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10. HALOGEN DERIVATIVES Can you recall ? CH, CH, - X CH, CH - X...

10. HALOGEN DERIVATIVES Can you recall ? CH, CH, - X CH, CH - X - ldentify the functional group (Haloalkane) (Haloalkene) in the following compounds. HC= C-X ii.CCI,E, i B (Haloalkyne) (Haloarene) (Benzylbromide) (Freon- 12) b. On the basis of number halogen atoms, of halogen derivatives are classified as mono, CI iv. CI-CH= CCI, di, tri or poly halogen compounds. (Westrosol) X CH,- CH, -X CI (Hexochlorobenzenc) Monohalogen compounds C X Dihalogen compounds CI (Hexachlorocyclohexane) The parent family of organic compounds is hydrocarbon. Replacement of aliphatic or aromatie hvdreg.ho tom's in Trihalogen compounds by halogen atom/s results in the formation of We will consider classification of mono halogen derivatives of hydrocartbons. halogen derivatives in more detail. In this chapter we will study halogen derivatives in a systematic way. 10.1.1 Classification of monohalogen compouds : Monohalogen compounds are Internetmy friend further classified on the basis of position of Find out the structures www halogen atom and the type of hybridization of of two thyroid hormones T3 carbon to which halogen is attached. (triiodothyronine) and T4 (thyroxine). a. Alkyl halides or haloalkanes : In alkyl How do these help our body ? halides or haloalkanes the halogen atom is 10.1 Classification of halogen derivatives : bonded to sp' hybridized carbon which is a part of saturated carbon skeleton. Alkyl Halogen derivatives of hydrocarbons are halides may be primary, secondaryor tertiary classified mainly in two ways. depending on the substitution state of the a. On hydrocarbon skeleton to the basis of carbon to which halogen is attached : (Refer which halogen atom is bonded, the halogen to Std. XI Chemistry Textbook, section 14.3). derivatives are classified as haloalkanes, haloalkenes, haloalkynes and haloarenes. 210 >oO  d. Vinylic halides :In vinylic halides halogen sp R - CH,- X R -ÇH- R R--X atom atom is bonded to a of aliphatic chain. Vinylic halide hybridized carbon is a haloalkene. Primary Secondary Tertiary halide halide halide (1° halide) (2° halide) (3° halide) CH,= CH - X b. Allylic halides : In allylic halides, halogen e. Haloalkyne : When a halogen atom is atom is bonded to a sp' hybridized carbon bonded to a sp hybridized carbon atom it is atom next to a carbon-carbon double bond. a haloalkyne. CH =C - X CH,- CH CH, - X - f. Aryl halides or haloarenes : In aryl halides, halogen atom is directly bonded to c. halide: In benzylic halides Benzylic the sp² hybridized carbon atom of an aromatic halogen atom is bonded to a sp' hybridized ring. carbon atom which is further bonded to an CH, aromatic ring. ÇH, Table 10.1 Names of some halogenderivatives Formula Common name IUPAC name |CH,CI, Methylenechloride Dichloromethane |CH,CH,Br Ethyl bromide Bromoethane CH,CH(CI)CH, Isopropyl chloride 2-Chloropropane |(CH),CH - CH,Br Isobutyl bromide 1-Bromo-2-methylpropane (CH),C Br Tert-butyl bromide |2-Bromo-2-methylpropane (CH), C CH,CI Neopentyl chloride 1-Chloro-2, 2-dimethyl pro pane CH, =CH -C1 Vinyl chloride |-Chloroethene | CH, =CH - CH,Br |Allyl bromide -Bromopropene CH= C-CI Chloro acetylene Chloroethyne CH,I Benzyl iodide IodophenyImethane p-lodotoluene |1-lodo-4-methyl benzeneor 4-lodotoluene |H,C m-dichlorobenzene 1, 3-dichlorobenzene 211 >>o 10.2 Nomenclature of halogen derivatives a wide variety. The hydroxyl group may be replaced by halogen atom using (a)halogen |Canyou recall acid, (b) phosphorous halide or (c) thionyl In IUPAC chloride. system of nomenclature does the a. By using halogen acid or hydrogen functional group "halogen' appear halide (HX) : The conditions for reaction asa suffix or prefix? of alcohol with halogen acid (HX) depend What arethe trivial names of laboratory on the structure of the alcohol and particular solvents CHCI, and CCI,? halogen acid used. The order of reactivity of alcohols with a given haloacid is 30>2>10. The common names of alkyl halides are (Refer to section 11.2.1 a) + HXonR suitable derived by naming the alkyl group followed R - OH - X +H,0 by the name of halogen as halide. For (Alcohol) (Alkyl halide) example, methyl iodide, tert-butyl chloride. According to IUPAC system of nomenclature Hydrogen chloride is used with zinc (Std. XI Chemistry Textbook Chapter 14, chloride (Grooves'process) for primary and section 144.7) alkyl halides are named as secondary alcohols, but tertiary alcohols haloalkanes. Aryl halides are named as readily react with concentrated hydrochloric haloarenes in common as well as IUPAC acid in absence of zinc chloride. For dihalogen of an arene, R- Cl +H,0 system. derivative prefix o-, m-, p- are used in common name R - OH + HCI system but inIUPAC system the numerals 1,2 ; 1,3 and 1,4 respectively are used. Do you know ? Common and IUPAC names of some halogen Zinc chloride is a Lewis acid derivatives are given in Table 10.1. and consequently can coordinate with the alcohol, weakening R -Obond. Use your brain power Mixtureof concentrated HCl and anhydrous Write IUPAC names of the ZnC1, is called Lucas reagent. following Constant boiling hydrobromic acid i. CH, - ÇH-CH, ii. CH, - CH - CH,I (48%) is used for preparing alkyl bromides. Br CH, Primary alkyl bromidescan also be prepared by reaction with NaBr and H,So,. Here HBr ii. CH, - CH=CH -CH,CI is generated in situ. NaBr, H,Sso iv. CH, -C=C- CH, -Br R-CH,-0H+HBr heot R-CH,-Br+H,0 Good yield of alkyl iodides may be Br Vi obtained by heating alcohols with sodium or potassium iodide in 95 % phosphoric acid. Here HI is generated in situ. R - OH+ HI Nal/H, PO, R-|+ H,0 10.3 Methods of preparationof alkyl halides 10.3.1 From alcohol : The most widely used Can you tell ? method of preparation of alkyl halide is Why phosphoric acid is preferred to replacement of hydroxylgroup of an alcohol H,SO, to prepare HI in situ? by halogen atom. Alcohols are available S in 212 >o b. By using phosphorous halide : An Addition of hydrogen halide to alkene alkyl halide may be prepared by action of Alkyl halides are formed on addition phosphorous halide on alcohol. Phosphorous of hydrogen halide to alkenes. Refer to Std tribromide and triiodide are usually generated XI ChemistryTextbook Chapter 15, section in situ (produced in the reaction mixture) by 15.2.4 for all the details including order of the action of red phosphorouson bromine and reactivity of HX, Markownikov rule and iodine respectively. Phosphorous pentachloride peroxide effect. reacts with alcohol to give alkyl chloride. 3R OH + PX, + 3R - X + H,P0, Problem 10.1 : How 1-bromo-1l-methylcyclohexane will you from alkene? obtain R - OH + PCI, R- CI+ HCI +POCI, Writepossible structures of alkene and the Do you know ? reaction involved. Some times during replacement Solution: of -OH by -X, alcohols tend to undergo rearrangement. This tendency CH CH, Br can be minimized by use of phosphorous + HBr halides. Straight chain primary alcohols react with phosphorous trihalide to give unrearranged alkyl halides. CH. CH, Br C. By using thionyl chloride Thionyl + HBr chloride reacts with straight chain primary alcohols to give unrearranged alkyl chloride. The byproducts obtained are gases. There Use your brain power is no need to put extra efforts for its separation. Therefore this method is preferred Rewrite the following reaction\ for preparation of alkyl chloride. by filling the blanks R - OH +SOCI, R - Cl+ So,f+ HCIt CH, - CH = CH, + HBr...... (major)(minor) Can you recall ? peroxide (CH),C-CHCH, +HBr t...... ldentify the products the (majorXminor) following reactions. CH, - CH =CH+HBr Proxide.t.... CH, + C1, Y? (majorXminor) i. ii. CH, - CH= CH ? Do vou know ? iii. CH, - CH= CH, + HBrPeroxide iv. CH, =CH -CH, + Br, C? Alkenes form additon vicinal dihalide, with product, chlorine or 10.3.2 From hydrocarbon bromine usually in inert solvent like CCI, at room temperature. Alkyl halides are formed from saturated as well as unsaturated hydrocarbons by various reactions. Halogenation of alkanes is not suitable for preparation of alkyl halides as a mixture of mono and poly halogen compounds is formed. (X= Cl, Br)  When toluene is brominated in presence Do you know ? of a mixture of iron, ortho and para bromo When alkenes are heated toluene is obtained. with Br,or Cl, at high temperature, hydrogen atom of allylic carbon is CH, CH, CH, substittued with halogen atom giving + Br, allyl halide. CH, =CH- CH, + CI, (0- Bromotoluene) Bi (p-Bromotoluene) CH,=CH -CH,CI+ HCI + HBr 10.3.3 Halogen exchange : Alkyl iodides Aromatic electrophilic substitution are prepared conveniently by treating alkyl with iodine is reversible. In this case use chlorides or bromides with sodium iodide in of HNO,/ 0HIO, removes HI by oxidation to methanol or acetone solution. The sodium L,, equilibrium is shifted to right and iodo bromide or sodium chloride precipitates from product is formed. E, being highly reactive, the solution and can be separated by filtration. fluoro compounds are not prepared by this method. R -Cl + Nal ctoneR - + NaCI 1 The reaction is known as Finkelstein reaction. 10.3.5 Sandmeyer's reaction : Aryl halides are most commonly prepared by replacement Alkyl fluorides are prepared by heating of nitrogen of diazonium salt. (For details alkyl chlorides or bromides with metal refer to Chapter 13 section 13.6). fluorides such as AgF, Hg,F,, AsF,, SbF, etc. R- Cl + Agf +R - F+ AgCI 10.4 of Physical alkyl halides properties: Physical properties are considerably different The reaction is known as Swartz reaction. from those of corresponding alkanes. The boiling points of alkyl halides are determined 10.3.4 Electrophilic substitution : by polarity of the C-X bond as well as the size of halogen atoms. Can you recall 10.4.1 Nature of intermnolecular forces:. Identify the product of the Halogens (X = F, CI, Br and I) are more following reaction. electronegative than carbon. CI, Carbon atom that carries halogen develops a partial positive charge while the Name the type of halide produced in halogen carries a partial negative charge. the above reaction. Thus carbon-halogen bond in alkylhalide is a What type of reactions are shown by polar covalent bond. Therefore alkyl halides benzene ? are moderately polar compounds. Aryl chlorides and bromides can be prepared by direct halogenation of benzene Size of the halogen at 224/ 364 m and its derivatives through electrophilic fluorine to iodine. Hence the gth substitution. It may be conveniently carried increases. The C-X bond strength decreases out in dark at ordinarytemperature in presence with an increase in size of halogen. This is of suitable Lewis acid catalyst like Fe, FeC1, because as the size of p-orbital of halogen or anhydrous AlCI, increases the p-orbital becomes more diffused GQ 214 >oo and the extent of overlap with orbital of Haloalkane Boiling point (K) carbon decreases. Some typical bond lengths, CH,CH,CHCH,Br 375 bond enthalpies and dipole moments of C-X bond are given in Table 10.2. CH, - CH -CH,- CH, 364 B Table 10.2 :Bond parameters of C-X bond CH, Bond Bond Bond Dipole 346 CH,-C-CH, length enthalpy moment Rr (pm) (kJ mol-)(debye) 139 452 1.847 10.4.3 Solubility : Though alkyl halides CH,- F are moderately polar, they are insoluble in CH, - CI 178 351 1.860 water. It is due to inability of alkyl halides to |CH, - Br 193 293 1.830 form hydrogen bonds with water. Attraction CH, - I 214 234 1.636 between alkyl halide molecules is stronger than attraction between alkyl halide and 10.4.2 Boiling point : Boiling points of alkyl water. Alkyl halides are soluble in non-polar halides are considerably higher than those of organic solvents. corresponding alkanesdue to higher polarity and higher molecular mass. Within alkyl Aryl halides are also insoluble in water halides, for a given alkyl group, the boiling but soluble in organic solvents. If aryl halides point increases with increasing atomic mass are not modified by presence of any other of halogen, because magnitude of van der functional group, they show properties similar Waals force increases with increase in size to corresponding alkyl halides. The isomeric and mass of halogen. dihalobenzenes have nearly the same boiling points, but melting points of these isomers Thus boiling point of alkyl halide show variation. Melting point of para isomer decreases in the order RI > RBr > RCI > RF is quite high compared to that ofortho or meta For example, : isomer. This is because of its symmetrical structure which can easily pack closely in Haloal CH,F CH,CICH,Br CHI the crystal lattice. As a result intermolecular kane forces of attraction are stronger and therefore Boiling 194.6 248.8 276.6315.4 greater energy is required to overcome point (K) lattice energy. For the given halogen, boiling point rises with increasing carbon number. For example, Haloalkane Boiling point (K) C CH,CI 248.8 CI CH,CH,CI 285.5 b.p./K 453 446 448 CH,CH,CH,CI 320.0 256 249 323 m.p./K CH,CH,CH,CH,CI 351.5 For isomeric alkyl halides, boiling point decrease with increased branching as surface area decreases on branching and van der Waals forces decrease. For example : GO 215 >  Let us, now, jot down the atoms/groups Problem 10.2 Arrange the following attached to each carbon in 2 - chlorobutane. compounds in order of increasing boiling points : bromoform, chloromethane, CH, - HCI- H,- H, dibromomethane,bromomethane. Solution : The comparative boiling points C-1 : - H, -H, -H, NCHN, H, of halogen derivatives are mainly related with van der Waals forcesof attraction c2: H, CI, CH, H,H, which depend upon the molecular size. In C-3 : -H, -H, H, HCHH, the present case all the compounds contain only one carbon. Thus the molecular size C-4 : -H, H. -H, H,HCIH, depends upon the size of halogen and It can be seen that the four groups bonded number of halogen atoms present. to C-2 are all different from each other. Thus increasing order ofboiling point is, Carbon atom in a molecule which carries CH,CI< CH,Br< CH,Br, < CHBr, four different groups/atoms is called chiral carbon atom. Thus,the C-2 in 2-chlorobutane is a chiral carbon. Chiral atom in a molecule 10.5 Optical somerism in halogen derivatives: is marked with asterisk (). For example, CH,-*CHCI-CH,-CH,. Can you recall ? When a molecule contains one chiral What is the relationship between atom, it acquires a unique property. Such a two compounds having the same molecule can not superimposeperfectly on its molecular formula? mirror image. It is called chiral molecule. A What is meant by stereoisomerism? chiral molecule and its mirror image are not identical (see Fig. 10.1). Isomers having the same bond connectivities, that is, structural formula are called stereoisomers. Knowledge of optical isomerism,which is a kind of stereoisomerism will be useful to understand nucleophilic substitution reactions of alkyl halides (see H mirror plane 10.6.3). C,H, 10.5.1 Chiral atom and molecular chirality CH, CH, Try this... Make a three dimensional H ICC C,H, C,H, model of 2 - chlorobutane. Make another model which is a mirror Fig. 10.1 :Nonsuperimposable mirror images image of the first model. A chiral molecule and its mirrorimage Try to superimposethe two models on both have the same structural formula and, each other. of course, the same molecular formula. The Do they superimpose on each other spatial arangement of the four different exactly ? groups around the chiral atom, however, is Comment on whether the two models different. In other words, a chiral molecule are identical or not. and its mirror image are stereoisomers of each SO 216}oo other. (Refer to Std. XI Chemistry Textbook, Do you know ? Chapter 14). Nicol prism is a specialtype of The relationship between a chiral prism made from pieces of calcite, molecule and its mirror image is similar to a crystalline form of CaCO,, arranged the relationship between and right hands. left in a specific manner. Nicol prism is also Therefore it is called handedness or chirality. called polarizer. (Origin : Greek word: Cheir means hand) The stereoisomerism in which the isomers 10.5.3 Optical activity : When an aqueous solution of certain organic compounds like have diferent spatial arrangements of groups/ sugar, lactic acid is placed in the path of atoms around a chiral atom is called optical plane polarized light, the transmitted light isomerism. The optical isomers differ from has oscillations in a different plane than each other in terms of a measurable property the original. In other words, the incident called optical activity. light undergoes rotation of its plane of To understand optical activity, we must polarization. The plane of polarization rotates know what is plane polarized light. either to the right (clockwise) or to the left (anticlockwise). This property of a substance Remember... by which it rotates plane of polarization of incident plane polarized light is known The phenomenon of optical\ as optical activity. The compoundsWnien isomerism in organic compounds rotate the plane of plane light are was observed first and its origin in called optically active compounds and those molecular chirality was recognized later. whichdo not rotate it are optically inactive compounds.Optical activity ofa substance 10.5.2 Plane polarized light :An ordinary is expressed numerically in terms of optical light consists of electromagnetic waves having rotation.The angle through which a substance oscillations of electric and magnetic field in rotates the plane of plane polarized light on all possible planes perpendicular to direction passing through it is called opticl rotation. of propagation of light. In accodance with the direction of optical of rotation an optically active substance is either When ordinary light is passed through dextrorotatory or laevorotatory.A compound Nicol's prism, oscillations only in one plane which rotates the plane of plane polarized emerge out. Such a light having oscillations light towards rightis called dextrorotatory only in one plane perpendicular to direction and designated by symbol d- or by (+) of propagation of light is known as plane polarized Jlight. sign. A compound which rotates plane of plane polarized light towards left is called laevorotatory and designated by symbol - or by (-) sign. Nicol Isomerism in which isomeric compounds Prism have different optical activity is known as optical isomerism. French scientist Louis Pasteur first recognized that optical activity is cross section of cross section of associated with certain type of 3-dimensional |ordinarylight plane polarized light structure of molecules. Pasteur introduced the term enantiomers for the optical isomers having equal and opposite optical rotation. G 217 >>o  Figure 10.2 indicates a few objects in our Enantiomers have identical physical day to day life which exhibit superimposable properties (Such as melting point, boiling and non-superimposable miror image points, densities, refractive index) except the relationship. sign of optical rotation. The magnitude of their optical rotation is equal but the sign of optical rotation is opposite. They have identical chemical properties except towards optically active reagent. An equimolar mixture of enantiomers (dextrorotatory and laevorotatory) is called racemic modification or racemic mixture. A racemic modification is optically inactive because optical rotation due to molecules of one enatiomer is cancelled by equal and Non superimposable Superimposable opposite optical rotation due to molecules of the other enantiomer. A racemic modification Fig. 10.2 :Superimposable and is designated as (dl) or by (t) sign. nonsuperimposable mirror image 10.5.5 Representation of configuration of molecules : Remember... Can you recall ? Optical activity is an Identify the type of following experimentally observable 3-D representation () and property compounds. Chirality is of (I) of a molecule and state a description of molecular structure. significance of the lines drawn. Optical activity is the consequence of W W chirality. X Molecules which contain one chiral atom are chiral, that is, they are nonsuperimposable on their mirror () (II) image. a. Fischer projection formula (cross The two image non-superimposable structures are called mirror pair of formula) :Two representations are used and to represent configuration of chiral carbon enantiomers. the 3-dimensional structure of optical isomers Enantiomers have equal and opposite on plane paper. These are (a) wedge formula optical rotation. Thus, enantiomers are and (b) Fischer projection formula (also called a kind of optical isomers. cross formula) (Std. XI Chemistry Textbook Chapter 14 section 14.2.3). 10.5.4 Enantiomers : The optical isomers C CI Bonds below Chiral carbon wthe plane which are non-superimposable mirror image Br of each other are called enantiomers or C-Br Bonds above enantiomorphs or optical antipodes. For H the plane H example, 2 - chlorobutane exists as a pair of Fischer projection Convention of vertical and horizontal lines enantiomers (Fig. 10.1). Fig. 10.3 Fischer projection formula G218 o b. Wedge formula : When a tetrahedral Can vou recall? carbon is imagined be present in the plane to What is meant by substitution of paper all the four bonds at this carbon reaction ? cannot lie in the same plane. The bonds in the plane of paper are represented by normal Can you identify substitution reaction lines, the bonds projecting above the plane from the following ? of paper are represented by solid wedges ()CH, CH, OH - - + HCI Znc (or simply by bold lines) while bonds going CH, - CH, - Cl + H,0 below the plane of paper are represented by broken wedges (or simply by broken lines). (i)CH, =CH, + HI+ CH, - CH, - I Br below the plane |In the plane Is the carbon carrying halogen in alkyl halide, an electrophilic or a nucleophilic centre? H Above the plane hybridization of that carbon the reaction is Try this... called substitution reaction. The C-X bond in alkyl halides is a polar covalent bond Draw structures ofenantiomers of lactic acid (CH,-ÇH-COOH) and the carbon in C-X bond is positively polarized. In other words, the C-X carbon is ÖH an electrophilic centre. It has, therefore, a using Fischer projection formulae. tendency to react with a nucleophile. (Refer to Draw structures of enantiomers of Std. XI Chemistry Textbook Chapter 14.) Alkyl 2-bromobutane using wedge formula. halides react with a variety of nucleophiles 10.6 to give nucleophilic substitution reactions Chemical properties : (S). The reaction is represented in general 10.6.1 Laboratory test of haloalkanes : form as shown below. Haloalkanes are of neutral type in aqueous medium. On warming with aqueous sodium -- Nu + x° or potassium hydroxide the covalently bonded halogen in haloalkane is converted to halide When a substrate reacts fast it is said to ion. be reactive. The reactivity of alkyl halides S, reaction depends upon two factors, + OH®AR- OH + X° in R -X namely, y, the substitution state (1°, 20 or 3) When this reaction mixture is acidified of the carbon and the nature of the halogen. by adding dilute nitric acid and silver nitrate The order of reactivity influenced by these solution is added a precipitate of silver halide two factors is as shown below. is formed which confims presence of halogen tertiary alkyl halide (30) > secondary alkyl in the original organic compound. halide (29) >primary alkyl halide (19) and R -I> R - Br > R -CI Ag (aq) + X° (aq) -AgX (s) Examples of some important nucleophilic 10.6.2 Nucleophilic substitution reactions of substitution reactions of alkyl halides are haloalkanes : shown in Table 10.3. When group bonded to a carbon in a a substrate replaced by another group to is get a product with no change in state of S219)oo  10.3 Nucleophilie substitution reactions of alkyl halides Sr. No. Alkyl halide Reagent Substitution product 1. R -X + NaOH(ag) (or KOH) A R -OH (alcohol) + (or NaX KX) 2. R-X NaOR'A R-0-R' + NaX (ether) (sodium alkoxide) 3. R-X R'--ÖÅg (silver carboxylate A R.c-OR + (ester) AgX NH,(ale.) R - NH, + HX 4. R-X + (excess) (primary amine) 5. R- X + KCN (alc.) A R -CN (nitrile)(alkyl cyanide) +KX 6. R-X AgCN (alc.)A+ R-NC AgX (isocyanide) 7. R-X K -N=0+ R-O-N=0 + KX (alkyl nitrite) (potassium nitrite) -0-N=0 8 R-X + Ag (silver nitrite) R- + AgX (nitroalkane) Can you tell ? Do you know ? Alkyl halides when treated with Cyanide ion is capable of alcoholic solution of silver nitrite give attacking through more than one nitroalkanes whereas with sodium nitrite site (atom). they give alkyl nitrites Explain. &NeC = N? 10.6.3 Mechanism of S, reaction : Such nucleophiles are called ambident Can you recall ? nucleophiles. KCN is predominantly ionic What is meant by order and (K°C®= N) and provides cyanide ions. Both molecularity of a reaction ? carbon and nitrogen are capable of donating electron pair. C-C Bond being stronger than What is meant by mechanism of chemical reaction ? C-N bond, attack occurs through carbonatom of cyanide group forming alkyl cyanides as It can be seen from the Table 10.3 that in major product. However AgCN (Ag-C N) = nucleophilic substitution reactions of alkyl is mainly covalent compound and nitrogen halides the halogen atom gets detached from is free to donate pair of electron. Hence the carbon and a new bond is formed between attack occurs through nitrogen resulting in that electrophilic carbon and nucleophile. formation of isocyanide. The covalently bonded halogen is converted into halide ion (X°). It means that the two Another ambident nucleophile is nitrite ion, which can attack through 0'or N electrons constituting the original covalent bond are carried away by the halogen along -N-0 with it. The halogen atom of alkyl halide is, therefore, called leaving group' in the O 220 >oo context of this reaction. Leaving group is the group which leaves the carbon by taking H, away the bond pair of electrons. The substrate H undergoes two changes during a S, reaction. H The originalC-X bond undergoes heterolysis Transition state and a new bond is formed between the carbon (T.S.) and the nucleophile using two electrons of the H nucleophile.These changes may occur in one or more steps. The description regarding the HO–C Br H sequence and the way in which these two changes take place in S, reaction is called mechanism of S,, reaction. The mechanism is Fig. 10.4 :S,2 mechanism deduced from the results of study of kinetics Salient features of S,2 mechanism of S,reactions. Two mechanisms are observed Single step mechanism with simultaneous i in various S, reactions. These are denoted as bond breaking and bond forming. s,l and s,2 mechanisms. i. Backside attack of nucleophile The a. S,2 Mechanism: The reaction between nucleophile attacks the carbon undergoing methyl bromide and hydroxide ion to give substitution from the side opposite to that methanol follows a second order kinetics, of the leaving group. This is to avoid steric that is, the rate of this reaction depends on repulsion (repulsion due to bulkyness of concentration of two reacting species, namely, the groups) and electrostatic repulsion methyl bromide and hydroxide. Hence it is between the incoming nucleophile and called subtitution nucleophilic bimolecular, the leaving group. s,2. iii. In the transition state (T.S.)the nucleophile CH,Br + OHe CH,OH + B and leaving groups are bonded to the carbon with bonds and carry rate = k [CH,Br][OH] partial partial negative charge. (Thus, the total Rate of a chemical reaction is influenced by negative charge is diffused.) the chemical species taking part in the slowest iv. The T.S. contains pentacoordinate step of its mechanism. In the above reaction only two reactants are present and both are carbon having three o (sigma) bonds in found to influence the rate of the reaction. one plane making bond angles of 120 with each other and two partial covalent This means that the reaction is a single step reaction which can also be called the slow bonds along a line perpendicular to this plane. step. This further implies that the two changes, namely, bond breaking and bond forming at V When S,2 reaction is brought about at the carbon take place simultaneously. This chiral carbon (in an optically active S,2 mechanism is represented as shown in substrate), the product is found to have Fig. 10.4. opposite configuration compared to that of the substrate. In other words, S2 reaction is found to proceed with inversion of configuration. This is like flipping of an umbrella (See Fig. 10.4). It is known as Walden inversion. The inversion in configuration is the result of backside attack of the nucleophile. O 221 >oo b. S,1 Mechanism : The reaction between iv. When S,1 reaction is carried out at chiral tert-butyl bromide and hydroxide ion to give carbon in an optically active substrate, tert-butyl alcohol follows a first-order kinetics, the product formed is nearly racemic. that is the rate of this reaction depends on This indicates that S I reaction proceeds concentration of only one species, which is mainly with racemization. This means the substrate molecule, tert-butyl bromide. both the enantiomersof product are formed Hence it is called substitution nucelophilic in almost equal amount. Racemization in unimolecular, S,1. s,1 reaction is the result of formation of planar carbocation intermediate (Fig. ÇH, CH, 10.5). Nucleophile can attack planar CH,- -Br + °OH CH,- -OH+ Be carbocationfrom either side which results CH, H, in formation of both the enantiomers of rate =k [(CH),CBr] the product. It can be seen in this reaction that Use vour brain power concentration of only substrate appears in the rate equation; concentration of the Draw the Fischer projection nucleophile does not influence the reaction formulae of two products rate. In other words, tert-butyl bromide reacts obtained when compound (A) reacts with hydroxide by a two step mechanism. with OH® by S,1 mechanism. In the slow step C-X bond in the substrate undergoes heterolysis and in the subsequent fast step the nucleophile uses its electron H,C-– Br (A) pair to form a new bond with the carbon n-C,H, undergoing change. This S,1 mechanism is Draw the Fischer projection formula of represented as shown in Fig. l0.5. the productformed when compound (B) Step reacts with OH° by S2 mechanism. I Slow Bf CH, H-¢-CI (B) H,C CH, C,H, (carbocation intermediate) 10.6.4 Factors influencing S,i and s,2 Step II mechanism : a. Nature of substrate : S2 : The T.S. of + °OH > (CH),C-OH s,2 mechanism is pentacoordinate and thus H,C CH, crowded (See Fig. 10.4). As a result S,2 mechanism is favoured in primary halides Fig. 10.5: S,I mechanism and least favoured in tertiary halides. Salient features of S,1mechanism: Two step mechanism. SI:A planar carbocation intermediate is formed in Sl reaction. It has no steric ii. Heterolyis of C-X bond in the slow crowding. Bulky alkyl groups can be easily and reversible first step to fom planar accommodated in planar carbocation See carbocation intermediate. (Fig. 10.5). As a esult S,1 mechanism is i. Attack of the nucleophile on the most favoured in tertiary halides and least carbocation intermediate in the fast favoured in primary halides. (Formation of second step to form the product. planar carbocation intermediate results in a O 222 o0  H H HcH HcH HcH (a) X Nu--..x Nuo --C--X Nu-C.X H H H H H H-S C-H H H H HHH crowding and destabilization increases (b) H H HH H H H H H-Ça H H-ça H HH -H H steric relief, stabilization by +I and hyperconjugationof a - hydrogens increases Fig. 10.6 :Influence of substrate in S,1 and S,2 (a)Transition states (T.S.) in S,2 (b)Carbocation intermediates in S,1 relief from steric crowding present in the Problem 10.4 : Primary allylic and tertiary halide substrate). primary benzylic halides show higher Secondly the carbocation intermediate is reactivity by Sl mechanism than stabilized by +I effect of alkyl substituents other primary alkyl halides. Explain. and also by hyperconjugation effect of alkyl Solution: S,i reaction involves substituents containing a-hydrogens. As a formation of carbocation result, S,1 mechanism is most favoured intermediate. The allylic and benzylic in tertiary halides and least favoured in carbocation intermediate formed are primary halides. This can be represented resonance stabilized, and hence Si diagramatically as shown below. mechanism is favoured. s,i rateincreases CH, - X 10 20 CH, CH;H, € H, -CH - CH, Resonance stabilization of allylic carbocation S.2 rate increases CH CH, Tertiary halides undergo nucleophilic substitution by SI mechanism while primary halides follow S,2 mechanism. Secondary halides react by either of the mechanism or CH, by mixed mechanism depending upon the exact conditions. Resonance stabilization of benzylic carbocation 223 >o b. Nucleophilicity of the reagent : carbocation is relatively poor and solvation of anion is particularly important. Anions are |Can you recall solvated by hydrogen bonding solvents, that Give Some examples of is, protic solvents. Thus S,I reaction proceeds more rapidly in polar protic solvents than in nucleophiles that are electrically aprotic solvents. neutral. Polar protic solvents usuallydecrease the Give some examples of anionic rate of S,.2 reaction. In the rate determining nucleophiles. step of S,2 mechanism substrate as well What is the difference between a base as nucleophile is involved. A polar solvent and a nucleophile ? stabilizes nucleophile (one of the reactant) by solvation. Thus solvent deactivates the A nucleophile is a species that uses its nucleophile by stabilizing it. Hence aprotic electron pair to form a bond with carbon. solvents or solvents of low polarity will Nucleophilic character of any species is favour S,2mechanism. expressed in its electron releasing tendency, which can be corelated to its strength as Lewis base. Problem 10.5 : Which of the following two compounds would react faster by S,2 A more powerfu