CHEM245.Lecture 9 Bromobutane 2014

5/26/2018 CHEM245.Lecture 9 Bromobutane 2014

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http://en.wikipedia.org/wiki/SN1_reaction

http://ull.chemistry.uakron.edu/organic_lab/butyl/index.html

http://www.cem.msu.edu/~reusch/VirtualText/intro1b.htm

http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/alhalrx2.htm#hal4

http://www.colby.edu/chemistry/OChem/DEMOS/Substitution.html

BromobutaneNucleophilic Substitutions:Preparation of Alkyl Halides

from Alcohols

CH3-CH2-CH2-CH2-OH

CH3-CH2-CH2-CH2-Br
http://en.wikipedia.org/wiki/SN1_reactionhttp://ull.chemistry.uakron.edu/organic_lab/butyl/index.htmlhttp://www.cem.msu.edu/~reusch/VirtualText/intro1b.htmhttp://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/alhalrx2.htmhttp://www.colby.edu/chemistry/OChem/DEMOS/Substitution.htmlhttp://www.colby.edu/chemistry/OChem/DEMOS/Substitution.htmlhttp://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/alhalrx2.htmhttp://www.cem.msu.edu/~reusch/VirtualText/intro1b.htmhttp://ull.chemistry.uakron.edu/organic_lab/butyl/index.htmlhttp://en.wikipedia.org/wiki/SN1_reaction


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NucleophilicSubstitution Reactions: SN1and SN2

The replacementof an atom/group (leaving group) in a

molecule by another atom/group (nucleophile).

Extremely important in organic synthesis!


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An electron deficient atom, ion or molecule that has anaffinity for an electron pair, and will bond to a base or

nucleophile.

An atom, ion or molecule with an electron pair that may be

donated to an electrophile (or Lewis acid) to form a

covalent bond.

Electrophile

Nucleophile


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Variable

R= 1 , 2 or 3; the a-carbon could be a chiral center, (R) or (S)

-X: Cl < Br < I (F is relatively unreactive)

Nu: anion vs. neutral; change basicity; change polarizability

Solvent: polar vs. non-polar; protic vs. non-protic

Mechanism: SN1 vs. SN2

Results

Products: substitution, elimination, no reaction.

Stereospecificity: for a chiral a-carbon,what happens to its configuration?

Reaction Rate: is a function of reactant concentration


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CH3Br + Cl ---> CH3Cl + Br

http://www.youtube.com/watch?v=ZtnAR3uOAbo

SN1

http://www.youtube.com/watch?v=YGSiUZ443xY&fea

ture=related
http://www.youtube.com/watch?v=ZtnAR3uOAbohttp://www.youtube.com/watch?v=YGSiUZ443xY&feature=relatedhttp://www.youtube.com/watch?v=YGSiUZ443xY&feature=relatedhttp://www.youtube.com/watch?v=YGSiUZ443xY&feature=relatedhttp://www.youtube.com/watch?v=YGSiUZ443xY&feature=relatedhttp://www.youtube.com/watch?v=YGSiUZ443xY&feature=relatedhttp://www.youtube.com/watch?v=YGSiUZ443xY&feature=relatedhttp://www.youtube.com/watch?v=YGSiUZ443xY&feature=relatedhttp://www.youtube.com/watch?v=YGSiUZ443xY&feature=relatedhttp://www.youtube.com/watch?v=YGSiUZ443xY&feature=relatedhttp://www.youtube.com/watch?v=YGSiUZ443xY&feature=relatedhttp://www.youtube.com/watch?v=ZtnAR3uOAbo


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The C-Cl bond in the halogenoalkane starts to lengthen and weaken

The HO-ion could attack from the left-hand side, leading to inversionof configuration

of the carbon skeleton (flipped umbrella); or attack could come from the right, leadingto retentionof configuration.

The use of a chiralhalogenoalkane in this reaction leads to production of a racemic

mixture if the amount of attack from each side is equal. However, hindrance caused by

the departing Cl-

gives about 40% attack from the right and 60% from the left.What favors SN1?How about the nature of the solvent?

SN1


7/38http://www.youtube.com/watch?v=yJlSTWED8Iw
http://www.youtube.com/watch?v=yJlSTWED8Iwhttp://www.youtube.com/watch?v=yJlSTWED8Iw


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SN2

The HO-anion approaches the chloromethane along the path of lowest

energy, colinear with the C-Cl axis

As the C-O bond starts to form, the C-Cl bond starts to break

The transition state has partial bonds between both C-O and the C-Cl

C-O bond shortens/strengthens as the C-Cl lengthens and weakens

In the final product the configuration of the carbon atoms has inverted;

use of a chiral halogenoalkane leads to an inversion of configuration,

called the Walden Inversion.What is the solvent effect?


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SN2


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SN2Mechanism: Nucleophilic attack of HO-on CH3I

http://user.mc.net/~buckeroo/ANIM.html


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SN1

racemization

2 steps, but unimolecular in

the rate determiningstep

Rate = kSN1[R3C-X]


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SN2: Inversion of configuration, one step,

synchronous, bimolecular

Rate = kSN2[R2CH-X][Nu-]


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why a

polar

solvent?

why no rxn?


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SN reaction may proceed in competition with eliminations:

E2(eliminationto form a double bond)


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In the lab: Mr. Butanol

CH3-CH2-CH2-CH2-OH


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HBr is produced in-situ from NaBr and H2SO4

CH3CH2CH2CH2OH + HBr ------> CH3CH2CH2CH2Br + H2O

This is a nucleophilic substitution reaction.

Since the products are impure, purification is required.

Chemistry

Start with 1-hydroxybutane and replace the -OH group

with a -Br using a substitution reaction.


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Bimolecular Nucleophilic Substitution SN2

What does each reagent do?

What is the stoichiometry, how are the amounts of

reagents calculated?

How is the yield calculated?


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Mechanism: protonation of the OH makes it a better

leaving group (H2O), more easily displaced by Br-.

Possible Reactions


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(1) HBR generation:

NaBr + H2SO4 HBr + NaHSO4

(2) Sulfuric Acid oxidizes HBr to Br2which dissociates into Br+and Br -

2HBr + H2SO4 Br2+ SO2+ 2H2O

(3) H2SO4protonates the alcohol. The oxonium ion is a better leaving group than OH. The

bromide ion nucleophilically displaces H2O.

CH3CH2CH2CH2OH + H2SO4 CH3CH2CH2CH2-OH2+/HSO4

-

(4) Sulfuric acid also catalyses the elimination(E2) of H2O a molecule of water intra-or inter-

molecularly to form a mixture of dibutyl ether and 1-butene.

CH3CH2CH=CH2+ H2O + H2SO4(intra)

or

CH3(CH2)3-O-(CH2)3CH3+ H2O + H2SO4(inter)

Possible Reactions


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OH

H2SO4

OH2

+

O

-H2O

Br-

Br

1-bromobutane

1-butene

dibutylether

Possible reactions

substitution

intramolecular

elimination

intermolecular

elimination

How do you separate them?

bp = -6 C

bp = 141 C

bp = 101.6 C

bp = 118 oC

Rfs?


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Reaction (reflux 45 min)

Procedure

What is the purpose of each step?

Distill at 115 CWash the distillate with water, NaHSO3or Na2S2O3

Wash with sulfuric acid

Wash with NaOH

Dry the organic layer with anhydrous MgSO4or CaCl2

Distill @99-103 C.

How do we calculate the yield?


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Preparat ion o f 1-B romobu tane


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Synthesis of 1-Bromobutane from 1-Butanol

Things to consider for the lab(macroscale, 16.2 )

1.Write the mechanism for the reaction performed in thisexperiment. Point out the rate-determining step if the reaction is

multi-stepped.

2.What are the roles of H2SO4in this reaction?

3.What is the purpose of using a 10% sodium bicarbonate solution

described in the procedure?

4.Why is it that the sodium bicarbonate solution is used after

washing the organic phase with 10 mL of water? What if one used

the sodium bicarbonate solution before washing the organic phase

with water?


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From the book procedure

13.3 g NaBr

10 mL butyl alcohol ( = 0.81)11.5 mL sulfuric acid ( at 100% strength = 1.8305 g/mLsay 1.22 at 30%)

Determine the nr of moles of each reagent Verify the reaction stoichiometry (is anything used in

excess and why?)

Calculate the yield as = moles bromobutane/moles

butanol.


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Carrying out the Reaction

Pour Butan-1-ol in a flask. Record the mass of thealcohol transferred. To that alcohol, add NaBr and water.

Add conc. Sulfuric Acid (30%) to the tap funnel

over a period of about 5 minutes, under cooling with acold water bath. The apparatus must also be swirled to

prevent the acid from reacting too fast, (exothermic

reaction) and splashing the acid.

The increased temperature may vaporize faster the

Butan-1-ol, and decrease the yield.

Side reactions ma happen at higher temperat res The side


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Side reactions may happen at higher temperatures. The side

arm in the apparatus (microscale) is needed for safety reasons, the

air which the acid displaces can escape through that arm.

The apparatus is heated under reflux using a condenser with

a guard tube, after having removed the funnel, for approximately

45 minutes. Add boiling chips. Reflux to complete the rxn, but

without loosing reactants through evaporation. The guard tube is

necessary to absorb any acid fumes or SO2gas.

As the reaction progresses, the liquid becomes less opaque

and a trace of yellow is observed. The reactants separate into two

layers because there are in fact several reactions taking place at the

same time and you have an organic and a water phase.


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Separate the Products

Due to the reactions mentioned earlier, there are several substances in

the reaction mixture. To get 1-Bromobutane, the organic layer is

sucked out carefully using a pipette.

Purify the Product

HCl or H2SO4 (30%) is added to the organic layer in a funnel. The

mixture is shaken vigorously, and the top is taken off from time to timeto release the pressure. After the mixture has completely reacted (no

more gas evolves), the organic layer is isolated and again placed in a

funnel. 5% NaHCO3 is added to the organic solution, the solution is

shaken, and the top is taken off from time to time to relieve pressure.

The mixture is left standing for 5 minutes while the layers separate;

the lower organic layer is carefully collected into a test tube.

Anhydrous Na2SO4is added to the test tube in small scoops to remove

the traces of water and the test tube is shaken after each addition.

When the liquid is totally clear it is ready for distillation.


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Add NaBr and water to the

butanol.

With a clean syringe/pipet

add concentrated sulfuric

acid.

The viscous acid moves

slowly through the pipette;

allow sufficient time for

complete transfer.


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Add a boiling chip, attach a condenser to the

flask, and reflux the mixture vigorously for 45

minutes.


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Add Sodium Sulfate to the distillate, and swirl the mixture. With a

syringe/pipet transfer the bromobutane (which layer is it?) to a 4-inch

test tube.


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Replace the condenser with a small-scale distillation setup, and distil

the bromobutane and water at a temperature below 110 into a 10-mL

flask, cooled in an ice-water bath.


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Using a clean syringe/pipette, add conc. sulfuric acid, and

mix the two layers thoroughly. Set the test tube in an ice-

water bath and allow the layers to separate.


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Draw off thesulfuric acid

(bottom layer),

and discard it

into a beaker

half- filled with

ice.


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Transfer the

bromobutane to a

clean10-ml flask. Use

clean adapters to setup the distillation

apparatus as before.


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Testing the purity of the product

Fractional distillation of the organic liquid is carried

out, and the fraction between 100 C and 104 C is collected

into a weighed tube. The % yield is calculated.

The reason why the % yield is not 100% is that there

are many side reactions, and a lot of reagent is lost as other

products. The steps and technique of purification will also

allow some products to escape.


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To ReadBromobutane:

4thCh 16: p 259 266; 5thCh. 16 p 325-332, 6th 311-317

SN1,2

4thCh 17: p 267-271; Ch. 17 p 333-340, 6th 318-325

Homework4thCh. 16: 2,4,7.; Ch. 17:2

5th, 6thCh. 16: 2,4,7; Ch. 17:2

Next Time:

Aspirin,

4thCh 41, p 501-506.

5thEsters: Ch 40, p 554-568, Aspirin: Ch 41, p 569-574

6

th

h 40 41

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CHEM245.Lecture 9 Bromobutane 2014

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