从理论角度看,挥发性甲基西洛大气氧化机制――西洛醇是如何形成的?
Mitchell W Alton1,2, Virginia L Johnson1, Sandeep Sharma1
1Department of Chemistry, University of Colorado, Boulder, Colorado 80309, United States.
The journal of physical chemistry. A
|November 27, 2023
概括
这项研究调查了循环挥发性甲基氧化物 (VMS) 的大气氧化. 理论计算显示,通过氧基和水反应形成氧醇过慢,而氧基反应在D3和D4VMS之间存在显著差异.
科学领域:
- 大气化学 大气化学
- 环境科学 环境科学
- 计算化学的计算化学
背景情况:
- 循环挥发性甲基氧化物 (VMS) 是生产量很高的人为化合物,大气氧化化学理解较差.
- 现有的机制不能充分解释氧醇的形成,这是主要观察到的氧化产物.
- 在VMS氧化途径的不确定性影响其环境命运和影响的评估.
研究的目的:
- 探索一种新的反应途径,用于涉及氧基和水的氧醇形成.
- 为了研究和比较激素中间体在六甲基环氧三 (D3) 和八甲基环氧三 (D4) 氧化过程中的反应速率.
- 阐明实验观察到的VMS氧化产物的形成机制.
主要方法:
- 理论量子化学计算被用来建模反应路径和动力学.
- 计算了关键中间体的反应速率,包括含HO2•和NO的过氧基 (RO2•).
- 计算速度与实验观测和大气条件的比较.
主要成果:
- 拟议的氧基与水反应以形成醇的途径在动力学上太慢,无法解释观察到的度.
- D3过氧基 (RO2•) 和HO2•之间的反应速率明显比以前假设的要慢 (3 × 10−13 cm3分子−1 s−1).
- D4过氧基 (RO2•) 和HO2•之间的反应速率也较慢 (2 × 10−11 cm3分子−1) 与典型的1 × 10−11 cm3分子−1 s−1.1的反应速率相比.
- 在城市环境中,预计VMS RO2•与NO的反应将占主导地位,而在更清洁的地区,异构化变得更加显著.
结论:
- 氧基与水的直接反应不是大气中氧醇的主要来源.
- 与HO2•一起的VMS过氧基的反应速度比预期的慢,需要重新评估大气模型.
- VMS氧化产物的命运高度依赖于大气条件,NO反应在污染地区普遍存在,在更清洁的环境中发生异构.
相关概念视频
02:44Oxidation of Alkenes: Syn Dihydroxylation with Osmium Tetraoxide
Alkenes are converted to 1,2-diols or glycols through a process called dihydroxylation. It involves the addition of two hydroxyl groups across the double bond with two different stereochemical approaches, namely anti and syn. Dihydroxylation using osmium tetroxide progresses with syn stereochemistry.
02:21Oxidation of Alkenes: Syn Dihydroxylation with Potassium Permanganate
Alkenes can be dihydroxylated using potassium permanganate. The method encompasses the reaction of an alkene with a cold, dilute solution of potassium permanganate under basic conditions to form a cis-diol along with a brown precipitate of manganese dioxide.
02:04Oxidation of Alkenes: Anti Dihydroxylation with Peroxy Acids
Diols are compounds with two hydroxyl groups. In addition to syn dihydroxylation, diols can also be synthesized through the process of anti dihydroxylation. The process involves treating an alkene with a peroxycarboxylic acid to form an epoxide. Epoxides are highly strained three-membered rings with oxygen and two carbons occupying the corners of an equilateral triangle. This step is followed by ring-opening of the epoxide in the presence of an aqueous acid to give a trans diol.
01:46Oxidative Cleavage of Alkenes: Ozonolysis
In ozonolysis, ozone is used to cleave a carbon–carbon double bond to form aldehydes and ketones, or carboxylic acids, depending on the work-up.
Ozone is a symmetrical bent molecule stabilized by a resonance structure.
Ozone is a symmetrical bent molecule stabilized by a resonance structure.
02:37Oxidation of Alcohols
In this lesson, the oxidation of alcohols is discussed in depth. The various reagents used for oxidation of primary and secondary alcohols are detailed, and their mechanism of action is provided.
The process of oxidation in a chemical reaction is observed in any of the three forms:
The process of oxidation in a chemical reaction is observed in any of the three forms:
01:20Radical Autoxidation
The oxidation of an organic compound in the presence of air or oxygen is called autoxidation. For example, cumene reacts with oxygen to form hydroperoxide. Autoxidation involves initiation, propagation, and termination steps. Many organic compounds are susceptible to autoxidation—especially ethers in the presence of oxygen, which form hydroperoxides. Even though this reaction is slow, old ether bottles contain small amounts of peroxide, which leads to laboratory explosions during ether...