Important reactions in organic chemistry for JEE Mains

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MEERWEIN-PONNDORF-VERLEY REDUCTION

About Meerwein-Ponndorf-Verley Reduction

When carbonyl compounds are heated with aluminum isopropoxide in an isopropanol solution, they are reduced to alcohols, and isopropoxide is oxidized to acetone, which can be easily removed by distillation. Aldehydes are reduced to primary alcohols and ketones to secondary alcohols. This method is generally used for the reduction of ketones to secondary alcohols. The reducing agent employed in the reaction is a specific reagent, used for reducing ketones or aldehydes in presence of reducible functional groups like double bond, nitro group etc.

Mechanism of Meerwein-Ponndorf-Verley Reduction

This reagent reduces only the carbonyl group, keeping other groups untouched.

The reverse of this reaction is called Oppenauer oxidation, in which alcohols are oxidized to carbonyl compounds. Primary alcohols are oxidized to aldehydes and ketones are obtained from secondary alcohols.

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PERKIN CONDENSATION

About Perkin Condensation

C6H5CHO + (CH3CO)2O→ C6H5CH = CHCO2H

The reaction between aromatic aldehydes and alkanoic anhydrides in presence of alkanoate is called Perkin reaction. The reaction is similar to [aldol condensation].

In this reaction, the carbanion is obtained by the removal of an Alpha hydrogen atom from acid anhydride by carboxylate (anion of the corresponding acid of the acid anhydride). The carbanion then attacks the aromatic aldehyde to yield alkoxide anion. The transfer of acetyl group then takes place from the carboxyl oxygen to alkoxy oxygen via a cyclic intermediate to give a more stable anion. Removal of an Alpha hydrogen from this anion by carboxylate results in the loss of good leaving group from the position to give anion of the Alpha, beta unsaturated acid. This on acidification gives Alpha,-unsaturated acid. For example, PhCHO on reaction with excess acetic anhydride in presence of sodium acetate followed by acidification gives cinnamic acid (3-phenyl propenoic acid).

Perkin Condensation Mechanism:

It is observed that the acetate ion abstracts a proton from the α-carbon of the anhydride producing a carbanion which then attacks the carbonyl group of the aldehyde. The product then abstracts a proton from the acid to form – aldol type compound. The latter then undergoes dehydration in the presence of hot acetic anhydride.

Prolonged heating (about 5 hours) and high temperature is required, since a weak base (acetate ion) has to react with a weak acid (anhydride).

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PINACOL-PINACOLONE REARRANGEMENT

About Pinacol-Pinacolone Rearrangement

The acid catalysed rearrangement of vic diols (1, 2-diols) to ketones or aldehydes with elimination of water is known as pinacol or pinacol-pinacolone rearrangement. The name was given from the classical example of the conversion of pinacol to pinacolone.

Reaction Mechanism of Pinacol-Pinacolone Rearrangement

The loss of water and migration of the alkyl group may be very rapid or simultaneous. Probably the migrating group does not become completely free before it is partially bonded.

(III) to the adjacent positively charged carbon, i.e., a type of intramolecular rearrangement is suggested.

Evidence in favor of this is the migrating group retains its configuration if chiral no cross-over products are obtained when a mixture of two nearly similar 1,2-diols is treated with acid.
Migratory aptitude: Migration order in general is H > aryl > alkyl . As the migrating group migrates with its electron pair, the more nucleophilic group might be expected to migrate. Thus, the order of migration amongst the aryl groups is p-anisyl > p-tolyl > phenyl > p-chlorophenyl, etc.

Example of Pinacol-Pinacolone Rearrangement

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REFORMATSKY REACTION

About Reformatsky Reaction

A similar reaction like the addition of organometallic compounds on carbonyl compounds that involves the addition of an organozinc reagent to the carbonyl group of an aldehyde or ketone. This reaction, called Reformatsky reaction, extends the carbon skeleton of an aldehyde or ketone and yields β-hydroxy esters. It involves treating an aldehyde or ketone with an α-bromo ester in the presence of zinc metal; the solvent most often used is benzene. The initial product is a zinc alkoxide, which must be hydrolysed to yield the β-hydroxy ester.

Reformatsky reaction Mechanism

The intermediate in the reaction appears to be an organozinc reagent that adds to the carbonyl group in a manner analogous to that of a Grignard reagent.

Example of Reformatsky reaction

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REIMER-TIEMANN REACTION

About Reimer-Tiemann Reaction

An alkaline solution of phenol is refluxed with chloroform at 60°C, distilling off the excess of chloroform and acidifying the residual liquid with sulphuric acid. As a result, o-hydroxy and p-;hydroxy benzaldehyde are formed, which are separated by steam-distillation.

Reaction Mechanism of Reimer-Tiemann Reaction

If ‘o’ both the o-positions are blocked, p-hydroxy benzaldehyde is the main product. With blocked p-position, o-hydroxy benzaldehyde and cyclohexadienones are formed.

Cyclohexadienone derivative remains unhydrolysed as it has a neopentylic system, which involves lot of steric crowding.

When phenol is refluxed with CCl4 in alkaline medium, salicylic acid is formed.

Example of Reimer-Tiemann Reaction

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SCHIMDT REACTION

About Schimdt Reaction

This is the reaction between a carbonyl compound and hydrazoic acid in the presence of a strong acid concentrated sulphuric acid. Aldehydes give a mixture of cyanide and formyl derivatives of primary amines, whereas ketones give amides. This reaction closely related to Hofmann and Curtius reaction, of all which involves formation of isocyanate intermediate through the migration of group from carbon to nitrogen. the rearrangement closely resembles the Beckmann rearrangement.

It has been shown to be intramolecular, and Smith (1948) has proposed the following mechanism, which is an example of the 1, 2-shift (from carbon to nitrogen); for ketones :

Reaction Mechanism of Schimdt Reaction

In ketones, if the two groups are not identical, then two geometrical isomers of (I) are possible. It is also reasonable to suppose that the anti group (to the diazonium nitrogen) is the group that migrates. In this way it is possible to explain how steric factors may influence the isomer ratio of amides formed:For aldehydes and so the reaction maybe formulated.

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SCHOTTEN BAUMANN REACTION

Reaction of Schotten Baumann Reaction

Schotten Baumann Reaction is a base-catalyzed reaction. The base is required to to shift the equilibrium towards the formation direction of of amides.The base also neutralizes the hydrochloric acid which is formed in the process, thereby preventing the further protonation of the amide product formed. Usually, aqueous NaOH is used as the base catalyst, but pyridine also can be used in this reaction. It is generally observed that the acyl chlorides are converted into acylating agents of superior power when pyridine is used as the base catalyst in Schotten Baumann Reaction The ‘Schotten Baumann conditions’ maintained in this reaction refer to the aqueous basic environment which is biphasic in nature.

Reaction Mechanism of Schotten Baumann Reaction

Example of Schotten Baumann Reaction

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STEPHEN’S REDUCTION

An alkyl or aryl cyanide dissolved in ether is reduced with stannous chloride and HCl to give aliphatic or aromatic aldehydes. The reaction proceeds by the formation of aldimine hydrochloride (present as stannichloride), which are not stable and hydrolyse to give aldehydes.

Mechanism of stephens Reactions

Following is the mechanism of Stephens reaction.

1. Gaseous hydrogen chloride(HCl) is added to the given nitrile, which reacts to provide its corresponding salt.
2. A single electron transfer from the tin(II) chloride reduces this salt. On further reduction with HCl, it forms tin (IV) tetrachloride and amine.
3. The salt obtained in step 2 precipitates quickly after as aldimine tin chloride.
4. The hydrolysis of aldimine tin chloride yields an amide.
5. The required aldehyde is formed from this amide. During this ammonium chloride is also formed.

Example of Stephen’s Reduction

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TISCHENKO REACTION

All aldehydes react with Al(OEt)3 to give an ester. The reaction is known as the Tischenko reaction.

Reaction Mechanism of Tischenko reaction

The 1,3-hydride shift generates the final product, which is an ester consisting of an R group from the alkoxide (1) and a possibly different R group from the aldehyde (2). The aldehyde that was bound with the (3) R group has now regenerated the original alkoxide reactant.

Example of Tischenko reaction

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WILLIAMSON’S SYNTHESIS

About Williamson’s synthesis

This is the most important method for formation of ethers. It is a nucleophilic substitution reaction, Nucleophilic (SN 2 ) attack by alkoxide ion on an alkyl halide/alkyl sulphate / alkyl sulphonato which are known as substrates.Substrates should have good leaving group like X–, —OSO2, —OSO2R and it must have a primary alkyl group for good yield if substrates is tertiary than elimination occurs, giving alkenes. With a secondary alkyl halide, both elimination and substitution products are obtained.

R⎯X + Na+ -O⎯R’ → R⎯O⎯R’ + Na+ X-

Reaction involves nucleophilic substitution of alkoxide ion for halide ion; it is strictly analogous to the formation of alcohols by treatment of alkyl halides with aqueous hydroxide.

Reaction Mechanism of Williamson’s synthesis

Since alkoxides and alkyl halides are both prepared from alcohols, the Williamson method ultimately involves the synthesis of ether from two alcohols.

If we wish to make unsymmetrical dialkyl ether, we have a choice of two combinations of reagents; one of these is nearly always better than the other. In the preparation of tert-butyl ethyl ether, for example, the following combinations are conceivable:

Alkoxides are not only nucleophiles, but also strong bases which tend to react with alkyl halides by elimination, to yield alkenes. Whenever we are trying to carry out nucleophilic substitution, one must be aware of the competing elimination reaction. The tendency of alkyl halides to undergo elimination is 3o > 2o > 1o.
In the above example, the use of the tertiary halide is rejected as it would be expected to yield mostly or all elimination product; hence the other combination is used.
Aromatic ethers are formed when phenoxides react with alkyl sulphates following SN2 mechanism.

C6H5O–Na+ + CH3—OSO2O—CH3 → C6H5OCH3 + NaSO2OCH3
Sodium Phenoxide Dimethyl sulphate ether Methyl phenyl

Example of Williamson’s synthesis

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WITTIG REACTION

About Wittig Reaction

Wittig reaction affords an important and useful method for the synthesis of alkenes by the treatment of aldehydes or ketones with alkyldinetripheylphosphorate (Ph3P =- CR2) or simply known as phosphorane.

The Wittig reagent, alkylidenetriphenylphosphorane, is prepared by treating trialkyl or triarylphosphine usually the latter with an alkyl halide in ether solution. The resulting phosphonium salt is treated with a strong base (such as C65Li, BuLi, NaNH2, NaH, C2H5ONa, etc.

Reaction Mechanism of Wittig Reaction

The reaction probably proceeds by the nucleophilic attack of the ylide on the carbonyl carbon. The dipolar complex (betain) so formed decomposes to olefin and triphenyphosphine oxide via a four centred transition state.

The mechanism is supported by the fact that an optically active phosphonium salt reacts to produce a phosphine oxide with retention of configuration.

Phosphorous yields react in the same manner with the C = O groups of ketones and isocyanates as also with the N = O and C = N groups of nitroso and imine compounds respectively.

Note: The aldehydes and ketones are converted into alkenes by using a special class of compounds called phos- phorus ylides, also called Wittig reagents.The Triphenyl group of phosphorane has a strong tendency to pull oxygen atom of the aldehyde or ketone via a cyclic transition state forming an alkene.

Example of Wittig Reaction

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WOLF KISHNER REDUCTION

Introducation to Wolf Kishner Reduction

About Wolf Kishner Reduction

This is a selective reduction it can reduce only carbonyl compounds unable to reduce the OH group. When a ketone or an aldehyde is heated in a basic solution of hydrazine, the carbonyl group is converted to a methylene group this process is called Deoxygenation because an oxygen is removed from the reactant. The reaction is known as the Wolf-Kishner Reduction.

Initially, the ketone reacts with hydrazine to form a hydrazone. Hydroxide ion and heat differentiate the Wolff-Kishner Reduction from ordinary hydrazone formation. After the hydrazone is formed, OH– removes a proton from the NH2 group. Heat is required because these protons are not easily removed. Resonance places some of the negative charge on carbon, which abstracts a proton from water. The last two steps are repeated to from the deoxygenated product and N2 gas

The mechanism for Wolff-Kishner

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ACY​LOIN CONDENSATION REACTION AND MECHANISM

Reactions and Introduction of Acyloin Condensation

When carboxylic acid esters are refluxed with metallic sodium in aprotic solvents such as ether, benzene, toluene or xylene. free from oxygen. a-hydroxy ketones called acyloins are formed. This is called acyloin condensation.

Mechanism of Acyloin Condensation

The mechanism of the condensation is not clearly known but it is observed that the reaction proceeds through a diketone intermediate, since diketone has been isolated in small amounts as a side product.As the reaction occurs in the presence of metallic sodium_ , a direct transfer of electron, i.e., a radical mechanism is suggested.The metallic sodium donates its electron to the carbonyl carbon to give (I) which subsequently dimerizes to yield (11). Loss of both the alkoxy groups from (II) produces 1, 2-diketone (Ill). Further reduction gives sodium salt of enediol (IV). Finally, addition of acid yields 1, 2-diol which tautomerizes to the stable acyloin (V).

Small traces of oxygen reduce the yield. Hence the reaction is carried out in an atmosphere of oxygen-free nitrogen.

Use of Acyloin Condensation

The condensation has considerable preparative value.

1.Acyloin Condensation is use in the Preparation of cyclic acyloins the condensation has been employed with great success for the preparation of _cyclic acyloin . Long-chain dicarboxylic esters have been converted to large-ring compounds without high dilution technique. The method 1s the best for closing rings of ten members or more.

The yields are as high as 60- 95% for 10 to 20 membered rings.

To account for the ready formation of large rings, it is suggested that the two ends of the ester are adsorbed, albeit weakly, to nearby sites on the surface of the sodium metal. Thus, the reactive ends are not

available for intermolecular coupling to compete with cyclisation.

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WURTZ REACTION

About Wurtz Reaction

An ethereal solution of an alkyl halide (preferably the bromide or iodide) is treated with sodium, when alkane is obtained. For example,

In this reaction, two R groups are coupled by reacting RBr, RCl or RI with Na or K. The yields of the product are best for 1° alkyl halide (60%) and least for 3° alkyl halides (10%).

Looking further in the above reaction, it was found that in addition to the desired alkane R1-R2, there will also be present the alkanes R1-R1 and R2-R2. Unsaturated hydrocarbons are also obtained. Obviously, then, the best yield of an alkane will be obtained when R1 & R2 are same, i.e., when the alkane contains an even number of carbon atoms and is symmetrical. It has been found that the Wurtz reaction gives good yields only for ‘even carbon’ alkanes of high molecular weight, and that the reaction generally fails with tertiary alkyl halides.
[Note: Metal other than sodium, which can be employed in Wurtz reaction are Ag and Cu in finely divided state]

Reaction Mechanism of Wurtz Reaction

The reaction involves the intermediate formation of free radicals. The suggested mechanism for the Wurtz reaction is shown as

Example of Wurtz Reaction

Limitation of Wurtz Reaction

The best yield of an alkane will be obtained when R and R are the same, i.e., when the alkane contains an even number of carbon atoms and is symmetrical. Experimentally, it is found that the Wurtz reaction gives good yields only for even carbon alkanes of high molecular mass. It fails with tertiary alkyl halides.

Application of Wurtz Reaction

Wurtz reaction is used to ascend the homologous series through the preparation of higher alkanes containing even number of carbon atoms.

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BIRCH REDUCTION MECHANISM

Reaction of Birch Reduction

When aromatic rings are reduced with sodium, potassium or lithium in liquid ammonia or amine in the presence of alcohol, addition of hydrogen takes place at posilions-1 and -4 to give an unconjugated diene. This is known as Birch reduction. Thus, benzene gives 1. 4-dihydro cyclohexadiene and naphthalene gives 1, 4-dihydronaphthalene.

Reaction mechanism of Birch Reduction

Liquid ammonia serves as solvent. Primary amines may also be used as solvent with advantage, since it permits higher temperature of reaction. (b.p. of ethyl amine is 19°C and b.p. of liquid ammonia is -33°C.)

The accepted mechanism of reduction involves the following sequential steps: The metal transfers one electron to the benzene ring to produce a resonance-stabilized anion radical (la-le) which accepts a proton from the alcohol to form a radical (II). The addition of an electron from the metal to the radical produces an anion (Ill) which subsequently takes up a proton from the alcohol to give the dihydro product. The repulsion between the anionic and radical centres Is minimum in (lb) which, therefore, adds a proton to give (II) and subsequently a 1, 4-dlhydro and not 1,2-dlhydro product Is formed.

At higher temperatures (50-120°C), ammonia becomes the proton source and alcohol need not be used. The amide Ion thus formed Is a strong base and isomerizes the 1, 4-dihydro product to 1,2-dihydro product.

Cyclohexene has a single olefinic bond which is unaffected by the reagent. The presence of electron-withdrawing gro1Jps in the aromatic rings makes the rings more electron-accepting and hence the reaction is facilitated. The presence of electron-releasing groups have, the reverse effect. With substituted benzene the electron-donating group remains on the unsaturated carbon and the electron-withdrawing group remain on the saturated carbon in the products.

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CURTIUS REACTION MECHANISM

About Curtius Rearrangement and Reaction

Curtius rearrangement involves the decomposition of acyl azides in an inert solvent (e.g., chloroform, benzene, etc.,) by gentle heat to isocyanate. Good yields of isocyanates have been obtained.

If the reaction is carried out in alcoholic or aqueous medium, the isocyanate further reacts to form urethane, amine or substituted urea.

Reaction Mechanism of Curtius Reaction

The mechanism of the rearrangement is very similar to Hofmann’s rearrangement to isocyanate. The driving force of the rearrangement is the electron-deficient nitrogen formed on elimination of nitrogen molecule on heating. Since there Is no evidence for the formation of nitrene, all the steps may be concerted.

Applications of Curtius Reaction

This simple one-step and mild reaction afford an important and easy preparative method for primary amines,-a ·amino acids, aldehydes, urethanes, etc. Preparation of. Primary amines-Primary amines, free from secondary and tertiary amines can be prepared, e.g.,

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REACTION AND MECHANISM OF DIECKMANN REACTION

About Dieckmann reaction

intramolecular Claisen condensation in dibasic acid esters is called the Dieckmann reaction. The resulting products are invariably cyclic 13-ketone derivatives. The condensing bases may be sodium, sodium ethoxide, sodium hydride, potassium t-butoxide, etc.

The reaction best proceeds with dibasic acid esters having 6, 7 or 8 carbon atoms which give stable rings with s, 6, or 7 carbons . Yields for rings of 9 to 12 carbons are very low. High-dilution technique is used for the formation of large-size rings.

Reaction Mechanism of Dieckmann reaction

The mechanism of the reaction is similar to that of Claisen condensation.

The base abstracts a proton from one of the a-carbons. The resulting carbanion then attacks the carbonyl carbon of the other ester group. Subsequent expulsion of the alkoxide ion gives the cyclic ketone derivative.The compound on hydrolysis and decarboxylation gives cyclic ketone,Esters of acids tower than adipic acid undergo more of intermolecular condensation with subsequent cyclisation. Thµs, ethyl succinate gives cyclohexandione derivative. This may be due to reasons of stability of six-membered rings.

Applications and extension of Dieckmann reaction

The reaction affords a useful route for the synthesis of cyclopentanone and cyclohexanone derivatives. Some examples are given for illustration.

The reaction has been used to buildup five- or six-membered rings in the synthesis of various natural products. The general process is given below.

Synthesis of steroid

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DIELS-ALDER REACTION AND MECHANISM

About Diels-Alder reaction

Diels-Alder reaction involves the 1, 4-addition of an alkene to a conjugated diene to form an adduct of six-membered ring. The double bond compound is called the dienophile. The reaction is initiated thermally or by Lewis acid catalyst with or without the use of solvents. Ethylene and simple olefins give poor yields even at high temperature.

Electron-withdrawing substituents in the dienophile, su·ch as >C=O, -CHO, -COOR, -CN, -N02, etc., promote the reaction. The reaction rate is also accelerated by the presence of electron-releasing groups in the dienes. Thus, the reaction between 2-methyl-1, 3-butadiene (isoprene) and acraldehyde is taste( than that between 1, 3-butadiene and acraldehyde. Since the reaction involves uniting of n orbitals, all carbon skeleton is not necessary in the dienophile, e.g.

Few Important points about Diels Alder Reaction

1.Besides acylic hydrocarbons, dienes may be alicyclic hydrocarbons, in which the conjugated double bonds may be wholly or partly inside the ring, some heterocvclic and some aromatic hydrocarbons.

2.Benzene, naphthalene and phenanthrene are quite unreactive but anthracene responds readily

3.The reaction occurs with the cisoid form (s-cis conformation) of the diene. Cyclic dienes are naturally in cisoid form and so react more rapidly than the acyclic dienes which normally exist in the more stable transoid forms (s-trans conformation).

4. When both the diene and dienophile are unsymmetrical, two products are possible. However, the 1, 2· and 1, 4-products predominate.

Mechanism of Diels Alder Reaction

There is little evidence in favour of stepwise polar or free radical mechanism. The other possibility is a concerted mechanism. Both the mechanisms are discussed. However, see note below.

(i)Stepwise polar and free radical mechanism The mechanism envisages that the bonds between the reactants are formed consecutively, i.e., one bond is formed followed by the other. The first bond gives a diion

C-C bond, as otherwise a mixture of cis and trans products (nonstereospecific) would be obtained with substituted reactants, which is contrary to observation. The reaction is strictly stereospecific. The orientation of the groups (i.e., cis or trans) in the reactant remain unaltered in the product. Hence the mechanism is not favourable.

(ii)Concerted mechanism The mechanism visualises a process in which there is simultanequs (not consecutive) breaking and making of bonds through a six-centred transition state with no intermediate. It is a 4n + 2n cycloaddition;md stereospecifically cis with respect to both reactants.

(ii)Concerted mechanism The mechanism visualises a process in which there is simultanequs (not consecutive) breaking and making of bonds through a six-centred transition state with no intermediate. It is a 4n + 2n cycloaddition;md stereospecifically cis with respect to both reactants. The addition of the dienophile is a1ways cis so that the groups cis in the olefin remain cis in the adduct. The addition is therefore stereospecific. There are two possible modes of addition, (a) exo and (b) endo. When the greater part of the dienophile is under the diene ring in the adduct, it is called endo and in the reverse case, it is called exo. The endo adduct is more stable.

Application of Diels alder reactions

1.This is because the cyclic transition state has further stabilization through secondary n-orbital overlaps for greater accumulation of double bonds in the endo adduct.

2.The reaction has proved of great value. Due to its high stereospecific nature, the reaction has been used to synthesize many natural products which otherwise would be difficult to prepare. The reaction can be used as a diagnostic test for conjugation in a system. Since the reaction is stereospecific, the configurations of the reactants can be determined by studying the adduct. The reaction has also been used to trap benzyne intermediate. Some reactions are given to illustrate its use.

1. Determination of configuration · Maleic and fumaric acids can be identified from the products.

2. In the synthesis of cantharidine The Diels-Alder adduct from luran and acetylene dicarboxylate is the starting product for a complicated synthesis of natural cantharidine.

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DIENONE PHENOL REARRANGEMENTS

About Dienone phenol Rearrangements

When 4, 4-dialkyl Cyclohexadienone is treated with acid, it is converted to phenol with migration of one of the alkyl groups to the adjacent carbon. This is known as dienone-phenol rearrangement. The dienone is dissolved in acetic anhydride and treated with catalytic amount of sulphuric acid. The product on hydrolysis gives the phenol.

Mechanism of Dienone phenol Rearrangements

On protonation of the oxygen, a carbocation is generated’ which is stabilized by de localization of the positive charge. In one of the canonical structures, the positive charge is on a carbon adjacent to a highly substituted carbon. Hence, a carbocation rearrangement occurs. Subsequent loss of a proton gives the 3, 4-disubstituted phenol. The ease of dienone-phenol rearrangement is due to the creation of a stable aromatic system.

When one of the alkyl group forms a part of the cyclic system, either the alkyl group or the ring methylene group may migrate. The course of the reaction depends on the structural or electronic factors and on the conditions of reaction. A reverse rearrangement, i.e., phenol–dienone rearrangement has been observed during the electrophilic substitution in phenols in some cases.

Applications of Dienone phenol Rearrangements

The rearrangement has useful applications. A classic example is the rearrangement of santonin to deisotope santonin.

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CHEMISTRY FORMULA FOR CLASS 11 CHAPTER- GENERAL ORGANIC CHEMISTRY

Chemistry Formulas

Class- 11 chapter- General Organic Chemistry

• Effect of Hybridization on bond length and bond enthalpy. The bond length and bond strength depend upon hybridization.
• sp-hybridised ‘C’ has 50% ‘s’ character, hence it is close to the nucleus and forms shorter and stronger bond. It is most electronegative.
• sp2–hybridised ‘C’ has 33% ‘s’ character, hence it is lessrclose to the nucleus and forms slightly longer and less strong bonds than ‘sp’-hybridised ‘C’. It is less electronegative and bigger than ‘sp’-hybridised.
• sp3–hybridised ‘C’ 25% ‘s’-character, hence it is least close to nucleus and forms longer and least strong bonds. It is bigger in size and less electronegative than sp2 and ‘sp’-hybridised ‘C’.
• Conformations. In ethane, the rotation of one CH3 fragment against another in a molecule doesi affect overlap in a sigma bond. Consequently, free rotation around a-bond is permissible. The carbon-carbon a-bond rotation results in different interconvertible forms of molecules known! Conformations.

Classification of Organic Compounds:

• Aliphatic compounds. Those compounds which are open chain compounds are called aliphatic compounds. They are also called acyclic compounds, e.g., propane, 1, 3-butadiene, butene, hexane, decane, etc.
• Alicyclic compounds. Compounds containing closed ring of carbon atoms are called Alicylic compounds, e.g., cyclopropane, cyclobutene, cyclohexane, etc.
• Aromatic compounds. Benzene and its derivatives are called aromatic compounds, e.g., benzene, toluene, phenol, aniline etc.

1. Benzenoid aromatic compounds
2. Non-benzenoid aromatic compounds

• Heterocyclic aromatic compounds

• Functional Group. Functional group is an atom or a group of atoms or reactive par of the compound which determines physical and chemical properties of compounds e.g., –OH, –CHO, –OR, –COOH groups etc.