Department of Chemistry, University of Nebraska-Lincoln , Lincoln, Nebraska 68588, United States.
Department of Chemistry and Biochemistry, University of California-San Diego , La Jolla, California 92093-0314, United States.
Acc Chem Res. 2016 May 17;49(5):877-83. doi: 10.1021/acs.accounts.6b00040. Epub 2016 Apr 13.
Hydrogen atom transfer (HAT) reactions are ubiquitous and play a crucial role in chemistries occurring in the atmosphere, biology, and industry. In the atmosphere, the most common and traditional HAT reaction is that associated with the OH radical abstracting a hydrogen atom from the plethora of organic molecules in the troposphere via R-H + OH → R + H2O. This reaction motif involves a single hydrogen transfer. More recently, in the literature, there is an emerging framework for a new class of HAT reactions that involves double hydrogen transfers. These reactions are broadly classified into four categories: (i) addition, (ii) elimination, (iii) substitution, and (iv) rearrangement. Hydration and dehydration are classic examples of addition and elimination reactions, respectively whereas tautomerization or isomerization belongs to a class of rearrangement reactions. Atmospheric acids and water typically mediate these reactions. Organic and inorganic acids are present in appreciable levels in the atmosphere and are capable of facilitating two-point hydrogen bonding interactions with oxygenates possessing an hydroxyl and/or carbonyl-type functionality. As a result, acids influence the reactivity of oxygenates and, thus, the energetics and kinetics of their HAT-based chemistries. The steric and electronic effects of acids play an important role in determining the efficacy of acid catalysis. Acids that reduce the steric strain of 1:1 substrate···acid complex are generally better catalysts. Among a family of monocarboxylic acids, the electronic effects become important; barrier to the catalyzed reaction correlates strongly with the pKa of the acid. Under acid catalysis, the hydration of carbonyl compounds leads to the barrierless formation of diols, which can serve as seed particles for atmospheric aerosol growth. The hydration of sulfur trioxide, which is the principle mechanism for atmospheric sulfuric acid formation, also becomes barrierless under acid catalysis. Rate calculations suggest that such acid catalysis play a key role in the formation of sulfuric acid in the Earth's stratosphere, Venusian atmosphere, and on heterogeneous surfaces. Over the past few years, theoretical calculations have shown that these acid-mediated double hydrogen atom transfers are important in the chemistry of Earth's atmosphere as well as that of other planets. This Account reviews and puts into perspective some of these atmospheric HAT reactions and their environmental significance.
氢原子转移 (HAT) 反应普遍存在，在大气、生物和工业中发生的化学反应中起着至关重要的作用。在大气中，最常见和传统的 HAT 反应是与 OH 自由基相关的反应，它通过 R-H + OH → R + H2O 从对流层中的大量有机分子中提取氢原子。这种反应模式涉及单个氢转移。最近，在文献中，出现了一种新的 HAT 反应的新兴框架，涉及双氢转移。这些反应广泛分为四类：(i) 加成，(ii) 消除，(iii) 取代和 (iv) 重排。水合和脱水分别是加成和消除反应的经典例子，而互变异构或异构化属于重排反应的一类。大气酸和水通常介导这些反应。有机酸和无机酸在大气中存在可观的水平，并能够与具有羟基和/或羰基官能团的氧合分子进行两点氢键相互作用。因此，酸影响氧合分子的反应性，从而影响其基于 HAT 的化学的能量学和动力学。酸的空间和电子效应在确定酸催化的效果方面起着重要作用。降低 1:1 底物···酸配合物的空间应变的酸通常是更好的催化剂。在一族单羧酸中，电子效应变得重要；催化反应的势垒与酸的 pKa 密切相关。在酸催化下，羰基化合物的水合作用导致二醇的无势垒形成，二醇可以作为大气气溶胶生长的种子颗粒。三氧化硫的水合作用，这是大气硫酸形成的主要机制，在酸催化下也变得无势垒。速率计算表明，这种酸催化在地球平流层、金星大气和异质表面硫酸形成中起着关键作用。在过去的几年中，理论计算表明，这些酸介导的双氢原子转移在地球大气以及其他行星的化学中非常重要。本综述回顾并探讨了一些大气 HAT 反应及其环境意义。
