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4.8 Preparation of Alkyl Halides from Alcohols and Hydrogen Halides ROH + HX RX + H 2 O.

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Presentation on theme: "4.8 Preparation of Alkyl Halides from Alcohols and Hydrogen Halides ROH + HX RX + H 2 O."— Presentation transcript:

1 4.8 Preparation of Alkyl Halides from Alcohols and Hydrogen Halides ROH + HX RX + H 2 O

2 Reaction of Alcohols with Hydrogen Halides ROH + HX RX + HOH Hydrogen halide reactivity HI HBr HCl HF most reactiveleast reactive

3 Reaction of Alcohols with Hydrogen Halides ROH + HX RX + HOH Alcohol reactivity R 3 COH R 2 CHOH RCH 2 OH CH 3 OH TertiarySecondaryPrimaryMethanol most reactiveleast reactive

4 Preparation of Alkyl Halides (CH 3 ) 3 COH + HCl (CH 3 ) 3 CCl + H 2 O 78-88% + H 2 O 73% CH 3 (CH 2 ) 5 CH 2 OH + HBr CH 3 (CH 2 ) 5 CH 2 Br + H 2 O 87-90% 25°C °C 120°C OH + HBr Br

5 Preparation of Alkyl Halides CH 3 CH 2 CH 2 CH 2 OH CH 3 CH 2 CH 2 CH 2 Br NaBr H 2 SO % heat A mixture of sodium bromide and sulfuric acid may be used in place of HBr.

6 4.9 Mechanism of the Reaction of Alcohols with Hydrogen Halides

7 CarbocationCarbocation The key intermediate in reaction of secondary and tertiary alcohols with hydrogen halides is a carbocation. A carbocation is a cation in which carbon has 6 valence electrons and a positive charge. C R R R+

8 CarbocationCarbocation The key intermediate in reaction of secondary and tertiary alcohols with hydrogen halides is a carbocation. The overall reaction mechanism involves three elementary steps; the first two steps lead to the carbocation intermediate, the third step is the conversion of this carbocation to the alkyl halide. C R R R+

9 ExampleExample (CH 3 ) 3 COH + HCl (CH 3 ) 3 CCl + H 2 O 25°C Carbocation intermediate is: C H3CH3CH3CH3C CH 3 + tert-Butyl alcohol tert-Butyl chloride tert-Butyl cation

10 Mechanism Step 1: Proton transfer from HCl to tert-butyl alcohol (CH 3 ) 3 C O H.. : H Cl : H O : + HCl :.... : – + fast, bimolecular tert-Butyloxonium ion

11 Mechanism Step 2: Dissociation of tert-butyloxonium ion + (CH 3 ) 3 C O H :H+ slow, unimolecular (CH 3 ) 3 C O H :H: tert-Butyl cation +

12 Mechanism Step 3: Capture of tert-butyl cation by chloride ion. fast, bimolecular + (CH 3 ) 3 C + tert-Butyl chloride (CH 3 ) 3 C Cl :.... Cl : :....–

13 4.10 Structure, Bonding, and Stability of Carbocations

14 Figure 4.8 Structure of methyl cation. Carbon is sp 2 hybridized. All four atoms lie in same plane.

15 Figure 4.8 Structure of methyl cation. Empty 2p orbital. Axis of 2p orbital is perpendicular to plane of atoms.

16 CarbocationsCarbocations Most carbocations are too unstable to be isolated. When R is an alkyl group, the carbocation is stabilized compared to R = H. C R R R+

17 CarbocationsCarbocations C H H H+ Methyl cation least stable

18 CarbocationsCarbocations C H3CH3CH3CH3C H H + Ethyl cation (a primary carbocation) is more stable than CH 3 +

19 CarbocationsCarbocations C H3CH3CH3CH3C CH 3 H + Isopropyl cation (a secondary carbocation) is more stable than CH 3 CH 2 +

20 CarbocationsCarbocations C H3CH3CH3CH3C CH 3 + tert-Butyl cation (a tertiary carbocation) is more stable than (CH 3 ) 2 CH +

21 Figure 4.9 Stabilization of carbocations via the inductive effect positively charged carbon pulls electrons in bonds closer to itself +

22 Figure 4.9 Stabilization of carbocations via the inductive effect positive charge is "dispersed ", i.e., shared by carbon and the three atoms attached to it

23 Figure 4.9 Stabilization of carbocations via the inductive effect electrons in CC bonds are more polarizable than those in CH bonds; therefore, alkyl groups stabilize carbocations better than H. Electronic effects transmitted through bonds are called "inductive effects."

24 Figure 4.10 Stabilization of carbocations via hyperconjugation electrons in this bond can be shared by positively charged carbon because the s orbital can overlap with the empty 2p orbital of positively charged carbon +

25 Figure 4.10 Stabilization of carbocations via hyperconjugation electrons in this bond can be shared by positively charged carbon because the s orbital can overlap with the empty 2p orbital of positively charged carbon

26 Figure 4.10 Stabilization of carbocations via hyperconjugation Notice that an occupied orbital of this type is available when sp 3 hybridized carbon is attached to C +, but is not availabe when H is attached to C +. Therefore,alkyl groups stabilize carbocations better than H does.

27 CarbocationsCarbocations The more stable a carbocation is, the faster it is formed. Reactions involving tertiary carbocations occur at faster rates than those proceeding via secondary carbocations. Reactions involving primary carbocations or CH 3 + are rare. C R R R+

28 CarbocationsCarbocations Carbocations are Lewis acids (electron-pair acceptors). Carbocations are electrophiles (electron-seekers). Lewis bases (electron-pair donors) exhibit just the opposite behavior. Lewis bases are nucleophiles (nucleus-seekers). C R R R+

29 Mechanism Step 3: Capture of tert-butyl cation by chloride ion. fast, bimolecular + (CH 3 ) 3 C + tert-Butyl chloride (CH 3 ) 3 C Cl :.... Cl : :....–

30 CarbocationsCarbocations The last step in the mechanism of the reaction of tert-butyl alcohol with hydrogen chloride is the reaction between an electrophile and a nucleophile. tert-Butyl cation is the electrophile. Chloride ion is the nucleophile. (CH 3 ) 3 C Cl : :....– + Cl :.... +

31 nucleophile (Lewis base) electrophile (Lewis acid) Fig Combination of tert-butyl cation and chloride ion to give tert-butyl chloride

32 4.11 Potential Energy Diagrams for Multistep Reactions: The S N 1 Mechanism

33 Potential energy Reaction coordinate Recall...Recall... H 2 O + HBr H 2 OH + Br – + H2OH2OH2OH2OHBr the potential energy diagram for proton transfer from HBr to water

34 The potential energy diagram for a multistep mechanism is simply a collection of the potential energy diagrams for the individual steps. Consider the mechanism for the reaction of tert-butyl alcohol with HCl. ExtensionExtension (CH 3 ) 3 COH + HCl (CH 3 ) 3 CCl + H 2 O 25°C

35 Mechanism Step 1: Proton transfer from HCl to tert-butyl alcohol (CH 3 ) 3 C O H.. : H Cl : H O : + HCl :.... : – + fast, bimolecular tert-butyloxonium ion

36 Mechanism Step 2: Dissociation of tert-butyloxonium ion + (CH 3 ) 3 C O H :H+ slow, unimolecular (CH 3 ) 3 C O H :H: tert-Butyl cation +

37 Mechanism Step 3: Capture of tert-butyl cation by chloride ion. fast, bimolecular + (CH 3 ) 3 C + tert-Butyl chloride (CH 3 ) 3 C Cl :.... Cl : :....–

38 proton transfer ROH ROH 2 + carbocation formation R+R+R+R+ carbocation capture RX

39 ROH ROH 2 + carbocation formation R+R+R+R+ carbocation capture RX (CH 3 ) 3 C O O H H H H Cl + + – –

40 proton transfer ROH ROH 2 + R+R+R+R+ carbocation capture RX (CH 3 ) 3 C + + O O H H H H + +

41 proton transfer ROH ROH 2 + carbocation formation R+R+R+R+ RX (CH 3 ) 3 C + + Cl – –

42 The mechanism just described is an example of an S N 1 process. The mechanism just described is an example of an S N 1 process. S N 1 stands for substitution-nucleophilic- unimolecular. S N 1 stands for substitution-nucleophilic- unimolecular. The molecularity of the rate-determining step defines the molecularity of the overall reaction. Mechanistic notation

43 The molecularity of the rate-determining step defines the molecularity of the overall reaction. Mechanistic notation (CH 3 ) 3 C + + O O H H H H + + Rate-determining step is unimolecular dissociation of alkyloxonium ion.

44 4.12 Effect of Alcohol Structure on Reaction Rate

45 slow step is: ROH 2 + R + + H 2 O The more stable the carbocation, the faster it is formed. Tertiary carbocations are more stable than secondary, which are more stable than primary, which are more stable than methyl. Tertiary alcohols react faster than secondary, which react faster than primary, which react faster than methanol.

46 Hammond's Postulate If two succeeding states (such as a transition state and an unstable intermediate) are similar in energy, they are similar in structure. Hammond's postulate permits us to infer the structure of something we can't study (transition state) from something we can study (reactive intermediate).

47 proton transfer ROH ROH 2 + carbocation formation R+R+R+R+ carbocation capture RX

48 proton transfer ROH ROH 2 + carbocation formation R+R+R+R+ carbocation capture RX Rate is governed by energy of this transition state. Infer structure of this transition state from structure of state of closest energy; in this case the nearest state is the carbocation.

49 4.13 Reaction of Primary Alcohols with Hydrogen Halides. The S N 2 Mechanism

50 Preparation of Alkyl Halides (CH 3 ) 3 COH + HCl (CH 3 ) 3 CCl + H 2 O 78-88% + H 2 O 73% CH 3 (CH 2 ) 5 CH 2 OH + HBr CH 3 (CH 2 ) 5 CH 2 Br + H 2 O 87-90% 25°C °C 120°C OH + HBr Br

51 Preparation of Alkyl Halides CH 3 (CH 2 ) 5 CH 2 OH + HBr CH 3 (CH 2 ) 5 CH 2 Br + H 2 O 87-90% 120°C Primary carbocations are too high in energy to allow S N 1 mechanism. Yet, primary alcohols are converted to alkyl halides. Primary alcohols react by a mechanism called S N 2 (substitution-nucleophilic-bimolecular).

52 Two-step mechanism for conversion of alcohols to alkyl halides: (1) proton transfer to alcohol to form alkyloxonium ion (2) bimolecular displacement of water from alkyloxonium ion by halide The S N 2 Mechanism

53 ExampleExample CH 3 (CH 2 ) 5 CH 2 OH + HBr CH 3 (CH 2 ) 5 CH 2 Br + H 2 O 120°C

54 Mechanism Step 1: Proton transfer from HBr to 1-heptanol H.. :HBr : H O : + H Br :.... : – + fast, bimolecular Heptyloxonium ion O CH 3 (CH 2 ) 5 CH 2

55 Mechanism Step 2: Reaction of alkyloxonium ion with bromide ion. ion. slow, bimolecular 1-Bromoheptane Br : :....– Br :.... CH 3 (CH 2 ) 5 CH 2 + H O :H+ + O H :H:

56 CH 2 OH Br – – CH 3 (CH 2 ) 4 CH 2 proton transfer ROH ROH 2 + RX

57 4.14 Other Methods for Converting Alcohols to Alkyl Halides

58 Thionyl chloride SOCl 2 + ROH RCl + HCl + SO 2 Phosphorus tribromide PBr 3 + 3ROH 3RBr + H 3 PO 3 Reagents for ROH to RX

59 CH 3 CH(CH 2 ) 5 CH 3 OH SOCl 2 K 2 CO 3 CH 3 CH(CH 2 ) 5 CH 3 Cl (81%) (pyridine often used instead of K 2 CO 3 ) (CH 3 ) 2 CHCH 2 OH (55-60%) (CH 3 ) 2 CHCH 2 Br PBr 3 ExamplesExamples


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