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From Wikipedia

Protic solvent

In chemistry a protic solvent is a solvent that has a hydrogen atom bound to an oxygen (as in a hydroxyl group) or a nitrogen (as in an amine group). More generally, any molecular solvent which contains dissociable H+, is called a protic solvent. The molecules of such solvents can donate an H+ (proton). Conversely, aprotic solvents cannot donate hydrogen.

Polar protic solvents are solvents that share ion dissolving power with aprotic solvents but have an acidic hydrogen. These solvents generally have high dielectric constants and high polarity.

Common characteristics of protic solvents :

Examples are water, methanol, ethanol, formic acid, hydrogen fluoride and ammonia.

Polar aprotic solvents are solvents that share ion dissolving power with protic solvents but lack an acidic hydrogen. These solvents generally have intermediate dielectric constants and polarity.

Common characteristics of aprotic solvents:

Examples are dimethyl sulfoxide, dimethylformamide, dioxane and hexamethylphosphorotriamide, tetrahydrofuran.

Polar protic solvents are favorable forSN1 reactions, while polar aprotic solvents are favorable for SN2 reactions.

Apart from solvent effects, polar aprotic solvents may be essential for reactions which use strong bases, such as reactions involving Grignard reagents or n-butyllithium. If a protic solvent were to be used, the reagent would be consumed by a side reaction with the solvent.

An example of a dipolar aprotic solvent is methylpyrrolidone.

Properties of common solvents

The solvents are grouped into non-polar, polar aprotic, and polar protic solvents and ordered by increasing polarity. The polarity is given as the dielectric constant. The properties of solvents that exceed those of water are bolded.


Organochloride

An organochloride, organochlorine, chlorocarbon, chlorinated hydrocarbon, or chlorinated solvent is an organic compound containing at least one covalently bondedchlorine atom. Their wide structural variety and divergent chemical properties lead to a broad range of applications. Many derivatives are controversial because of the effects of these compounds on the environment.

Physical properties

Chloride substituents modify the physical properties of organic compounds in several ways. They are typically denser than water due to the presence of high atomic weight of chlorine. Chloride substituents induce stronger intermolecular interactions than hydrogen substituents. The effect is illustrated by trends in boiling points: methane (-161.6 °C), methyl chloride (-24.2 °C), dichloromethane (40 °C), chloroform (61.2 °C), and carbon tetrachloride (76.72 °C). The increased intermolecular interactions is attributed to the effects of both van der Waals and polarity.

Natural occurrence

Although rare compared to non-halogenated organic compounds, many organochlorine compounds have been isolated from natural sources ranging from bacteria to humans. Chlorinated organic compounds are found in nearly every class of biomolecules including alkaloids, terpenes, amino acids, flavonoids, steroids, and fatty acids. Organochlorides, including dioxins, are produced in the high temperature environment of forest fires, and dioxins have been found in the preserved ashes of lightning-ignited fires that predate synthetic dioxins. In addition, a variety of simple chlorinated hydrocarbons including dichloromethane, chloroform, and carbon tetrachloride have been isolated from marine algae. A majority of the chloromethane in the environment is produced naturally by biological decomposition, forest fires, and volcanoes. The natural organochloride epibatidine, an alkaloid isolated from tree frogs, has potent analgesic effects and has stimulated research into new pain medication.

Preparation

From chlorine

Alkanes and arylalkanes may be chlorinated under free radical conditions, with UV light. However, the extent of chlorination is difficult to control. Aryl chlorides may be prepared by the Friedel-Crafts halogenation, using chlorine and a Lewis acid catalyst.

The haloform reaction, using chlorine and sodium hydroxide, is also able to generate alkyl halides from methyl ketones, and related compounds. Chloroform was formerly produced thus.

Chlorine adds to the multiple bonds on alkenes and alkynes as well, giving di- or tetra-chloro compounds.

Reaction with hydrogen chloride

Alkenes react with hydrogen chloride (HCl) to give alkyl chlorides. For example, the industrial production of chloroethane proceeds by the reaction of ethylene with HCl:

H2C=CH2 + HCl → CH3CH2Cl

Secondary and tertiary alcohols react with the Lucas reagent (zinc chloride in concentrated hydrochloric acid) to give the corresponding alkyl halide; this reaction a method for classifying alcohols:

Other chlorinating agents

In the laboratory, alkyl chlorides are most easily prepared by reacting alcohols with thionyl chloride (SOCl2), phosphorus trichloride (PCl3), or phosphorus pentachloride (PCl5):

ROH + SOCl2→ RCl + SO2 + HCl
3 ROH + PCl3→ 3 RCl + H3PO3
ROH + PCl5→ RCl + POCl3

In the laboratory, thionyl chloride is especially convenient, because the byproducts are gaseous.

Alternatively, the Appel reaction:

Reactions

Alkyl chlorides are versatile building blocks in organic chemistry. While alkyl bromides and iodides are more reactive, alkyl chlorides tend to be less expensive and more readily available. Alkyl chlorides readily undergo attack by nucleophiles.

Heating alkyl halides with sodium hydroxide or water gives alcohols. Reaction with alkoxides or aroxides give ethers in the Williamson ether synthesis; reaction with thiols give thioethers. Alkyl chlorides readily react with amines to give substituted amines. Alkyl chlorides are substituted by softer halides such as the iodide in the Finkelstein reaction. Reaction with other pseudohalides such as azide, cyanide, and thiocyanate are possible as well. In the presence of a strong base, alkyl chlorides undergo dehydrohalogenation to give alkenes or

Thin layer chromatography

Thin layer chromatography (TLC) is a chromatography technique used to separate mixtures. Thin layer chromatography is performed on a sheet of glass, plastic, or aluminum foil, which is coated with a thin layer of adsorbent material, usually silica gel, aluminium oxide, or cellulose (blotter paper). This layer of adsorbent is known as the stationary phase.

After the sample has been applied on the plate, a solvent or solvent mixture (known as the mobile phase) is drawn up the plate via capillary action. Because different analytes ascend the TLC plate at different rates, separation is achieved..

Thin layer chromatography can be used to:

  • Monitor the progress of a reaction
  • Identify compounds present in a given substance
  • Determine the purity of a substance

Specific examples of these applications include:

A number of enhancements can be made to the original method to automate the different steps, to increase the resolution achieved with TLC and to allow more accurate quantitation. This method is referred to as HPTLC, or "high performance TLC".

Plate preparation

TLC plates are usually commercially available, with standard particle size ranges to improve reproducibility. They are prepared by mixing the adsorbent, such as silica gel, with a small amount of inert binder like calcium sulfate (gypsum) and water. This mixture is spread as a thick slurry on an unreactive carrier sheet, usually glass, thick aluminum foil, or plastic. The resultant plate is dried and activated by heating in an oven for thirty minutes at 110 Â°C. The thickness of the adsorbent layer is typically around 0.1 – 0.25 mm for analytical purposes and around 0.5 – 2.0 mm for preparative TLC.

Technique

The process is similar to paper chromatography with the advantage of faster runs, better separations, and the choice between different stationary phases. Because of its simplicity and speed TLC is often used for monitoring chemical reactions and for the qualitative analysis of reaction products.

To run a TLC, the following procedure is carried out:

  • A small spot of solution containing the sample is applied to a plate, about 1.5 centimeters from the bottom edge. The solvent is allowed to completely evaporate off, otherwise a very poor or no separation will be achieved. If a non-volatile solvent was used to apply the sample, the plate needs to be dried in a vacuum chamber.
  • A small amount of an appropriate solvent (elutant) is poured in to a glass beaker or any other suitable transparent container (separation chamber) to a depth of less than 1 centimeter. A strip of filter paper is put into the chamber, so that its bottom touches the solvent, and the paper lies on the chamber wall and reaches almost to the top of the container. The container is closed with a cover glass or any other lid and is left for a few minutes to let the solvent vapors ascend the filter paper and saturate the air in the chamber. (Failure to saturate the chamber will result in poor separation and non-reproducible results).
  • The TLC plate is then placed in the chamber so that the spot(s) of the sample do not touch the surface of the elutant in the chamber, and the lid is closed. The solvent moves up the plate by capillary action, meets the sample mixture and carries it up the plate (elutes the sample). When the solvent front reaches no higher than the top of the filter paper in the chamber, the plate should be removed (continuation of the elution will give a misleading result) and dried.

Different compounds in the sample mixture travel at different rates due to the differences in their attraction to the stationary phase, and because of differences in solubility in the solvent. By changing the solvent, or perhaps using a mixture, the separation of components (measured by the Rf value) can be adjusted. Also, the separation achieved with a TLC plate can be used to estimate the separation of a flash chromatography column.

Separation of compounds is based on the competition of the solute and the mobile phase for binding places on the stationary phase. For instance, if normal phase silica gel is used as the stationary phase it can be considered polar. Given two compounds which differ in polarity, the more polar compound has a stronger interaction with the silica and is therefore more capable to dispel the mobile phase from the binding places. Consequently, the less polar compound moves higher up the plate (resulting in a higher Rf value). If the mobile phase is changed to a more polar solvent or mixture of solvents, it is more capable of dispelling solutes from the silica binding places and all compounds on the TLC plate will move higher up the plate. It is commonly said that "strong" solvents (elutants) push the analyzed compounds up the plate, while "weak" elutants barely move them. The order of strength/weakness depends on the coating (stationary phase) of the TLC plate. For silica gel coated TLC plates, the elutant strength increases in the following order: From Yahoo Answers

Question:

Answers:http://www.ipfw.edu/chem/261/261Fall03/261nt02o.htm Polar aprotic solvents are solvents that share ion dissolving power with protic solvents but lack an acidic hydrogen. These solvents generally have high dielectric constants and high polarity. Examples are dimethyl sulfoxide, dimethylformamide, dioxane and hexamethylphosphorotriamide. Polar protic solvents are favorable for SN1 reactions, while polar aprotic solvents are favorable for SN2 reactions. Apart from solvent effects, polar aprotic solvents may also be essential for reactions which use strong bases, such as reactions involving Grignard reagents or n-butyl lithium. If a protic solvent were to be used, the reagent would be consumed by a side reaction with the solvent.

Question:how can i state whether a polar protic or a polar aprotic solvent would increase the yield for a given reaction, and why it has the effect it does?

Answers:Depends on the particular reaction.

Question:The best nucleophile for an Sn2 reaction in a polar aprotic solvent is... a) F- b) NH3 c) H2O d) HO- The best leaving group is ... a) HO- b) TosO- c) CH3- d) NH2- The best solvent for an Sn2 reaction is... a) (CH3)2S=O b) CH3CH2OH c) CCl4 d) H2O

Answers:Since the solvent is aprotic, nucleophilicity follows basicity - the stronger the base, the better the nucleophile. For the first part of the question, I'd suggest identifying the strongest base (hydroxide is your best bet). Finding the best leaving group is a bit easier - para-toluenesulfonate (TosO-) is probably the best leaving group you will encounter in a general organic chemistry course. However, if you were not to know that from experience, the best leaving group is typically the weakest base (since bases have high energy electrons, the are far less likely to leave). For example, water is always a better leaving group than hydroxide, and the relative leaving capability of the halogens goes I > Br > Cl > F. The last part of the question requires you to recognize that aprotic solvents are ALWAYS favored when running an SN2 reaction. Otherwise, your nucleophile will get solvated which lowers reaction rate (since SN2 rate depends upon both the nucleophile and the leaving group). So, ethanol and water wouldn't be a good choice. The only difference between a) and c) is the fact that one is polar and the other is polar. I'll leave you to decide which one is which, but it should be known that the polar aprotic solvent would be your best choice.

Question:

Answers:The best solvent to have in an SN2 reaction is a polar aprotic solvent. If it is protic, then it will form a solvent cage around the nucleophile and essentially hide it from the electrophile. Obviously the solvent would have to be polar because the nucleophile is ionic and, as the old addage goes, like dissolves like. In an SN1 reaction, the solvent works best when it is polar protic, because it stabilizes the intermediates, and lowers the transition state energy.

From Youtube

Some SN2, SN1, E2, and E1 topics (1) :Organic chemistry: Polar protic vs. aprotic solvents. Carbocation rearrangements. E2 and E1 regiochemistry (what to do when there's more than one beta carbon--Zaitsev vs. Hofmann products). Antiperiplanar transition state for E2 reaction; E2 and cyclohexane This is a recording of a tutoring session, posted with the students' permission. These videos are offered on a "pay-what-you-like" basis. You can pay for the use of the videos at my website: www.freelance-teacher.com For a printable document containing the handout discussed in this video series, go to my website. For a list of all the available video series, arranged in suggested viewing order, go to my website. For a playlist containing all the videos in this series, click here: www.youtube.com (1) Polar protic vs. polar aprotic solvents (2) Continued (3) Review of factors affecting SN2 and SN1 (4) Carbocation rearrangements (5) Continued (6) E2 regiochemistry: Zaitsev vs. Hofmann (7) Continued. E1 regiochemistry (8) Antiperiplanar transition state for E2; consequences for E2 and chair cyclohexane (9) Continued (10) Continued