Report 2
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TOLLENS: Ag+ ions react with OH- of aqueous ammonia to produce a brown precipitate of AgOH which is dissolved on addition of an excess of aqueous ammonia under the formation of [Ag(NH3)2]+. The silver diamine complex is reduced by glucose to metallic silver. Glucose is oxidized to gluconic acid.
Fig. 1: Redox reaction The Tollens' test is important in carbohydrate chemistry, for proof of structure. The test is specific for reducing sugars. Fructose is also capable of reducing Tollens' solution, and is thus classified a "reducing sugar". Under basic conditions fructose is converted into glucose. Note: The silver that is adhered to the flask dissolves readily in a small amount of conc. nitric acid. Aqueous solutions of glucose, silver nitrate plus ammonium nitrate, and sodium hydroxide are mixed in a Florence flask. The flask is stoppered and swirled to evenly coat the flask with the mixture. A silver mirror soon coats the flask as the Ag(I) ion is reduced to Ag metal and glucose is oxidized to gluconic acid. Discussion A silver mirror coats the flask because the Ag(I) ion is reduced to Ag(0) as Ag(I) oxidizes glucose to gluconic acid.
Safety note: Tollens’ reagent may explode on storage. Demonstrators repeating this should use only freshly prepared reagent and dispose of the reactions’ products immediately after the demonstration. Reference: http://jchemed.chem.wisc.edu/JCESoft/CCA/CCA5/MAIN/1ORGANIC/ORG12/TRAM 12/D/0384525/THUMBS.HTM March 17, 2009 2:02PM ----------------------------------------------------------------------------------------------------------LUCAS: Lucas' reagent
Lucas' reagent is a solution of zinc chloride in concentrated hydrochloric acid, used to classify alcohols of low molecular weight. The reaction is a substitution in which the chlorine replaces the hydroxy group. Even though this reaction is normally very unfavorable, the zinc ion complexes with the hydroxy group (by accepting a lone electron pair from O of -OH), making it a better leaving group. The remaining carbonium ion then combines with the chloride ion to form an alkyl halide.
Lucas Test Lucas test in alcohols is a test to differentiate between primary, secondary and tertiary alcohols. It is based on the difference in reactivity of the three classes of alcohols with hydrogen halides. When Lucas' reagent (ZnCl2 in concentrated HCl solution) is added to the alcohol, H+ from HCl will protonate the -OH group of alcohol, so that the leaving group H2O, being a much weaker nucleophile than OH-, can be substituted by nucleophile Cl-. Lucas' reagent offers a polar medium in which SN1 mechanism is favored. In unimolecular nucleophilic substitution, the reaction rate is faster when the carbocation intermediate is more stabilized by greater number of electron donating alkyl group (R-) bonded to the positively charged carbon atom. Tertiary alcohols react immediately with Lucas reagent to produce turbidity while secondary alcohols do so in five minutes. Primary alcohols do not react appreciably with Lucas reagent at room temperature. The reagent dissolves the alcohol, removing the OH group, forming a carbocation. The speed of this reaction is proportional to the energy required to form the carbocation, so tertiary, benzylic, and allylic carbocations react quickly, while smaller, less substituted, alcohols react more slowly. The cloudiness observed is caused by the carbocation immediately reacting with the chloride ion creating an insoluble chloroalkane.
Hence, the time taken for turbidity to appear is a measure of the reactivity of the class of alcohol with Lucas
• • •
reagent, and this is used to differentiate between the three classes of alcohols: no visible reaction: primary alcohol solution turns cloudy in 3-5 minutes: secondary alcohol solution turns cloudy immediately, and/or phases separate: tertiary, allyl, or benzyl alcohol The temperature at which the test is conducted is usually room temperature.
Reference: http://www.chemie.de/lexikon/e/Lucas'_reagent/
Reference: http://therealmoforganicsynthesis.blogspot.com/2008/10/lucas-test.html ----------------------------------------------------------------------------------------------------------DNPH: Shows positive test for: aldehydes and ketones Reactions: reacts with the carbonyl group of aldehydes and ketones
How to perform the test: Five drops of the compound to be tested are mixed with 5 drops of the dinitrophenylhydrazine reagent (an orange solution) in 2 ml of ethanol and the tube shaken. If no positive test is observed immediately, the mixture should be allowed to stand for 15 minutes. A positive test is indicated by: The formation of a yellow, orange or orange-red precipitate.
a negative test (left) and a positive test (right) Reference: http://www.harpercollege.edu/tmps/chm/100/dgodambe/thedisk/qual/dnp.htm Narrative A solution of 2,4-dinitrophenylhydrazine is added to each of three test tubes containing 2propanol, 2-propanone (acetone), and propionic acid. An orange precipitate forms with the 2-propanone indicating the presence of an aldehyde or ketone functional group. Discussion Orange 2,4-dinitrophenylhydrazine forms with the 2-propanone indicating the presence of an aldehyde or ketone.
----------------------------------------------------------------------------------------------------------IODOFORM Narrative Aqueous solutions of iodine and potassium iodide are added to basic solutions of 2pentanone and 3-pentanone. The iodoform reaction is a classical test for methyl ketones. A light yellow precipitate of iodoform forms immediately with the methyl ketone 2pentanone. Discussion
Shows positive test for: acetaldehyde and methyl ketones Reactions: the methyl group of the ketone is removed from the molecule and produces iodoform (CHI3)
How to perform the test: Three drops of the compound to be tested are added to 3 ml of water and 10 drops of KI/I2 solution (a dark purple-brown solution). 10% NaOH solution is added dropwise until the dark color of the solution fades to yellow. The solution is heated at 60oC. If the color of the solution becomes colorless, more KI/I2 solution is added. Indications of a positive test: The formation of a yellow precipitate or suspension of iodoform is a positive test.
a negative test (left) and a positive test (right) ----------------------------------------------------------------------------------------------------------DICHROMATE:
ALKENES and POTASSIUM MANGANATE(VII)
This page looks at the reaction of the carboncarbon double bond in alkenes such as ethene with potassium manganate(VII) solution (potassium permanganate solution).
Oxidation of alkenes Experimental details Alkenes react with potassium manganate(VII) solution in the cold. The colour change depends on whether the potassium manganate(VII) is used under acidic or alkaline conditions. If the potassium manganate(VII) solution is acidified with dilute sulphuric acid, the purple solution becomes colourless. If the potassium manganate(VII) solution is made slightly alkaline (often by adding sodium carbonate solution), the purple solution first becomes dark green and then produces a dark brown precipitate. Chemistry of the reaction We'll look at the reaction with ethene. Other alkenes react in just the same way. Manganate(VII) ions are a strong oxidising agent, and in the first instance oxidise ethene to ethane1,2diol (old name: ethylene glycol). Looking at the equation purely from the point of view of the organic
reaction:
Note: This type of equation is quite commonly used in organic chemistry. Oxygen written in square brackets is taken to mean "oxygen from an oxidising agent". The reason for this is that a more normal equation tends to obscure the organic change in a mass of other detail as you will find below! The full equations are given below, although you probably won't need them.
The full equation depends on the conditions. Under acidic conditions, the manganate(VII) ions are reduced to manganese(II) ions.
Note: If you want to know how to write equations for redox reactions like this you could follow this link, and explore in the redox section of this site. Use the BACK button (or HISTORY file or GO menu) on your browser to return to this page later.
Under alkaline conditions, the manganate(VII) ions are first reduced to green manganate(VI) ions . . .
. . . and then further to dark brown solid manganese(IV) oxide
(manganese dioxide).
This last reaction is also the one you would get if the reaction was done under neutral conditions. You will notice that there are neither hydrogen ions nor hydroxide ions on the lefthand side of the equation. Note: You might possibly remember that further up the page it says that potassium manganate(VII) is often made slightly alkaline by adding sodium carbonate solution. Where are the hydroxide ions in this? Carbonate ions react with water to some extent to produce hydrogencarbonate ions and hydroxide ions. It is the presence of these hydroxide ions that gives sodium carbonate solution its pH in the 10 11 region.
Complications (. . . and you thought this was already complicated enough?) The product, ethane1,2diol, is itself quite easily oxidised by manganate(VII) ions, and so the reaction won't stop at this point unless the potassium manganate(VII) solution is very dilute, very cold, and preferably not under acidic conditions. That means that this reaction has little use as a way of preparing ethane1,2diol. Its only real use is in testing for carboncarbon double bonds and even then it isn't very good! Note: Ethane1,2diol is an alcohol, although unlike simple ones like ethanol it contains two OH groups. The oxidation of alcohols is explored on another page if you are interested.
That page deals with the oxidation of alcohols by acidified potassium dichromate(VI) solution a slightly less powerful oxidising agent than potassium manganate(VII). The essential chemistry will be the same, although manganate(VII) ions eventually oxidise ethane1,2diol all the way to carbon dioxide and water. You only need read the first part of that page about oxidation of primary alcohols. Use the BACK button on your browser to return to this page if you choose to follow this link.
Using the reaction to test for carboncarbon double bonds If an organic compound reacts with dilute alkaline potassium manganate(VII) solution to give a green solution followed by a dark brown precipitate, then it may contain a carboncarbon double bond. But equally it could be any one of a large number of other compounds all of which can be oxidised by manganate(VII) ions under alkaline conditions. The situation with acidified potassium manganate(VII) solution is even worse because it has a tendency to break carboncarbon bonds. It reacts destructively with a large number of organic compounds and is rarely used in organic chemistry. You could use alkaline potassium manganate(VII) solution if, for example, all you had to do was to find out whether a hydrocarbon was an alkane or an alkene in other words, if there was nothing else present which could be oxidised. It isn't a useful test. Bromine water is far more clear cut.
Fig. 1: Redox reaction The Tollens' test is important in carbohydrate chemistry, for proof of structure. The test is specific for reducing sugars. Fructose is also capable of reducing Tollens' solution, and is thus classified a "reducing sugar". Under basic conditions fructose is converted into glucose. Note: The silver that is adhered to the flask dissolves readily in a small amount of conc. nitric acid. Aqueous solutions of glucose, silver nitrate plus ammonium nitrate, and sodium hydroxide are mixed in a Florence flask. The flask is stoppered and swirled to evenly coat the flask with the mixture. A silver mirror soon coats the flask as the Ag(I) ion is reduced to Ag metal and glucose is oxidized to gluconic acid. Discussion A silver mirror coats the flask because the Ag(I) ion is reduced to Ag(0) as Ag(I) oxidizes glucose to gluconic acid.
Safety note: Tollens’ reagent may explode on storage. Demonstrators repeating this should use only freshly prepared reagent and dispose of the reactions’ products immediately after the demonstration. Reference: http://jchemed.chem.wisc.edu/JCESoft/CCA/CCA5/MAIN/1ORGANIC/ORG12/TRAM 12/D/0384525/THUMBS.HTM March 17, 2009 2:02PM ----------------------------------------------------------------------------------------------------------LUCAS: Lucas' reagent
Lucas' reagent is a solution of zinc chloride in concentrated hydrochloric acid, used to classify alcohols of low molecular weight. The reaction is a substitution in which the chlorine replaces the hydroxy group. Even though this reaction is normally very unfavorable, the zinc ion complexes with the hydroxy group (by accepting a lone electron pair from O of -OH), making it a better leaving group. The remaining carbonium ion then combines with the chloride ion to form an alkyl halide.
Lucas Test Lucas test in alcohols is a test to differentiate between primary, secondary and tertiary alcohols. It is based on the difference in reactivity of the three classes of alcohols with hydrogen halides. When Lucas' reagent (ZnCl2 in concentrated HCl solution) is added to the alcohol, H+ from HCl will protonate the -OH group of alcohol, so that the leaving group H2O, being a much weaker nucleophile than OH-, can be substituted by nucleophile Cl-. Lucas' reagent offers a polar medium in which SN1 mechanism is favored. In unimolecular nucleophilic substitution, the reaction rate is faster when the carbocation intermediate is more stabilized by greater number of electron donating alkyl group (R-) bonded to the positively charged carbon atom. Tertiary alcohols react immediately with Lucas reagent to produce turbidity while secondary alcohols do so in five minutes. Primary alcohols do not react appreciably with Lucas reagent at room temperature. The reagent dissolves the alcohol, removing the OH group, forming a carbocation. The speed of this reaction is proportional to the energy required to form the carbocation, so tertiary, benzylic, and allylic carbocations react quickly, while smaller, less substituted, alcohols react more slowly. The cloudiness observed is caused by the carbocation immediately reacting with the chloride ion creating an insoluble chloroalkane.
Hence, the time taken for turbidity to appear is a measure of the reactivity of the class of alcohol with Lucas
• • •
reagent, and this is used to differentiate between the three classes of alcohols: no visible reaction: primary alcohol solution turns cloudy in 3-5 minutes: secondary alcohol solution turns cloudy immediately, and/or phases separate: tertiary, allyl, or benzyl alcohol The temperature at which the test is conducted is usually room temperature.
Reference: http://www.chemie.de/lexikon/e/Lucas'_reagent/
Reference: http://therealmoforganicsynthesis.blogspot.com/2008/10/lucas-test.html ----------------------------------------------------------------------------------------------------------DNPH: Shows positive test for: aldehydes and ketones Reactions: reacts with the carbonyl group of aldehydes and ketones
How to perform the test: Five drops of the compound to be tested are mixed with 5 drops of the dinitrophenylhydrazine reagent (an orange solution) in 2 ml of ethanol and the tube shaken. If no positive test is observed immediately, the mixture should be allowed to stand for 15 minutes. A positive test is indicated by: The formation of a yellow, orange or orange-red precipitate.
a negative test (left) and a positive test (right) Reference: http://www.harpercollege.edu/tmps/chm/100/dgodambe/thedisk/qual/dnp.htm Narrative A solution of 2,4-dinitrophenylhydrazine is added to each of three test tubes containing 2propanol, 2-propanone (acetone), and propionic acid. An orange precipitate forms with the 2-propanone indicating the presence of an aldehyde or ketone functional group. Discussion Orange 2,4-dinitrophenylhydrazine forms with the 2-propanone indicating the presence of an aldehyde or ketone.
----------------------------------------------------------------------------------------------------------IODOFORM Narrative Aqueous solutions of iodine and potassium iodide are added to basic solutions of 2pentanone and 3-pentanone. The iodoform reaction is a classical test for methyl ketones. A light yellow precipitate of iodoform forms immediately with the methyl ketone 2pentanone. Discussion
Shows positive test for: acetaldehyde and methyl ketones Reactions: the methyl group of the ketone is removed from the molecule and produces iodoform (CHI3)
How to perform the test: Three drops of the compound to be tested are added to 3 ml of water and 10 drops of KI/I2 solution (a dark purple-brown solution). 10% NaOH solution is added dropwise until the dark color of the solution fades to yellow. The solution is heated at 60oC. If the color of the solution becomes colorless, more KI/I2 solution is added. Indications of a positive test: The formation of a yellow precipitate or suspension of iodoform is a positive test.
a negative test (left) and a positive test (right) ----------------------------------------------------------------------------------------------------------DICHROMATE:
ALKENES and POTASSIUM MANGANATE(VII)
This page looks at the reaction of the carboncarbon double bond in alkenes such as ethene with potassium manganate(VII) solution (potassium permanganate solution).
Oxidation of alkenes Experimental details Alkenes react with potassium manganate(VII) solution in the cold. The colour change depends on whether the potassium manganate(VII) is used under acidic or alkaline conditions. If the potassium manganate(VII) solution is acidified with dilute sulphuric acid, the purple solution becomes colourless. If the potassium manganate(VII) solution is made slightly alkaline (often by adding sodium carbonate solution), the purple solution first becomes dark green and then produces a dark brown precipitate. Chemistry of the reaction We'll look at the reaction with ethene. Other alkenes react in just the same way. Manganate(VII) ions are a strong oxidising agent, and in the first instance oxidise ethene to ethane1,2diol (old name: ethylene glycol). Looking at the equation purely from the point of view of the organic
reaction:
Note: This type of equation is quite commonly used in organic chemistry. Oxygen written in square brackets is taken to mean "oxygen from an oxidising agent". The reason for this is that a more normal equation tends to obscure the organic change in a mass of other detail as you will find below! The full equations are given below, although you probably won't need them.
The full equation depends on the conditions. Under acidic conditions, the manganate(VII) ions are reduced to manganese(II) ions.
Note: If you want to know how to write equations for redox reactions like this you could follow this link, and explore in the redox section of this site. Use the BACK button (or HISTORY file or GO menu) on your browser to return to this page later.
Under alkaline conditions, the manganate(VII) ions are first reduced to green manganate(VI) ions . . .
. . . and then further to dark brown solid manganese(IV) oxide
(manganese dioxide).
This last reaction is also the one you would get if the reaction was done under neutral conditions. You will notice that there are neither hydrogen ions nor hydroxide ions on the lefthand side of the equation. Note: You might possibly remember that further up the page it says that potassium manganate(VII) is often made slightly alkaline by adding sodium carbonate solution. Where are the hydroxide ions in this? Carbonate ions react with water to some extent to produce hydrogencarbonate ions and hydroxide ions. It is the presence of these hydroxide ions that gives sodium carbonate solution its pH in the 10 11 region.
Complications (. . . and you thought this was already complicated enough?) The product, ethane1,2diol, is itself quite easily oxidised by manganate(VII) ions, and so the reaction won't stop at this point unless the potassium manganate(VII) solution is very dilute, very cold, and preferably not under acidic conditions. That means that this reaction has little use as a way of preparing ethane1,2diol. Its only real use is in testing for carboncarbon double bonds and even then it isn't very good! Note: Ethane1,2diol is an alcohol, although unlike simple ones like ethanol it contains two OH groups. The oxidation of alcohols is explored on another page if you are interested.
That page deals with the oxidation of alcohols by acidified potassium dichromate(VI) solution a slightly less powerful oxidising agent than potassium manganate(VII). The essential chemistry will be the same, although manganate(VII) ions eventually oxidise ethane1,2diol all the way to carbon dioxide and water. You only need read the first part of that page about oxidation of primary alcohols. Use the BACK button on your browser to return to this page if you choose to follow this link.
Using the reaction to test for carboncarbon double bonds If an organic compound reacts with dilute alkaline potassium manganate(VII) solution to give a green solution followed by a dark brown precipitate, then it may contain a carboncarbon double bond. But equally it could be any one of a large number of other compounds all of which can be oxidised by manganate(VII) ions under alkaline conditions. The situation with acidified potassium manganate(VII) solution is even worse because it has a tendency to break carboncarbon bonds. It reacts destructively with a large number of organic compounds and is rarely used in organic chemistry. You could use alkaline potassium manganate(VII) solution if, for example, all you had to do was to find out whether a hydrocarbon was an alkane or an alkene in other words, if there was nothing else present which could be oxidised. It isn't a useful test. Bromine water is far more clear cut.
