Preparation of 2-chloro-2-methylbutane

.. An SN1 reaction

Substitution Reactions

Two mechanisms possible –

1. SN1 – unimolecular transition state – formation of carbocation intermediate– Preferred for tertiary carbons and polar

solvents.

2.SN2 – bimolecular transition state – no intermediate formed– Preferred by primary carbons and less polar

solvents.

1. Tertiary alcohol and polar solvent2. Acid catalyzed – creates good leaving group (i.e. water)3. Tertiary carbocation intermediate does not rearrange –

accepts the good nucleophile chloride anion

CH3CH2C-OH

CH3

CH3

CH3CH2C

CH3

CH3

OH2

+

CH3

CH3

CH3CH2C +

+ HCl

- H2O

+ Cl -

CH3CH2C-Cl

CH3

CH3

and Cl -

- HCl

+ H2O

- Cl -

Reaction Mechanism

Competition? • SN1 favored over SN2, if…

• Sterics effects : Tertiary (3°) carbon in C-LvG bond (bulky alcohol)• Electronic effects: More alkyl groups on the carbon atom of C-LvG bond to stabilize carbocation formed.• Polar solvent is used.

• Principle side reaction for SN1 in a polar solvent is E1

• SN1 favored over E1, if…• Nucleophile is weakly basic and highly polarizable (in other words, won’t “want” to pull of a hydrogen/proton as much, and will instead favor nucleophilic attack on the carbocation).• Nucleophile is not very bulky.

• Under the reaction conditions used today, however, ELIMINATION IS REVERSIBLE!

• Excess of HCl• Chloride (Cl-) is a good nucleophile (weakly basic, highly polarizable)

Procedure – pg.475-476

1. Mix 10 mL of 2-methyl-2-butanol and 25 mL of conc. HCl in a separatory funnel. Swirl to mix without the top. Note whether there are 2 layers or not.

2. Put the top on and shake for 1 minute – then vent. (expect a good “burp”)

3. Continue shaking and venting for about 5 minutes.4. Allow the layers to separate & determine which is the organic layer. 5. Wash to remove impurities –

1st wash with 10 mL aq. NaCl2nd wash with 10 mL cold aq NaHCO3 (expect gas!!)3rd wash with 10 mL water4th wash with 10 mL aq. NaCl

6. Remove organic layer and dry over small amount anhydrous sodium sulfate.

7. Carefully separate the product from the sodium sulfate, bottle, determine %-yield and submit. (no simple distillation!)

8. If time permits, conduct Silver Nitrate classification test (procedure for which is on middle of pg. 870 in book).

How do we know that a chemical change has occurred?

• Both the reagent and the product are clear, colorless liquids.

• The alcohol (starting material) is soluble in the aqueous HCl ... Is the alkyl chloride (product)?

Silver Nitrate Classification Test (see pg. 870 in book):Tertiary alkyl halides react with ethanolic AgNO3 to produce a precipitate of AgCl.

RCl + AgNO3 = AgCl(s) + RONO2

curdly white precipitate

Is the intended product a tertiary alcohol?

To prepare your NB…• Table of 9 Chemicals: water, concentrated HCl, 2-chloro-2-

methylbutane, 2-methyl-2-butanol, saturated sodium bicarbonate solution, saturated sodium chloride solution, anhydrous sodium sulfate, silver nitrate, ethanol

• 2 Chemical Reactions:

• Figures: Just reference pgs. in your NB from Extraction experiment to illustrate how to use a Separatory Funnel.

+ H2O

+ AgNO3 + AgCl

ONO2

ethanol

Calculations

• Theoretical yield of product – requires balanced equation. - What is the limiting reagent? (Used excess HCl, so…?) - Use unit analysis (calc. shown below) to find Theoretical Yield of alkyl chloride (RCl):

mL ROH * g ROH * mole ROH * mole RCl * g RCl

mL ROH g ROH mole ROH mole RCl

= Theoretical yield of RCl in grams!!!

• Percent Yield = grams RCl obtained × 100 theo. yield of RCl