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10. Halogen Derivatives 12th Science 12th Chemistry Syllabus ~ 1. Classification of halogen derivatives 2. Nomenclature of halogen derivatives 3. Methods of preparation of alkyl halides 4. Physical properties 5. Optical isomerism in halogen derivatives 6. Chemical properties 6.1 Reaction with active metals 7. Uses and environmental effects of some polyhalogen compounds ] | Theory Notes Q. What are halogen deriva- tives of alkanes? Q. What are halogen deriva- tives of arenes? Q. Give the classification of halogen derivatives. iv) Polyhalogen derivatives Halogen Derivatives of Alkanes (Haloalkanes) The eempounds obtained by replacing one or more hydrogen atoms from alkane (saturated hydrocarbon) with equal number of halogen atoms (Cl, Br, I) are called as halogen derivatives of alkanes or haloalkanes. Ex. i) C.HsBr (ethy! bromide) ii) CHs— Cl(methy! cloride) Halogen Derivatives of Arenes (Haloarenes) The compounds obtained by replacing one or more hydrogen atoms from arene (aromatic hydrocarbon) with equal number of halogen atoms (Cl, Br, 1) are called as halogen derivatives of arenes OT haloarenes. Ex. i) CsHs— Br (bromo benzene) ii) CoHs— Cl (chloro benzene) Classification of Halogen Derivatives 1) On the basis of hydrocarbon skeleton to which halogen atom is bonded, the halogen derivatives are classified into the following four types - i) Haloakanes Ex. CH;— CH»— X ii) Haloalkenes Ex. CH, = CH~ X iii) Haloalkynes Ex. CH = C-X iv) Haloarenes a Ex. SK 2) On the basis of number of halogen atoms, halogen derivatives are classified into the following four types - i) Monohalogen derivatives x Ex. CH3—CH:—X or CY ii) Dihalogen derivatives ~ iii) Trihalogen derivatives Ex. CH,—CH- CH: lo xX xX xX Ex. CH, — CH — CH—- CH i {it J | xX xX XX X s Ri EX. Gy,-CH-X CH,—CH: x | or | | 7 x x Xx x Xx x | | r x x x 0 x x Q. What are alkyl halides? Q. What are the different types of alkyl halides? Q. Explain common system of nomenclature of alkyl halides. Alkyl Halides 1) Monohaloge! halides. 2) They conta are obtained by replacing one hydrogen one halogen atom. ; 3) They are represented as R — X where'R' is alkyl group and —'x' is halogen atom (functional group). 4) Their general formula is Cy Hao xX. 5) Ex. i) CH;— Cl (methyl chloride) ii) C2Hs— Br {ethyl bromide) n derivatives of alkanes are also known as alkyl in only one halogen atom in one molecule and they atom from alkane wit), Classification of Alkyl Halides Alkyl halides are classified into following types on the basis of position of halogen atom. 1. Primary Alkyl Halides (1°) 1) The alkyl halides in which halogen atom is attached to primary carbon atom, are called as primary alkyl halides. 2) They are represented asR—- CH:~ X. Ex. CH:— CH:— CH:— Cl (n - propyl chloride) 2. Secondary Alkyl Halides (2°) 1) The alky halides in which halogen atom is attached to secondary carbon atom, are called as secondary alkyl halides. R-CH-X 2) They are represented as R,—~ CH X or | R' (R and R’ may be same or different) CHs— CH — CH; | (Gs0~ propyl chloride) 3, Tertiary Alkyl Halides (3°) 1) The alkyl halides in which halogen atom is attached to tertiary carbon atom, are called as tertiary alkyl halides. R 2) They are represented as R;C—XorR-C-X R (R’ and R” may be same or different) CHs | Ex. CHa CCH T (ter—buty! bramide) Common System (Trivial System) of Nomenclature of Alkyl Halides 1) In common system, name of the alkyl group is written first followed by name of the halogen atom as halide. 2) Ex. Clas chloride, Br as bromide and I as iodide. 3) If alkyl group contains more than two carbon atoms, then prefix n, iso, neo, pri, sec or ter is used as follows- a) prefix n for primary alkyl halides having straight chain of carbon atoms. Q. How is ethyl bromide prepared from ethyl alcohol? Q. What is the action of HBr on iso-propyl alcohol? Q. What is the action of Q. What is the action of following on 2-methyl propan-2-olP b)NaBr and cone. H2SO. (Oct, 2008- Mark 1) Q. What is the action of con. Hl on ethyl alcohol in pres- ence of H;PO.? Q. What is the action of con. Hl on iso-propyl alcohol in presence of HsPO.? Q. How will you obtain alkyl chlorides from alcohols? TT CH; CHs \ CHs— a CH, + con. HCl —+ CH; ~ C > CHi + H20 | H (tor butyl aleohol) (verbal ehloride) Formation of Alkyl Bromides (R— Br) 1) When alcohol is treated wi ri i ea ed with HBr, it forms corresponding alkyl 2) In this reaction, -OH group from alcohol is replaced with Br 3) HBr is prepared from NaBr and conc. H,SO,in situ. Examples i) When ethyl alcohol is treated with HBr, in presence of NaBr and H,SQ, it forms ethyl bromide. CH) — CH)— OH + HBr a5-* CHs — CH: ~ Br + H2O {e1hlalooks) HO. (eshyl bromide) ii) When iso-propyl alcohol is treated with HBr, it forms iso-propyl bromide. CH;— CH- CHs oe CHa CH - CH | +HBit—iear* | +H,0 | [ Br {isopropyi aleokol) {iso“prpyl bromide) iii) When ter-butyl alcohol is treated with HBr, it forms ter-butyl bromide. CHa CHs NaBr es | | CHy—O~ CH Bt Sa —p-Gie Br (ser—butyl aleahol) (ter—butyl bromide) Formation of Alkyl Iodides (R~!) 1) When alcohol is treated with concentrated HI, it forms corre- sponding alky] iodide. 2) In this reaction, -OH group from alcohol is replaced with | atom. 3) HI is prepare from Nal and HPO, in situ. Examples i) When ethyl alcohol is treated with HI in presence of H,PO,, it forms ethyl iodide. CH;— CH»~ OH + con.HI CH;- CH,-1+H,0 {ethyl aleohel) (ethyl iodide) ii) When iso-propy! alcohol is treated with HI in presence of H,PO,, it forms iso-propyl iodide. Nal/ioPOs CH;— CH~ CHs ayinino, CHa CH-CHs | + con.H] —“—— | +H,O (eH at (isopropy! iodide) Preparation of Alkyl Halides From Alcohols (R— OH) and Halogenating agents ‘Alcohol is treated with halogenatin; agents like phosphorus trihalide (PX) or phosphorus pentahalide tps) or thionyl chloride (SOC!:) to form corresponding alkyl halide. A) Preparation of Alkyl Chloride (R— Cl) 1) Alcohol is treated with PCI; (phosphorus trichloride), PCl; (phos- phorus pentachloride) or SOCl, (thionyl chloride or sulphonyl chloride) to form corresponding alkyl! chloride. 2) In this reaction hydroxyl group (-OH) is replaced with chlorine atom (Cl). ; 3) Reaction of alcohol with thionyl chloride is the most suitable method to prepare alkyl chloride, since the byproducts SO, and HCl are in gaseous state and alkyl halide is obtained in pure form. This reaction is known as Darzen’s method. 5 10. Halogen Derivatives Q. How will you obtain bromoethane from ethanol? Q. What is the action of the following reagents on ethanol? b) Mixture of red phosphorus and bromine (Sept.2009-Mark 1) Q. How will you obtain alicyl iodides from alcohols? Q. How will you obtain iodoethane from ethanol? Q. Explain halogen exchange method to prepare alkyl halides. *Q. Write a note on Finkelstein reaction. *Q. Convert 1-Chlorobutane into 1odobutane. Chea) Ae General Reaction 4» SS. 3R — Br + HPO; Gres bxomide) PBr, and SOBn cannot be formed. Therefore reaction of alcohol with PBr; and SOBn is not carried out. Examples Ethyl alcohol (ethanol) is treated with mixture of red phosphorus and bromine to form ethyl bromide. Br 3CxHs— OH + PBry (ey aleohol) 3C,H; — Br + H3PO; (ethyl bromide) C) Preparation of Alkyl Iodides(R — 1) 1) Alcohol is treated with mixture of red phosphorus and iodine to form corresponding alkyl iodide. 2) In this reaction hydroxyl group (-OH) is replaced with iodine atom (I). General Reaction 3R—OH + Pl; 2 (alcohol) 3R-I+Hs; (ey iodide) _ POs! PI, and SOI, cannot be formed. Therefore reaction of alcohol with Pl; and SOL is not carried out. Examples Ethyl alcohol (ethanol) is treated with mixture of red phosphorus and iodine to form ethyl iodide. Red Pls ry 3C.H;—1+ HsPOs (ech! iodige) 3C,H; ~ OH + Pl; (ethyl aleohal) Halogen Exchange Method of Preparation of Alkyl Halides The reaction in which one halogen atom in alkyl halide is replaced with another halogen atom is known as halogen exchange. i) Finkelstein Reaction +t In this reaction, alkyl chloride or alkyl bromide is treated with Nal in acetone to prepare alkyl iodide. General Reaction _ R-Cl+Nal —““~ R-1+NaCl atte) 7 (shat) OR R-Br+Nal “=~ R-I + NaBr| (atkyl bromide) {alkyl iodide) nC) Example Ethy] bromide is treated with Nal to prepare ethyl iodide. CoH;— Br+ Nal — C,H; ~ 1+ NaBr (ethyl bromide) (ethyl iodide) L ob , . Cote J {-Chlorobutane is treated with Nal to prepare 1-lodobutane. CHsCH2CH,CH:CI + Nal —~ CH3CH2CH2CH21 + NaCl (~chloroibutane) (-iodobutane) \di) Swarts Reaction : In this reaction, alkyl chloride or alkyl bromide is treated with metallic fluorides like AgF,CoF,,SbF: or Hg:F., to prepare alkyl fluoride. Q. Write down the physical properties of alkyl halides. OQ. Explain the solubility of alkyl halides and ary! halides solubility. Q. Give the laboratory test to confirm the presence of halogen in the organic compound. Q. Define substitution reaction. Q. What is leaving group? General Reaction R-Cl+ Agr — R-F +AgCl {alkyl chloride) (alkyl fvaride) OR ap R-F + ABB — _ iT Rabe Ast — Baakat M8 ith AgF to prepare ethyl fluoride. Example Ethyl bromide is treated wil FF —> CoHs— F + AgBr (ethy! Muoride) C.H;— Brt Ag! {ethyl bromide) Physical Properties of Alkyl Halides 1) Tat room temperature, lower alkyl halides are gases and higher alkyl halides are liquids or solids. 2) Volatile alkyl halides have sweet smell, i, , 3) Boiling point of alkyl halides increases with increase in molecular weight. . o=, 4) For the same alkyl group, poiling point is in the order- R-1>R-Br>R-Cl>R-F 5) For isomeric alkyl halides, boiling point decreases as branching increases. 6) Alkyl halides are slightly so! organic solvents. Juble in water but readily soluble in Solubility 1) Alkyl halides are insoluble in water but soluble in non-polar organic solvents. This is because alkyl halides can not form hydrogen bonds with water. 2) Aryl halides are also insolub! solvents. 3) Para isomer has high ortho or meta isomer. T! which can easily pack close intermolecular forces of attra greater energy is required to overcome its la’ le in water but soluble in organic er melting point as compared to that of his is because of its symmetrical structure ly in the crystal lattice. As a result ction are stronger and therefore ttice energy. Ans. Alkyl halide is warmed with aq. NaOH or KOH to R-X+OH —R-OH+X ixture is acidified by adding dilute nitric acid e of silver halide is ound. produce halide ion. When this reaction m: and silver nitrate solution is added, a precipitat formed which confirms presence of halogen in the organic comp Agia) + X (ay —> AgX 1 (s) Substitution Reaction The reaction in which an atom or group of atoms is replaced with another atom or group of atoms is called substitution reaction. Leaving Group During nucleophic substitution reaction of alkyl halides, halogen e covalent atom leaves the alkyl group along with two electrons in th bond. Hence halogen atom in alkyl halide is called leaving group. R-X——R*+ (leaving group) Q. What is the action of following reagents on ethyl iodide? i) Ale. KCN (Feb. 2009-Mark 1) Q. What happens when methyl chloride is boiled with ale, KCN? Q. Convert 1-Chlorobutane into CHsCH:CH2CH2CN. Q. How does alkyl halide react with alcoholic silver cyanide? Q. How is —X group replaced by -NC? Q. What happens when ethyl bromide is boiled with ale. AgCN? Q. What happens when methyl chloride is boiled with ale. AgCN? Q. How does alkyl halide react with alcoholic ammonia? *Q. How is -X group replaced by — NH»? Q. What happens when ethyl bromide is boiled with alc. NH,? Q. What happens when methyl chloride is boiled with ale. NH; ? Examples i) When nae cyanide (propa » C,H; — I+ alc.KCN (ethyl iodide) 1 iodide is boiled with alcoholic KCN, it forms ethy, itrile or propanonitrile). a eKCN = C:Hs—- CN + Kl (etiyl cyanide) ide i i i Icoholic KCN, it for; ii hyl chloride is boiled with alc« hi ne " tid eyectidh (ethane nitrile or acetonitr ile). CHs— Cl + ale-KCN + CHs— CN+KCI (methyl chloride) ’ iii) When 1-Chlorobutane is boiled with alc n-butyl cyanide. vot : cil “ , 4 CH;CH»CH.CH,Cl + ale. KCN CH;CHCH:CH:CN KCl (I ehlorobutane) oholic KCN, it forms es with Alcoholic Silver Cyanide (AgCN) boiled with alcoholic solution of AgCN, it anide (alkyl carbyl amine). f alkyl halide is replaced with Reaction of Alkyl Halid 1) When alkyl halide is d forms corresponding alkyl iso-cy: 2) In this reaction, halogen atom 0! isocyanide (-NC) group. General Reaction R~X+aleAgCN —“~ R—NC + AgX (alkyl halide) Examples ; . i) When ethyl bromide is boiled with alcoholic AgCN, it forms ethyl iso-cyanide. C.Hs— Brt alc.AgCN —"— C.Hs— NC + AgBr (ethyl bromide). (ethyl isocyanide) ii) When methyl chloride is boiled with alcoholic AgCN, it forms methyl iso-cyanide. CH; — Cl + ale.AgCN —“"- CH,;— NC + AgCl (metiy! ehoride) (meth! isocyanide) Reaction of Alkyl Halides with Alcoholic Ammonia (NH;) 1) When alkyl halide is boiled with excess of alcoholic solution of NHs, under pressure, it forms corresponding primary amine. 2) This reaction is known as ammonolysis. 3) In this reaction, halogen atom of alkyl halide is replaced with amino (—NH:) group. General Reaction ae R-NH+HX 7 (ps.amine) R~-X + (excess) alc.NH; (alkyl halide) i) When ethyl bromide is boiled with alcoholic NH it forms ethyl amine. ° C.Hs — Br + (excess) alc. NHy (ethyl bromide) —\ C:Hs— NH + HBr (ethylamine) ii) When methyl chloride is boiled with alcoholic NH, it forms methyl amine. ° CH: — Cl + (excess) alc. NH; —*—. (methyl chloride) pressure CH; — NH. + HCl (methyl amine) Q. How does alkyl halide react with alcoholic sodium alkoxide? Q. Write a note on Williamson’s synthesis. (Feb. 2009-Mark 2) | Q. What happens when ethyl bromide is boiled with alc. sodium ethoxide? Q. What happens when ethyl bromide is boiled with ale. sodium methoxide ? Q. What happens when methyl chloride is boiled with alc. sodium methoxide? Q. Predict the expected product of substitution reaction. isobutyl chloride + sodium ethoxide General Reaction Examples Reaction of Alkyl Halides with Alcoholic Sodium Alkoxide (R-O-Na) 1) When alkyl halide is boiled with alcoholic solution of sodium 2 pikoxidelR —O-Na), it forms corresponding ether. 2) 1 is also known as Williamson’s Synthesis. ) In this reaction, halogen atom of alkyl hal alkoxy (—OR) group. mil R-Q-R'+Nax (ethen lide is replaced with R! i) When ethyl bromide is boiled with alcoholic sodium ethoxide, it forms diethyl ether. CoH.— Br + ale.Na— O— CGsHs “= CH) O= CoH + NaBr (ethyl bromide) (cotium ethniée) (ciety ether) ii) When ethyl bromide is boiled with alcoholic sodium methoxide, it forms ethyl methyl ether. C:Hs— Br+ale.Na—O- CH: + Cr (ethyl bromide) (sodium methoxide) H,—-O- CH; + NaBr (ethyl methyl ether) iii) When methyl chloride is boiled with alcoholic sodium methoxide, it forms dimethyl ether. CH,— C1 + ale.Na—O—- CH; —“~ CH,— 0 = CHa + NaCl (coethyl chloride) (sodium methoxide) (imethy) ther) iv) When isobutyl chloride is boiled with alcoholic sodium ethoxide, it forms isobutyl ethyl eth s CH i + alc NaOC:H: + NaCl CH-CH-cH-cl CH.- CH- CH, ~ OC+Hs ot Seam a Q. How does alkyl halide react with alcoholic solution of silver salt of carboxylic acid? Q. What happens when methyl bromide is boiled with alc. silver acetate? Q. How will bring about | following conversion? | Ethyl iodide to ethyl acetate. (Oct. 2008- Mark 1) Q. What is the action of following reagents on ethyl iodide? i) Silver acetate (Feb. 2009-Mark 1) Q. Write balanced chemical equation for the action of methyl bromide on silver Dropanoate. (Mar. 13, Mark 1) Q. Convert 1-Chlorobutane into CH.CH.CH:CH:COOCH: Reaction of Alkyl Halides with Silver Salt of Carboxylic Acid (R=COO-A 2) 4) When alkyl halide is boile of carboxylic acid, it forms corresponding ester. 5) In this reaction halogen atom of alkyl halide is replaced with carboxylate (R - COO") group. General Reaction alc.R'— COO- Ag+ X-R——R'~ (ey! halide) d with alcoholic solution of silver salt COO-R+AgX ‘cia (silver salt of cabonylic acid) i) When methyl bromide is boiled with alcoholic silver acetate, it forms methyl acetate. alc.CH; - COO ~ Ag+ Br- CH, + CHs—C (methyl bromide) (00 — CH: + AgBr (methyl ecetate) th alcoholic silver acetate, it (aver acetate) ii) When ethyl iodide is boiled wit forms ethyl acetate. ale.CHs— COO - Ag + 1— C,H; (cilver acetate) (ethyl iodide) boil CH;-— COO — C:H; + Agl (ethyl acetate) iii) When methyl bromide is treated with alcoholic silver propa- noate, it forms methyl propanoate. alc.C:H— COO — Ag + Br— CH, ~*~ C:Hs— COO ~ CH: + AgBr + fropavonia) (methyl bromide) (methy! propanoste) iv) When 1-Chlorobutane is treated with alcoholic silver acetate, it forms n-butyl acetate. OOAg + CHsCH,CH.CH:C] =~ CH:CHy (CH»CH,COOCHs+ AgCl (ebotytoctate) 10. Halogen Derivatives cq rda > Ot Q. How can you obtain ethane from methyl iodide? Q. How is butane prepared from ethyl bromde? Q. What will happen, if mixture of methyl iodide and ethyl iodide is treated with sodium metal in presence of dry ether? Q. Predict the expected product of substitution reaction- n-butyl cloride+sodium. Q. Define Grignard reagent. Q. How is Grignard reagent formed? to +g Q. What happens when methyl iodide is treated with Mg metal in presence of dry ether? Q. What happens when ethyl bromide is treated with Mg metal in presence of dry ether? us ails Examples i ) When methyl iodide is treated with Na metal, in presence of ry ether, it forms ethane. 2CHa— 1+ 2Na —32—+ CH + 2Nal (methyl ioaide) a (eave) ii) ware ethyl bromide is treated with sodium metal in presence ry ether, it forms n-butane. 2C,H,— Br + 2Na 2 (uayl bromide) ether + 2NaBr a) ted with sodium metal If mixture of two different alkyl halides is trea ts of mixture of three in presence of ' . ‘ alkanes. dry ether, then product consis‘ Example i) When mixture of methyl iodide and ethy! sodium metal in presence of dry ether, of ethane, propane and n-butane. CH 1+ 2Na+ CHs =I —atu> Coot CsHe+ Cold + 2Nal (methyl iodide) {ethyl iodide) whee (propane) (n~butane) Self coupling products C,H» and C,H» are major products while cross coupling product C;Hs is minor product. Jiodide is treated with then it forms mixture (ethene) ii) When n-butyl chloride is treated with sodium metal in presence of dry ether, it forms n-octane. 2CHsCH,CHCH,Cl + 2Na > CsHis + 2NaCl (n= butyl chloride) her (a= ontane) Grignard Reagent 1) Alkyl magnesium halide is known as Grign: represented as R- Mg-X Where'R' is alkyl group 'Mg'is magnesium metal 'X'is halogen atom. 2) Ex. i) CH, — Mg — ii) CoH,— Mg-Br (ethyl 3) It is an organometallic compound It is very reactive. 4) It is used in the preparation of alcohols, aldehydes, ketones, carboxylic acids, amines etc. ard Reagent. It is I (methyl magnesium iodide) magnesium bromide) discovered by Victor Grignard. Formation of Grignard Reagent When alkyl halide is treated with of dry ether, then it forms correspon magnesium halide). General Reaction R-X g = R-Mg-X . {(etkyl halide) (alkyl mognesi ) oY - magnesium metal, in presence ding Grignard Reagent (Alkyl Examples i): When methyl iodide is treated with Mg metal, in presence of dry ether, then it forms m« ethyl magnesium iodide. —*- CH:- Mg tbe’ {methy! mognesium iodide) CH;—I+Mg (methyl iodide) When ethyl bromide is treated with Mg metal in presence of dry ether, then it forms ethyl magnesium bromide. CH, — CH»— Br + Mg —aar> CHs~ CHa~ Mg — Br her {ethy! bromide) (ethyl magnesium bromide) ii) Q. Define ordir lary light and monochromatic Tight. " Q. Explain the term plane Polarized light. (Feb. 2009-Mark 1) Q. Define optical activity. (Mar. 2008-Mark 1) Q. Explain the term optical activity. (Mar. 16, Mark 1) Ordinary Light . ~ The light which contains light rays of different wavelengths vibray in all possible planes perpendicular to the direction of Propagate’ is known as Ordinary Light. ion Monochromatic Light , The light which contains light rays of single wavelen, colour) vibrating in all possible planes perpendicular tot of propagation is known as Monochromatic Light. gth (sin . it he direction ee Plane Polarised Light (PPL) ; a 1) The light which contains light rays vibrating in a single plan, perpendicular to the direction of propagation is known as Plane Polarised Light. . 2) Plane polarized light is obtained by passing ordinary light o, monochromatic light through Nicol prism. 3) Nicol prism is made up of two prisms of calcite crystals (CaCo,) joined diagonally by Canada balsam. a--- MJ ~ohoeek eb _ | Direction of Propagation Nicol prism ordinary light Polarised light Fig:- Plane polarised light Optical Activity The property of an organic compound to rotate the plane of plane polarised light in clockwise (right) or anticlockwise (left) direction is known as optical activity and such compound is called as Optically Active Compound. The angle through which a substance rotates the plane of plane polarized light is called optical rotation. e 8 ed > if Hi i angle of rotation i coe ia o- t H-C-OH H-C-a HG" OH CH; CH; H-C-—OH (lactic acid) (2chloro butane) | H-C-—OH | H-C-—OH | CH:OH (glucose) Q. Explain optical activity of lactic acid. Q. Discuss the optical activity of lactic acid, (Mar. 14m Mark 3) (Oct. 13, Mark 2) Q. Define nucleophilic substitution reaction. Q. Define mechanism of reaction. Q. Explain Sx2_ mechanism in detail. O. With the help of energy profile diagram explain the mechanism of alkaline hydrolysis of bromomethane. (Mar. 2008-Mark 4) Q. Draw energy profile diagram of Sw2 reaction. (Sept. 2009-Mark 2) ; Q. Discuss the mechanism of alkaline hydrolysis of bromomethane. (Oct. 15, Mark 4) Optical Activity of Lactic acid Lactic acid is optically active compoun follows. 1. Presence of Asymmetric Carbon ators 1) The structure of lactic acid e shows elow- d. This can be explaineg as | H-C’-OH | CHs 2) This shows that lactic acid has one asymmetric carbon atom, which is attached to four different atoms or groups. i.e. H, COOK, OH and CH. . ; - 3) Asymmetric carbon atom is responsible for optical activity, 2. Non Super Imposable Mirror Images ae 1) Due to presence of one asymmetric carbon atom, lactic acid has two different structures which have same molecular formulg and same structural formula but opposite action on the plane of plane polarised light. - F 2) These structures are non super imposable mirror images of each other. COOH mee | H-C-—OH HO- 7 —-H | CHs HC (lactic acid) (tactic acid) minor 3) d-lactic acid rotates the plane of plane polarised light in clockwise (right) direction (dextro isomer) while |-lactic acid rotates the plane of plane polarised. light in anticlockwise (left) direction (laevo isomer). 3. Racemic Mixture Equimolar mixture ofd—lactic acid and ]—lactic acid is optically inactive due to external compensation. Nucleophilic Substitution Reaction 1) The substitution reaction in which one nucleophile is replaced with another nucleophile is known as nucleophilic substitution reaction. 2) It is of two types, Syl and Sy2. The step by step detail description of a chemical reaction is known as its mechanism. Sn2 Mechanism (Mechanism of Alkaline Hydrolysis of Methyl Bromide) 1) Sy2 means substitution nucleophilic bimolecular reaction. 2) When methyl bromide is boiled with aq. alkali (KOH or NaOH),it forms methyl alcohol (methanol). CH,~ Br +aq.KOH “> CH,—- OH+KBr (methy! bromide) (methanol) Kinetics 1) It is a bimolecular nucleophilic substitution reaction because rate of chemical reaction depends upon concentration of methyl bromide and nucleophile OH’. 2) It is second order reaction. r &(CH3—Brl[OH"] r= K[CH,~ Br] [OH] Where, r is rate of chemical reaction. 16 Q. Explain Sy1 mechanism in detail. Q. Explain mechanism of alkaline hydrolysis of ter. butyl bromide with energy brofile diagram. (Feb. 2009-Mark 4) 7 Write the mechanism of i reaction when t-butyl caenae is treated with ous KOH with ener Profile diagram. “ ica rate constant [OH isco concentration of methyl bromide. Seren centration of nucleophile. Itisa si . of Bort eae mechanism, as breaking of bond a 2) mechanism, = place simultaneously. It is calle e nuc i P avo een OH’ attacks carbon atom from the pack side to 3) In TS eric repulsion and forms transition state. hydro 2 pentacoordinate carbon atom is attac' maxim n atoms, one bromine atom and one ~ OH group. 4) 1 pane energy and minimum stability. -S., carbon atom and three hydrogen atoms lie in one plane 5) Peace to HO-~-C---Br axis. 6) Afte nH bond angle is 120°. the bons Abemation of T.S., the bond C-Br is brok ; alcohol, is formed to produce the final product } ee ine has opposite structure ie. i i ° takes place, inversion of configuration nd formation d concerted hed to three It has en down and methyl to that of methyl (Walden inversion) n H 4H H 3 \/ -3 H-C~Br+on —-|Ho--C-~-Br|—+ HO-C-H+ Br \ (oxitol roadie] Hy (methyl nleohol) ee aan) TS. potential energy—> rate of reaction > Fig:- Energy profile diagram for Sy2 mechanism Where, T.S. is transition state, AHis heat of reaction, AE is activa- tion energy. Syl Mechanism (Mechanism of Alkaline Hydrolysis of Ter.- Butyl Bromide) 1) Sylmeans substitution nucleophilic unimolecular reaction. 2) When ter.-butyl bromide is boiled with aq. alkali (KOH or NaOH) it forms ter.-butyl alcohol. (CHs),— C ~ Br + aq.KOH ti . (CHs),- C - OH + KBr (ter butyl aleohal) (ter butyl bromide) Kinetics ays 1) It is unimolecular nucleophilic substitution reaction because rate of the chemical reaction depends upon concentration of only ter.- butyl bromide. 2) It is first order reaction. ra.[(CHs),— C— Br] 1 =K[(CHs),— C- Br] P pxplain the following ems y profile diagram a) activation energy ‘ eat of reaction nsition state on 2009-Mark 1 each) g. Write the difference petween Sx land Sw mechanisms. . Distinguish betweenSn 1 and Sx2 mechanisms. (Mar. 14, Mark 2) Q. Explain the factors influencing Sw1 and Sx2 mechanism. Where, AHi _—__—____—~ - is heat of rez 5 2 sation ene! 5y and T. S. is transition foe AE, and AE» are activation re _ ergy Profile Diagram ate of The graph . sraph obtained by plotting potential enerBy against T reaction is called 4 i Activation Energy (Ae) profile diagram. The diff reateaaete eclne ee energy of transition state and energy of . clive . Ferg Reaction an Energy. ec. is walled Heat petween energy of products and energy of reactants taal of Reaction. ‘Tretioe State (T.S.) n e : vonctante meat state which is formed during conve inieaa into the products and which has maximum energ. um stability is called Transition State. rsion of y and Dift a ifference between S,land Sx2 Mechanism —_——| Sxl Mechanism 1, It is unimolecular nucleo- philic substitution reaction. 2. It is two step reaction. 3, Both front side as well as backside attack takes place. 4, 50% inversion and 50% retention of configuration may take place. Sy2 Mechanism 1. It is bimolecular nucleop- hilic substitution reaction. 2. It is one step reaction. 3. Only backside attack takes place. 4, 100% inversion of configu- ration takes place. 5. Only one T.S. is formed. 6. There is no formation of stable carbonium ion. 5. Two T.S. are formed. 6. There is formation of stable carbonium ion. 7. Itis favoured by non-polar 7. It is favoured by polar solvents. solvents. Factors influencing SylandSy2 mechanism 1) Nature of substrate Sn2 mechanism is most favoured in favoured in tertiary halides. Snl mechanism is most favoured in tertiary halides and least favoured in primary halides. ‘Syl rate increases CH-X 1 2° 3 ‘Sad rate increases primary halides and least imary halides follow Sn2 mechanism tertiary halides follow mand secondary halides follow Sw 1 or Sxn2 mechanism the exact conditions. Thus pr: Swl mechanis depending upon 2) Nucleophilicity ofthe reagent As the nucleophilicity of the reagent increases, Sx2 mechanism is favoured. The rate of Sw] mechanism is independent of the nature of nucleophile. 3) Solvent polarity . . Polar protic solvents favour Syl mechanism while aprotic solvent favour Sn2 mechanism. Q. For each pair of compounds, state which compound is better Sx2 substrate? i) 2-Bromobutane, isopropyl bromide ti) 1-lodo-2,2-dimethyl propane or isopropyl iodide Q. Give the classification of haloarenes. i) 2-Bromobutane, isopropyl bromide ; Isopropyl bromide has two methyl groups on —carbon while 2-Bromobutane has one methyl and one ethyl group on @~carbon, Therefore isopropyl bromide has less steric hindrance and hence it is better S»2 substrate. ii) 1-Iodo-2, 2-dimethyl propane or isopropyl! iodide 1-lodo-2,2-dimethyl propane has one methyl group on O&~ carbon while isopropyl iodide has two methyl groups on O— carbon, Therefore 1-lodo-2,2-dimethyl propane has less steric hindrance and hence it is better 5x2 substrate. Classification of Haloarenes On the basis of number of halogen atoms a ring, haloarenes are classified as below- 1) Monohaloarenes- Only one halogen atom 1s attached to the benzene ring. cl Ex, (chlorobenzene) ttached to the benzene 2) Dihaloarenes- Two halogen atoms are attached to the benzene ring. cl | cl Ex. (p - dichlorobenzene) 3) Trihaloarenes- Three halogen atoms are attached to the benzene ring. Br ; Br Br Ex. (1,2,3 tribrom obenzene) 4) Polyhaloarenes- More than three halogen atoms are attached to the benzene ring. cl cl : cl cl Ex. (1,2,4,6 tetra chlorobenzene) 20