怎样讲述键的极性

在现行高中化学课本中,有关极性键和非极性键的概念是这样阐述的：“在单质分子中,同种原子形成的共价键,两个原子吸引电子的能力相同,共用电子对不偏向任何一个原子,这两个电子在键的中央出现的机会最多,成键的原子都不显电性。这样的共价键叫做非极性共价键,简称非极性键。”“在化合物的分子中,不同种原子形成的共价键,由于不同原子吸引电子的能力不同,共用电子对必然偏向吸引电子能力强的原子一方,也就是说,靠近吸引电子能力强的原子一方电子云比较密集。因而吸引电子能力强的原子就带部分负电荷,吸引电子能力较弱的原子就带部分正电荷,这样的共价键叫极性共价键,简称极性键”。     

主族金属元素的氢-元素键及碳-元素键的键能计算

考察化合物能量水平的基本参数是分子中的键能,七十年代初期,R.T.Sanderson 对键能的计算问题,提出分享键能(Contributing Bond Energy,CBE)的概念,他把极性共价键的能量分解为非极性共价键的 E_c 和离子键的 E_i,它们在该键中所占的比量分别用系数 t_c 和 t_i 表示,则键能的计算公式是:CBE=t_cE_c t_iE_i     

共价键稳定性的影响因素

虽然小分子中的共价键强度可以很方便地通过高斯计算而相当准确地得到,一些手册和数据库中也可以直接查出部分键能/离解能数据,但共价键的强弱变化的影响因素分析在化学教学中仍然显得非常重要。共价键的强弱与成键原子及其环境密切相关,其中成键原子因素主要包括原子半径、成键类型、成键轨道类型、相对论效应、电负性、成键数量、反馈效应和孤电子对效应,而成键环境因素包括键间张力效应、离域效应、次级化学键效应、诱导效应和位阻效应。在教学中,我们可以通过对化学键影响因素的分析帮助学生理解共价键键能的变化规律。本文分析了影响共价键强弱的主要因素,并介绍了这种分析思路在化学教学中的应用。     

基于计算化学的探究式共价键教学设计

针对共价键教学中存在的问题,探讨了将计算化学技术作为探究工具,引入到共价键探究教学过程中。通过共价键形成过程中轨道重叠程度、键长和体系能量变化等数据,深刻揭示共价键成键的本质,给学生专家级的探究体验。同时强调化学学科方法思想和知识准备的作用,使探究活动更具科学价值。     

二元化合物鍵的极性程度的计算

分子中原子间化学键的极性程度,是指两个成键原子之间的电子偏移的数量,即与共价键相比较电子云(指价电子而言)重心从原子间距离的中心偏移的百分率。这数值可以用来表示化学键的极性程度,Pauling称之为“离子性百分率”,也可以叫做键极性指数。关于键的极性程度的计算,三十年来,提出的经验     

加速溶剂萃取-在线衍生-气相色谱-质谱法同时分析土壤中有机氯农药和酸性除草剂

建立了土壤中13种极性除草剂与20种非极性有机氯农药的同时萃取检测的方法.利用加速溶剂萃取过程中的温度环境与溶剂环境,使强极性氯代酸性除草剂与五氟苄基溴在萃取池内反应,酯化产物的极性显著降低.利用丙酮溶剂使之与非极性的有机氯农药同时萃取;利用衍生产物与有机氯农药的高电负性使除草剂与有机氯农药在气相色谱负化学源质谱上同时...     

沉淀制备双酚Aπ-π堆积自组装印迹聚合物及其特异性识别机理研究

以类雌性激素双酚A为模板分子,选取4-乙烯基吡啶作为功能单体,在极性溶剂甲醇中,通过非共价键完成自组装过程,沉淀聚合制备对双酚A分子具有高度选择性识别和亲和力的分子印迹聚合物.采用紫外-可见光谱研究了双酚A和4-乙烯基吡啶的配合方式以及配位比,结果表明双酚A与4-乙烯基吡啶以配位比1∶2的方式,在极性溶剂中,无法以常用的氢键形成超分子配合物,却可以π-π堆积这种非共价键完成自组装过程.通过红外谱图,验证了沉淀聚合法制备的印迹聚合物可以完成双酚A的印迹和可逆分离.通过扫描电镜和等温吸附实验考察了分子印迹聚合物对模板分子的特异性识别和选择性吸附机理,印迹聚合物的特异性吸附容量为37.39μmol/g,印迹指数为4.31;对其结构类似物四溴双酚A和苯酚的分离因子分别为2.51和2.06,同时对莠去津,萘普生和槲皮素的分离因子分别为2.14,2.26和2.63,证实了分子印迹聚合物在极性溶剂中可以对双酚A分子进行很好的特异性识别,并且识别环境更接近天然分子环境.     

非共价键功能化石墨烯/碳纳米管负载型金属配合物催化剂及催化反应中的应用

碳材料（石墨烯、碳纳米管）具有超大的比表面积、高机械强度、化学稳定性、环境友好等特点，使得其作为一类新型非均相催化剂的优良载体，固载的金属配合物催化剂在许多催化反应中得到了广泛的应用.由于弱相互作用（π-π键、氢键、静电）功能化可以有效的保护碳材料的完整性，从而更好地发挥碳材料本身的优异性能.通过改变温度、溶液极性、外场力来调控非共价键功能化碳材料催化剂在催化反应中载体与催化剂的吸附与分离，使其具有均相催化剂优良的催化活性和多相催化剂的可回收性.综述了近些年来非共价键功能化石墨烯和碳纳米管固载的金属配合物催化剂在催化反应中的研究进展.     

点击化学法修饰氧化石墨烯及其在高分子中的应用研究

点击化学是一种操作简单方便、灵活高效的化学合成方法,对石墨烯的改性具有高效和活性位点可控等特点,是一种新型高效修饰石墨烯的改性法。本文结合点击化学改性石墨烯的特点及在高分子中的应用,将点击化学功能化修饰石墨烯分为共价键点击功能化和非共价键点击功能化,其中共价键结合又可细分为边缘点击功能化改性和表面点击功能化改性。本文介绍了叠氮功能改性剂的制备方法及其修饰石墨烯的点击反应原理,总结了点击功能化石墨烯及氧化石墨烯高分子复合材料的功能特性和应用前景。     

共价键理论发展史及其教育价值

共价键理论是化学学科的核心内容,基于认识视角梳理共价键的发展史,将共价键理论的形成和发展分为4个阶段,现象阶段：初识物质结合方式;表征阶段：探索微粒结合过程;本质阶段：揭示共价键形成实质;发展阶段：解释物质结构与性质。共价键历史的发展阶段与教材的编排一脉相承,结合学生的认知发展,有以下教育价值：(1)凸显学科思想方法;(2)持续发展学生认识进阶;(3)促进学生素养形成。     

二氧化碳和丙烯直接合成甲基丙烯酸的NiPMo/TiO2催化剂的研究

用溶胶-凝胶法以磷钼酸(MPA)的镍盐溶液水解钛酸四丁酯制备了NiPMo/TiO2催化剂.使用ICP、 XRD、 TG-DTA、 IR、 TPD-MS和微反应技术研究了催化剂的化学组成、热稳定性、化学吸附性质和催化反应性能.杂多钼酸盐与TiO2通过O2-在TiO2表面发生了键合.在623 K下,杂多阴离子仍保持原有的Keggin结构.CO2在Lewis酸位Ni(Ⅱ)和Lewis碱位Ni-O-Mo的桥氧协同作用下生成CO2卧式吸附态Ni(Ⅱ)←O-(CO)←(O--Ni).丙烯有多种吸附态在催化剂上吸附.在563 K、 1 MPa和空速1500 h-1的反应条件下,丙烯的摩尔转化率为3.2%,产物MAA选择性为95%.     

Toward new camptothecins. Part 6: Synthesis of crucial ketones and their use in Friedländer reaction

In the context of the preparation of camptothecin and luotonin A analogs, the synthesis of some key keto-precursors and their use in Friedländer condensation are described. This paper also focuses on the stability of these keto intermediates and emphasizes the major differences between indolizinones and pyrroloquinazolinones series. Noteworthy is also the report of some original structures isolated as by-products of some experiments.     

CoFe2O4自形成磁性液体场致结构化对磁化的影响

The Langevin paramagnetic theory can’t describe the relation between magnetization of ferrofluids and applied magnetic field. The structuralization of ferrofluids, which is considered the main influence factor of the magnetization, is regarded. The part of magnetization works is deposited when the structure is forming. This action influences the magnetization of ferrofluids directly or indirectly. On the base of the “compressing” model, the Langevin function that usually describes the magnetization of ferrofluid is modified, and a well-fitted curve is obtained. An equation of the relation between the equivalent volume fraction after being “compressed” and the intensity of magnetic field is discovered, which approximately describes the process of magnetization. The relation between the approximate initial susceptibility and the volume fraction can be obtained from modified formula.     

Highly regioselective Buchwald–Hartwig amination at C-2 of 2,4-dichloropyridine enabling a novel approach to 2,4-bisanilinopyridine (BAPyd) libraries

The highly regioselective Buchwald–Hartwig amination at C-2 of the cheap and readily accessible reagent, 2,4-dichloropyridine with a range of anilines and heterocyclic amines is described. This new methodology is robust and provides a facile access to 4-chloro-N-phenylpyridin-2-amines on 0.25 mol scale. These intermediates undergo a further Buchwald–Hartwig amination at higher temperature to enable rapid exploration of the chemical space at C-4 and to provide a library of 2,4-bisaminopyridines.     

KMnO4-mediated oxidative CN bond cleavage of tertiary amines: Synthesis of amides and sulfonamides

KMnO₄-mediated oxidative CN bond cleavage of tertiary amines producing secondary amine was introduced, which was trapped by electrophiles (acyl chloride and sulfonyl chloride) to form amides and sulfonamides. The reaction could take place at mild condition, tolerating a wide range of function groups and affording products in moderate to excellent yields.     

Chemistry of heterocyclic compounds at the A. E. Favorsky Irkutsk Institute of Chemistry, Siberian Branch of the Russian Academy of Sciences, over 50 years (Review)

The review contains a concise historical account and information on the most significant researches undertaken by the staff at the A. E. Favorsky Irkutsk Institute of Chemistry, Siberian Branch of the Russian Academy of Sciences on the Chemistry of Heterocyclic Compounds. Dedicated to Academician of the Russian Academy of Sciences B. A. Trofimov on his 70th jubilee. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 10, pp. 1443–1502, October, 2008.     

Sulfur-directed metal-free and regiospecific methyl C(sp3)–H imidation of thioanisoles

Regiospecific and direct imidation of the methyl C(sp³)–H bond of thioanisoles is realized under mild and metal-free conditions with N-fluorobis(benzenesulfonyl)imide as an oxidant and nitrogen source. Proposed mechanism suggests that thionium ion intermediates and a Pummerer-type reaction are involved. The imidation has advantages such as high step-economy, excellent functionality tolerance, and regiospecificity, giving structurally diverse imidation products.     

Selective syntheses of 2H-1,3-oxazines and 1H-pyrrol-3(2H)-ones via temperature-dependent Rh(II)-carbenoid-mediated 2H-azirine-ring expansion

The Rh(II)-catalyzed reaction of 2-carbonyl-substituted 2H-azirines with ethyl 2-cyano-2-diazoacetate or 2-diazo-3,3,3-trifluoropropionate provides an easy access to 2H-1,3-oxazines and 1H-pyrrol-3(2H)-ones. These compounds can be selectively prepared from the same starting material using temperature as the only varied parameter. The 2-azabuta-1,3-diene intermediate, a common precursor for both heterocyclic products, isomerizes into 2H-1,3-oxazine under kinetic control, while 1H-pyrrol-3(2H)-one is the sole product of the reaction at elevated temperatures. According to DFT-calculations a one-atom oxazine ring contraction involving ring-opening to a 2-azabuta-1,3-diene intermediate, followed by a 1,5- and 1,2-prototropic shift leads to the consecutive formation of imidoylketene and azomethine ylide, which then further undergo cyclization to the pyrrole derivative.     

Synthesis of 1-benzyloxypyrazin-2(1H)-one derivatives

Different approaches for the synthesis of 1-benzyloxypyrazin-2(1H)-one derivatives from simple amino acids have been investigated. A library of 33 precursors for the preparation of N-hydroxy pyrazinones was obtained in moderate to good yields.