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The Preparation of 1- Bromobutane

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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, 0-110°C

* 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
should be very careful with and make sure that all of the safety
precautions are followed all the time.

1. Sulphuric Acid ( H2SO4 )

When contacted with skin or eyes it can cause serious burns and harm.
Concentrated Sulphuric Acid that I will be using poses a real threat
since solutions with a molarity stronger than 1.5 M are very
corrosive. Therefore, I'll need to take general precautions such as
goggles, a lab coat, wearing gloves all the time during the
experiment. Whenever, any spill occurred it should be wiped strongly
and quickly with a damp cloth.

2. Hydrochloric Acid ( HCl )

Hydrochloric acid also can cause serious harm and burns on skin and
eye if contacted. Also it irritates the respiratory system. Solutions
of HCl with a molarity equal to or more than 6.5 M are corrosive,
those less are only irritants. While I will be using Concentrated
Hydrochloric Acid, I will carry out stages in a fume cupboard to be
safe from any fumes that may be given off.

3. Nitric Acid (HNO3 )

Nitric Acid is a strong oxidising agent, and can cause burns and harm
if contacted with skin. A contact between Nitric Acid and any
combustible compound including organic compounds such as Ethanol, and
Ethanoic Acid may cause a fire. The Nitric Acid solution that I will
be using for the experiment is a 0.5 M solution, which can only be
labelled as an irritant. However, I still have to be careful since I
will use it to test Bromobutane, which is an organic compound.

Risk Assessment (continued)

4. Sodium Hydroxide (NaOH)

When contacted with skin or eyes it can cause serious burns. Solutions
of NaOH with a molarity stronger than 0.5 M are corrosive. If even a
small amount of water is present, an exothermic reaction will take
place and a hot ejection may occur.

5. Hydrogen Bromide (HBr)

Hydrogen Bromide can cause irritation to the lining of the lungs and
serious burns on skin. Therefore, this will be produced in a fume
cupboard, to be safe in case of any escapes.

Implementing…

Method

After searching on the internet for a suitable method of preparing
1-Bromobutane I chose the most suitable one for me. I adjusted it
slightly according to my own goals, such as a 70% yield and totally 10
g of 1-Bromobutane (product) to be produced. Also some areas of the
experiment were made safer.

General Information About Chemicals Used

Chemical

Boiling Point (oC )

Density (g cm -3)

Water

100

1.0

Butan-1ol

117

0.8

1-Bromobutane

101.5

1.27

Sulphuric Acid

100

1.4

All of these chemicals stated above are liquids as it shows clearly
from the boiling points.

Since all needed points are went through now, I can carry out the
experiment.

The Procedure

Preparation:

1. Weigh out 15.5 g of Butan-1-ol on a balance.

2. Weight about 20 g of Sodium Bromide.

3. Measure out 20 cm3 of Water in a measuring cylinder.

4. Place 20 cm3 of water, 20 g of powdered Sodium Bromide and 15.5 g
of butan-1-ol in a pear shaped (round bottomed) flask and shake
well to completely dissolve and homogenise the Sodium Bromide.

5. Fit a tap funnel to the flask via a stillhead.

6. Place 20 cm3 of moderately concentrated sulphuric acid in the tap
funnel.

7. Allow acid to fall drop by drop into the flask, keeping the
contents well shaken and cooled occasionally in an ice-water bath.

8. When the addition is complete, leave to cool and set up the
reflux apparatus (replace the tap funnel with a reflux water
condenser).

9. Gently boil the mixture over a sand-bath for about 45 minutes,
shaking the flask gently from time to time.

10. Leave to cool down.

Purification:

1. Once the solution has cooled, remove the reflux condenser and
rearrange the apparatus for distillation.

2. Distil off the crude 1-Bromobutane (about 20 cm3)

3. Shake the distillate with water in a separating funnel, and run
off the lower layer of 1-bromobutane; reject the aqueous layer.

4. Return the 1-Bromobutane to the funnel, add about half its volume
(10 cm3) of Concentrated hydrochloric acid, and shake well to
react any unreacted Butanol.

5. Run off and discard the lower layer of acid into a measuring
cylinder.

6. Clean the Bromobutane by adding a base to neutralise the acid by
shaking the 1-Bromobutane cautiously with dilute sodium carbonate
solution (added about 10 cm3), cautiously relasing the pressure at
intervals.

7. Run off the lower layer of 1-Bromobutane and add some granular
anhydrous Calcium Chloride. Swirl the mixture until the liquid is
clear.

8. Filter the 1-Bromobutane into a clean dry flask, and distil it,
collecting the fraction boiling between 99-102 oC.

9. Weigh how much Bromobutane is made

10. Test product with a series of small tests in order to make sure
what I made was Bromobutane.

11. Work out percentage yield.

Notes About Preparation

* Sulphuric Acid when diluted with water gives out a great deal of
heat, enough sometimes to raise steam which would cause dangerous
splashing. So it's added slowly, and we cooled it to prevent any
harm in an ice-cold water bath.

* At step 9, a sand-bath is used for heating. Because, the sand
spreads the heating uniformly over the base of the flask. This
reduces the likelihood of cracking, and of unwanted side reactions
occurring owing to hot-spots.

Notes About Purification

· At step 2 of the purification, the mixture is distilled to remove
the liquid 1-bromobutane from the involatile sodium salts (mostly
sodium hydrogen sulphate at the end of the reaction) and the much less
volite sulphuric acid. The 1-bromobutane will be contaminated with
water, unchanged butan-1-ol, and some sulphuric acid carried over as
tiny droplets during the distillation.

· At step 3, it's shaken with water to remove sulphuric acid and some
of the butan-1-ol. According to the density of 1-Bromobutane, which is
1.27 g cm-3, the layers are separated.

· Later Concentrated Hydrochloric acid is added to protonate the
butan-1-ol, giving an ionic species that is much more soluble in water
than the alcohol itself.

· Mixture is shaken with Sodium Carbonate solution to remove
hydrochloric acid dissolved in 1-bromobutane.

[ Na2CO3 + 2HCl → 2NaCl + CO2 + H2O ]

· Calcium Chloride is a drying agent.

· 1-Bromobutane has a boiling temperature of 101.5 oC, so the range is
narrow enough to ensure that this is the distillate.

Results

Mass of clean beaker= 55.73 g

Mass of beaker and Bromobutane = 74.34 g

Mass of Bromobutane = 18.61 g

I have made 18.61 g of 1-Bromobutane from 15.5 g of Butan-1-ol and
Hydrogen Bromide which was produced "on-site".

Testing the Product…

To prove that what I produced was 1-Bromobutane I carried out a series
of tests. I compared the results of the tests with results achieved
when test is carried out with the pure bottled 1-Bromobutane.

Testing My Product

Test 1

- Add;

Silver Nitrate, 2 cm3

Ethanol, 2 cm

1-Bromobutane, a few drops (my product)

- Observation: Solution changed colour to white with a brown
precipitate formed at the bottom.

Test 2

- Add;

Nitric Acid, 2 cm3

Sodium Hydroxide, 2 cm3

Silver Nitrate, 2 cm3

1-Bromobutane, a few drops (my product)

- Observation: Solution became creamy yellow in colour.

Testing with Bottled Bromobutane

Test 1

- Add,

Silver Nitrate, 2 cm3

Ethanol, 2 cm3

1-Bromobutane, a few drops (bottled-pure)

- Observation: Solution changed colour to white with a brown
precipitate formed at the bottom.

Test 2

- Add,

Nitric Acid, 2 cm3

Sodium Hydroxide, 2 cm3

Silver Nitrate, 2 cm3

1-Bromobutane, a few drops (bottled-pure)

- Observation: Solution became creamy yellow in colour.

Interpreting…

The Reaction

1) Sulphuric acid gives the Butan-1-ol an H+ , acting like a proton
donor in this reaction. H+ joins with OH- and forms Water, leaving a
Carbocation (a carbon atom with a partially positive charge).

C4H9OH (aq) + H+ C4H9+ (aq) + H2O (l)

2) The water acts as a solvent dissolving the Sodium Bromide so that
it can be fully ionised.

NaBr (s) + H2O (l) Na+ (aq) + Br+ (aq)

3) Now NaBr exists as two different ions, Na+and Br+. Br+ will be
attracted to C4H9+ ion.

C4H9+ (aq) + Br -(aq) C4H9Br (aq) + Na+ (aq)

4) Since Sulphuric Acid has given one of it's proton to Butan-1-ol,
now it is a Hydrogen Sulphate ion, with a negative charge and is
attracted to the Sodium ion that holds a positive charge, and forms
Sodium Hydrogen Sulphate.

HSO4- (aq) + Na+ (aq) NaHSO4 (aq)

5) Then the solution is refluxed to ensure that it's mixed up properly
since this provides molecules the activation energy they need to reach
without Butanol evaporating.

6) Then the solution is distilled to get Bromobutane separated.

7) Butan-1-ol, which has not reacted, may be present in the solution,
so concentrated hydrochloric acid is added to remove this unreacted
butanol.

C4H9OH (aq) + HCl (aq) C4H9Cl (aq) + H2O (l)

8) Then we add a base, Sodium Carbonate, to neutralise the acid and
clean the Bromobutane. Due to the reaction between the base and the
acid, a salt is always produced at this stage, in our case salt that
is produced is Sodium Chloride.

Na2CO3 (aq) + HCl (aq) NaCl (aq) + H2O (l) + CO2 (g)

9) The Anhydrous Calcium Chloride is added to dry to bromobutane by
removing water.

C4H9Br (aq) + H2O (l) + Na2SO4 (aq) C4H9Br (aq) + Na2SO4 . 10H2O (s)

10) In final distillation, temperature range was 99-102 oC,
considering that boiling point of water is 100oC, and boiling point of
Bromobutane is 101.5oC, I believe it was a suitable temperature range.
Any extra water left is separated.

Analysis of Results

How much did I make?

Aim of my experiment was to make 20 g of 1-Bromobutane from 15.5 g of
Butan-1-ol and Hydrogen Bromide which was produced and reacted in
site.

My percentage Yield

%yield = Mass of Bromobutane Made / Mass of Bromobutane Expected

18.61 / 20 = 0.9305

0.9305 x 100 = 93.05 %

So, 93.05% yield of the expected 70% is achieved.

True yield that was achieved was:

If at 70% yield, product is 20 g,

At x%, product is 18.61 g ?

(70 x 18.61) / 20 = 65%

The expected yield was 70%, achieved yield was 65%.

Amount wanted to produce was 20 g, I produced 18.61 g.

So,

20-18.61 = 1.39 g is missing.

( 1.39 / 20 ) x 100 = 6.95% of the product is missing. Only;

100-6.95 = 93.05% of the expected amount could be achieved.

What did I make?

After the tests I carried out with both my product and bottled
Bromobutane, I can certainly say that what I made is Bromobutane.
Because, the tests carried out showed exactly the same results (same
colour changes and precipitates).

Sources of Errors

While carrying out the experiment, some errors occurred, without my
knowledge and fault.

Equipment Error

One of the errors that might have occurred is possible related to
equipment used to measure quantities of chemicals needed for the
experiment.

% error = Accuracy of equipment / How much was measured in it

Balance: - Accurate to 0.05 g

This was used to weigh 15.5 g of Butanol

%error = ( 0.05 / 15.5 ) x 100 = 0.32% to 2 d.p.

This shows that my results could have been 0.32% higher or lower and
indicates that other errors were the mainly cause of the fall in the
yield and the missing 6.95%, since the percentage error (0.32%) is not
big enough to account for the missing 6.95% of my product.

More %error could be found if we involved the measuring cylinder here.
But, chemicals measured in measuring cylinder were in excess so
accuracy does not count for these. Therefore none of the %errors
count.

An error could have occurred during the transfer of chemicals from
different containers, beakers and measuring cylinders. For example,
some of the chemical might have stayed inside the container because it
got stuck to the edge.

Some points where some errors could have occurred:

1) Although many precautions were made to keep the solution cool, when
the sulphuric acid was placed into the pear shaped flask, due to the
quick exothermic reaction releasing a great deal of hotness quickly,
some of the Butanol might have evaporated.

2) Butanol I used was somewhat old, and was used before for several
times. This is an important factor, because when the bottle is opened
air is able to get into the bottle. An oxidation of the Butanol can
take place producing (forming) Butanoic Acid. So an old bottle of
Butanol may contain impurities of Butanoic Acid. Therefore, although I
weighed and used the correct mass of Butanol, I might not have
supplied the full amount of Butanol needed, since Butanoic Acid will
be present instead of Butanol in the weighed amount. This means that
my reaction will not react as how it should be and so affect my
results causing some inaccuracy.

3) Butanol, Sodium Bromide and Water were measured on the balance and
then placed in different beakers, and then poured to the round
bottomed flask (pear shaped flask). During the transfer and placement
some of the chemicals might have stayed inside the beaker. Also it is
possible that some of the Butanol might have evaporated during
transfer, considering that Butanol is volatile. These all means that
less amount of the chemicals were transferred and so everytime a
transfer took place, even less amount of chemical was transferred and
was supplied to the reaction, which means that less Bromobutane will
be produced at the end.

All of these points would have caused a fall in the amount of
1-Bromobutane produced at the end. To overcome these errors, some of
the unnecessary transfers could be cut, and more improved professional
apparatus could be used for the experiment, I tried to do my best with
the resources I have that is available.

Conclusion…

My Aim was to produce 20 g of 1-Bromobutane, with a 70% yield. At the
end of the experiment, I produced 18.61 g of 1-bromobutane with a 65%
yield. The tests I carried out with my product gave exactly same
results that they gave with the bottled 1-Bromobutane, the same colour
changes and precipitates, showing that what I made was certainly
1-Bromobutane.

The percentage error of the equipment error cant be responsible for
the 6.95% of the product, 1-Bromobutane, that wasn't produced at the
end. I believe the amounts of chemicals reduced as they were being
transferred, and the age of Butan-1-ol used can be responsible for
this 6.95% lost. Some improvements can be done in the choice of
apparatus, for example use of a more sensitive balance (with an
accuracy of 0.005 g for example), burette, and pipettes, since these
have a great degree of accuracy. Also If experiment is to be repeated
again, some unnecessary transfers of chemicals from different beakers
can be cut. For example, Butan-1-ol was weighed out in a beaker, and
then placed to the pear shaped flask. Weighing it out straight in the
pear shaped flask would provide no lose of Butan-1-ol in the first
stage of the experiment. In addition to this, using a new bottle of
Butan-1-ol would certainly be better and increase the quality of the
reaction, considering that it would not contain any Butanoic Acid
impurities unlike the old bottle of Butan-1-ol I used. Also some
further more improvements can be done in the experiment, for example
in keeping the external factors (such as temperature) constant or low
according to the step.

I believe if these improvements were to be done, they would for sure
help to an achievement of the desired amount of 1-butanol with a
greater percentage yield since there would be less errors occurring
during the experiment.

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