Isomers Different compounds with the same molecular formula. Geometric isomers

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1 Exercise 6 rganic qualitative analysis. identification of unsaturated compounds, alcohols, phenols, aldehydes, ketones, carboxylic acids and amines. Theoretical part According to the simplest definition, organic chemistry is the study of the compounds of carbon. Perhaps the most remarkable feature of organic chemistry is that it is the chemistry of carbon and only a few other elements chiefly, hydrogen, oxygen, and nitrogen. Chemists have discovered or made well over ten million compounds of carbon and these three other elements. rganic compounds are everywhere around us in our foods, flavours, and fragrances; in our medicines, toiletries, and cosmetics; in our plastics, films, fibres, in our bodies and those of all living things. Isomers Isomers Different compounds with the same molecular formula Constitutional isomers Isomers with a different order of attachement of atoms in their molecules Stereoisomers Isomers with the same order of attachement of atoms in their molecules, but a different orientation of their atoms or groupof their atoms in space Geometric isomers ptical isomers Constitutional isomerism ( structural) two or more compound with the same molecular formula but different properties are called isomers. Those with different sequences of bonded 1

2 atoms are structural isomers. Alkanes at given number of C atoms but different skeletons are examples. The smallest alkane to exhibit structural isomerism has four C atoms: two different compounds have the formula C4H10. The unbranched one is butan (common name, n-butane0 and the other is 2-methylpropane 9common name, isobutene). CH 3 CH 2 CH 2 CH 3 n- butane CH 3 CH CH 3 CH 3 isobutan Geometric isomerism geometric isomers have different orientations of groups around a double bond. The alkene 2-butene, for example, has two geometric isomers. cis-2-butene has the CH3 groups at the same side of the C=C bond, and trans-2-butene has them on opposite sides of the C=C bond. This type of geometric isomerism is also called cis-trans isomerism. H 3 C CH 3 C C H H cis-2-butene H 3 C CH 3 C C H H trans-2-butene ptical isomerism is a type of stereoisomerism that arises when an object and its mirror image cannot be superimposed on each other. An asymmetric molecules is called chiral. In most cases, an organic molecules is chiral if it contains a C atom that is bonded to four different groups. Unlike structural isomers, optical isomers are identical in all but two respect: 1. In their physical properties, optical isomers differ only in the direction that each isomer rotates the plane of polarized light. A polarimeter is used to measure the angle that the plane is rotated. An optical isomer is optically active because it rotates the plane of this polarized light. The dextrorotary isomer (designated d or +) rotates the plane of light to the right; the levorotary isomer (designated l or - ) is the mirror image of the first and rotates the plane to the left. An equimolar mixture of the two isomers does not rotate the plane at all because the dextrorotation cancels the levorotation. 2

3 2. In their chemical properties, optical isomers differ only in a chiral chemical environment, one that distinguishes right-handed and left-handed molecules. Typically, one isomer of an optically active reagent is added to a mixture of optical isomers of another compound. ptical isomerism plays a vital role in living cells. Nearly all carbohydrates and amino acids are optically active, but only one of the isomers is biologically usefull. For example, D-glucose is metabolised for energy, but l-glucose would be excreted unused. Similarly, l-alanine is incorporated naturally into proteins, but d-alanine is not. Experimental 1. Lassaigne s test. Transfer a little amount of a tested substance to a special test-tube and add piece of sodium. Heat carefully on fire until it completely charred and next transfer to evaporating dish with distilled water. Stir and then filtrate the solution and divide into three pieces. Detection of sulphur: Transfer some drops of CH3C)2Pb solution. Na2S + (CH3C)2Pb PbS ( black precipitate) + 2CH3CNa Detection of nitrogen: Transfer a few drops of FeS4 solution, heat to boiling, then cool and add HCl in order to acidity the solution. Next add solution of FeCl3. 2NaCN + FeS4 Fe(CN)2 + Na2S4 Fe(CN)2 + 4NaCN Na4[Fe(CN)6] 3Na4[Fe(CN)6] + 4FeCl3 Fe4[Fe(CN)6]3 (blue precipitate) + 12NaCl Detection of chloride: Transfer HN3 to the third piece of detected solution. Boil the solution and next cool. Add drop by drop solution of AgN3. NaCl + AgN3 AgCl (white precipitate) + NaN3 2. Properties and reactivity of common functional group of organic compounds 3

4 Detection of the double bonds: - reaction with KMn4 solution Transfer to the tested solution a few drops of KMn4 solution. When KMn4 is mixed with a compound which has a double bond it adds to the bond and its purple color disappears. 2KMn4 + 3C6H5-CH=CH-CH + 4H2 3C6H5-CH(H) CH(H)-CH + 2Mn2 + 2KH Detection of the alcohol functional group: Alcohols are structural derivatives of alkanes, in which an alcohol contains one hydroxyl group in place of one of the hydrogen atoms in the corresponding alkane. We classify alcohols as primary (1 o ), secondary (2 o ), or tertiary (3 o ) depending on whether the H group is on a primary, secondary, or tertiary carbon. R H C H R R' C H R R' C H H H R" Primary (1 o ) Secondary (2 o ) Tertiary (3 o ) A compound containing two hydroxyl groups is named as a diol, one containing three hydroxyl groups is named a triol, and so on. If we want to determine whether a given alcohol is primary, secondary, or tertiary we should used the Lucas reagent ( a solution of fused zinc chloride in concentrated hydrochloric acid in 1 : 1 mole ratio). Tertiary alcohols react immediately with the Lucas reagent, and the tertiary chlorides separates as a distinct layer; secondary alcohols react within 5 min. at 20 0 C, giving first a cloudy liquid and then a separate layer; primary alcohols do not react, and the solution remains clear. - esterification reaction Using alcohols we can prepare an ester which has a characteristic odour. In this reaction an alcohol is treated a carboxylic acid in the presence of an acid catalyst, most commonly concentrated sulphuric acid. H 2 S 4 CH 3 C H CH 3 CH 2 H CH 3 C CH 2 CH 3 H 2 Transfer to 1cm 3 ethanol alcohol a few drops of anhydride acetate acid and sulphuric acid. Carrefully boil the solution with a boiling stone. After cooling smell the solution. 4

5 - test for ethanol (the haloform reaction) ne of the methods to identify ethanol is conversion into iodoform. A small quantity of a dilute aqueous solution of ethanol (2 cm 3 ) alkaline with sodium hydroxide (4cm 3 5% NaH) and then add drop by drop Lugol solution until a pale yellow color appears. The mixture warm to about 60 0 C, if the yellow color disappears, add more of the iodine solution. The iodoform is recognized by its characteristic odour and by its yellow crystals. CH3CH2H + NaI CH3CH + NaI + H2 CH3CH + 3NaI CI3CH + 3Na H CI3CH + NaH CHI3(s) + HCNa Detection of the phenol functional group: The functional group of a phenol is a hydroxyl group bonded to benzene ring. Phenols and alcohols both contain a hydroxyl group, -H. Phenols, however, are grouped as a separate class of compounds because their chemical properties are quite different from those of alcohols. ne of the most important of these differences is that phenols are significantly more acidic than alcohols are. Phenols are weak acids and react with strong bases such as NaH to form water-soluble salts (Alcohol do not react with NaH). H NaH Na H 2 Phenol Sodium hydroxide Sodium phenoxide - reaction with FeCl3 Add a few drop of FeCl3 solution to 1cm 3 of tested solution of phenol. The phenols complex with Fe is recognized by its characteristic violet color. 5

6 H 3 FeCl 3 Fe + 3HCl Detection of the aldehyde functional group: The functional group of an aldehyde is a carbonyl group ( C=) bonded to a hydrogen atom. - oxidation tests: a) Tollens s reaction Add to about 1cm 3 Tollens reagent 1cm 3 of aldehyde. Mix well and place the tube in the boiling water bath for 5 minutes. Silver mirror becomes in the presence of aldehyde. (Tollens reagent contains silver oxide with ammonia). 2[Ag(NH3)2]H + HCH + H - 2Ag + HC - + 4NH3 + 2H2 b) Trommer s reaction Add 3 drops of CuS4 solution to about 2cm 3 of detected solution of sugar. Mix and then introduce drop by drop 2M solution. Stir incessantly until obtained Cu(H)2 dissolve. Heat the blue solution to boiling. Red precipitate appears in the presence of reducing sugar. (Trommer reagent contains copper sulphate with sodium hydroxide). 2Cu(H)2 + R-CH Cu2 + R-CH + 2H2 - reaction with KMn4 Transfer to the test solution some drops of KMn4 solution. When KMn4 is mixed with an aldehyde purple color disappears. 2Mn R-CH + H2 3R-C - + 2MN2 + H - 6

7 Detection of the ketone functional group: The functional group of a ketone is a carbonyl group bonded to two carbon atoms. xidation of ketones is brought about only by strong oxidizing agent (ketons do not reduce Tollens and Trommer solution). - Legal reaction Add a few drop of sodium nitropruside (Na2[Fe(CN)5N] 2H2) solution to the solution of methylketone. Next add NaH to the basic ph. In presence of methylketone red color appears which changes to yellow color. Color changes to violet-red when is added concentrated acetate acid. - reaction with hydroxylamine Ketones react with hydroxylamine to form oximes which are called ketoximes. Transfer 2-3 drop of methyl orange to the test tube containg hydroxylamine (NH2NH2 HCl) and then while mixing continuosly add 0.1 M NaH until color change from red to onion. Next add a few drops of tested solution. The red color return in the presence of keton group in solution. R2C= + (NH3H) + Cl - R2C=NH + H + + Cl - - iodoform reaction Substances that undergo the iodoform reaction are acetaldehyde (CH3CH); ethanol (CH3CH2H); any secondary alcohols of the structure CH3CHHR; and any methyl ketone, CH3CR. This reaction was described above (the alcohols reactions). Detection of the carboxyl functional group: The functional group of carboxylic acids is a carboxyl group, so named because it is made up of a carbonyl group and a hydroxyl group. C H Carboxylic acids are weak acids. - reaction with sodium hydrogen carbonate (NaHC3) Add 0.5cm 3 of tested solution to test tube which contains 1-2cm 2 solution of sodium hydrogen carbonate (5% solution) and observe the solution. 7

8 Each carboxylic acid is converted to its sodium salt and next carbonic acid is formed, which immediately decomposes to carbon dioxide and water. CH3CH + NaHC3 CH3CNa + H2 + C2 - reaction of esterification This reaction was described previously (alcohols). Detection of the amine functional group: Amines are classified as primary (1 o ), secondary (2 o ), or tertiary (3 o ) depending on the number of hydrogens atoms of ammonia which are replaced by alkyl or aryl groups. CH 3 NH 3 CH 3 NH 2 CH 3 NH CH 3 N CH CH 3 3 Ammonia Methylamine (a 1 o amine) Dimethylamine (a 2 o amine ) Trimethylamine (a 3 o amine) Amines are further divided into aliphatic amines and aromatic amines. In an aliphatic amine, all the carbon bonded directly to nitrogen are derived from alkyl groups; in an aromatic amine, one or more of the groups bonded directly to nitrogen are aryl groups. NH 2 CH 3 N H Aniline (a 1 o aromatic amine) N-Methylaniline (a 2 o aromatic amine) Primary, secondary and tertiary amines differently react with nitric acid (III). Aliphatic primary amines in reaction with nitric acid (III) give alcohol and nitrogen. NaN 2 + HCl R CH 2 NH 2 R CH 2 H N 2 H 2 Aliphatic secondary amines in reaction with nitric acid (III) give N-nitrozamines. Aliphatic tertiary amines do not react with nitric acid (III). - acylation reaction only for primary and secondary amines Transfer 1cm 3 20% sodium acidic solution and a few drops of anhydric acidic into a solution of amine. Shake and leave for 5 minutes. After this time it is obtained white, crystal precipitate. 8

9 R-NH2 + CH3CNa R-NH-CCH3 + NaH - reaction with NH4SCN and CuS4 - only for aromatic amines Transfer drop by drop 1% solution NH4SCN (3-4 drops) to a amine solution and next add a few drops of 10% CuS4 solution. It is obtained olive color precipitate. +2 NH 2 N H 2 Cu 2 SCN - NH 2 H 2 N - preparation of diazonium salts and coupling with phenols. Add about 2cm 3 2M HCl into 1cm 3 solution of aromatic amine, cool solution and add about 1cm 3 7% solution of sodium nitric (III). Stir and divide onto two pieces. The first part get warm to liberate a nitrogen, in the solution remains phenol which has a characteristic odour. Transfer 3-4cm 3 solution of β-naphtol in 5% NaH into second piece. It is obtained red dye known as azo dye. H H N + N + N N - biuret reaction Biuret is obtained by heating urea. Solution of biuret with solution of cuprum sulfide give blue-violet color. All proteins, peptides (above dipeptides) give a positive result in this test. This is a typical reaction for compounds which have peptide bonds. C NH Heat on fire 2-3 little spatula of urea in a dry test tube. Next cool test tube and add 5cm 3 of water. 2cm 3 of the obtained solution transfer to the next test tube and add a few drops of 9

10 sodium base and then a few drops of cuprum sulfic solution. Solution takes a red-violet color. H H C C alken elimination HX addition H 2 addition elimination dehydratation reduction R R R H RC R X - lub HX substitution H 2 R X substitution CN - substitution NH 3 R C N reduction ether RH alcohol substitution esther amine R - CH reduction substitution R 1 NH 2 RC NH amide R R H oxydation oxydation reduction substitution H 2 RH R C R keton R C H aldehyde R C H carboxylic acid oxydation 10

17 Alcohols H H C C. N Goalby chemrevise.org 1 H H. Bond angles in Alcohols. Boiling points. Different types of alcohols H 2 C CH 2 CH 2

17 Alcohols H H C C. N Goalby chemrevise.org 1 H H. Bond angles in Alcohols. Boiling points. Different types of alcohols H 2 C CH 2 CH 2 17 Alcohols General formula alcohols n 2n+1 Naming Alcohols These have the ending -ol and if necessary the position number for the group is added between the name stem and the ol If the compound has an