Synthesis of Alkenes 1: Miniscale Synthesis of Synthesis of Alkenes 1: Miniscale Synthesis of Alkenes Via the Acid-catalyzed Dehydration of Alkenes Via the Acid-catalyzed Dehydration of

3,3-dimethyl-2-butanol3,3-dimethyl-2-butanol

Organic Chemistry Lab II, Spring 2009Dr. Milkevitch

March 1 & 3, 2010

C C

Elimination ReactionsElimination Reactions

TodayToday: Conduct an elimination reaction: Conduct an elimination reaction– One of the fundamental reactions of organic One of the fundamental reactions of organic

compoundscompounds

What you will learn/observe:What you will learn/observe:Review elimination reactionsReview elimination reactions

Learn how to synthesize an alkeneLearn how to synthesize an alkene– Acid catalyzed dehydration of an alcoholAcid catalyzed dehydration of an alcohol

Look at the products formedLook at the products formed

See which product will predominateSee which product will predominate

Elimination ReactionsElimination Reactions

Let’s Review:Let’s Review:– Elimination reaction:Elimination reaction: a fundamental organic reaction a fundamental organic reaction

Two species are eliminated from a substrateTwo species are eliminated from a substrate

Elimination mean’s they are gone, gone, goneElimination mean’s they are gone, gone, gone– NOT a substitutionNOT a substitution

Basic mechanism:Basic mechanism:

Another Kind of Elimination Reaction

A dehydration reactionA dehydration reaction– Multiple step reactionMultiple step reaction– Loss of water, forming a carbocationLoss of water, forming a carbocation– Alkene formation results Alkene formation results

Mechanism: E1 mechanismMechanism: E1 mechanism– E1 elimination of a protonated alcoholE1 elimination of a protonated alcohol

Reaction of alcoholsReaction of alcohols– Acid catalyzedAcid catalyzed– Forms an alkene and waterForms an alkene and water

General Mechanism: Acid-Catalyzed General Mechanism: Acid-Catalyzed Dehydration of an AlcoholDehydration of an Alcohol

Step 1: Protonation of hydroxyl group

Step 2: Ionization to carbocation

Step 3: Deprotonation to give the alkene

Mechanism for This ReactionMechanism for This ReactionStep 1: Protonation of the alcohol

Step 2: Formation of the carbocation

Rearrangement

Hydrogens available for elimination

Products Products

Products: Don’t Forget Products: Don’t Forget

Formed from secondary carbocation

Which Product Prevails?Which Product Prevails?

•More substituted alkene predominates

•Called Zaitsev’s Rule

ProcedureProcedure

Construct your simple distillation apparatusConstruct your simple distillation apparatusUse a 25 ml RB flaskUse a 25 ml RB flaskPlace 6 ml of 3,3-dimethyl-2-butanol into this flaskPlace 6 ml of 3,3-dimethyl-2-butanol into this flask– Include a magnetic stir barInclude a magnetic stir bar

Add 6 ml of 85% phosphoric acid to this flaskAdd 6 ml of 85% phosphoric acid to this flaskPlace on apparatusPlace on apparatusStart stirring and heating Start stirring and heating Collect distillate in a graduated cylinderCollect distillate in a graduated cylinder– In an ice bathIn an ice bath

Distillate should be cloudyDistillate should be cloudyDon’t let the temperature rise above 75 deg C!!Don’t let the temperature rise above 75 deg C!!

Procedure IIProcedure II

Complete distillationComplete distillationTurn off heat, let apparatus coolTurn off heat, let apparatus coolDisassemble distillation apparatusDisassemble distillation apparatus– Clean with acetoneClean with acetone– Set aside to drySet aside to dry

Transfer distillate to a small separatory funnelTransfer distillate to a small separatory funnel– Drain lower aqueous layer into a beakerDrain lower aqueous layer into a beaker

Wash organic layer with 20 ml of saturated salt solnWash organic layer with 20 ml of saturated salt solnDrain off lower aqueous layerDrain off lower aqueous layerPour off organic layer into a 25 ml erlenmeyer flaskPour off organic layer into a 25 ml erlenmeyer flaskDry with anhydrous magnesium sulfateDry with anhydrous magnesium sulfate

Procedure IIIProcedure III

Gravity filter into a clean, dry 10 ml RB flaskGravity filter into a clean, dry 10 ml RB flaskReassemble distillation apparatusReassemble distillation apparatusDistill again Distill again – Cool receiving flask in an ice bathCool receiving flask in an ice bath

Weigh a small vial, record weight of this flaskWeigh a small vial, record weight of this flaskWhen distillation is done, transfer distillate to this When distillation is done, transfer distillate to this vialvialReweigh the distillate in the vialReweigh the distillate in the vialDo a GC analysisDo a GC analysis– Determine the % of your alkenesDetermine the % of your alkenes– Determine the % of your starting material (3,3-dimethyl-2-Determine the % of your starting material (3,3-dimethyl-2-

butanol)butanol)

Accomplish a IR spectrumAccomplish a IR spectrum

Your ReportYour ReportFormal Report RequiredFormal Report RequiredMake sure to include the mechanism of the reaction Make sure to include the mechanism of the reaction in your report (separate section)in your report (separate section)Results Section:Results Section:– Make sure to Make sure to statestate all results all results

Prove that you made the alkenesProve that you made the alkenes

– Include GC chromatogram (annotated)Include GC chromatogram (annotated)– Determine % of your alkenes Determine % of your alkenes – IR spectrum, annotated (if completed)IR spectrum, annotated (if completed)

Conclusions section:Conclusions section:– Was the experiment successful?Was the experiment successful?

How much of the alkenes are produced?How much of the alkenes are produced?Any leftover 3,3-dimethyl-2-butanol?Any leftover 3,3-dimethyl-2-butanol?

– If unsuccessful, any ideas why?If unsuccessful, any ideas why?

Miniscale Apparatus for DistillationMiniscale Apparatus for Distillation