The Preparation of 1- Bromobutane

The Preparation of 1- Bromobutane

Introduction...

Aim : To prepare 1-Bromobutane.

Background : The most common way of preparing alkyl halides, which are

very useful intermediates in syntheses, is the replacement of the OH

group of an alcohol by a halogen.

This replacement is a nucleophilic substitution reaction, and alcohols

do not undergo nucleophilic substitution reactions because hydroxide

ison is strongly basic and a poor leaving group. However, alcohols

readily undergo nucleophilic substitutions if the hydroxyl group is

first activated to produce a better leaving group, so reaction is

carried out in the presence of a strong acid. The acid protonates the

alcohol to create a suitable leaving group, water, for the SN2

reaction.

[IMAGE]

Designing...

In this experiment 1-butanol will be converted to 1-bromobutane by an

SN2 reaction.

CH3CH2CH2CH2OH (aq) + HBr (aq) -----> CH3CH2CH2CH2Br (aq) + H2O (l)

CH3CH2CH2CH2-OH + HBr [IMAGE] CH3CH2CH2CH2-OH2 (+) Br(-) [IMAGE] CH3CH2CH2CH2-Br

+ H2O SN2

Reaction is nucleophilic substitution, and the products are impure,

and so various stages of purification are required before a sample of

reasonable purity can be obtained.

I'm planning to produce 20 g of 1-Bromobutane as a result of this

experiment.

Amount of Butan-1-ol needed:

C4H9OH (aq) + HBr (aq) -----> C4H9Br (aq) + H2O (l)

74 : 70 : 137 : 18 ß Relative Molecular Mass.

Proportion is 74 to 137 at 100% yield, so 137 g of Bromobutane will be

formed from 74 g of Butan-1-ol.

I will produce 20 g of 1-Bromobutane, my planned yield is 70%.

So;

At 70% yield : (137/100) x 70 = 95.9 g (1 mole)

I want 20 g of C4H9Br so;

( 20 x 74 ) / 95.9 = 15.5 g , so :

Production of 1-Bromobutane will require 15.5 g of Butan-1-ol to

obtain the full amount at 70% yield.

Chemicals Needed:

* Butan-1-ol, 7.72 g. (C4H9OH)

* Sodium Bromide (powdered). (NaBr)

* Concentrated Sulphuric Acid. (H2SO4)

* Concentrated Hydrochloric Acid. (HCl)

* Dilute Sodium Carbonate. (Na2CO3)

* Calcium Chloride. (CaCl2)

* Ethanol. (C2H5OH)

* Dilute Nitric Acid. (HNO3)

* Dilute Sodium Hydroxide. (NaOH)

* 0.1 M Silver Nitrate Solution. (Approx. 2.5 g in 250 cm3)

Apparatus Needed:

* Round flask, 250 cm3

* Tap funnel to fit still head

* Ice-water bath

* Reflux condenser

* Thermometer, 0-110oC

* Still head

* Conical flask, cm3

* Test tubes

* Beaker, 250 cm3

* Filter

* Teat pipette

* Holder

Mechanism...

Mechanism for this synthesis is a nucleophilic substitution. Here are

steps:

1. Since alcohols do not undergo a nucleophilic substitution without

the presence of a strong acid, these are usually called as acid

catalised reactions, because acid acts like a catalyst.

2. Sulphuric Acid attacks to the lone pairs of electrons on the

oxygen.

3. Since Sulphuric Acid acted as a proton donor, Hydrogen joins onto

the Oxygen. Water is released (condensation). As a result Carbon

will have a positive charge, now turned into a Carbocation.

4. Sodium Bromide will dissolve in water, which will act as a

solvent. As a result, aqueous solution of NaBr will be present, in

which, NaBr will exist as Na+ and Br -. The Bromide ion, having a

negative charge, will be attracted to Carbocation, bond will form

between, and 1-Bromobutane is formed.

Risk Assessment

Many chemicals used in the experiment pose a hazard so I will need to

be very careful during the experiment. Therefore, I will be carrying

out a risk assessment using the hazcards to find out which chemicals I