4 8 preparation of alkyl halides from alcohols and hydrogen halides n.
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4.8 Preparation of Alkyl Halides from Alcohols and Hydrogen Halides. ROH + HX  RX + H 2 O. Reaction of Alcohols with Hydrogen Halides. R OH + H X  RX + H OH. Hydrogen halide reactivity HI HBr HCl HF most reactive least reactive.

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slide2

Reaction of Alcohols with Hydrogen Halides

ROH + HX  RX + HOH

Hydrogen halide reactivity HI HBr HCl HFmost reactiveleast reactive

slide3

Reaction of Alcohols with Hydrogen Halides

ROH + HX  RX + HOH

Alcohol reactivityR3COH R2CHOH RCH2OH CH3OHTertiary Secondary Primary Methanolmost reactiveleast reactive

slide4

OH + HBr

Br

Preparation of Alkyl Halides

25°C

(CH3)3CCl + H2O

78-88%

(CH3)3COH + HCl

80-100°C

+ H2O

73%

120°C

CH3(CH2)5CH2OH + HBr

CH3(CH2)5CH2Br + H2O

87-90%

slide5

Preparation of Alkyl Halides

A mixture of sodium bromide and sulfuric acid may be used in place of HBr.

NaBrH2SO4

CH3CH2CH2CH2Br

CH3CH2CH2CH2OH

heat

70-83%

slide7

+

R

R

C

R

Carbocation

The key intermediate in reaction of secondary and tertiary alcohols with hydrogen halides is a carbocation.

A carbocation is a cation in which carbon has6 valence electrons and a positive charge.

slide8

+

R

R

C

R

Carbocation

The key intermediate in reaction of secondary and tertiary alcohols with hydrogen halides is a carbocation.

The overall reaction mechanism involves threeelementary steps; the first two steps lead to thecarbocation intermediate, the third step is the conversion of this carbocation to the alkyl halide.

slide9

+

H3C

CH3

C

CH3

Example

25°C

(CH3)3CCl + H2O

(CH3)3COH + HCl

tert-Butyl alcohol

tert-Butyl chloride

Carbocation intermediate is:

tert-Butyl cation

slide10

..

+

:

H

Cl

..

H

..

+

+

:

:

:

O

Cl

(CH3)3C

..

H

Mechanism

Step 1: Proton transfer from HCl to tert-butyl alcohol

..

:

O

(CH3)3C

H

fast, bimolecular

tert-Butyloxonium ion

slide11

H

+

:

O

(CH3)3C

H

H

:

:

O

H

Mechanism

Step 2: Dissociation of tert-butyloxonium ion

slow, unimolecular

+

+

(CH3)3C

tert-Butyl cation

slide12

..

:

:

Cl

..

..

:

Cl

(CH3)3C

..

Mechanism

Step 3: Capture of tert-butyl cation by chloride ion.

+

+

(CH3)3C

fast, bimolecular

tert-Butyl chloride

figure 4 8 structure of methyl cation
Figure 4.8 Structure of methyl cation.

Carbon is sp2 hybridized.All four atoms lie in same plane.

figure 4 8 structure of methyl cation1
Figure 4.8 Structure of methyl cation.

Empty 2p orbital.

Axis of 2p orbital is perpendicular to plane of atoms.

slide16

+

R

R

C

R

Carbocations

Most carbocations are too unstable to beisolated.

When R is an alkyl group, the carbocation isstabilized compared to R = H.

slide17

+

H

H

C

H

Carbocations

Methyl cationleast stable

slide18

Carbocations

+

H

H3C

C

H

Ethyl cation(a primary carbocation) is more stable than CH3+

slide19

Carbocations

+

CH3

H3C

C

H

Isopropyl cation(a secondary carbocation) is more stable than CH3CH2+

slide20

Carbocations

+

CH3

H3C

C

CH3

tert-Butyl cation(a tertiary carbocation) is more stable than (CH3)2CH+

slide21

Figure 4.9 Stabilization of carbocationsvia the inductive effect

positively chargedcarbon pullselectrons in  bondscloser to itself

+

slide22

Figure 4.9 Stabilization of carbocationsvia the inductive effect

positive charge is"dispersed ", i.e., sharedby carbon and thethree atoms attachedto it









slide23

Figure 4.9 Stabilization of carbocationsvia the inductive effect

electrons in C—Cbonds are more polarizable than thosein C—H bonds; therefore, alkyl groupsstabilize carbocationsbetter than H.

Electronic effects transmitted through bonds are called "inductive effects."









slide24

Figure 4.10 Stabilization of carbocationsvia hyperconjugation

electrons in this bond can be sharedby positively chargedcarbon because thes orbital can overlap with the empty 2porbital of positivelycharged carbon

+

slide25

Figure 4.10 Stabilization of carbocationsvia hyperconjugation

electrons in this bond can be sharedby positively chargedcarbon because thes orbital can overlap with the empty 2porbital of positivelycharged carbon





slide26

Figure 4.10 Stabilization of carbocationsvia hyperconjugation

Notice that an occupiedorbital of this type isavailable when sp3hybridized carbon is attached to C+, but is not availabe when His attached to C+. Therefore,alkyl groupsstabilize carbocationsbetter than H does.





slide27

+

R

R

C

R

Carbocations

The more stable a carbocation is, the faster it isformed.

Reactions involving tertiary carbocations occurat faster rates than those proceeding via secondarycarbocations. Reactions involving primary carbocations or CH3+ are rare.

slide28

+

R

R

C

R

Carbocations

Carbocations are Lewis acids (electron-pairacceptors).

Carbocations are electrophiles (electron-seekers).

Lewis bases (electron-pair donors) exhibit just theopposite behavior. Lewis bases are nucleophiles(nucleus-seekers).

slide29

..

:

:

Cl

..

..

:

Cl

(CH3)3C

..

Mechanism

Step 3: Capture of tert-butyl cation by chloride ion.

+

+

(CH3)3C

fast, bimolecular

tert-Butyl chloride

slide30

..

..

:

Cl

:

:

Cl

(CH3)3C

..

..

Carbocations

+

+

(CH3)3C

The last step in the mechanism of the reaction oftert-butyl alcohol with hydrogen chloride is the reaction between an electrophile and a nucleophile.

tert-Butyl cation is the electrophile. Chloride ionis the nucleophile.

fig 4 11 combination of tert butyl cation and chloride ion to give tert butyl chloride

+

Fig. 4.11 Combination of tert-butyl cation andchloride ion to give tert-butyl chloride

nucleophile

(Lewis base)

electrophile

(Lewis acid)

recall

H2O

H

Br

+

H2O—H + Br –

Recall...





the potential energy diagram for proton transfer from HBr to water

Potentialenergy

H2O + H—Br

Reaction coordinate

extension

25°C

(CH3)3CCl + H2O

(CH3)3COH + HCl

Extension

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 oftert-butyl alcohol with HCl.

slide35

..

+

:

H

Cl

..

H

..

+

+

:

:

:

O

Cl

(CH3)3C

..

H

Mechanism

Step 1: Proton transfer from HCl to tert-butyl alcohol

..

:

O

(CH3)3C

H

fast, bimolecular

tert-butyloxonium ion

slide36

H

+

:

O

(CH3)3C

H

H

:

:

O

H

Mechanism

Step 2: Dissociation of tert-butyloxonium ion

slow, unimolecular

+

+

(CH3)3C

tert-Butyl cation

slide37

..

:

:

Cl

..

..

:

Cl

(CH3)3C

..

Mechanism

Step 3: Capture of tert-butyl cation by chloride ion.

+

+

(CH3)3C

fast, bimolecular

tert-Butyl chloride

slide38

+

ROH2

carbocation formation

carbocation capture

R+

proton transfer

ROH

RX

slide39

–

+

O

H

Cl

(CH3)3C

H

+

ROH2

carbocation formation

carbocation capture

R+

ROH

RX

slide40

H

+

+

O

(CH3)3C

H

+

ROH2

carbocation capture

R+

proton transfer

ROH

RX

slide41

+

–

Cl

(CH3)3C

+

ROH2

carbocation formation

R+

proton transfer

ROH

RX

mechanistic notation
Mechanistic notation

The mechanism just described is an example of an SN1 process.

SN1 stands for substitution-nucleophilic-unimolecular.

The molecularity of the rate-determining step defines the molecularity of theoverall reaction.

mechanistic notation1

H

+

+

O

(CH3)3C

H

Mechanistic notation

The molecularity of the rate-determining step defines the molecularity of theoverall reaction.

Rate-determining step is unimoleculardissociation of alkyloxonium ion.

slide45
slow step is: ROH2+  R+ + H2O

The more stable the carbocation, the fasterit is formed.

Tertiary carbocations are more stable thansecondary, 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 fasterthan methanol.

slide46

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).

slide47

+

ROH2

carbocation formation

carbocation capture

R+

proton transfer

ROH

RX

slide48

+

ROH2

carbocation formation

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.

carbocation capture

R+

proton transfer

ROH

RX

slide50

OH + HBr

Br

Preparation of Alkyl Halides

25°C

(CH3)3CCl + H2O

78-88%

(CH3)3COH + HCl

80-100°C

+ H2O

73%

120°C

CH3(CH2)5CH2OH + HBr

CH3(CH2)5CH2Br + H2O

87-90%

slide51

Preparation of Alkyl Halides

Primary carbocations are too high in energy to allow SN1 mechanism. Yet, primary alcohols are converted to alkyl halides.

Primary alcohols react by a mechanism called SN2 (substitution-nucleophilic-bimolecular).

120°C

CH3(CH2)5CH2OH + HBr

CH3(CH2)5CH2Br + H2O

87-90%

the s n 2 mechanism
The SN2 Mechanism

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

slide53

Example

120°C

CH3(CH2)5CH2OH + HBr

CH3(CH2)5CH2Br + H2O

slide54

CH3(CH2)5CH2

O

H

Br

..

:

:

Br

..

Mechanism

Step 1: Proton transfer from HBr to 1-heptanol

..

..

+

:

:

..

H

fast, bimolecular

H

+

+

:

O

CH3(CH2)5CH2

H

Heptyloxonium ion

slide55

H

..

+

:

:

O

:

Br

CH3(CH2)5CH2

..

H

H

..

:

:

:

CH3(CH2)5CH2

Br

O

..

H

Mechanism

Step 2: Reaction of alkyloxonium ion with bromide

ion.

+

slow, bimolecular

+

1-Bromoheptane

slide56

+

–

Br

CH2

OH2

CH3(CH2)4 CH2

proton transfer

ROH

+

ROH2

RX

reagents for roh to rx
Reagents for ROH to RX

Thionyl chloride

SOCl2 + ROH  RCl + HCl + SO2

Phosphorus tribromide

PBr3 + 3ROH  3RBr + H3PO3

examples
Examples

SOCl2

CH3CH(CH2)5CH3

CH3CH(CH2)5CH3

K2CO3

Cl

OH

(81%)

(pyridine often used instead of K2CO3)

PBr3

(CH3)2CHCH2OH

(CH3)2CHCH2Br

(55-60%)