v nydrogm for he presence of he 高4 number Aga6aohsaae2musthaveacomtnatond niogtomnree the umber of 11-7 CatalicRelative The heat of hydrogenation is a measure of stability. t8aan ww =-103kea md and has #-H, HH -cimt ho的b8me四inthe omrtablea n bc o om ©- c-0 口X□ 7 7 A fully saturated hydrocarbon will have 2n+2 hydrogen atoms for every n carbon atom. Consider the compounds in the class C5H8. This compound is 4 hydrogens short of being saturated, so its degree of unsaturation is 4/2 = 2. All molecules having this formula must have a combination of rings and π bonds adding up to 2. The presence of heteroatoms may affect the calculation. The presence of a halogen atom decreases the number of hydrogens by one. The presence of a nitrogen atom increases the number of hydrogens by one. The presence of oxygen or sulfur does not affect the number of hydrogens. To determine the degree of unsaturation: Step 1: Hsat = 2nC + 2 – nX + nN Step 2: Degree of unsaturation = (Hsat – Hactual)/2 Catalytic Hydrogenation of Alkenes: Relative Stability of Double Bonds 11-7 Hydrogen gas and an alkene will react when mixed in the presence of a catalyst such as platinum or palladium. Two hydrogen atoms are added to the alkene in a reaction called hydrogenation, which is very exothermic. The heat released is called the “heat of hydrogenation” and has a typical value of about -30 kcal mol-1 per double bond. The heat of hydrogenation is a measure of stability. The relative stabilities of related alkenes can be determined by measuring their heats of combustion. The thermodynamic stability of the butenes increases in the order: 1-butene < cis-2-butene < trans-2-butene. Highly substituted alkenes are most stable; trans isomers are more stable than cis. The relative stability of the alkenes increases with increasing substitution (hyperconjugation), and trans isomers are usually more stable than cis isomers (crowding). An exception to this stability rule is in medium-ring and smaller cycloalkenes. The trans isomers of cycloalkenes are much more strained than are the corresponding cis isomers. The smallest isolated simple trans cycloalkene is trans￾cyclooctene which is 9.2 kcal mol-1 less stable than the cis isomer is very twisted