Synthesis of an Alkyl Halide

Michelle Joy L. Fermaran and Lovelyn Marie M. NievalesInstitute of Chemistry, university of the Philippines, Diliman, Quezon City 1101 Philippines

Date/s Performed: April 25,2013; Date Submitted: April 30,2013

Alkyl Halides are formed when alcohol reacts with hydrogen halide (HX; X is a halogen). Halogens like Cl, Br and I are the commonly used in this reaction. This experiment shows the use of HCl to convert tert-butyl alcohol into tert-butyl chloride via a separatory funnel and then purifying it by means of distillation. The main boiling point for tert-butyl alcohol was 50.7ᵒC when HCl was mixed the boiling point lessens. Expect that the yield and its percentage will be less than the actual volume or weight of the compound due to some factors.

Introduction

Alkyl halides belong to the class of compounds where a halogen atom or atoms were bound to an sp³ orbital of an alkyl group.They are also called as haloalkanes or halogenoalkanes which is composed of an organic group containing one or more hydrogens and have been replaced by halogens.

Alkyl halides are shown by means of the formula RX wherein R is a carbon chain and X is the representation of a halogen.

Mostly, these alkyl halides are colorless, odorless, hydrophobic or insoluble in water and denser than H₂O, soluble in low polarity solvents and liquid at room temperature. They have higher boiling point (BP) than alkanes of the same number of carbons. As their molecular weight (MW) increases, the boiling point also increases. As the branching increases the BP decreases.

They are classified the same as hydrogens. Their classification depends on the number of neighboring carbons attached to them. So, if there is one carbon attached to the halide it is classified as a primary carbon (1ᵒ). Considered as a secondary carbon (2ᵒ) if two carbons were attached; tertiary carbon (3ᵒ) if three carbons surround the halide. As the neighboring carbon increases, the classification of halide also increases; as the classification increases, its stability also increases (Figure 1). These halides are also group depending on the number of halogens attached to them. They are called organofluorine, organochlorine, organobromine and organoiodine compounds.

They are widely used in industry,agriculture and pharmaceuticals. Examples of this are polyvinyl chloride (PVC), polytetrafluoroethene (Teflon), fluorocarbonanesthetics and many more. Because of its benefits, systematic techniques have been formulated to synthesize them. One of the common technique is the reaction between an alcohol and a hydrogen halide in an acidic environment (HX where X= Cl, Br, I) yielding an alkyl halide and a water. “The order of decreasing reactivity of HX towards an alcohol is HI>HBr>HCl. HCl reacts readily with tertiary alcohols while the presence of a Lewis acid like zinc chloride is required for the reaction to primary and secondary alcohols. Therefore, tert-butyl alcohol is readily converted to tert-butyl chloride at room temperature by concentrated HCl” (OrganicChemistry Laboratory Manual, 2008 ed.).

Figure1. Classification of Alkyl Halides according to the number of carbon/s attached to them.

Formulas used in calculations:

1

Mmole = mass(g) x molecular weight ( gmol ) x

1 mol1mmol

Theoritical yield = mas s tert−butyl ch loridemass tert−butyl alco h ol

% yield = mass tert−butyl c hloridemass tert−butyl alcoh ol

x 100

Experimental Details

Materials:Tert-butyl alcohol separatory funnel (30mL) hotplate Concentrated HCl distillation set-up (semi-micro) oil bathSolid NaHCO₃ Erlenmeyer flask (50 mL) rubber tubingIce bath thermometer aluminium foil

The experiment was done first by mixing 10 ml tert-butyl alcohol and 20ml cold concentrated HCl in a 30ml separatory funnel and swirling it gently. Both components which are HCl and alcohol are in their liquid phase so, the reaction will also be in the same phase. The product which is an alkyl halide (alkyl chloride in the experiment) will be obtained because it is insoluble in aqueous solution so it will separate from the liquid portion of the mixture. The reaction of the two substances produces a carbocation intermediate. Using of concentrated HCl means faster carbocation production and faster carbocation fastens the rate of reaction.

The reaction taking place in the experiment is nucleophilic substitution reaction. In this reaction, halogen, (Cl in the experiment) from acid which is nucleophilic dissociates completely in an aqueous environment producing an alkyl halide product. This means that high concentration of acids used in the experiment permits the formation of the alkyl chloride which is the expected product of the experiment.

Acid used in the experiment must be cold because usage of hot acid may be a key to other reactions and may lead to formation of other products instead of alkyl halides. After mixing the different substances, internal pressure must be reduced by means of slowly opening the stopcock of the separatory funnel. Then the class allowed it to stand for 20 mins. As mentioned above, alkyl halides re insoluble in water and letting it stand for some time gives it the chance to separate from the aqueous solution. Double layered solution will be observed after the reaction (at the top is the organic solution while below is the aqueous solution). The aqueous solution was discard to be able to extract the organic layer and then was transferred to another flasks with solid NaHCO ₃ again swirling the mixture gently.

Solid NaHCO₃ was used in the experiment instead of the aqueous one because aqueous solution contains water and the reaction of the organic compound with water might reverse the reaction. For example, in the experiment, resulting organic compound which is HCl if mixed with water might give alcohol as product which is tert-butyl. Then the mixture again underwent the process of decantation.

Crude alkyl halide which is the filtrate from the solution was then dried with the use of anhydrous CaCl₂. Anhydrous compounds remove water from the said filtrate and removal of water from alkyl halides prevents its decomposition to form alcohol through the process of hydrolysis. Then the compound will again be decanted into a 25ml round bottom flask.Tert- butyl chloride will now be distilled and boiling chips will be added to it while boiling.

Boiling chips are usually added to distillation set-ups to provide nucleation sites (place where liquids may turn to gas) so liquid will boil smoothly without becoming superheated or

2

bumping. In simple words, boiling chips make place for bubble formation. During the distillation process, water must be continuously flowing in the condenser to prevent the total or 100% evaporation of the sample. During distillation, liquid evaporates to a pure gaseous form and with the help of water, the vapor will be condensed bringing it back again to its liquid form. The pure tert-butyl chloride will then be collected in a 10ml graduated cylinder and then placed in an ice bath to avoid it from evaporating because it is highly volatile. Collect the product that boils at 49-52ᵒ C. Formation of 2-methylpropene as byproduct during the reaction is possible when tert- butyl carbocation undergo elimination of beta hydrogen.

Set-ups:

Figure 2.1 Holding and venting a separatory funnel

Figure 2.2 Draining the lower layer

Slightly open to facilitate separation less dense layer

More dense layer

3

Figure 3. Simple distillation set-up

Note the position of thermometer bulb

Heat source

Results and Discussion

Tert-butyl alcohol Tert-butyl chlorideMass 10 g 1.5 gMolecular weight 74.04 g/mol 92.57 g/molDensity 0.775 g/cm³ 7.809 g/cm³Mmol 135.06 mmol 16.20 mmolColor colorless colorlessSolubility in water insoluble InsolubleBoiling point 50.7ᵒC 49ᵒCTheoretical yield 0.15% yield 15%

Only 15% was retrieved after purifying the tert-butyl chloride by means of distillation. This is maybe due to human error during conducting the experiment. Loss of product was due to deficiency in precision during the separation and imperfections in the distillation set-up. Pipette was used to get or separate the organic compound from the aqueous solution because there is a white precipitate clogged on the separatory funnel. The exactness of getting the compound totally and purely without mixing with the solution wasn’t able to perform due to repeating attempts of getting it but instead of pure there is a solution carried together with the compound so again putting it back to the separatory funnel to separate again the compound and solution.

Tert –butyl chloride was formed from the reaction of tert-butyl alcohol and HCl. Electrophilic oxygen gains the proton of the hydrogen and detaches itself from the parent compound forming water (figure3.1). With the hydroxide group gone, an electrophilic, carbocation intermediate is formed which attracts the electron of the chlorine (figure3.2). The product are tert-butyl chloride and H₂O (figure3.3) and the reaction is an Sn1 reaction.

4

Figure 3.1

Figure 3.2

Figure 3.3

HCl was added in excess to push the reaction toward the formation of the products.

Conclusion

In the experiment we performed, the boiling point of the yielded product showed that the experiment produced an alkyl halide (tert- butyl chloride). The boiling point of the product was at 49ᵒC which agrees with the theoretical data so we can say that the objective of the experiment which is to synthesize and purify tert- butyl alcohol into tert- butyl chloride was achieved.

References

(n.d).Chemistry1.Chapter5:Alkyl Halides.Retrieved from http://www.chemtech.org/cn/cn2323/2323-5. htm

Organic Chemistry Lab Manual (2008), Institute of Chemistry: UP Diliman, pp. 34-37

Kokv_goh. 2011. One Part of Chemistry. Retrieved from http://1chemistry.blogspot.com/ 2011/06/synthesis-of-tert-butyl-chloride.html

(n.d). Alkyl Halides. Retrieved from http://crab.rutgers.edu/~alroche/Ch06.pdf

(n.d). Alkyl Halides(Haloalkanes). Retrieved from http://www.chem.sc.su.ac.th/elearning/ 513250/2.pdf

Herland, Brittany. Synthesis of an alkyl halide. Retrieved from http://12-herlandbrittany.simmons labsciences.wikispaces.net

(n.d.) Retrieved from www.tunqa.com/chemistry/432-Chemistry-4html .

5