11th Chemistry Volume · Part

OH . . Isomerism in organic compounds: The term ‘isomerism’ was given by Berzelius, and its represents of existence of two or more compounds with the same molecular formula but different structure and properties (physical, chemical, or both). Compounds exhibiting this isomerism are called isomers.

The difference in properties of two isomers is due to difference in (bond connectivity or spatial arrangement) the arrangement of atoms within their molecules. Isomerism is broadly divided into two types. i. Constitutional isomerism, ii.

stereoisomerism. ISOMERS (same molecular formula) Constitutional Isomers (Formerly structural isomers) (different in bond connectivity) Steroisomers (same in bond connectivity) Configurational Optical isomers Conformational Chain Position Functional Metamers Tautomers Ring Chain Geometrical isomers . . Constitutional isomers (Formerly structural isomers): This type of isomers have same molecular formula but differ in their bonding sequence.

Structural or constitutional isomerism is further classified into following types. (a) Chain or nuclear or skeletal isomerism: These isomers differ in the way in which the carbon atoms are bonded to each other in a carbon chain or in other words isomers have similar molecular formula but differ in the nature of the carbon skeleton (ie. Straight or branched) n-Pentane Isopentane -methyl butane Neopentane , -dimethyl propane (b) Position isomerism: If different compounds belonging to same homologous series with the same molecular formula and carbon skeleton, but differ in the position of substituent or functional group or an unsaturated linkage are said to exhibit position isomerism. Example: (i) and Pent- -ene Pent- -ene Molecular formula C H (ii) -chlorobutane and -chlorobutane Mol.

formula C H Cl (iii) Pent- -one Pent- -one and Mol. formula C H O (c) Functional isomerism: Different compounds having same molecular formula but different functional groups are said to exhibit functional isomerism. Example: (i) C H O CHO Propanone (keto group) Propanal (aldehyde group) (ii) C H O COOH Propanoic acid acid group COOCH Methyl acetate (ester group) (d) Metamerism: This type of isomerism is a special kind of structural isomerism arises due to the unequal distribution of carbon atoms on either side of the functional group or different alkyl groups attached to the either side of the same functional group and having same molecular formula. This isomerism is shown by compounds having functional group such as ethers, ketones, esters and secondary amines between two alkyl groups.

(i) C H O C H Methyl propyl ether C H C H diethyl ether (ii) C H O C H C H diethyl ketone C H Methyl propyl ketone Methyl isopropyl ketone -methoxypropane ethoxyethane pentan- -one pentan- -one Methyl iso-propyl ether -methoxypropane -methylbutan- -one (e) Tautomerism: It is a special type of functional isomerism in which a single compound exists in two readily inter con- vertible structures that differ markedly in the relative position of atleast one atomic nucleus, generally hydrogen. The two dif- ferent structures are known as tautomers. There are several types of tautomerism and the two important types are dyad and triad systems. (i) Dyad system: In this system hydrogen atom oscillates between two directly linked polyvalent atoms.

Eg: N N (hydrogencyanide) (hydrogen isocyanide) In this example hydrogen atom oscillates between carbon & nitrogen atom ) Write all the possible isomers of molecular formula C H O and identify the isomerisms found in them. ? Evaluate Yourself (ii) Triad system: In this system hydrogen atom oscillates between three polyvalent atoms. It involves , migration of hydrogen atom from one polyvalent atom to other within the mole- cule.

The most important type of triad system is keto–enol tautomerism and the two groups of tautomers are ketoform and enol-form. The polyvalent atoms involved are one oxygen and two carbon atoms. Enolisation is a process in which keto-form is converted to enol form. Both tautomeric forms are not equally stable.

The less stable form is known as labile form Example: Acetaldehyde OH H C Enol form (trace amount) Keto form ( %) ª Nitro-aci tautomerism. N N (Nitrite form) (nitro form) (f) Ring chain isomerism: In this type of isomerism, compounds having same molecular formula but differ in terms of bonding of carbon atom to form open chain and cyclic structures for eg: C H HC H C Propene and H C Cyclopropane H C CH CH Cyclobutane H C Methylcyclopropane . . Stereoisomerism: The isomers which have same bond connectivity but different arrangement of groups or atoms in space are known as stereoisomers.

This branch of chemistry dealing with the study of three-dimensional nature (spactial arrangement) of molecules is known as stereo chemistry. The metabolic activities in living organisms, natural synthesis and drug synthesis involve various stereoisomers. Steroisomerism: . .

Geometrical isomerism: Geometrical isomers are the stereoisomers which have different arrangement of groups or atoms around a rigid frame work of double bonds. This type of isomerism occurs due to restricted rotation of double bonds, or about single bonds in cyclic compounds. In alkenes, the carbon-carbon double bond is sp hybridized. The carbon-carbon double bond consists of a σ bond and a π bond.

The σ bond is formed by the head on overlap of sp hybrid orbitals. The π bond is formed by the side wise overlap of ‘p’ orbitals. The presence of the π bond lock the molecule in one position. Hence, rotation around C=C bond is not possible.

This restriction of rotation about C-C double bond is responsible for geometrical isomerism in alkenes. H C H C cis - -butene Trans -butene These two compounds are termed as geometrical isomers and are distinguished from each other by the terms cis and trans. The cis isomer is one in which two similar groups are on the same side of the double bond. The trans isomers is that in which the two similar groups are on the opposite side of the double bond, hence this type of isomerism is often called cis-trans isomerism.

The cis-isomer can be converted to trans isomer or vice versa is only if either isomer is heated to a high temperature or absorbs light. The heat supplies the energy (about 62kcal/ mole) to break the π bond so that rotation about σ bond becomes possible. Upon cooling, the reformation of the π bond can take place in two ways giving a mixture both cis and trans forms of trans- -butene and cis- -butene. heat H C H C Reform H C trans H C Generally the trans isomer is more stable than the corresponding cis isomers.

This is because in the cis isomer, the bulky groups are on the same side of the double bond. The steric repulsion of the groups makes the cis isomers less stable than the trans isomers in which bulky groups are on the opposite side. These cis and trans isomers have different chemical property is. They can be separated by fractional distillation, gas chromatography etc., All alkenes with identical substrate do not show geometrical isomerism.

Geometrical isomerism is possible only when each double bonded C atom is attached to two different atoms or groups eg. In propene no geometrical isomers are possible because one of the double bonded carbon has two identical H atoms. Cis-trans isomerism is also seen around single bond. For eg: , -butadiene has two double bonds in conjugation.

CH =CH-CH=CH . It can exist in infinite number of conformations, but the following two extreme conformations are important. ii) Oximes and azo compounds: Restricted rotation around C=N (oximes) gives rise to geometrical isomerism in oximes. Here ‘syn’ and ‘anti’ are used instead of cis and trans respectively.

In the syn isomer the H atom of a doubly bonded carbon and –OH group of doubly bonded nitrogen lie on the same side of the double bond, while in the anti isomer, they lie on the opposite side of the double bond. For eg: N OH H C syn (cis) acetaldoxime N OH H C anti (trans) acetaldoxime . . Optical Isomerism Compounds having same physical and chemical property but differ only in the rotation of plane of the polarized light are known as optical isomers and the phenomenon is known as optical isomerism.

Some organic compounds such as glucose have the ability to rotate the plane of the plane polarized light and they are said to be optically active compounds and this property of a compound is called optical activity. The optical isomer, which rotates the plane of the plane polarised light to the right or in clockwise direction is said to be dextro rotatary (dexter means right) denoted by the sign (+), whereas the compound which rotates to the left or anticlockwise is said to be leavo rotatary (leavues means left) denoted by sign(-). Dextrorotatory compounds are represented as ‘d’ or by sign (+) and lavorotatory compounds are represented as ‘l’ or by sign (-). Enantiomerism and optical activity An optically active substance may exist in two or more isomeric forms which have same physical and chemical properties but differ in terms of direction of rotation of plane polarized light, such optical isomers which rotate the plane of polarized light with equal angle but in opposite direction are known as enantiomers and the phenom- enon is known as enantiomerism.

Isomers which are non-super imposable mirror images of each other are called enantiomers. Conditions for enantiomerism or optical isomerism A carbon atom whose tetra valency is satisfied by four different substituents (atoms or groups) is called asymmetric carbon or chiral carbon. It is indicated by an asterisk as C*. A molecule possessing chiral carbon atom and non-super imposable to its own mirror image is said to be a chiral molecule or asymmetric, and the property is called chirality or dissymmetry.

OH COOH COOH HO COOH OH non superimpossible mirror images (Lactic Acid) * Assymetric Carbon atom Mirror . Detection of elements in organic compounds Introduction The first step in the analysis of an organic compound is the detection of elements present in it. The principal elements are carbon, hydrogen and oxygen In addition to these they may contain nitrogen, sulphur and halogens. Phosphorous.

Metals like Li, Mg, Zn are present in certain organometalic compounds. Detection of carbon and hydrogen If the compound under investigation is or- ganic, there is no need to test for carbon. This test is performed only to establish whether a given compound is organic or not. With the exception of few compounds like CCl , CS all organic compounds also contain hydro- gen.

The presence of both these elements is confirmed by the following common test. Copper(II)oxide test: The organic substance is mixed with about three times its weight of dry copper oxide by grinding. The mixture is then placed in a hard glass test tube fitted with a bent delivery tube. The other end of which is dipping into lime water in an another test tube.

The mixture is heated strongly and the following reaction take place. CO CuO H O + Cu 2CuO 2Cu 2H Thus if carbon is present, it is oxidized to CO which turns lime water milky. If hydrogen is also present, it will be oxidized to water which condenses in small droplets on the cooler wall of the test tube and inside the bulb. Water is collected on anhydrous CuSO which turns anhydrous CuSO blue.

This confirms the presence of C and H in the compound. Detection of nitrogen by lassaigne sodium fusion test: This is a good test for the detection of nitrogen in all classes of nitrogenous compound and it involves the preparation of sodium fusion extract This method involves the conversion of covalently bonded N, S or halogen present in the organic compounds to corresponding water soluble ions in the form of sodium salts For this purpose a small piece of Na dried by pressing between the folds of a filter paper is taken in a fusion tube and it is gently heated. When it melts to a shining globule, put a pinch of the organic compound on it. Heat the tube till reaction ceases and becomes red hot.

Plunge it in about mL of distilled water taken in a china dish and break the bottom of the tube by striking against the dish. Boil the contents of the dish for about mts and filter. This filtrate is known as lassaignes extract or sodium fusion extract and it used for detection of nitrogen, sulfur and halogens present in organic compounds. ii) Test for Nitrogen: If nitrogen is present it gets converted to sodium cyanide which reacts with freshly prepared ferrous sulphate and ferric ion followed by conc.

HCl and gives a Prussian blue color or green colour or precipitate. It confirms the presence of nitrogen. HCl is added to dissolve the greenish precipitate of ferrous hydroxide produced by the excess of NaOH on FeSO which would otherwise mark the Prussian blue precipitate. The following reaction takes part in the formation of Prussian blue.

Na +C N NaCN FeSO + Fe(OH) + Na SO from organic compounds (from excess of sodium) 2NaOH + Fe(OH) Na [Fe(CN) ] + 2NaOH 3Na [Fe(CN) ] + 4FeCl Fe [Fe(CN) ] + NaCl ferric ferrocyanide Prussian blue or green ppt sod.ferrocyanide 6NaCN Incase if both N & S are present, a blood red colour is obtained due to the following reactions. Na + C + N + S Heat NaCNS 3NaCNS + FeCl Fe(CNS) + sodium sulphocyanide ferric sulphocyanide (Blood red colour) 3NaCl iii) Test for sulphur: a) To a portion of the lassaigne's extract, add freshly prepared sodium nitro prusside solution. A deep violet or purple colouration is obtained. This test is also used to detect S - in inorganic salt analysis Na S+Na [Fe (CN ) NO]→Na4 [Fe (CN ) NOS] sodium nitro prusside b) Acidify another portion of lassaigne's extract with acetic acid and add lead acetate solution.

A black precipitate is obtained. (CH COO) Pb +Na S PbS (black ppt) + 2CH COONa c) Oxidation test: The organic substances are fused with a mixture of KNO and Na CO . The sulphur, if present is oxidized to sulphate. Na CO + S +3O Na SO + CO The fused mass is extracted with water, acidified with HCl and then BaCl solution is added to it.

A white precipitate indicates the presence of sulphur. BaCl + Na SO BaSO + 2NaCl iv) Test for halogens: To another portion of the lassaigne’s filtrate add dil HNO warm gently and add AgNO solution. a) Appearance of curdy white precipitate soluble in ammonia solution indicates the presence of chlorine. b) Appearance of pale yellow precipitate sparingly soluble in ammonia solution indicates the presence of bromine.

c) Appearance of a yellow precipitate insoluble in ammonia solution indicates the presence of iodine. Na + X heat NaX (Where x= Cl, Br, I) from organic compound Nax + AgNO AgX + NaNO If N or S is present in the compound along with the halogen, we might obtain NaCN and Na S in the solution, which interfere with the detection of the halogen in the AgNO test Therefore we boil the lassaignes extract with HNO which decomposes NaCN and Na S as NaCN + HNO NaNO + HCN Na S + 2HNO 2NaNO + H S NaCN + AgNO AgCN + NaNO Na S + AgNO Ag S + NaNO further white ppt confusing with AgCl black ppt V) Test for phosphorous: A solid compound is strongly heated with a mixture of Na CO & KNO . phosphorous present in the compound is oxidized to sodium phosphate. The residue is extracted with water and boiled with Conc.

HNO . A solution of ammonium molybdate is added to the above solution. A canary yellow coloration or precipitate shows the presence of phosphorous. .

Estimation of elements After detecting the various elements present in a given organic compound by qualitative analysis it is necessary to determine their composition by weight. The estimation of carbon, hydrogen, nitrogen, sulphur halogens are discussed here. No dependable method is however available for determination oxygen and hence its amount is always determined by difference. Estimation of carbon and hydrogen: Both carbon and hydrogen are estimated by the same method.

A known weight of the organic substance is burnt in excess of ox- ygen and the carbon and hydrogen present in it are oxidized to carbon dioxide and wa- ter, respectively. C x H y + O (in excess) xCO + H O y The weight of carbon dioxide and water thus formed are determined and the amount of carbon and hydrogen in the organic substance is calculated. The apparatus employed for the purpose consists of three units (i) oxygen supply ( ) combustion tube ( ) absorption apparatus. (see Fig.

. ) ( ) Oxygen supply: To remove the moisture from oxygen it is allowed to bubble through sulphuric acid and then passed through a U-tube containing sodalime to remove CO . The oxygen gas free from moisture and carbondioxide enters the combustion tube. ( ) Combustion tube: A hard glass tube open at both ends is used for the combustion of the organic substance.

It contains (i) an oxidized copper gauze to prevent the backward diffusion of the products of combustion (ii) a porcelain boat containing a known weight of the organic substance (iii) coarse copper oxide on either side and (iv) an oxidized copper gauze placed towards the end of the combustion tube. The combustion tube is heated by a gas burner. ( ) Absorption Apparatus: The combustion products containing moisture and carbon-dioxide are then passed through the absorption apparatus which consists of ( ) a weighed U-tube Fig . Estimation of Carbon and Hydrogen packed with pumice soaked in Conc.

H SO to absorb water (ii) a set of bulbs containing a strong solution of KOH to absorb CO and finally (iii) a guard tube filled with anhydrous CaCl to prevent the entry of moisture from atmosphere. Procedure: The combustion tube is heated strongly to dry its content. It is then cooled slightly and connected to the absorption apparatus. The other end of the combustion tube is open for a while and the boat containing weighed organic substance is introduced.

The tube is again heated strongly till the substance in the boat is burnt away. This takes about hours. Finally, a strong current of oxygen is passed through the combustion tube to sweap away any traces of carbon dioxide or moisture which may be left in it. The U-tube and the potash bulbs are then detached and the increase in weight of each of them is determined.

Calculation: Weight of the organic substance taken = w g Increase in weight of H O = x g Increase in weight of CO = y g g of H O contains 2g of hydrogen