Dr. Anand Gupta Mr Mahesh Kapil

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1 Dr. Anand Gupta Mr Mahesh Kapil

2 Preparation of Haloalkanes From Alkanes Alkenes Alcohols Carboxylic Acids (Hundsdicker Reaction) Halide Exchange Method»Finkelstein Reaction»Swarts Reaction

3 Physical Properties of Alkyl Halides: Boiling point Solubility in water

4 Effect of Structure on Boiling Point Molecular weight Boiling point, C Dipole moment, D CH 3 CH 2 CH 3 CH 3 CH 2 F

5 Boiling point increases with increasing number of halogens Compound Boiling Point CH 3 Cl -24 C CH 2 Cl 2 40 C CHCl 3 61 C CCl 4 77 C Even though CCl 4 is the only compound in this list without a dipole moment, it has the highest boiling point. Induced dipole-induced induced dipole forces are greatest in CCl 4 because it has the greatest number of Cl atoms. Cl is more polarizable than H.

6 PHYSICAL PROPERTIES Boiling point Increases with molecular size due to increased van der Waals forces M r bp / C chloroethane chloropropane bromopropane Boiling point also increases for straight chain isomers. Greater branching = lower inter-molecular forces bp/ C 1-bromobutane CH 3 CH 2 CH 2 CH 2 Br bromobutane CH 3 CH 2 CHBrCH bromo -2-methylpropane (CH 3 ) 3 CBr 73

7 Solubility in water Alkyl halides are insoluble in water. Methanol, ethanol, isopropyl alcohol are completely miscible with water. µ = 1.9 D

8 Chemical Properties I. Nucleophilic Aliphatic Substitution A. Mechanisms of nucleophilic substitution 1. S N 2 mechanism 2. S N 1 mechanism B. Factors influencing S N 1 and S N 2 reactions 1. Nucleophile 2. Substrate structure 3. Leaving group 4. Solvent II. Elimination A. Mechanisms of elimination 1. E2 mechanism 2. E1 mechanism III. Reaction with Active metals

9 I. Nucleophilic Aliphatic Substitution R X + Y R Y + X substrate nucleophile product leaving group e.g., H O CH 3 Br CH 3 OH + Br

10 I. Nucleophilic Aliphatic Substitution R X + HO alkyl halide (sp 3 only) R'O HS R'S R OH R O R' R SH R S R' alcohol ether thiol thioether CN R C N nitrile I R I halide NH 3 R NH 3 + X - alkylammonium salt

11 Nucleophilic Aliphatic Substitution B. Mechanisms of nucleophilic substitution Two mechanisms: General: Rate = k 1 [RX] + k 2 [RX][Y ] k 1 increases RX = CH 3 X 1º 2º 3º k 2 increases k 1 ~ 0 Rate = k 2 [RX][Y ] (bimolecular) S N 2 k 2 ~ 0 Rate = k 1 [RX] (unimolecular) S N 1

12 1. S N 2 mechanism Nucleophilic Aliphatic Substitution e.g., CH 3 I + OH CH 3 OH + I find: Rate = k[ch 3 I][OH ], i.e., bimolecular both CH 3 I and OH involved in RDS concerted, single-step mechanism: [HO---CH 3 ---I] Transition state CH 3 I + OH CH 3 OH + I

13 Nucleophilic Aliphatic Substitution 1. S N 2 mechanism Stereospecific reaction: H Br HO H NaOH (R)-( )-2-bromooctane (S)-(+)-2-octanol Reaction proceeds with inversion of configuration.

14 Nucleophilic Aliphatic Substitution 1. S N 2 mechanism back-side attack: inversion of configuration H δ + δ - HO C I H H HO H C H H I HO H C H H + I HO C I HO C I HO C I sp 3 sp 2 sp 3 high energy TS

15 2. S N 1 mechanism II. Nucleophilic Aliphatic Substitution e.g., H 3 C CH 3 CH 3 C Br CH 3 OH + H 3 C C O CH 3 + HBr CH 3 CH 3 Find: Rate = k[(ch 3 ) 3 CBr] Unimolecular RDS depends only on (CH 3 ) 3 CBr

16 2. S N 1 mechanism II. Nucleophilic Aliphatic Substitution CH 3 CH 3 RLS: H 3 C C Br CH 3 H 3 C C + Br CH 3 H 3 C CH 3 CH 3 H C HOCH 3 H 3 C C O CH 3 CH 3 CH 3 H 3 C CH 3 H C O CH 3 CH 3 -H + H 3 C CH 3 C O CH 3 + HBr CH 3

17 II. Nucleophilic Aliphatic Substitution 2. S N 1 mechanism Two-step mechanism: carbocation intermediate R + RBr + CH 3 OH ROCH 3 + HBr

18 II. Nucleophilic Aliphatic Substitution 2. S N 1 mechanism stereochemistry: stereorandom CH 3 CH 2 Br C H CH 3 H 2 O OH CH CH 3 CH 3 2 C + CH 3 CH H C H 2 CH 3 OH racemic OH 2 CH 3 CH 2 C H CH 3 + OH 2 sp 2, trigonal planar, achiral (optically inactive)

19 C. Factors influencing S N 1 and S N 2 reactions 1. Nucleophile Nucleophile: good moderate poor I, Br HS, RS HO, RO RCO 2 H 2 S, RSH NH 3, RNH 2, R 2 NH H 2 O, ROH RCO 2 H Increasing reactivity toward S N 2 (no effect on rate of S N 1) increases k 2 Rate = k 1 [RX] + k 2 [RX][Y ]

20 C. Factors influencing S N 1 and S N 2 reactions 2. Substrate structure S N 2 mechanism: governed by steric factors steric hindrance = hindrance to back side attack on the carbon by the nucleophile owing to the size of the groups on that carbon less steric hindrance faster rate of S N 2 S N 1 mechanism: governed by electronic effects more stable cation faster rate of S N 1

21 C. Factors influencing S N 1 and S N 2 reactions 2. Substrate structure S N 2 mechanism: steric effects e.g., R Br + I R I + Br (S N 2) Compound Rel. Rate methyl CH 3 Br 150 1º RX CH 3 CH 2 Br 1 2º RX (CH 3 ) 2 CHBr º RX (CH 3 ) 3 CBr ~0 increasing steric hindrance

22 C. Factors influencing S N 1 and S N 2 reactions 2.Substrate structure H I H H C Br minimal steric hindrance I H H C H C H C H H C H Br H maximum steric hindrance

23 C. Factors influencing S N 1 and S N 2 reactions 2. Substrate structure S N 1 mechanism: electronic effects R + stability: 3º > 2º >> 1º > CH + 3 R-X reactivity toward S N 1: 3º > 2º >> 1º > CH 3 X CH 3 + 1º R + 2º R + 3º R +

24 C. Factors influencing S N 1 and S N 2 reactions 2. Substrate structure Summary: Rate = k 1 [RX] + k 2 [RX][Nu] rate of S N 1 increases (carbocation stability) RX = CH 3 X 1º 2º 3º rate of S N 2 increases (steric hindrance) react primarily by S N 2 (k 1 ~ 0, k 2 large) may go by either mechanism reacts primarily by S N 1 (k 2 ~ 0, k 1 large)

25 C. Factors influencing S N 1 and S N 2 reactions 2. Substrate structure Which compound in each group would react fast by S N 2? Which by S N 1? a) Br Br Br b) I I I

26 3. Leaving group C. Factors influencing S N 1 and S N 2 reactions weaker base = better leaving group reactivity as leaving group increases I > Br > Cl >> F > RCO 2 >> HO, RO > H 2 N base strength increases good leaving groups poor leaving groups never leaving groups

27 3. Leaving group C. Factors influencing S N 1 and S N 2 reactions R X + Y R Y + X stronger weaker base base K > 1 Br + NaOH OH + NaBr SB WB Br acetone + NaI + NaBr (s) I WB SB precipitate drives rxn (Le Châtelier)

28 4. Solvent C. Factors influencing S N 1 and S N 2 reactions nonpolar aprotic: hexane, benzene moderately polar aprotic: ether, acetone, ethyl acetate polar protic: H 2 O, ROH, RCO 2 H polar aprotic: DMSO DMF acetonitrile CH 3 O S CH 3 S N 1 mechanism promoted by polar protic solvents H O C N(CH 3 ) 2 CH 3 C N stabilize R +, X relative to RX R + X in less polar solvents in more polar solvents RX

29 4. Solvent C. Factors influencing S N 1 and S N 2 reactions S N 2 mechanism promoted by moderately polar & polar aprotic solvents destabilize Nu s, make them more nucleophilic e.g., OH in H 2 O: strong H-bonding to water makes OH less reactive OH in DMSO: weaker solvation makes OH more reactive (nucleophilic) in DMSO in H 2 O RX + OH ROH + X

30 II. Nucleophilic Aliphatic Substitution C. Factors influencing S N 1 and S N 2 reactions 5. Summary rate of S N 1 increases (carbocation stability) RX = CH 3 X 1º 2º 3º rate of S N 2 increases (steric hindrance) react primarily by S N 2 may go by either mechanism reacts primarily by S N 1 S N 2 promoted good nucleophile (Rate = k 2 [RX][Nu]) -usually in polar aprotic solvent S N 1 occurs in absence of good nucleophile (Rate = k 1 [RX]) -usually in polar protic solvent (solvolysis)

31 S N1 and S N2 Reactions Substrate Nucleophile Leaving group Solvent Rate Carbocation intermediate? Stereochemistry Rearrangement 3 >2 >1 Unimportant, but usually weak Excellent Polar and ionizing =k[rx] Yes mix S N 1 ~H, ~ CH3 possible S N 2 CH 3 X>1 >2 >3 Strong and unhindered Better than nucleophile Polar aprotic =k[rx][nuc:-] No Inversion of configuration No rearrangements

32 Elimination B: - is a species acting as a base. Since HX is lost, this particular reaction is called a dehydrohalogenation.

33 II. Elimination Y C Z C C C + Y Z Elimination reaction β-elimination or 1,2-elimination H X C C + B C C + BH + X dehydrohalogenation strong base, usually HO or RO Br EtONa EtOH Cl KOH EtOH

34 II. Elimination Follows the Zaitsev rule (Saytzeff):the more stable alkene predominates (more substituted alkene; more trans than cis) Br EtONa EtOH % 20% 19% Br EtONa EtOH + 71% 29% What would be the product distribution from the following reaction? Br KOH EtOH

35 A. Mechanisms of elimination 1. E 2 mechanism Single step, concerted mechanism: X X X C C C C C C B H B H B H bimolecular Rate = k[rx][b ] Br + OH - Zaitsev

36 Mechanisms of elimination 2. E 1 mechanism Occurs in the absence of a strong base: Br EtOH major + minor Rate = k[rbr] unimolecular (no involvement from B ) Reactivity: RI > RBr > RCl > RF (RDS involves R X breaking) and: 3º > 2º > 1º (RDS invloves R + )

37 Elimination 2. E 1 mechanism Two step mechanism: Step 1: (RLS) unimolecular Br + Br EtOH + HBr Step 2: + EtOH 2 EtOH H Also undergoes S N 1 reaction; can t control the ratio of S N 1 to E1.

38 Bimolecular Reactions: S N 2 and E2 (usually the preferred way) require a good nucleophile or a strong base promoted by polar aprotic solvents Substrate 1º 2º 3º Good Nu, Weak B I, Br, Cl, RS, R 3 N S N 2 S N 2 no reaction Good Nu, Strong B HO, RO, H 2 N mostly S N 2 mostly E2 E2

39 ELIMINATION v. SUBSTITUTION The products of reactions between haloalkanes and OH are influenced by the solvent SOLVENT ROLE OF OH MECHANISM PRODUCT WATER NUCLEOPHILE SUBSTITUTION ALCOHOL ALCOHOL BASE ELIMINATION ALKENE Modes of attack Aqueous soln OH attacks the slightly positive carbon bonded to the halogen. OH acts as a nucleophile Alcoholic soln OH attacks one of the hydrogen atoms on a carbon atom adjacent the carbon bonded to the halogen. OH acts as a base (A BASE IS A PROTON ACCEPTOR) Both reactions take place at the same time but by varying the solvent you can influence which mechanism dominates.

40 C. Reaction With Metals Wurtz Reaction Frankland Reaction Reaction with Magnesium

Essential Organic Chemistry. Chapter 9

Essential Organic Chemistry. Chapter 9 Essential Organic Chemistry Paula Yurkanis Bruice Chapter 9 Substitution and Elimination Reactions of Alkyl Halides 9.1 How Alkyl Halides React Substitution Reactions One group takes the place of another.