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Experiment 12: BASE PROMOTED ELIMINATION OF HBR FROM AN ALKYL HALIDE + + Base ROH Br

BASE PROMOTED ELIMINATION OF HBR FROM AN ALKYL HALIDE

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Objectives To synthesize an isomeric mixture of alkenes from “E2” base promoted elimination of HBr from 2-bromoheptane under reflux. To purify the products through simple distillation. To study the effect of base size on product distribution using GC analysis. To identify characteristic absorptions in the IR spectra of reactants and products.

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Page 1: BASE PROMOTED ELIMINATION OF HBR FROM AN ALKYL HALIDE

Experiment 12:BASE PROMOTED

ELIMINATION OF HBR FROM AN ALKYL HALIDE

+ +BaseROH

Br

Page 2: BASE PROMOTED ELIMINATION OF HBR FROM AN ALKYL HALIDE

Objectives To synthesize an isomeric mixture of

alkenes from “E2” base promoted elimination of HBr from 2-bromoheptane under reflux.

To purify the products through simple distillation.

To study the effect of base size on product distribution using GC analysis.

To identify characteristic absorptions in the IR spectra of reactants and products.

Page 3: BASE PROMOTED ELIMINATION OF HBR FROM AN ALKYL HALIDE

CHEMICAL EQUATION

+ +BaseROH

2-BromoheptaneMF: C7H15BrMW: 179.1bp: 180 oCd: 1.14 g/mL

MF: C7H14MW: 98.2bp: 94 oCd: 0.697 g/mL

MF: C7H14MW: 98.2bp: 98-99 oCd: 0.708 g/mL

MF: C7H14MW: 98.2bp: 98 oCd: 0.701 g/mL

Br

Dehydrohalogenation of 2-bromoheptane.

Page 4: BASE PROMOTED ELIMINATION OF HBR FROM AN ALKYL HALIDE

MECHANISM

C

Br

+OH

+

a

b

+

a a b

ROHH

HH HH

a a bb

b

2. …C=C bond forms…

1. Base oxygen attacks and removes a proton

from carbon adjacent to bromine…

3. …bromine is eliminated from the opposite side of the molecule.

ALL STEPS OCCUR IN A CONCERTED FASHION!

Page 5: BASE PROMOTED ELIMINATION OF HBR FROM AN ALKYL HALIDE

MECHANISM (MOST SUBSTITUTED ALKENE ISOMER)

C

Br

H H

HHH

Base

+ HBase Br+

• Removal of more hindered proton, leading to more stable, more highly substituted alkene.• MAJOR product for small base, MINOR product for large base!

Page 6: BASE PROMOTED ELIMINATION OF HBR FROM AN ALKYL HALIDE

MECHANISM (LEAST SUBSTITUTED ALKENE ISOMER)

C

Br

H H

HHH

Base

+ HBase Br+

• Removal of least hindered proton, leading to a less substituted alkene product.• MAJOR product for large base, MINOR product for small base.

Page 7: BASE PROMOTED ELIMINATION OF HBR FROM AN ALKYL HALIDE

STERIC HINDRANCE The reason that the

bulkier (larger) base gives more of the less substituted alkene is that steric hindrance prevents it from approaching a hydrogen on a more highly substituted carbon.

Page 8: BASE PROMOTED ELIMINATION OF HBR FROM AN ALKYL HALIDE

OVERVIEW Heat alkyl bromide and assigned base in

solvent under reflux to synthesize products.

Purify products by simple distillation to remove unreacted starting materials.

Prepare GC sample. Analyze and determine product ratio

using GC results.

Page 9: BASE PROMOTED ELIMINATION OF HBR FROM AN ALKYL HALIDE

EXPERIMENTAL PROCEDURE(SYNTHESIS)

water out

water in

heating mantle

iron ring

to voltage regulator

• Add 2-bromoheptane and alcohol solvent to 25 mL flask w/3 boiling chips.• Clamp flask to ring stand.• Add solid base to flask using powder funnel.•Place the condenser on the flask.•Using a thermometer adapter, place a CaSO4 drying tube in the top of the condenser.• Begin water flow, apply heat.• Reflux the solution for 60 minutes.• Cool flask using a beaker of tap water.

CaSO4 drying tube in

thermometer adapter

Page 10: BASE PROMOTED ELIMINATION OF HBR FROM AN ALKYL HALIDE

EXPERIMENTAL PROCEDURE(PURIFICATION)

Heating Mantle

to voltage regulator

water out

water in

iron ring

• Once cooled, arrange a simple distillation apparatus.• Remember to clamp both flasks to ring stand!• Apply water flow and heat.• When the first drop reaches the receiving flask, mark the temperature (Ti).• Collect ~5 mL in the receiving flask.• Read the temperature again (Tf). Remove the heat.

• Prepare a GC sample.

Keck clips here!

Page 11: BASE PROMOTED ELIMINATION OF HBR FROM AN ALKYL HALIDE

Table 12.1

• Remember to get data from the opposite alcohol/base pair!

Compound

GC Retention Times (min) and Adjusted Area Percent

StandardRt

SMALL BASE(methanol/sodium

methoxide)

LARGE BASE(t-butanol/potassium t-

butoxide)

Sample Rt

Area Percen

t

Adjusted Area Percent

Sample Rt

Area Percen

t

Adjusted Area

Percent

methanolt-butanol ---1-heptenetrans-2-heptene

cis-2-heptene

Page 12: BASE PROMOTED ELIMINATION OF HBR FROM AN ALKYL HALIDE

EXPERIMENTAL PROCEDURE

(IR Analysis)

Br

IR spectra on page 103 of lab manual!

Page 13: BASE PROMOTED ELIMINATION OF HBR FROM AN ALKYL HALIDE

Table 12.2

FunctionalGroup

BaseValues(cm-1)

2-bromohept

ane

1-heptene

cis-2-heptene

trans-2-heptene

Frequency (cm-1)

Frequency

(cm-1)

Frequency

(cm-1)

Frequency (cm-1)

C-Br stretch500-700

sp3 CH stretch2850-

3000

sp2 CH stretch3000-

3100

Alkene C=C stretch

1600-1680

Page 14: BASE PROMOTED ELIMINATION OF HBR FROM AN ALKYL HALIDE

SAFETY CONCERNS Sodium methoxide and potassium t-

butoxide are strong bases and corrosive! Use gloves when handling!

Methanol and t-butanol are flammable! Wear safety goggles at all times and use extreme caution when heating!

Page 15: BASE PROMOTED ELIMINATION OF HBR FROM AN ALKYL HALIDE

WASTE MANAGEMENT After adding water to the reaction

flask, pour this and product solution into container labeled “LIQUID ORGANIC WASTE”.

Page 16: BASE PROMOTED ELIMINATION OF HBR FROM AN ALKYL HALIDE

CLEANING… Clean round bottom flasks with

soap/water/wash acetone.

Clean all remaining reflux/distillation glassware with wash acetone ONLY while in your lab hood! DO NOT REMOVE THIS GLASSWARE FROM YOUR HOOD!

Funnel and beaker can be cleaned with soap/water/wash acetone.

Page 17: BASE PROMOTED ELIMINATION OF HBR FROM AN ALKYL HALIDE

LABORATORY NOTEBOOK(Pre-lab)

• OBJECTIVE (Must clearly state…)•What compounds you will make and how• How you will purify the compound• How you will determine the purity of your compound

• CHEMICAL EQUATION Include the general chemical equation from page 99.

•TABLE OF PHYSICAL DATA (Complete the following table using MSDS sheets from a site on WWW Links ONLY. Wikipedia is unacceptable)

• REFERENCE TO PROCEDURE (Must include…)•full title, including edition and authors•page numbers where actual procedure can be found

Compound MW (g/mol)

bp (Co) d (g/mL) HAZARDS

2-bromoheptane 180 1.14 Flammable, irritant

cis-2-heptenetrans-2-heptene1-heptenemethanolt-butanolsodium methoxide XXX XXXpotassium t-butoxide XXX XXX

Page 18: BASE PROMOTED ELIMINATION OF HBR FROM AN ALKYL HALIDE

LABORATORY NOTEBOOK(In-lab)

• DATA/CALCULATIONS • Initial weight of 2-bromoheptane used• Initial weight and identity of base used• Theoretical yield calculation (not just the value!)• Physical state and color of product• GC vial slot # • GC sample solvent identity• Give an example of an adjusted area % calculation

• EXPERIMENTAL PROCEDURE• In paragraph form, briefly describe the procedure that you

actually followed during the lab. • Paragraph must be written in PAST TENSE, PASSIVE VOICE.

• Include ACTUAL volumes or weights of chemicals used during the experiment.• Include any mistakes, accidents, or observations if necessary.