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24 Organic and Biological Chemistry Solutions to Exercises (d) 2,4-dimethylhexane (e) methylcyclobutane 24.19 Assuming that each component retains its effective octane number in the mixture (and this isn't always the case), we obtain: octane number = 0.35(0) + 0.65(100) = 65. 24.20 Octane number can be increased by increasing the fraction of branched-chain alkanes or aromatics, since these have high octane numbers. This can be done by cracking. The octane number also can be increased by adding an anti-knock agent such as tetraethyl lead, Pb(C₂H₅)₄ (no longer legal); methyl t-butyl ether (MTBE); or an alcohol, methanol, or ethanol. Alkenes, Alkynes, and Aromatic Hydrocarbons (section 24.3) 24.21 (a) Alkanes are said to be saturated because they contain only single bonds. Multiple bonds that enable addition of H₂ or other substances are absent. The bonding capacity of each carbon atom is fulfilled with single bonds to C or H. (b) No, C₄H₆ is not saturated. Alkanes are saturated hydrocarbons. The maximum number of hydrogen atoms for 4 C atoms in an alkane is [(2x4)+2] = 10. C₄H₆ does not contain the maximum possible hydrogen atoms and is unsaturated. 24.22 (a) The molecule is unsaturated because it contains a double bond. It is possible to add more hydrogen to the molecule. (b) The formula has too many H atoms bound to the right-most C atom; the formula as it stands implies 5 bonds to this atom. A correct formula is with 2 H atoms on the right-most C atom. 24.23 Analyze/Plan. Consider the definition of the stated classification and apply it to a compound containing five atoms. Solve. (a) (b) CH₂ CH₂ / , (c) (d) C₅H₈ saturated: (a), (b); unsaturated: (c), (d) 24.24 cycloalkane, ,C₆H₁₂ saturated CH₂ C H CH₃ cycloalkene, HC=CH ,C₆H₁₀, unsaturated CH₂ C H CH₃ 725