第一章卤化反应
周子豪药学院08级 2008302290026
1.卤化反应在有机合成中的应用？为什么常用一些卤代物作为反映中间体？
a)制备特定活性的化合物：药物、兽药、农药、杀虫剂除草剂等
b)构成新的有机化合物：成链，成环，官能团转换等
c)提高有机物的反应性能：取代、加成、消除、缩合、聚合等
d)引入卤素原子作为保护基、阻断基等
X原子的引入可以使有机分子的理化性质、提高反应性能，为重要的有机合成中间体, C-X很容易转化成其它官能团
2.归纳常用的氯化剂、溴化剂有哪些？它们的主要理化性质及适用对象和范围？
工商注册查询系统
答：氯化剂主要是氯气Cl2，具有足够的电负性，它本身在反应中发生极化而参加反应；也可用
柠檬树的养殖方法和注意事项HOCl,CH3CO2Cl，氯化硫S2Cl2、硫酰氯SO2Cl2和次氯酸叔丁酯t-BuOCl，均可以释放氯正离子作为亲电试剂。
用分子溴的取代反应，通常在醋酸中进行，若在反应中加入碘，可以提高反应速度；其他溴化剂包括NBS、HOBr、酰基次溴酸酐（AcOBr、CF3CO2Br等），尤其后者特别有活性。
3.比较X2,HX,HOX对双键离子型加成反应机理有什么异同，如何判断加成产物的立体构型？
答：X2：卤素负离子进攻碳正离子；HX对双键加成；HOX中的卤素正离子对烯烃的双键做亲电攻击，形成桥卤三圆环过渡态，再由水分子或者OH对其亲核进攻。
立体构型一般取决于反应中间体的结构，若中间体为离子形式，则最终产物正反各一半，如果中间体形成溴鎓离子，则最后产物为反式。
区域选择性卤化反应
In chemistry, regiolectivity is the preference of one direction of chemical bond making or breaking over all other possible directions. It can often apply to which of many possible positions a reagent will affect, such as which proton a strong ba will abstract from an organic molecule, or where on a substituted benzene ring a further substituent will add.
A specific example is a halohydrin formation reaction with 2-propenylbenzene:
The reaction product is a mixture of two isomers and the regiolectivity is said to be poor.
Regiolectivity in ring-closure reactions is subject to Baldwin's rules.
参考文献酸萝卜怎么泡
Reactivity–lectivity principle
In chemistry the reactivity–lectivity principle or RSP states that a more reactive chemical compound or reactive intermediate is less lective in chemical reactions. In this context lectivity reprents the ratio of reaction rates.
This principle was generally accepted until the 1970s when too many exceptions started to appear. The principle is now considered obsolete .
A classic example of perceived RSP found in older organic textbooks concerns the free radical halogenation of simple alkanes. Whereas the relatively unreactive bromine reacts with
2-methylbutane predominantly to 2-bromo-2-methylbutane, the reaction with much more reactive chlorine results in a mixture of all four regioisomers.
Another example of RSP can be found in the lectivity of the reaction of certain carbocations with azides and water. The very stable triphenylmethyl carbocation derived from solvolysis of the corresponding triphenylmethylchloride reacts a 100 times faster with the azide anion than with water. When the carbocation is the very reactive tertiary adamantane carbocation (as judged from diminished rate of solvolysis) this difference is only a factor of 10.
倍的认识教案Constant or inver relationships are just as frequent. For example a group of 3- and 4-substituted pyridines in their reactivity quantified by their pKa show the same lectivity in their reactions with a group of alkylating reagents.
The reason for the early success of RSP was that the experiments involved very reactive intermediates with reactivities clo to kinetic diffusion control and as a result the more reactive intermediate appeared to react slower with the faster substrate.
General relationships between reactivity and lectivity in chemical reactions can successfully explained by the Hammond postulate.
When reactivity-lectivity relationships do exist they signify different reaction modes. In one study the reactivity of two different free radical species (A, sulfur, B carbon) towards addition to simple alkenes such as acrylonitrile, vinyl acetate and acrylamide was examined.
黄金时代海报The sulfur radical was found to be more reactive (6*108 vs. 1*107 mole-1.s-1) and less lective (lectivity ratio's 76 vs 1200) than the carbon radical. In this ca the effect can be explained by extending the Bell–Evans–Polanyi principle with a factor accounting for transfer of charge from the reactants to the transition state of the reaction which can be calculated in silico:
with the activation energy and the reaction enthalpy change. With the electrophilic sulfur radical the charge transfer is largest with electron-rich alkenes such as acrylonitrile but the resulting reduction in activation energy (βis negative) is offt by a reduced enthalpy. With the nucleophilic carbon radical on the other hand both enthalpy and polar effects have the same direction thus extending the activation energy range.
[edit] External links
Reactivity–lectivity principle Gold Book Link
[edit] References
王健林小目标
1.^ Minireview The Reactivity-Selectivity Principle: An Imperishable Myth in Organic
Chemistry Herbert Mayr, Armin R. Ofial Angewandte Chemie International Edition Volume 45, Issue 12 , Pages 1844 - 1854 Abstract寿司简笔画
2.^ Search for High Reactivity and Low Selectivity of Radicals toward Double Bonds: The
Ca of a Tetrazole-Derived Thiyl Radical Jacques Lalevée, Xavier Allonas, and Jean Pierre Fouassier J. Org. Chem.; 2006; 71(26) pp 9723 - 9727; (Article) doi:10.1021/jo061793w
3.^ Sulfur tetrazole radical derived from photolysis of disulfide and carbon radical derived
from photolysis of t-butylperoxide followed by proton abstraction from triethylamine
Electrophilic halogenation
In organic chemistry, an electrophilic aromatic halogenation is a type of electrophilic aromatic substitution. This organic reaction is typical of aromatic compounds and a very uful method for adding substituents to an aromatic system.
A few types of aromatic compounds, such as phenol, will react without a catalyst, but for typical benzene derivatives with less reactive substrates, a Lewis acid catalyst is required. Typical Lewis acid catalysts include AlCl3, FeCl3, FeBr3, and ZnCl2. The work by forming a highly electrophilic complex which attacks the benzene ring.
[edit] Reaction mechanism
The reaction mechanism for chlorination of benzene is the same as bromination of benzene. Ferric bromide and ferric chloride become inactivated if they react with water, including moisture in the air. Therefore, they are generated in situ by adding iron fillings to bromine or chlorine.
The mechanism for iodination is slightly different: iodine (I2) is treated with an oxidizing agent such as nitric acid to obtain the electrophilic iodine (2 I+). Unlike the other halogens, iodine does not rve as a ba since it is positive. In one study the iodinization reagent is a mixture of iodine and iodic acid.[1]
In another ries of studies the powerful reagent obtained by using a mixture of iodine and potassium iodate dissolved in concentrated sulfuric acid was ud. Here the iodinating agent is the tri-iodine cation I3+ and the ba is HSO4-. In the studies both the kinetics of the reaction and the preparative conditions for the iodination of strongly deactivated compounds, such as benzoic acid and 3-nitrobenzotrifluoride, were investigated.[2][3]桥梁施工方案
Halogenation of aromatic compounds differs from the halogenation of alkenes, which do not require a Lewis Acid catalyst. The formation of the arenium ion results in the temporary loss of aromaticity, which has a higher activation energy compared to carbocation formation in alkenes. In other words, alkenes are more reactive and do not need to have the Br-Br or Cl-Cl bond weakened.
[edit] Scope
If the ring contains a strongly activating substituent such as -OH, -OR or amines, a catalyst is not ne
cessary, for example in the bromination of p-cresol:[4]
However, if a catalyst is ud with excess bromine, then a tribromide will be formed.
Halogenation of phenols is faster in polar solvents due to the dissociation of phenol, with phenoxide ions being more susceptible to electrophilic attack as they are more electron-rich.
Chlorination of toluene with chlorine without catalyst requires a polar solvent as well such as acetic acid. The ortho to para lectivity is low:[5]
No reaction takes place when the solvent is replaced by tetrachloromethane. In contrast, when the r
eactant is 2-phenyl-ethylamine, it is possible to employ relatively apolar solvents with exclusive ortho- regiolectivity due to the intermediate formation of a chloramine making the subquent reaction step intramolecular.
The food dye erythrosine can be synthesized by iodination of another dye called fluorescein:
