作业帮有机化学中碱性的比较,六氢吡啶和三甲胺谁的碱性大?为什么?
来源：学生作业帮 编辑：作业帮 分类：英语作业 时间：2024/05/14 20:26:22
有机化学中碱性的比较,六氢吡啶和三甲胺谁的碱性大?为什么?
这个问题其实很纠结.
我查到的数据是 哌啶 pka=11.22.三甲胺 pka=9.87. 很明显哌啶的碱性大.
一般认为烷基是给电子集团,所以烷基取代越多,应该碱性越大.从以上的数据来看,这种观点不对.
我从论坛上查看了一下类似的问题.比较氨,甲胺,二甲胺,三甲胺的碱性大小.其实这类问题要考虑两个方面：极化性和溶剂化效应.我把原帖转过来,希望对你有所帮助：
The basicity trends of amines are most easily explained by looking at the acidity of ammonium-ions, which (in solution) follows the trend given below:
NH4+ > (CH3)3NH+ > CH3NH3+ > (CH3)2NH2+
That is NH4+-ions are the most acidic (least basic) ones, whereas (CH3)2NH2+-ions are the least acidic (most basic) ones.
This trend can be explained by two conflicting effects - polarizability and solvation.
If you look at gas phase acidities the trend is as follows:
NH4+ > CH3NH3+ > (CH3)2NH2+ > (CH3)3NH+
Which is what we would normally expect since more methyl-groups donates more electrons spreading out the positive charge and therefore stabilizing the ammonium-ion (making it less acidic, and the conjugate base more basic).
On the other hand solvation effects leads to the following trend:
(CH3)3NH+ > (CH3)2NH2+ > CH3NH3+ > NH4+
This trend can be explained by the size of the ammonium-ion - the larger, the least well solvated, therefore least stable and hence more acidic (less basic).
A combination of these two trends gives the acidity (and therefore basicity) trend of ammonium-ions (amines) in solution.
我查到的数据是 哌啶 pka=11.22.三甲胺 pka=9.87. 很明显哌啶的碱性大.
一般认为烷基是给电子集团,所以烷基取代越多,应该碱性越大.从以上的数据来看,这种观点不对.
我从论坛上查看了一下类似的问题.比较氨,甲胺,二甲胺,三甲胺的碱性大小.其实这类问题要考虑两个方面：极化性和溶剂化效应.我把原帖转过来,希望对你有所帮助：
The basicity trends of amines are most easily explained by looking at the acidity of ammonium-ions, which (in solution) follows the trend given below:
NH4+ > (CH3)3NH+ > CH3NH3+ > (CH3)2NH2+
That is NH4+-ions are the most acidic (least basic) ones, whereas (CH3)2NH2+-ions are the least acidic (most basic) ones.
This trend can be explained by two conflicting effects - polarizability and solvation.
If you look at gas phase acidities the trend is as follows:
NH4+ > CH3NH3+ > (CH3)2NH2+ > (CH3)3NH+
Which is what we would normally expect since more methyl-groups donates more electrons spreading out the positive charge and therefore stabilizing the ammonium-ion (making it less acidic, and the conjugate base more basic).
On the other hand solvation effects leads to the following trend:
(CH3)3NH+ > (CH3)2NH2+ > CH3NH3+ > NH4+
This trend can be explained by the size of the ammonium-ion - the larger, the least well solvated, therefore least stable and hence more acidic (less basic).
A combination of these two trends gives the acidity (and therefore basicity) trend of ammonium-ions (amines) in solution.