Unit 12 Basic concepts of organic reactions

Learning Objectives

After learning this unit, students will be able to

•• understand the concept of organic reaction mechanism Otto dielsdiels andand Kurtkurt Alderalder •• describe homolytic and heterolytic fission of bonds describesdescribe anan important reaction mechanism for •• identify free radicals, nucleophiles and electrophiles, the reaction between a conjucated diene and a •• classify organic reactions into substitution, substituted alkene. For this elimination, addition, oxidation and reduction work they were awarded •• describe electron movement in organic reactions nobel prize in in 1950 DielsDiels - -Alder Alder reaction reaction is •• explain the electronic effects in co-valent bonds isaa powerful powerful tool tool inin synthetic organic chemistry.

12.1 Introduction

A chemical reaction can be treated as a process by which some existing bonds in the reacting molecules are broken and new bonds are formed. i.e., in a chemical reaction, a reactant is converted into a product. This conversion involves one or more steps. A In general an organic reaction can be represented as

Substrate + Reagent [Intermediate state (and/or) Transition State] Product

Here the substrate is an organic molecule which undergoes chemical change. The reagent which may be an organic, inorganic or any agent like heat, photons etc., that brings about the chemical change

Many chemical reactions are depicted in one or more simple steps. Each step passes through an energy barrier, leading to the formation of short lived intermediates or transition states. The series of simple steps which collectively represent the chemical change, from substrate to product is called as the mechanism of the reaction. The slowest step in the

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Unit 12 Final - jagan.indd 161 4/14/2020 6:27:13 PM mechanism determines the overall rate of of UV light in a compound containing the reaction. non polar covalent bond formed between atoms of similar . In such 12.1.1 Fundamental concepts in organic molecules, the cleavage of bonds results reaction mechanism into free radicals. They are short lived and are highly reactive. The type of reagents that The mechanism is the theoretical promote homolytic cleavage in substrate pathway which describes the changes are called as free radical initiators. For occurring in each step during the course example Azobisisobutyronitrile (AIBN) of the chemical change. An organic and peroxides such as benzoyl peroxide reaction can be understood by following are used as free radical initiators in the direction of flow of electrons and the polymerisation reactions. type of intermediate formed during the course of the reaction. The direction of hυ flow of electron is represented by curved CO O O CO arrow. The movement of a pair of electron is represented by a double headed arrow 2 CO + which starts from the negative and ends 2 with the atom to which the electrons needs to be transferred. As a free radical with an unpaired electron is neutral and unstable, it has a 12.1.2 Fission of a covalent bond tendency to gain an electron to attain All organic molecules containstability. Organic reactions involve covalent bonds which are formed by homolytic fission of C-C bonds to form the mutual sharing of electrons between alkyl free radicals. The stability of alkyl atoms. These covalent bonds break in two free radicals is in the following order different ways, namely homolytic cleavage ˙C(CH ) > ˙CH(CH ) >˙CH CH >˙CH (symmetrical splitting) and heterolytic 3 3 3 2 2 3 3 cleavage (unsymmetrical splitting). The Heterolytic Cleavage cleavage of a bond in the substrate is influenced by the nature of the reagent Heterolytic cleavage is the (attacking agent). process in which a covalent bond breaks unsymmetrically such that one of the Homolytic Cleavage bonded atoms retains the bond pair of electrons. It results in the formation Homolytic cleavage is the processof a cation and an anion. Of the two in which a covalent bond breaks bonded atoms, the most electronegative symmetrically in such way that each of atom becomes the anion and the other the bonded atoms retains one electron. atom becomes the cation. The cleavage is It is denoted by a half headed arrow (fish denoted by a curved arrow pointing towards hook arrow). This type of cleavage occurs the more electronegative atom. under high temperature or in the presence

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Unit 12 Final - jagan.indd 162 4/14/2020 6:27:14 PM For example, in tert-butyl bromide, Empty the C-Br bond is polar as bromine is more p orbital electronegative than carbon. The bonding Sp2–S electrons of the C-Br bond are attracted Overlap more by bromine than carbon. Hence, the H C-Br undergoes heterolytic cleavage to H C+ form a tert-butyl cation during hydrolysis. H CH3 CH3 δ δ H O + H C C Br 2 H3C C Br 3 CH3 CH3 CH3 Fig 12.1(a) Shape of Carbocation Thecarbanions are generally Let us consider the cleavage in a pyramidal in shape and the lone pair carbon-hydrogen (C-H) bond of aldehydes occupies one of the sp3 hybridised orbitals. or ketones We know that the carbon is more An alkyl free radical may be either pyramidal electronegative than hydrogen and hence or planar. the heterolytic cleavage of C-H bonds results in the formation of carbanion (carbon Lone Pair in bears a negative charge). For example in sp3 Orbital aldol condensation the OH- ion abstracts a α-hydrogen from the aldehyde, which leads Sp3–S to the formation of the below mentioned Overlap carbanion. C– H H OH (aq) H CH2 C H CH2 C H + H2O H –: O O CH3 Fig 12.1(b) Shape of Carbanion Hybridisation of carbon in carbocation: Unpaired electron In a carbocation, the carbon bearing positive in p orbital charge is sp2 hybridised and hence it has a Sp2–S planar structure. In the reaction involving Overlap H such a carbocation, the attack of a negatively charged species (nucleophiles) take place H C on either side of the carbocation as shown H below. . CH3 Fig 12.1(c) Shape of Carbon radical

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Unit 12 Final - jagan.indd 163 4/14/2020 6:27:14 PM The relative stability of the alkyl carbocations 12.1.3 Nucleophiles and elctrophiles and carbanions are given below. Nucleophiles are reagents that has Relative stability carbocations. high affinity for electro positive centers. +C(CH ) > +CH(CH ) > +CH CH > +CH They possess an atom has an unshared pair 3 3 3 2 2 3 3 of electrons, and hence it is in search for an relative stability of carbanions electro positive centre where it can have an opportunity to share its electrons to form a –C(CH ) < –CH(CH ) < –CH CH < –CH 3 3 3 2 2 3 3 covalent bond, and gets stabilised. They are The energy required to bring aboutusually negatively charged ions or electron homolytic splitting is greater than that of rich neutral molecules (contains one or heterolytic splitting. more lone pair of electrons). All Lewis bases act as nucleophiles.

Electron Types Examples rich site

Ammonia (NH3) and amines (RNH2) N: Neutral molecules having Water (H2O), alcohols (ROH) and ethers (R-O-R) :O: unshared pair of electron Hydrogen sulphide (H2S) and thiols (RSH) :S: Chlorides (Cl–), bromides (Br–) and iodides (I–) X- Negatively charged Hydroxide (HO– ), alkoxide (RO-) and Carboxlate ions O- nucleophiles (RCOO–) Cyanide (CN–) N-

Electrophiles are reagents that are attracted towards negative charge or electron rich center. They are either positively charged ions or electron deficient neutral molecules. All Lewis acids

act as electrophiles. Neutral molecules like SnCl4 can also act as an electrophile, as it has vacant d-orbitals which can accommodate the electrons from others.

Electron deficient Types Examples entity Carbon dioxide (CO ), dichlorocarbene (:CCl ) 2 2 C Neutral electrophiles Aluminium chloride (AlCl ), boron trifluoride 3 Metal (M) (BF3) and ferric chloride (FeCl3) Carbocations (R+) C+ Proton (H+) H+ Positively charged Alkyl halides (RX) X+ electrophiles Nitrosonium ion (NO+) O+

+ + Nitronium ion ( NO2) N

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Unit 12 Final - jagan.indd 164 4/14/2020 6:27:15 PM Human body produces Type 1: A lone pair to a bonding pair free radicals when it is exposed to x-rays, ·· ··

·· H H O H cigarette smoke, H O·· ·· industrial chemicals H and air pollutants. Free radicals ·· ·· ·· CH O C can disrupt cell membranes, O·· 2 ·· increase the risk of many forms H of cancer, damage the interior lining of blood vessels and lead Type 2: A bonding pair to a lone pair to a high risk of heart disease and stroke. Body uses vitamins and ·· H O H H O ·· H minerals to counter the effects ·· ·· ·· of free radicals. Fruits contains H antioxidants which decrease the ··

·· ·· C O CH2 effects of free radicals. O·· ·· H 12.1.4 Electron movement in organic reactions Type 3: A bonding pair to an another bonding pair All organic reactions can be understood by following the electron H movements, i.e. the electron redistribution H BH BH3 HH during the reaction. The electron movement 3 depends on the nature of the substrate,

reagent and the prevailing conditions. The CH2 H2C flow of electrons is represented by curved CH2 arrows which show how electrons move as shown in the figure. These electron H H movements result in breaking or formation H2CCH2 of a bond (sigma or pi bond). The movement H2CCH2 of single electron is indicated by a half H -headed curved arrows. H There are three types of electron movement H2CCH H2CCH2 viz., H

• lone pair becomes a bonding pair. 12.1.5 Electron displacement effects in • bonding pair becomes a lone pair co-valent bonds

• a bond breaks and becomes another Some of the properties of organic bond. molecules such as stability, reactivity,

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Unit 12 Final - jagan.indd 165 4/14/2020 6:27:15 PM basicity etc., are affected by the displacement decreases rapidly, as we move away from

of electrons that takes place in its covalent C1 and is observed maximum for 2 carbons bonds. This movement can be influenced and almost insignificant after 4 bonds from by either the atoms/groups present in close the active group. proximity to the bond or when a reagent approaches a molecule. The displacement δ δ+ δ+ δ– effects can either be permanent or a CH > CH >> Cl 2 3 1 2 temporary. In certain cases, the electron displacement due to an atom or a substituent group present in the molecule cause a It is important to note that permanent polarisation of the bond and it the does not transfer leads to fission of the bond under suitable electrons from one atom to another but conditions. The electron displacements the displacement effect is permanent. are catagorised into inductive effect (I), The inductive effect represents the ability effect (R), electromeric effect (E) of a particular atom or a group to either and hyper conjugation. withdraw or donate electron density to the attached carbon. Based on this ability the Inductive effect (I) substituents are classified as +I groups and -I groups. Their ability to release or withdraw Inductive effect is defined as thethe electron through sigma covalent bond is change in the polarisation of a covalent called +I effect and -I effect respectively. bond due to the presence of adjacent bonds, atoms or groups in the molecule. This is a Highly electronegative atoms and permanent phenomenon. groups with an atom carrying a positive charge are electron withdrawing groups Let us explain the inductive effector –I groups. by considering ethane and ethylchloride as examples. The C-C bond in ethane is Example: -F, -Cl, -COOH, -NO2, -NH2 non polar while the C-C bond in ethyl chloride is polar. We know that chlorine Higher the electronegativity of the is more electronegative than carbon, and substitutent, greater is the -I effect. The hence it attracts the shared pair of electron order of the –I effect of some groups are between C-Cl in ethyl chloride towards given below. itself. This develops a slight negative charge

on chlorine and a slight positive charge on NH3> NO2> CN > SO3H > CHO >

carbon to which chlorine is attached. To CO > COOH > COCl > CONH2> F > Cl > Br > I > OH > OR > NH > C H > H compensate it, the C1 draws the shared 2 6 5

pair of electron between itself and C2. This polarisation effect is called inductive effect. Highly electropositive atoms and This effect is greatest for the adjacent bonds, atoms are groups which carry a negative but they also be felt farther away. However, charge are electron donating or +I groups. the magnitude of the charge separation Example. Alkali metals, alkyl groups

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Unit 12 Final - jagan.indd 166 4/14/2020 6:27:16 PM such as methyl, ethyl, negatively charged to the carboxyl group. – – – groups such as CH3O­, C2H5O , COO etc Trichloro acetic acid > Dichloro Lesser the electronegativity of the elements, acetic acid > Chloro acetic acid > acetic acid greater is the +I effect. The relative order of +I effect of some alkyl groups is given below O

Cl CH2 C O <<<< H –C(CH3)3> –CH(CH3)2>–CH2CH3>–CH3 Cl O < Let us understand the influence CH <

Cl< O Reactivity: Cl <

CH3 CH3 O CH3 O δ δ CH3 C C OH < CH C OH

H3C C Br H3C < CH3 O O CH3 HCOH CH3 C OH< If a -I group is attached nearer to a Electromeric effect (E) carbonyl carbon, it decreases the availability of electron density on the carbonyl Electromeric is a temporary effect carbon, and hence increases the rate of the which operates in unsaturated compounds nucelophilic addition reaction. (containing >C=C<, >C=O, etc...) in the presence of an attacking reagent. Acidity of carboxylic acids: Let us consider two different compounds When a halogen atom is attached(i) to compounds containing carbonyl group the carbon which is nearer to the carboxylic (>C=O) and (ii) unsaturated compounds acid group, its -I effect withdraws the such as alkenes (>C=C< ). bonded electrons towards itself and makes the ionisation of H+ easy. The acidity of When a nucleophile approaches various chloro acetic acid is in the following the carbonyl compound, the π electrons order. The strength of the acid increases with between C and O is instantaneously shifted increase in the -I effect of the group attached to the more electronegative oxygen. This

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Unit 12 Final - jagan.indd 167 4/14/2020 6:27:17 PM makes the carbon electron deficient and Whenπ electron the is transfered thus facilitating the formation of a new bond away from the attacking reagent, it is called, between the incoming nucleophile and the -E (negative electromeric) effect carbonyl carbon atom.

Nu XY XY CN + CONC CO·· Nu

On the other hand when an - + The attack of CN on a carbonyl carbon, as electrophile such as H approaches an shown above, is an example of -E effect. alkene molecule, the π electrons are instantaneously shifted to the electrophile Resonance or Mesomeric effect and a new bond is formed between carbon and hydrogen. This makes the other carbon The resonance is a chemical electron deficient and hence it acquires a phenomenon which is observed in certain positive charge. organic compounds possessing double bonds at a suitable position. Certain organic δ + δ - compounds can be represented by more

H2CCH2 + H H2CCH2 + H than one structure and they differ only in the position of bonding and lone pair of electrons. Such structures are called H CCH 2 3 resonance structures (canonical structures) and this phenomenon is called resonance. The electromeric effect, is denoted This phenomenon is also called mesomerism as E effect. Like the inductive effect, the or mesomeric effect. electromeric effect is also classified as +E and -E based on the direction in which the For example, the structure of aromatic pair of electron is transfered to form a new compounds such as benzene and conjugated bond with the attacking agent. systems like 1,3-butadiene cannot be represented by a single structure, and their Whenπ electron the is transferred observed properties can be explained on the towards the attacking reagent, it is called + basis of a resonance hybrid. E (positive electromeric) effect. In 1,3 buta diene, it is expected that the bond 1 2 3 4 X Y between C -C and C –C should be shorter than that of C2-C3, but the observed bond E lengths are of same. This property cannot be explained by a simple structure in which 1 2 3 The addition of H+ to alkene as shown two π bonds localised between C -C and C 4 above is an example of +E effect. –C . Actually the π electrons are delocalised as shown below.

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Unit 12 Final - jagan.indd 168 4/14/2020 6:27:18 PM substituents are attached to the conjugated 1 2 3 4 1 2 3 4 H2CCC CH2 H2CCC CH2 system. In such cases, the attached group has H H H H a tendency to withdraw electrons through resonance. These electron withdrawing 1 2 3 4 groups are usually denoted as -R or -M H2CCC CH2 H H groups. Examples : NO2, >C=O, -COOH,- C≡N etc These resonating structures are called Resonance is useful in explaining canonical forms and the actual structure lies certain properties such as acidity of phenol. between these three resonating structures, The phenoxide ion is more stabilised than and is called a resonance hybrid. The phenol by resonance effect(+M effect) resonance hybrid is represented as below. and hence resonance favours ionisation of phenol to form H+ and shows acidity. H2CCC CH2 H H O H O H Similar to the other electron displacement effect, mesomeric effect is also classified into positive mesomeric effect (+M or +R) and negative mesomeric effect (-M O H O H O H of -R) based on the nature of the present adjacent to the multiple bond.

Positive Mesomeric Effect:

O O Positive resonance effect occurs, when the electrons move away from substituent attached to the . It occurs, if the electron releasing substituents are attached to the conjugated O O O system. In such cases, the attached group has a tendency to release electrons through resonance. These electron releasing groups are usually denoted as +R or +M groups.

Examples : -OH, -SH, -OR,-SR, -NH2, -O- The above structures shows that etc. there is a charge separation in the resonance structure of phenol which needs energy, Negative Mesomeric Effect where as there is no such hybrid structures Negative resonance effect occurs,in the case of phenoxide ion. This increased when the electrons move towards the stability accounts for the acidic character of substituent attached to the conjugated phenol. system. It occurs if the electron withdrawing

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Unit 12 Final - jagan.indd 169 4/14/2020 6:27:18 PM Hyper conjugation order to supply electrons for delocalisation giving rise to 3 new canonical forms. In the The delocalisation of electronscontributing of canonical structures: (II), (III) σ bond is called as hyper conjugation. It is & (IV) of propene, there is no bond between a special stabilising effect that results due an α-carbon and one of the hydrogen atoms. to the interaction of electrons of a σ-bond Hence the hyperconjugation is also known (usually C-H or C-C) with the adjacent, as “no bond resonance” or “Baker-Nathan empty non-bonding p-orbital or an anti- effect”. The structures (II), (III) & (IV) are bonding σ* or π*-orbitals resulting in polar in nature. an extended molecular orbital. Unlike electromeric effect, hyper conjugation is a Example 2: permanent effect. Hyper conjugation effect is also It requires an α-CH group or observeda when atoms / groups having lone lone pair on atom like N, O adjacent to a pair of electrons are attached by a single π bond (sp2 hybrid carbon). It occurs by bond, and in conjugation with a π bond. The the overlapping of the σ-bonding orbital or lone pair of electrons enters into resonance the orbital containing a lone pair with the and displaces π electrons resulting in more adjacent π-orbital or p-orbital. than one structure Example 1: ·· H2CCCl ·· In propene,σ-electrons the of C-H H ·· bond of methyl group can be delocalised into the π-orbital of doubly bonded carbon

as represented below. ·· H2CCCl ·· H ·· H H H H Example 3: H CC C H CC C ·· H H H H H H When electronegative atoms or group of (I) (II) atoms are in conjugation with a π -bond,they pull π - electrons from the multiple bond.

H H H H H2CCC N ·· H

·· H CC C ·· H CC C

H H H H H H (IV) (III) H2CCC N ·· No bond resonance structures shown by H ·· propene are due to hyperconjugation

In case of carbocations, greater the In the above structure the sigma number of alkyl groups attached to the bond is involved in resonance and breaks in

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Unit 12 Final - jagan.indd 170 4/14/2020 6:27:19 PM carbon bearing positive charge, greater is Here Y– is the incoming nucleophile or – number of the hyper conjugate structure. and attacking species and X is the leaving thus the stability of various carbocations group. decreases in the order Example: Hydrolysis of alkyl halides

30 Carbocation > 20 Carbocation > 10 Carbocation aqueous OH CH3Br CH3OH + Br 12.2 Different types of organic reactions Aliphatic nucleophilic substitution reactions Organic compounds undergo many take places either by S 1 or S 2 mechanism. number of reactions, however in actual N N Detailed study of the mechanisms is given in sense we can fit all those reactions into the unit 14. below mentioned six categories. Electrophilic Substitution • Substitution reactions • Addition reactions AX+ Y AY+X • Elimination reactions + • Oxidation and reduction reactions Here Y is an electrophile • Rearrangement reactions Example: Nitration of Benzene

• Combination of the above NO2 + + 12.2.1 Substitution reaction + NO2 + H (Displacement reaction)

In this reaction an atom or a group of Mechanism of aromatic electrophilic atoms attached to a carbon atom is replaced substitution reactions (EAS) is discussed in by a new atom or a group of atoms. Based detail in unit 13. on the nature of the attacking reagent, this Free radical substitution reactions can be classified as AX+ Y AY+X i) Nucleophilic substitution CH4 + Cl CH3+HCl ii) Electophilic substitution Aliphatic electrophilic substitution iii) Free radical substitution A general aliphatic electrophilic substitution Nucelophilic substituion: is represented as This reaction can be represented as R- X + E R - E + X + + R2NH + NO → R2N–NO + H Y + AX AY+ X

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Unit 12 Final - jagan.indd 171 4/14/2020 6:27:20 PM 12.2.2 Addition reactions

Br It is a characteristic reaction of H CCH + an unsaturated compound (compounds 2 2 Br Br H2CCH2 + Br containing C-C localised double or triple bond). In this reaction two molecules H2CCH2 combine to give a single product. Like substitution this reaction also can be Br Br classified as nucleophilic, electrophilic and freeradical addition reactions depending Nucleophilic addition reaction the type of reagent which initiates the reaction. During the addition reaction the hydridisation of the substrate changes (from 2 3 sp → sp in the addition reaction of alkenes AB+ Y W AB+ W or sp → sp2 in the addition reaction of alkynes) as only one bond breaks and two Y new bonds are formed. AB I W Y H H - I H H

H H Example: addition of HCN to acetaldehyde OH H H H - OH CN H H OH H2SO4 H3CCH + H CN H CCH H 3 H H O OH Acetaldehyde Acetaldehyde cyanohydrin

Electrophilic Addition reaction Free radical addition Reaction:

A general electrophilic addition A General freeradical addition reaction can be represented as below. reaction can be represented as below.

AB+ Y W AB+ W AB+ Y W AB+ W

Y Y AB+ W AB

AB Y W Y

W Y

Brominatin of alkene to give bromo alkane is an example for this reaction. 172

Unit 12 Final - jagan.indd 172 4/14/2020 6:27:21 PM (o) CH CHO CH COOH 3 Acidic 3 Acetaldihyde dichromate Acetic acid Benzoyl Peroxide H2CCH2 + HBr CH3 CH2 Br

Pt /H2 In the above reaction, Benzoyl benzene cyclohexane peroxide acts as a radical initiator. The OH O mechanism involves free radicals. K2Cr2O7 H2SO4 Elimination reactions:

In this reaction two substituents OH O are eliminated from the molecule, and a 4-Hydroxy Phenol P-benzoquinone new C=C double bond is formed between the carbon atoms to which the eliminated atoms/groups are previously attached. Apples contain an enzyme Elimination reaction is always accompanied called polyphenol oxidase with change in hybridisation. (PPO), also known as tyrosinase. Cutting an Example: n-Propyl bromide on reaction apple exposes its cells to with alcoholic KOH gives propene. In this the atmospheric oxygen and oxidizes the reaction hydrogen and Br are eliminated. phenolic compounds present in apples. This is called the enzymatic browning αβ Alcoholic OH - CH3 CH CH2 Br that turns a cutapple brown. In addition to apples, enzymatic browning is also H evident in bananas, pears, avocados and - CH3 CH CH2 + H2O+ Br even potatoes

Oxidation and reduction reactions: 12.3 Functional Group inter conversion Many oxidation and reduction Organic synthesis involves functional reactions of organic compounds fall into one group inter conversions. A particular of the four types of reaction that we already functional group can be converted into other discussed but others do not. Most of the functional group by reacting it with suitable oxidation reaction of organic compounds reagents. For example: The carboxylic acid involves gain of oxygen or loss of hydrogen group (–COOH) presents in organic acids Reduction involves gain of hydrogen and can be transformed to a variety of other loss of oxygen. functional group such as – CH2–OH, –

Examples: CONH2, – COCl by treating the acid with

LiAlH4, NH3 and SOCl2 respectively. (o) CH CHO CH COOH 3 Acidic 3 Acetaldihyde dichromate Acetic acid

Pt /H2 benzene cyclohexane 173 OH O K2Cr2O7 H2SO4

Unit 12 Final - jagan.indd 173 4/14/2020 6:27:22 PM

OH O 4-Hydroxy Phenol P-benzoquinone Some of the important functional group interconversions of Organic compounds are summarised in the below mentioned Flow chart.

Oxidation

acid K2Cr2O7 Primary alcohol Aldehyde LiAlH reduction R–CH2OH 4 R–CHO Alcohol

Oxidation acid K Cr O Secondary alcohol 2 2 7 Ketone

R2–CHOH R–CO R LiAlH4 reduction

+ H2O/H

LiAlH4 LiAlH4 ≡ R–CH2–NH2 R–C N [H] ether R–C–NH2 10 amine O amide

+ H2O/H R–C– OH O Carboxylic acid aqeous OH– R–OH substitution Alkylhalide R-Cl H2SO4/H2O alcoholic OH– alkene elimination

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Unit 12 Final - jagan.indd 174 4/14/2020 6:27:23 PM 5. Homolytic fission of covalent bond Evaluation leads to the formation of

I. Choose the best answer. (a) electrophile (b) nucleophile 1. For the following reactions (c) Carbo cation (d) free radical

→ 6. Hyper Conjugation is also known as (A) CH3CH2CH2Br + KOH CH –CH= CH + KBr +H O 3 2 2 (a) no bond resonance → (b) Baker - nathan effect (B) (CH3)3CBr + KOH (CH3)3 COH + KBr (c) both (a)and (b) (d) none of these Br → (C) + Br2 7. Which of the group has highest +I Br effect? Which of the following statement is correct? (a) CH3- (b) CH3-CH2- (c) (CH ) -CH- (d) (CH ) -C- (a) (A) is elimination, (B) and (C) are 3 2 3 3 substitution 8. Which of the following species does not exert a resonance effect? (b) (A) is substitution, (B) and (C) are elimination (a) C6H5OH (b) C6H5Cl + (c) C6H5NH2 (d) (c) (A) and (B) are elimination and (C) C6H5NH3 is addition reaction 9. -I effect is shown by

(d) (A) is elimination, B is substitution (a) -Cl (b) -Br and (C) is addition reaction. (c) both (a) and (b) (d) -CH3

2. What is the hybridisation state of 10. Which of the following carbocation ­benzyl carbonium ion? will be most stable?

2 2 (a) sp (b) spd + + 3 2 (a) Ph3C- (b) CH -CH - (c) sp (d) spd + 3 2 + (c) (d) (CH3)2-CH CH2= CH - CH2 3. Decreasing order of nucleophilicity is 11. Assertion: Tertiary Carbocations are - - - generally formed more easily than (a) OH > NH2 > OCH3 > RNH2 - - - primary Carbocations ions. (b) NH2 > OH > OCH3 > RNH2 (c) NH - > CH O- > OH- > RNH 2 3 2 Reason: Hyper conjugation as well as (d) CH O- > NH - > OH- > RNH 3 2 2 inductive effect due to additional alkyl group stabilize tertiary carbonium 4. Which of the following species is not ions. electrophilic in nature?

+ + + (a) Cl (b) BH3 (c) H3O (d) NO2 175

Unit 12 Final - jagan.indd 175 4/14/2020 6:27:23 PM (a) both assertion and reason are true15. The geometrical shape of carbocation and reason is the correct explanation of is assertion. (a) Linear (b) tetrahedral (b) both assertion and reason are (c) Planar (d) Pyramidal true but reason is not the correct explanation of assertion. II. Write brief answer to the following questions. (c) Assertion is true but reason is false 16. Write short notes on (d) Both assertion and reason are false (a) Resonance 12. Heterolytic fission of C–C bond results (b) Hyperconjucation in the formation of 17. What are electrophiles and (a) free radical(b) Carbanion nucleophiles? Give suitable examples for each. (c) Carbocation (d) Carbanion and Carbocation 18. Show the heterolysis of covalent bond by using curved arrow notation and 13. Which of the following represent a set complete the following equations. of nuclephiles? Identify the nucleophile is each case.

2- (i) CH - Br + KOH → (a) BF3, H2O, NH 3 (ii) CH - OCH + HI → (b) AlCl3, BF3, NH3 3 3

- - 19. Explain inductive effect with suitable (c) CN , RCH2 , ROH example. + + (d) H , RNH3 , :CCl2 20. Explain electromeric effect. 14. Which of the following species does not acts as a nucleophile? 21. Give examples for the following types of organic reactions (a) ROH (b) ROR (i) β - elimination

(c) PCl3 (d) BF3 (ii) electrophilic substitution.

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Unit 12 Final - jagan.indd 176 4/14/2020 6:27:23 PM CONCEPT MAP

Organic Compounds

Organic reaction

Reaction mechanism Types of Reaction Electron movements in Organic Compounds

1. Substitution Inductive Effect Fission of covalent bond 2. Addition 3. Elimination Resonance Effect 4. Oxidation and homolytic fission heterolytic fission Electromeric effect Reduction Carbocation Hyper conjugation freeradical

Carbanion

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Unit 12 Final - jagan.indd 177 4/14/2020 6:27:23 PM