The objectives of this experiment are to synthesis 1-bromobutane from n-butanol under reflux setup via SN2 mechanism

by desmondseahtm (student)

University Degree Physical Sciences

Singapore Polytechnic

School of Chemical and Life Sciences

Diploma in Applied Chemistry with Pharmaceutical Science

Experiment 4

Preparation of 1-bromobutane

Desmond Seah

1006812

DACP/FT/2A/21

Date of Experiment: 28 January 2012

Date of Submission: January 2012

Introduction

1 Aims

The objectives of this experiment are to synthesis 1-bromobutane from n-butanol under reflux setup via SN2 mechanism, and crude purification is carried out using distillation followed by series of solvent extraction. The yield and density of 1-bromobutane will then be determined.

2 Introduction

The mechanism of this reaction belongs to SN2 – Bimolecular nucleophilic substitution.

Figure 1: Reaction Scheme (ChemDraw)

The starting material is a primary monohydric alcohol (n-butanol) and can be prepared be the reactions below:

Figure 2: Preparation of Alcohol (Mehta, 2006)

Based on the figure 2, the addition of water-hydrolysis of but-1-ene will not be able to give us a primary butanol instead it will give us a secondary butanol. It is because reaction is an electrophilic addition reaction and follows Markovnikov’s rule. Therefore, we have to use oxymercuration-demecuration process in alkene or hydroboration oxidation in but-1ene to prepare the primary alcohol.

In a bimolecular mechanism, the attack of nucleophile (Br-) on sp3 hybridized carbon of protonated n-butanol and removal of water (H2O+), (the leaving group) occurs simultaneously in a single step. Thus, nucleophile (Br-) has no other option but to attack the carbon from a side opposite to water (leaving group).

The reaction proceeds through a transition state, which has partial attachment of nucleophile (Br-) and partial detachment of leaving group-water.

The SN2 mechanism is stereospecific in nature and always leads to the formation of substituted product with a configuration opposite to that of the initial “haloalkane” (in this case, n-butanol). In other words, SN2 mechanism involves the inversion of configuration (also known as Walden Inversion). This is illustrated by the diagram below:

Figure 3: Inversion process (ChemDraw)

The order of reactivity of alkyl halide is CH3X>1o>2o>3o.

Since the substrate in this reaction is n-butanol, a primary substrate, it will favor SN2 reaction instead of SN1 reaction.

Reflux

Just about 80% of the reactions in organic lab involve a step called refluxing. You use a reaction solvent to keep materials dissolved and at a constant temperature by boiling the solvent, condensing it, and returning it to the flask.

To execute a reflux,

1. Place the reagents in a round-bottomed flask. The flask should be large enough to hold both the reagents and enough solvent to dissolve them, without being much more than half full.

2. You should now choose a solvent that

a. Dissolves the reactants at the boiling temperature.

b. Does not react with the reagents.

c. Boils at a temperature that is high enough to cause the desired reaction to go at a rapid pace.

3. Dissolve the reactants in the solvent. Sometimes the solvent itself is a reactant. Then don't worry.

4. Place a condenser, upright, on the flask, connect the condenser to the water faucet, and run water through the condenser (Fig. below). Remember —in at the bottom and out at the top.

5. Put a suitable heat source under the flask and adjust the heat so that the solvent condenses no higher than halfway up the condenser. You'll have to stick around and watch for a while, since this may take some time to get started. Once the reaction is stable, though, go do something else. You'll be ahead of the game for the rest of the lab.

6. Once this is going well, leave it alone until the reaction time is up. If it's an overnight reflux, wire the water hoses on so they don't blow off when you're not there.

Figure 4: Reflux Setup

7. When the reaction time is up, turn off the heat, let the setup cool, dismantle it, and collect and purify the product.

In this experiment

We have chosen water as a solvent (although it is polar protic solvent does not favor SN2) because it is able to dissolve our starting materials effectively and able to reflux at 100oC for 30 mins (sufficient for this reaction) and this condition is sufficient to break all the necessary strong covalent bonds in this reaction.

Distillation

For separation of liquid boiling below 150oC at 1 atm from

1. Nonvolatile impurities

2. Another liquid boiling 25oC higher than the first liquid. They must dissolve in each other

In any distillation the flask should not be more than two-thirds full at the start. Great care should be taken not to distill to dryness because, in some cases, high boiling explosive peroxides can become concentrated.

Figure 5: Simple Distillation

Assemble the apparatus for simple distillation in the figure above starting with the support ring, followed by the electric flask heater and then the flask. One or two boiling stones are put in the flask o promote ...