THE PREPARATION OF 1-BROMOBUTANE FROM 1- BUTANOL

Objective ;

The purpose of this experiment is to prepare 1- butanol

Introduction ;

            The most generally uses classes of synthetic organic reactions is nucleophilic substituition. This is a second order nucleophilic substituition, S_N2. As we know that the reaction required a nucleophile , an electrophile and a leaving group in order to apply the experiment in nucleophile second order sustituition .

CH₃CH₂CH₂CH₂-OH₂⁺ + Br^-               CH₃CH₂CH₂CH₂-Br + H₂O

The  mechanism S_N2 shown as below:

            Bromide is an exellent nucleophile and the electrophile is a 1˚ alkyl group, but hydroxide is poor leaving group due to its negative charge and its basicity. We a few choices to make the OH^- become better leaving group. First, we react the alcohol with p-toluenesulfonyl chloride which will convert OH^-  into sulfonic acid ester to react with sodium bromide to produce 1-bromobutane. Second , we react with the alcohol with phosphorus tribromide(PBr₃), which convert the OH^- to “P(OH)X₂” leaving group and also which produces free bromide ions which react with electrophile , replacing the new leaving group .Lastly, we are using a strong acid to protonate the OH^- group in the presence of the bromide ion , which changes the leaving group in the presence of the bromide ion , which changes the leaving the group from hydroxide to water , and allows the bromide to react in the same mixture . In this experiment we are using the third method to prepare the 1-bromobutane.We will separate and purify the product using simple distillation.

            We will carry out a reaction with the specific purpose of making new compound which is a synthetic reaction .It is our responsibility to really understand the experimentcarefully. In this experiment , sodium bromide and 1-Butanol are dissolved in water . Sulphuric acid is added cautiously which generates hydrobromic acid , which turn reacts with the alcohol upon heating to make 1- Bromobutane.

RESULT AND OBSERVATION

Melting point 98 ˚c

Mass of 100-ml round bottom flask=41.6120g

Mass of anhydrous CaCl₂ = 3.3874-2.3858

                                          = 1.0016 g

Mass of 1 – bromobutane = 43.5112 g – 41.6120 g

                           = 1.8992 g

CH₃CH₂CH₂CH₂-OH₂⁺ + Br^-               CH₃CH₂CH₂CH₂-Br + H₂O

Molar mass of 1-butanol = 136.904

Mass sodium bromide = 17.0405 g

No of mol 1-butanol = 17.0405 / 136.904

                               = 0.1245 mol

1 mol of 1-butanol need to react with 1 mol Br^- to produce 1 mol 1-bromobutane

0.1245 mol of 1-butanol need to react with 0.1245 mol Br^- to produce o.1245 mol 1-bromobutane

Theoretical yield = 0.1245 x 136.904

                          = 17.0405 g

Actual yield = 1.8992 g

Percentage yield = (1.8992 g / 17.0405 g) x 100%

                            = 11.15%

DICUSSION

           

            S_N2 mechanisms was shown as above. S_N2 reaction always occur in with inversion configuration at the substrate carbon.The nucleophile approaches the substracte carbon from the back side with respect to the leaving group.In this experiment we are using water as an solvent which called as protic solvent because it has a hydrogen atom which it attached to a strongly to electroneagative element.Hydrogen bonding encumbers a nucleophile and hinders its reactivity in a substituion reaction.

The boiling point of this product is 95˚C-98˚C , so the boiling point is nearly to 1- Bromobutane which has 101˚C as it boiling point. As we know alcohol do not undergo nucleophilic substituition reactions because hydroxide are ion is strongly basic and poor leaving group.The leaving group is is a group of atom which depart  with the electron pair used to bond them with the substrate.However , alcohols readily undergo S_N2 because the sulphuric acid was added which protonate the hydroxyl group in the presence of the bromide ion which chenges the leaving group from hydroxide to water , and allows bromide ions to react with it in the same mixture .

            The sulphuric acid serves as two purposes which is to increases the amount of protonated alcohols present in the reaction mixture and to help tie up the water molecules generated in the reaction shifting the equilibrium in favor of the alkyl bromide .An alternative and more covenient method involves the in situ generation oh hydrobromic acid by the addition of concentrated sulphuric acid to an aqueous soluition sodium bromide.

            From the calculation , the percatage yielid for this experiment is 11.15% which is very large different from 100%. It is because while we are doing an experiment the set up of the distillation apparatus is not correct so small amount of  gas from the 100 ml round bottom flask was release to surrounding so we do not get an accurate an actual mass in this experiment. We also need to wear gloves while doing an experiment because 1- Butanol and 1-Bromobutane both are flammable fluid which ca irritate our skin.Sulphuric acid also very concentrated acid which can effect our skin.Makesure we wash our hand and gloves after handling the substances .

CONCLUSION

            The experiment was involving second order nucleophilic substituition, S_N2.The 1-Bromobutane was prepared from 1-Butanol.The theoretical yield for this experiment is 17.0405 g while the actual yield is 1.8992 g. The percentage yield is 11.15% . the melting point is 95˚C-98˚C nearly to the boiling point of 1-Bromobutane which is 101˚C , so the product in this experiment is 1-Bromobutane