 kJ mol−1, on hydrogenating benzene only 208 kJ mol−1 is given out, which is much less than the 360 kJ mol−1 that we would have predicted by multiplying the figure for cyclohexene by 3. Benzene has something to make it stable which cycloctatetraene does not have.