An ion is a stable structure in which an atom loses or gains one or more electrons due to its own or external effects, making it reach the outermost electron number of 8 or 2.
The charged particles formed after atoms lose or gain electrons are called ions, such as sodium ion Na+。The charged atomic group is also called "ion", such as sulfate ion.Some molecules can also form ions under special circumstances.
Generally speaking, the outermost electron number of the atomic nucleus is less than 4, which usually forms a cation, and the number greater than 4, which usually forms an anion. Another rule is that the outermost electron number of the atomic nucleus also determines the valence of the element, which is the number before the plus or minus sign.
Cation refers to the stable structure in which an atom loses one or more electrons due to external action, making the number of its outermost electrons reach 8 or 2.The larger the atomic radius, the stronger the ability of losing electrons, and the stronger the metallicity.The strongest metal element is francium (Fr).
Cationic polymerized monomerMonomers capable of cationic polymerization include olefins, aldehydes, cyclic ethers and cyclic amides.Different monomers have different cationic polymerization activities.
In principle, all olefin monomers with electron pushing substituents can undergo cationic polymerization.The electron pushing substituent makes the electron cloud density of carbon carbon double bond increase, which is beneficial to the attack of cationic active species;On the other hand, it makes the generated carbon cation charge disperse and stable.
Ethylene has no side groups, and the electron cloud density on the double bond is low.It is not easy to polarize.Low affinity for cationic active species.Therefore, cationic polymerization is difficult.Methyl and ethyl groups on propylene and butene are electron pushing groups, and the density of double bond electron cloud has increased, but the power supply of one alkyl group is not strong, and the polymerization growth rate is not too fast. The generated carbon cation is a secondary carbon cation, and the charge cannot be well dispersed and is not stable enough, so side reactions such as rearrangement are likely to occur, resulting in more stable i-level carbon cation.Take 3-methyl-1-butene as an example:
The rearrangement results in branching. Therefore, only low molecular weight oil can be obtained through cationic polymerization of propylene and butylene.
isobutene There are two methyl groups supplying power, which makes the density of double bond electron cloud increase a lot, vulnerable to the attack of cationic active species, and initiates cationic polymerization.Generated ～ CHtwoC+(CHthree)twoIt's Level III C+, relatively stable.In chain - CHtwoThe hydrogen on - is protected by four methyl groups on both sides and is not easy to be captured, reducing side reactions such as rearrangement and branching.Therefore, linear polymers with high relative molecular weight can be generated.More advanced replacementα-Olefins can only be polymerized into dimers due to steric hindrance.Isobutene is actually the only monomer in a-olefin that can undergo cationic polymerization.Another vinyl monomer capable of cationic polymerization is alkyl vinyl ether.Although alkoxy group has the induction effect of electron absorption, which will reduce the density of double bond electron cloud, there is no shared electron pair energy on oxygen and the double bond forms P-π conjugation.Conjugation effect is dominant, resulting in an increase in electron cloud density.The conjugation effect of unshared electron pairs on alkoxy oxygen can also disperse the charge of the formed carbon cation.As a result, alkyl vinyl ether can only be cationic polymerized.The initiators used for cationic polymerization areElectrophilic reagent。Common cationic polymerization initiators include proton acid and cation source/Lewis acid based initiator systems.[1]
Qualitative analysis of cations
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Preparation of cation analysis test solution
Select appropriate solvent to dissolve the sample according to the results of solubility test.Attention shall be paid to:
1. When the sample can be dissolved in two solvents, such as water and acid (or dilute acid and concentrated acid), water should be used first and then acid, and dilute first and then concentrated.If you can dissolve it with the former, do not use the latter.
2. When the sample can be partially dissolved in water, first dissolve it in water, and then treat the insoluble part with acid.Several test solutions obtained should be analyzed separately, and finally the results obtained should be judged comprehensively to get the results.
3. If the sample is dissolved with concentrated acid, the solution obtained shall be evaporated nearly dry to remove excess acid;After cooling, the residue is dissolved with water to make analytical solution.[2]
Analytical characteristics of cations
The analytical characteristics of ions refer to some properties of ions related to identification and separation, such as color, acid-base property, redox property, solubility and volatility of corresponding compounds.
Under certain range and conditions, the same properties of some ions are called commonness, and the significantly different properties are called characteristics.The separation characteristics can be used for the identification of this ion, while the commonness of some ions can be used for the separation of this group of ions, and then the identification can be carried out in a small range, simplifying the complex analysis.
Preliminary Test of Cation Qualitative Analysis
Through the preliminary test, it can be judged whether some cations exist, and then determine the analysis steps and select the test method.Common preliminary tests include HCl, HtwoSOfour、NaOH、NHthree·HtwoO. Chromate, sulfide, etc.[4]
1. HCl test: take a little test solution, add 2 drops of 0.1 moI/L HCl, only Ag+、Pb2+、Hgtwo2+、Tl+It can interact with HCl and generate white precipitates. AgCI precipitates can be soluble in ammonia water, and can also be partially soluble in concentrated HCl.PbCItwoThe solubility of is relatively large, only in Pb2+Precipitation occurs only when the concentration is high, PbCltwoPrecipitation is also soluble in hot water.In the test solution, if there is no white precipitation after adding HCl, it can only prove that there is no Ag+、Hgtwo2+And Tl+Exists, but cannot prove no Pb2+Exists.It must be noted that when the acidity of the solution is low, Bi3+、Sb3+、Sn2+And Sn4+Alkaline salt precipitation is resolved by water.
2、HtwoSOfourTest: take a little test solution and add 2 drops of HtwoSOfourSolution, only Ba2+、Sr2+、Ca2+、Pb2+、Hgtwo2+And HtwoSOfourFunction, forming white precipitate.CaSOfourThe solubility of Ca is relatively high, only when Ca2+Precipitation occurs only when the concentration is high.If appropriate ethanol is added to the solution towel, CaSOfourThe solubility of is greatly reduced.PbSOfourPrecipitation is soluble in NHfourAc，BaSOfour、SrSOfourAnd HgtwoSOfourIt is slightly soluble in strong acid.
3. NaOH and NHthree·HtwoO Test The following is the dissolution rule of metal hydroxide:
(1) In monovalent metal ions, alkali hydroxide can be soluble in water, but Cu+、Hgtwo2+、Ag+And Au+It can generate hydroxide or oxide precipitation.
(2) In divalent metal ions, except Ca2+、Sr2+、Ba2+Only hydroxide precipitates can be formed in concentrated NaOH solution. Hydroxides of other metals are insoluble in water, but Pb2+、Sn2+、Be2+、Zn2+It is obviously amphoteric and soluble in the most NaOH solution.
(3) Both trivalent and tetravalent metal hydroxides will be insoluble in water, similar to Al3+、Cr3+、Sb3+、Ga3+、ln3+And Sn4+It is amphoteric and soluble in excess NaOH solution.
（4）Ag+、Cu2+、Zn2+、Cd2+、Co2+And Ni2+Can form complex ammonia ion, so its hydroxide is soluble in NHthree·HtwoO。[3]
The test can be carried out in a small test tube. Take 10 drops of test solution and add 3-5 drops of 0.1 mol/L NaOH or 0.1 mol/L NHthree·HtwoO. Observe whether there is precipitation and the color of precipitation, and then drip 2 mol/L NaOH or 2mol/L NH into the precipitationthree·HtwoO. Then observe the change of precipitation.[2]
Identification method of cation
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Aluminium ion
1. Aluminium reagent (0.1% aqueous solution): react with reagent in ammonium acetate ammonia aqueous solution;The phenomenon is the formation of red sediment.Sensitivity is 0.16μg. Interference ion is Ca2+, can be eliminated by adding ammonium carbonate.
2. Chromium solid blue B (5% aqueous solution): add MgCO in acid test solutionthreeFine powder until no CO is producedtwo, add test agent and apply (1+1) HtwoSOfourAcidification, and then ether extraction of excess colored substances;The phenomenon is that the solution appears magenta or pink.Sensitivity is 0.1μg
Silver ion
1. Bromo pyrogallol red (0.1 mol/L aqueous solution): react with reagent in the presence of 0.1 mol/L EDTA, 0.001 mol/L phenanthroline, 20% ammonium acetate;The phenomenon is the formation of a blue silver phenanthroline dibromo pyrogallol red ternary complex.Sensitivity is 0.05μg
2. Hydrochloric acid: take 2 drops of test solution and add 1 drop of hydrochloric acid,;Separating precipitation, washing, adding ammonia water and then adding nitric acid;The phenomenon is white precipitation, dissolved after adding ammonia, and precipitated after adding nitric acid.Sensitivity is 0.5μg. The interfering ion is Fe2+Other ions can be eliminated with o-phenanthroline and shielded with EDTA.[2]
