Organic Chemistry – Some Basic Principles and Techniques

(i) CH = CH + Br → CH Br – CH Br Substrate Reagent Product (ii) Nucleophiles and Electrophiles Reagents attack the reactive site of the substrate. The reactive site may be electron deficient portion of the molecule (a positive reactive site) e.g., an atom with incomplete electron shell or the positive end of the dipole in the molecule. If the attacking species is electron rich, it attacks these sites. If attacking species is electron deficient, the reactive site for it is that part of the substrate molecule which can supply electrons, e.g., π electrons in a double bond.

A reagent that brings an electron pair to the reactive site is called a nucleophile (Nu:) i.e., nucleus seeking and the reaction is then called nucleophilic . A reagent that takes away an electron pair from reactive site is called electrophile (E + ) i.e., electron seeking and the reaction is called electrophilic . During a polar organic reaction, a nucleophile attacks an electrophilic centre of the substrate which is that specific atom or part of the substrate which is electron deficient. Similarly, the electrophiles attack at nucleophilic centre, which is the electron rich centre of the substrate.

Thus, the electrophiles receive electron pair from the substrate when the two undergo bonding interaction. A curved-arrow notation is used to show the movement of an electron pair from the nucleophile to the electrophile. Some examples of nucleophiles are the negatively charged ions with lone pair of electrons such as hydroxide (HO – ), cyanide (NC – ) ions and carbanions (R C: – ). Neutral molecules such as etc., can also act as nucleophiles due to the presence of lone pair of electrons.

Examples of electrophiles include carbocations (C + H ) and neutral molecules having functional groups like carbonyl group (>C=O) or alkyl halides (R C-X, where X is a halogen atom). The carbon atom in carbocations has sextet configuration; hence, it is electron deficient and can receive a pair of electrons from the nucleophiles. In neutral molecules such as alkyl halides, due to the polarity of the C-X bond a partial positive charge is generated on the carbon atom and hence the carbon atom becomes an electrophilic centre at which a nucleophile can attack. Problem .

Using curved-arrow notation, show the formation of reactive intermediates when the following covalent bonds undergo heterolytic cleavage. (a) CH –SCH , (b) CH –CN, (c) CH –Cu Problem . Giving justification, categorise the following molecules/ions as nucleophile or electrophile: Nucleophiles: HS – ,C H O – ,(CH ) N:H N: – These species have unshared pair of electrons, which can be donated and shared with an electrophile. Electrophiles : BF ,C1 + H –C + =O,N + O .

Reactive sites have only six valence electrons; can accept electron pair from a nucleophile. Problem . Identify electrophilic centre in the following: CH CH=O, CH CN, CH I. Among CH HC * =O, H CC * ≡ N, and H C * –I, the starred carbon atoms are electrophilic centers as they will have partial positive charge due to polarity of the bond.

. . Electron Movement in Organic Reactions The movement of electrons in organic reactions can be shown by curved-arrow notation. It shows how changes in bonding occur due to electronic redistribution during the reaction.

To show the change in position of a pair of electrons, curved arrow starts from the point from where an electron pair is shifted and it ends at a location to which the pair of electron may move. Presentation of shifting of electron pair is given below : (i) from π bond to adjacent bond position (ii) from π bond to adjacent atom (iii) from atom to adjacent bond position Movement of single electron is indicated by a single barbed ‘fish hooks’ (i.e. half headed curved arrow). For example, in transfer of hydroxide ion giving ethanol and in the dissociation of chloromethane, the movement of electron using curved arrows can be depicted as follows: .

. Electron Displacement Effects in Covalent Bonds The electron displacement in an organic molecule may take place either in the ground state under the influence of an atom or a substituent group or in the presence of an appropriate attacking reagent. The electron displacements due to the influence of an atom or a substituent group present in the molecule cause permanent polarlisation of the bond. Inductive effect and resonance effects are examples of this type of electron displacements.

Temporary electron displacement effects are seen in a molecule when a reagent approaches to attack it. This type of electron displacement is called electromeric effect or polarisability effect. In the following sections we will learn about these types of electronic displacements. .

. Inductive Effect When a covalent bond is formed between atoms of different electronegativity, the electron density is more towards the more electronegative atom of the bond. Such a shift of electron density results in a polar covalent bond. Bond polarity leads to various electronic effects in organic compounds.

Let us consider cholorethane (CH CH Cl) in which the C–Cl bond is a polar covalent bond. It is polarised in such a way that the carbon- gains some positive charge ( δ + ) and the chlorine some negative charge ( δ – ). The fractional electronic charges on the two atoms in a polar covalent bond are denoted by symbol δ (delta) and the shift of electron density is shown by an arrow that points from δ + to δ – end of the polar bond. δδ + δ + δ − CH → CH → Cl In turn carbon- , which has developed partial positive charge ( δ + ) draws some electron density towards it from the adjacent C-C bond.

Consequently, some positive charge ( δδ + ) develops on carbon- also, where δδ + symbolises relatively smaller positive charge as compared to that on carbon – . In other words, the polar C – Cl bond induces polarity in the adjacent bonds. Such polarisation of σ -bond caused by the polarisation of adjacent σ -bond is referred to as the inductive effect . This effect is passed on to the subsequent bonds also but the effect decreases rapidly as the number of intervening bonds increases and becomes vanishingly small after three bonds.

The inductive effect is related to the ability of substituent(s) to either withdraw or donate electron density to the attached carbon atom. Based on this ability, the substitutents can be classified as electron-withdrawing or electron donating groups relative to hydrogen. Halogens and many other groups such as nitro (- NO ), cyano (- CN), carboxy (- COOH), ester (COOR), aryloxy (-OAr, e.g. – OC H ), etc.

are electron-withdrawing groups. On the other hand, the alkyl groups like methyl (–CH ) and ethyl (–CH –CH ) are usually considered as electron donating groups. ­ Problem . Which bond is more polar in the following pairs of molecules: (a) H C-H, H C-Br (b) H C-NH , H C-OH (c) H C-OH, H C-SH (a) C–Br, since Br is more electronegative than H, (b) C–O, (c) C–O Problem .

In which C–C bond of CH CH CH Br, the inductive effect is expected to be the least? Magnitude of inductive effect diminishes as the number of intervening bonds increases. Hence, the effect is least in the bond between carbon- and hydrogen. .

. Resonance Structure There are many organic molecules whose behaviour cannot be explained by a single Lewis structure. An example is that of benzene. Its cyclic structure containing alternating C–C single and C=C double bonds shown is inadequate for explaining its characteristic properties.

As per the above representation, benzene should exhibit two different bond lengths, due to C–C single and C=C double bonds. However, as determined experimentally benzene has a uniform C–C bond distances of pm, a value intermediate between the C–C single( pm) and C=C double ( pm) bonds. Thus, the structure of benzene cannot be represented adequately by the above structure. Further, benzene can be represented equally well by the energetically identical structures I and II.

benzene cannot be adequately represented by any of these structures, rather it is a hybrid of the two structures (I and II) called resonance structures. The resonance structures (canonical structures or contributing structures) are hypothetical and individually do not represent any real molecule. They contribute to the actual structure in proportion to their stability. Another example of resonance is provided by nitromethane (CH NO ) which can be represented by two Lewis structures, (I and II).

There are two types of N-O bonds in these structures. However, it is known that the two N–O bonds of nitromethane are of the same length (intermediate between a N–O single bond and a N=O double bond). The actual structure of nitromethane is therefore a resonance hybrid of the two canonical forms I and II. The energy of actual structure of the molecule (the resonance hybrid) is lower than that of any of the canonical structures.

The difference in energy between the actual structure and the lowest energy resonance structure is called the resonance stabilisation energy or simply the resonance energy . The more the number of important contributing structures, the more is the resonance energy. Resonance is particularly important when the contributing structures are equivalent in energy. The following rules are applied while writing resonance structures: The resonance structures have (i) the same positions of nuclei and (ii) the same number of unpaired electrons.

Among the resonance structures, the one which has more number of covalent bonds, all the atoms with octet of electrons (except hydrogen which has a duplet), less separation of opposite charges, (a negative charge if any on more electronegative atom, a positive charge if any on more electropositive atom) and more dispersal of charge, is more stable than others. Benzene Therefore, according to the resonance theory (Unit ) the actual structure of Problem . Write resonance structures of CH COO – and show the movement of electrons by curved arrows. First, write the structure and put unshared pairs of valence electrons on appropriate atoms.

Then draw the arrows one at a time moving the electrons to get the other structures. Problem . Write resonance structures of CH =CH–CHO. Indicate relative stability of the contributing structures.

The two structures are less important contributors as they involve charge separation. Additionally, structure I contains a carbon atom with an incomplete octe t. . .

Resonance Effect The resonance effect is defined as ‘the polarity produced in the molecule by the interaction of two π -bonds or between a π -bond and lone pair of electrons present on an adjacent atom’. The effect is transmitted through the chain. There are two types of resonance or mesomeric effect designated as R or M effect. (i) Positive Resonance Effect (+R effect) In this effect, the transfer of electrons is away from an atom or substituent group attached to the conjugated system.

This electron displacement makes certain positions in the molecule of high electron densities. This effect in aniline is shown as : Stability: I > II > III [I: Most stable, more number of covalent bonds, each carbon and oxygen atom has an octet and no separation of opposite charge II: negative charge on more electronegative atom and positive charge on more electropositive atom; III: does not contribute as oxygen has positive charge and carbon has negative charge, hence least stable]. Problem . Explain why the following two structures, I and II cannot be the major contributors to the real structure of CH COOCH .

(ii) Negative Resonance Effect (- R effect) This effect is observed when the transfer of electrons is towards the atom or substituent group attached to the conjugated system. For example in nitrobenzene this electron displacement can be depicted as : The atoms or substituent groups, which represent +R or –R electron displacement effects are as follows : +R effect: – halogen, –OH, –OR, –OCOR, –NH , –NHR, –NR , –NHCOR, – R effect: – COOH, –CHO, >C=O, – CN, –NO The presence of alternate single and double bonds in an open chain or cyclic system is termed as a conjugated system. These systems often show abnormal behaviour. The examples are , - butadiene, aniline and nitrobenzene etc.

In such systems, the π -electrons are delocalised and the system develops polarity. . . Electromeric Effect (E effect) It is a temporary effect.

The organic compounds having a multiple bond (a double or triple bond) show this effect in the presence of an attacking reagent only. It is defined as the complete transfer of a shared pair of π -electrons to one of the atoms joined by a multiple bond on the demand of an attacking reagent. The effect is annulled as soon as the attacking reagent is removed from the domain of the reaction. It is represented by E and the shifting of the electrons is shown by a curved arrow ( ).

There are two distinct types of electromeric effect. (i) Positive Eelctromeric Effect (+E effect) In this effect the π− electrons of the multiple bond are transferred to that atom to which the reagent gets attached. For example: system or to an atom with an unshared p orbital. The σ electrons of C—H bond of the alkyl group enter into partial conjugation with the attached unsaturated system or with the unshared p orbital.

Hyperconjugation is a permanent effect. To understand hyperconjugation effect, let us take an example of CH C + H (ethyl cation) in which the positively charged carbon atom has an empty p orbital. One of the C-H bonds of the methyl group can align in the plane of this empty p orbital and the electrons constituting the C-H bond in plane with this p orbital can then be delocalised into the empty p orbital as depicted in Fig. .

(a). (ii) Negative Electromeric Effect (–E effect) In this effect the π - electrons of the multiple bond are transferred to that atom to which the attacking reagent does not get attached. For example: When inductive and electromeric effects operate in opposite directions, the electomeric effect predominates. .

. Hyperconjugation Hyperconjugation is a general stabilising interaction. It involves delocalisation of σ electrons of C—H bond of an alkyl group directly attached to an atom of unsaturated Fig. .

(a) Orbital diagram showing hyperconjugation in ethyl cation This type of overlap stabilises the carbocation because electron density from the adjacent σ bond helps in dispersing the positive charge. In general, greater the number of alkyl groups attached to a positively charged carbon atom, the greater is the hyperconjugation interaction and stabilisation of the cation. Thus, we have the following relative stability of carbocations : Hyperconjugation is also possible in alkenes and alkylarenes. D e l o c a l i s a t i o n o f e l e c t r o n s b y hyperconjugation in the case of alkene can be depicted as in Fig.

. (b). Problem . Explain why (CH ) C + is more stable than CH C + H and C + H is the least stable cation.

Hyperconjugation interaction in (CH ) C + is greater than in CH C + H as the (CH ) C + has nine C-H bonds. In C + H , vacant p orbital is perpendicular to the plane in which C-H bonds lie; hence cannot overlap with it. Thus, C + H lacks hyperconjugative stability. .

. Types of Organic Reactions and Mechanisms Organic reactions can be classified into the following categories: (i) Substitution reactions (ii) Addition reactions (iii) Elimination reactions (iv) Rearrangement reactions You will be studying these reactions in Unit and later in class XII. . Methods of Purification of Organic Compounds Once an organic compound is extracted from a natural source or synthesised in the laboratory, it is essential to purify it.

Various methods used for the purification of organic compounds are based on the nature of the compound and the impurity present in it. The common techniques used for purification are as follows : (i) Sublimation (ii) Crystallisation (iii) Distillation (iv) Differential extraction and (v) Chromatography Finally, the purity of a compound is ascertained by determining its melting or boiling point. Most of the pure compounds have sharp melting points and boiling points. New methods of checking the purity of an organic compound are based on different Fig.

. (b) Orbital diagram showing hyperconjugation in propene There are various ways of looking at the hyperconjugative effect. One of the way is to regard C—H bond as possessing partial ionic character due to resonance. The hyperconjugation may also be regarded as no bond resonance.

types of chromatographic and spectroscopic techniques. . . Sublimation You have learnt earlier that on heating, some solid substances change from solid to vapour state without passing through liquid state.

The purification technique based on the above principle is known as sublimation and is used to separate sublimable compounds from non- sublimable impurities. . . Crystallisation This is one of the most commonly used techniques for the purification of solid organic compounds.

It is based on the difference in the solubilities of the compound and the impurities in a suitable solvent. The impure compound is dissolved in a solvent in which it is sparingly soluble at room temperature but appreciably soluble at higher temperature. The solution is concentrated to get a nearly saturated solution. On cooling the solution, pure compound crystallises out and is removed by filtration.

The filtrate (mother liquor) contains impurities and small quantity of the compound. If the compound is highly soluble in one solvent and very little soluble in another solvent, crystallisation can be satisfactorily carried out in a mixture of these solvents. Impurities, which impart colour to the solution are removed by adsorbing over activated charcoal. Repeated crystallisation becomes necessary for the purification of compounds containing impurities of comparable solubilities.

. . Distillation This important method is used to separate (i) volatile liquids from nonvolatile impurities and (ii) the liquids having sufficient difference in their boiling points. Liquids having different boiling points vaporise at different temperatures.

The vapours are cooled and the liquids so formed are collected separately. Chloroform (b.p K) and aniline (b.p. K) are easily separated by the technique of distillation (Fig . ).

The liquid mixture is taken in a round bottom flask and heated carefully. On boiling, the vapours of lower boiling component are formed first. The vapours are condensed by using a condenser and the liquid is collected in a receiver. The vapours of higher boiling component form later and the liquid can be collected separately.

Fractional Distillation: If the difference in boiling points of two liquids is not much, simple distillation cannot be used to separate them. The vapours of such liquids are formed within the same temperature range and are condensed simultaneously. The technique of fractional distillation is used in such cases. In this technique, vapours of a liquid mixture are passed through a fractionating column before condensation.

The fractionating column is fitted over the mouth of the round bottom flask (Fig. . , page ). Vapours of the liquid with higher boiling point condense before the vapours of the liquid with lower boiling point.

The vapours rising up in the fractionating column become richer in more volatile component. By the Fig. . Simple distillation.

The vapours of a substance formed are condensed and the liquid is collected in conical flask. time the vapours reach to the top of the fractionating column, these are rich in the more volatile component. Fractionating columns are available in various sizes and designs as shown in Fig. .

. A fractionating column provides many surfaces for heat exchange between the ascending vapours and the descending condensed liquid. Some of the condensing liquid in the fractionating column obtains heat from the ascending vapours and revaporises. The vapours thus become richer in low boiling component.

The vapours of low boiling component ascend to the top of the column. On reaching the top, the vapours become pure in low boiling component and pass through the condenser and the pure liquid is collected in a receiver. After a series of successive distillations, the remaining liquid in the distillation flask gets enriched in high boiling component. Each successive condensation and vaporisation Fig.

. Different types of fractionating columns. unit in the fractionating column is called a theoretical plate . Commercially, columns with hundreds of plates are available.

One of the technological applications of fractional distillation is to separate different fractions of crude oil in petroleum industry . Fig. . Fractional distillation.

The vapours of lower boiling fraction reach the top of the column first followed by vapours of higher boiling fractions. Distillation under reduced pressure: This method is used to purify liquids having very high boiling points and those, which decompose at or below their boiling points. Such liquids are made to boil at a temperature lower than their normal boiling points by reducing the pressure on their surface. A liquid boils at a temperature at which its vapour pressure is equal to the external pressure.

The pressure is reduced with the help of a water pump or vacuum pump (Fig. . ). Glycerol can be separated from spent-lye in soap industry by using this technique.

Fig. . Distillation under reduced pressure. A liquid boils at a temperature below its vapour pressure by reducing the pressure.

Steam Distillation: This technique is applied to separate substances which are steam volatile and are immiscible with water. In steam distillation, steam from a steam generator is passed through a heated flask containing the liquid to be distilled. The mixture of steam and the volatile organic compound is condensed and collected. The compound is later separated from water using a separating funnel.

In steam distillation, the liquid boils when the sum of vapour pressures due to the organic liquid ( p ) and that due to water ( p ) becomes equal to the atmospheric pressure ( p ), i.e. p = p + p . Since p is lower than p , the organic liquid vaporises at lower temperature than its boiling point. Thus, if one of the substances in the mixture is water and the other, a water insoluble substance, then the mixture will boil close to but below, 373K.

A mixture of water and the substance is obtained which can be separated by using a separating funnel. Aniline is separated by this technique from aniline – water mixture (Fig. . , Page ).