Formal Report Chem 31.1

Preparation and Purification of an Alkyl Halide Abstract Alkyl halides consist of an alkyl group linked to a halogen atom (X) by a single bond. It can be prepared by reacting it with a hydrohalogen (HX). In this experiment, tert-butyl chloride was synthesized through the reaction of tert-butyl alcohol and concentrated hydrochloric acid following SN1 mechanism. It was then separated from its aqueous solution using a separatory funnel given the knowledge that is insoluble in water. A simple distillation set-up was equipped to purify the obtained compound. The experiment was successfully able to synthesize an alkyl halide as the end product tert-butylchloride obtained was found to be colorless, having a boiling point of 51° C, which agrees with the theoretical boiling point of tert-butyl chloride. However, due to some errors, only 5.26 grams of the product was obtained, giving a percent yield of 53.93 %, suggesting a more careful execution of the procedures.

I. Introduction Alkyl halides, also called haloalkanes, simply have a halogen atom bonded to one of the sp3 hybrid carbon atoms of an alkyl group. The halogen is more electronegative than carbon, and the C – X bond is polarized with a partial positive charge on carbon and a partial negative charge on the halogen. A generic formula of R-X where X = fluorine, chlorine, bromine or iodine would be suitable for these compounds. These are classified according to the nature of the carbon atom bonded to the halogen. If the halogen – bearing carbon is bonded to one carbon, it is called primary halide (1°). If there are two carbons bonded to the halogen-bearing

carbon, then it is a secondary halide (2°). If three carbons are bonded to the halogenbearing carbon, then it is a tertiary halide (3°). Figure1. Classification of Alkyl Halides according to the number of carbon/s attached to them[2]. Alkyl halides had been very useful to us since a long time ago until now. Most syntheses use alkyl halides as useful reagents in making more complex molecules. Freons, (also called chlorofluorocarbons, or

CFCs) are also alkyl halides that was once a popular refrigerant. Alkyl halides are also widely used as insecticides such as DDT (DichloroDiphenyltrichloroethane), a chlorinated insecticide and as anesthetics like CHCl3 or more commonly known as chloroform[4]. These applications wouldn’t be possible without the preparation of alkyl halides. Synthesis of alkyl halides are of major importance in industry as well as in the laboratory. As a matter of fact, synthesis of alkyl halides had been one of the three most useful classes of organic reactions[1]. There are many different ways on how to synthesize these compounds. These can be prepared from alkanes through freeradical halogenation but this is useful only in certain cases. These can also be obtained from alkanes and alkynes through allylic bromination and even from other halides. But the most common laboratory method for the preparation of alkyl halides is the replacement of the hydroxyl group of an alcohol with a halogen atom[2]. Alcohols react with hydrohalogen (HX) but the reaction works well only for tertiary alcohols. Tertiary alcohols almost react instantaneously at room temperature, while primary and secondary alcohols need other reagents like SOCl2, PBr3 or HF-pyridine for them to react and form the desired alkyl halide[2].

This experiment aimed to synthesize tert-butyl chloride through a nucleophilic substitution reaction - an SN1 reaction to be more specific - using tert-butyl alcohol and concentrated hydrochloric acid. Other than executing methods concerning synthesis and distillation, methodology particulars and elaboration of data presented from the calculations were also given emphasis.

solid NaHCO3 and then swirled. This was decanted into another dry flask. The collected filtrate was dried using a small amount of anhydrous CaCl2. A small amount of anhydrous CaCl 2 is still added until it clumped together.

II. Methodology

In the distillation process, a distillation set up was equipped as shown in Figure 3. The water should continuously flow into the bottom of the condenser’s cooling jacket and out from the top to avoid pressure build up in the condenser. A thermometer bulb is placed just below the side arm of the distillation head. The chosen round bottom flask to be used is big enough to hold a sample that is 1/3 to 2/3 of its volume. To regulate the temperature, water bath was used in this experiment.

10 mL of tert- butyl alcohol and 20 mL of cold concentrated HCl was placed in a dry 50 mL separatory funnel. It was gently swirled. The internal pressure was safely relieved from time to time by slowly opening the stopcock. The mixture was allowed to stand undisturbed for twenty minutes. It is important for the aqueous layer to be discarded and this can only be done through the separation of layers in the separatory funnel. Adding three to five mL of 6 M sodium chloride solution facilitates this layer separation. Afterwards, one to two drops of water was added to any of the separated layers. This step helped in determining which the organic layer is.

The crude tert-butyl chloride was decanted into a dry 25 mL round bottom flask. Few boiling chips were added before distilling.

Figure 2a. Draining the Lower Layer

[8]

Figure 3. Simple Distillation Set-up[5]

Figure 2b. Holding Separatory Funnel[8]

and

Venting

a

The organic layer is then transferred into a dry flask containing a small amount of

The sample is then placed inside the flask. Some pieces of boiling chips were added into the flask. It is very important to take note that it is not safe to add boiling chips to a hot liquid because if a solution is at or near the boiling point, then the addition of boiling chips will cause the flask to boil up suddenly and some of the boiling liquid will escape uncontrollably from the flask. The water flow through the condenser is then checked by turning on the water supply. All the ground glass joints are checked, and it is necessary that they are well fitted. The sample was heated in the flask to a gentle boil. But this time, the temperature reading on the thermometer rose rapidly.

However, it will remain constant at its boiling point. The boiling point was taken note. The vapors and condensate are observed to pass through the side arm and into the condenser, where most of the vapour condenses into a liquid. Finally, it dripped from the adapter into the receiving flask. The heat was adjusted such that the distillation occurs at a rate of 2 drops distillate per second. The first few millilitres of the distillate was discarded; but the fraction that distilled at the desired temperature was collected. The constant temperature was the boiling point of the volatile component. It was distilled first to dryness. After all of these steps, the sample was waited until it started to boil. Then, it was immediately removed from the heat source. The whole set-up was allowed to cool before dismanting it. Because the boiling point of tert-butyl chloride is 51° C, only the fraction that boils at 49-52 °C was collected. This pure product is placed in a pre-weighed vial and was cooled in an ice bath since the product is highly volatile. Lastly, the yield for the experiment was calculated. III. Results and Discussion The presence of a strongly electrophilic carbon center makes the alkyl halides susceptible to nucleophilic attack whereby a nucleophile displaces the halogen as a nucleophilic halide ion. This is called the nucleophilic substitution reaction – one of the major reactions undergone by an alkyl halide.

In this experiment, the desired product which is the tert-butyl chloride was synthesized via SN1 reaction. The requirements for an SN1 reaction to be possible have been met since tert-butyl chloride is tertiary and the solvent used is an alcohol which is a protic solvent. Protic solvents such as alcohols and water are even more effective solvents in the SN1 reactions because anions form hydrogen bonds with the –OH hydrogen atom. The SN1 reaction involves a two-step mechanism. The first one is the slow formation of the carbocation or the so-called rate limiting step, and the second one is the fast nucleophilic attack on the carbocation.

Step 1. Formation of the carbocation (slow) [4] .

Step 2. Nucleophilic attack carbocation (fast)

on

the [4] .

As for the experiment, we can see a convincing evidence that for tertiary alcohols, the reaction proceeds via S N1 (substitution-nucleophilic-unimolecular) mechanism. For tert-butyl alcohol, the SN1 reaction with concentrated hydrochloric acid is illustrated as follows:

I. Figure 4. Reaction[4].

Nucleophilic

Substitution

Under this type of reaction are two kinds of mechanism – SN1 and SN2 reactions. Under protic solvent conditions with nonbasic nucleophiles, the SN1 mechanism is preferred and the order of reactivity is as follows [3]: 3° > 2° > 1° It is fair to say that nucleophilic substitution of tertiary alkyl halides will take place under the SN1 mechanism while primary and secondary alkyl halides will undergo SN2 mechanism.

II. III.

In step I, the alcohol accepts the proton to form the protonated alcohol. This dissociates into a carbocation and water as seen in step II. This formation of carbocation is favoured for tertiary alkyl halide since it relieves the steric strain in its crowded tetrahedral structure[3].And finally, the chloride ion is

combined with the carbocation (step III) to form the product tert-butyl chloride. In order for the experiment to be carried out successfully, many particulars were stressed out. Without these, failure of experiment may occur and the desired product may not be synthesized. The first detail that needs not to be neglected is the use of cold hydrochloric acid. This is done to prevent the formation of isobutylene (CH3)2C=CH2 via E1 mechanism. This isobutylene or 2-methylpropene is formed as a side product. This is believed to be inevitable because for almost all cases, a fraction of the intermediate carbonium ions lose a proton instead of adding halide ions. The mechanism is shown below:

Figure 5. Isobutylene as a minor product [6]. In order for the reaction to limit or prevent the formation of this side product, a cold concentrated hydrochloric acid was used, as already mentioned. Also, these side products can also be easily eliminated since it has a very low boiling point compared to the major product. Hydrochloric acid was added in excess to ensure that an alkyl halide will be formed in a reasonable yield. This is possible knowing the fact that the formation of tert-butyl chloride follows a first order rate reaction, meaning, it is dependent only on the concentration of tert-butyl alcohol and not with the amount of hydrochloric acid used. The tert-butyl alcohol and hydrochloric acid solution was separated from the aqueous layer in a separatory funnel. After it was done, the organic layer is transferred to a dry flask with a small amount of solid NaHCO3. Solid NaHCO3 was used instead of aqueous NaHCO3 to neutralize the excess HCl adequately. Doing another way around will just expose tert-butyl chloride to a lot of water and this risks tert-butyl chloride to becoming an alcohol again. After this, anhydrous CaCl2 was used to dry the crude product before distillation. This

step must not be missed so that the traces of water that may impede the formation of desired products may be avoided [7]. In purifying further the target compound from possible impurities, simple distillation process was done. Here, the separation of liquids by vaporization and condensation of the vapor back to the liquid phase occurs. The use of boiling chips was emphasized in this step as it ensures even boiling and prevents bumping. Bumping occurs when part of the solvent becomes superheated and leads to sudden bursts. Without boiling chips, the process could be dangerous and may cause some loss of the product [7]. The continuous flow of water in the condenser during distillation is necessary to prevent all the liquid from evaporating and to prevent pressure build up that may be explosive and dangerous. Cautiously following the mentioned particulars, the purpose of the experiment which is the preparation of tert-butyl chloride from tert-butyl alcohol and concentrated hydrochloric acid was successfully employed based the following equation: (CH 3)3COH + HCl  (CH3)3CCl + H20 The balanced equation above shows that hydrochloric acid and tert-butyl alcohol has a stoichiometric ratio of 1:1. Now, it is safe to conclude that tert-butyl alcohol is the limiting reactant because it has a smaller amount than hydrochloric acid. Therefore, tert-butyl alcohol will determine the rate of formation of tert-butyl chloride. Applying this principle, the following data were obtained: Table 1. Properties of tert-butyl alcohol Mass 7.809 g Molecular Weight 74.12 g/mol Density 0.7809 g/cm3 Mmol 105.36 mmol Color clear/colorless Solubility in water Soluble Boiling point 82° C Table 2. Properties of tert-butyl chloride Mass Molecular Weight Density Mmol

5.26 g 92.57 g/mol 0.84 g/cm3 105.36 g

Color clear/ colorless Solubility in water insoluble Boiling point 51° C Table 3. Summary of Experimental Results Mass of tert-butyl 7.809 g alcohol Mass of tert-butyl 5.26 g chloride Theoretical yield 9.75 g Percent yield 53.93 % Boiling point 51° C By the end of the experiment, the desired product was gathered and it was found that it has a percent yield of 53.93 % and a boiling point of 51° C, which is almost similar with its theoretical boiling point. IV. Conclusion With a percent recovery of 53.93 % and with a boiling point of 51° C, the experiment is reckoned a success as it was able to obtain the desired product tert-butyl chloride via SN1 reaction of tert-butyl alcohol and concentrated hydrochloric acid. Improvement in the experimental procedures such as carefully separating the layers may help to increase the yield of the desired product because it is possible that some amount of the organic layer were discarded together with the aqueous layer. This experiment’s errors could be further minimized by ensuring the completeness of the distillation process because it is also possible that not all of the sample compound was converted to tert-butyl chloride. All in all, it is always best to follow the experimental procedures with caution to improve the results. V. References:

=(weight of vial+tert-butyl chloride)-weight of vial =27.52 g – 22.26g = 5.26 g  Millimole of tert-butyl chloride The equation (CH3)3COH + HCl (CH3)3CCl + H20 gives us a 1:1 ratio: =

105.36 mmolt−butyl alcohol ×



1 mmoltert butyl c h loride 1mmol tert −butyl alco h ol

= 105.36 mmol tert-butyl chloride VI. Appendices Calculations:  Mass of tert-butyl alcohol

¿ 0.7809

 Theoretical Yield of tert-butyl chloride

1 mol 1000 mmol

g ×10 ml ml

¿ 105.36 mmolt−butyl chloride(

= 7.809 g tert-butyl alcohol Millimole of tert-butyl alcohol

=

105.36

mmol

 Mass of tert-butyl chloride

=9.75 g tert-butyl chloride  Percent yield = (actual/theoretical) x 100 = (5.26g/9.75g) x 100 = 53.93 %

1000 mmol 1 mol ) 1 mol ¿ 7.809 g ( )¿ 74.12 g

alcohol

) 92.57 g )¿ 1mol

tert-butyl

Note: The datasheet is stapled together with this paper.