一、巧用烷烃通式C_nH_(2n+2),速求取代基个数或不饱和度例1某有机物分子中有a个一CH_3,n个-CH_2-,m个-CH-,其余为-OH,则该物质中的-OH个数可能为()(A)m-a(B)n+m+a(C)m+1-a(D)m+2-a解析:由题意可知所有碳原子均达到饱和.故-OH数量等于为烷烃时所缺的氢原子数,即2(m+n+a)+2-3a-2n-m=m+2-a.所以答案为(A).例2分子式为C_5H_2的烃,其结构不可能 1. Using the alkane formula C_nH_(2n+2), find the number of substituents or the degree of unsaturation quickly. Example 1 There are a CH_3, n -CH_2-, m -CH- in an organic molecule, and the rest are -OH, the number of -OH in the substance may be ()(A)ma(B)n+m+a(C)m+1-a(D)m+2-a All carbon atoms are saturated. So the number of -OH is equal to the number of hydrogen atoms missing for the alkane, ie 2(m+n+a)+2-3a-2n-m=m+2-a. So the answer is ( A). Example 2 Hydrocarbons of the formula C_5H_2, whose structure is impossible