Preparation of T-Butyl-Chloride

Alkyl halides can be synthesized when alcohols react with hydrogen halides. An alkyl halide is a halogen-substituted alkane, and a hydrogen halide is a compound consisting of a hydrogen bonded to a halogen (H-X). Alkyl halides are classified as primary, secondary, or tertiary depending on the number of alkyl substituents directly attached to the carbon bearing the halogen atom. The purpose of this laboratory experiment was to prepare t-butyl-chloride, an alkyl halide, by dissolving t-butyl alcohol in concentrated hydrochloric acid. The reaction occurs via nucleophilic substitution, in which a nucleophile replaces the leaving group in the substrate. In this case, the hydroxyl group of t-butyl alcohol is replaced by a chlorine atom. The reaction proceeds via Sn1mechanism. The second part of the experiment consisted of purification of t-butyl chloride using the distillation process. A nucleophile is any neutral or uncharged molecule with an unshared pair of electrons. In the substitution reaction, the nucleophile donates an electron pair to the substrate, leading to the formation of a new bond to the nucleophile, while breaking the existing bond to the leaving group.

The two types of nucleophilic substitution reactions, Sn1 and Sn2, are identified based on whether these events occur simultaneously or in two separate steps. To synthesize t-butyl chloride, the t-butyl alcohol undergoes first order nucleophilic substitution, also known as SN1. To understand why t-butyl reacts via Sn1 pathway, the kinetics of the reaction mechanisms must be observed. The steps of the nucleophilic substitution involved in the preparation of t-butyl chloride can be identified in the experiment. When concentrated hydrochloric acid is added to the t-butyl-alcohol and mixed, t-butyl chloride forms. This product is not soluble in aqueous hydrochloric acid and forms a distinct separate layer. The t-butyl chloride is less dense then water and therefore is the top layer. An excess of hydrochloric acid was used in this step to drive reaction towards equilibrium to ensure an adequate amount of t-butyl-chloride was formed.

The bottom aqueous layer is discarded from the funnel, separating t-butyl-chloride from aqueous hydrochloric acid. At this point the crude product contains hydrochloric acid, unreacted alcohol, and traces of water. To remove excess acid, water is added is t-butyl-chloride. This is a physical method, and purification is based on solubility. Next, the last traces of acid are removed by repeating the wash process with aqueous sodium carbonate, which acts to neutralize the solution. As a result, sodium chloride salt, water, and carbon dioxide gas are formed. Being highly soluble in water, sodium chloride is discarded with aqueous layer. The wash process is then repeated with water to remove sodium carbonate. Lastly, anhydrous calcium chloride is added as a drying agent to remove the trace amounts of water droplets. This addition creates dry crude t-butyl-chloride.

Crude t-butyl-chloride is purified by distillation where substances of different volatilities and boiling points are separate from each other. Purification is achieved by collecting distillate product at forty-eight and fifty-two degrees Celsius, which is within boiling point range of t-butyl-chloride. Because the product was collected between these specific temperatures, all impurities lower and higher than the range were removed, leaving pure t-butyl-chloride.

Main and Side Reactions:
T-butyl-chloride is prepared from t-butyl-alcohol and concentrated hydrochloric acid. The main overall reaction is shown below:

There are side reactions that take place during the preparation of t-butyl chloride. One side product formed is isobutylene. This product forms when the tert-butyl carbocation intermediate undergoes a first order elimination reaction. The formation of isobutylene is shown in the reaction below.

In addition, tert-butyl ethyl ether is formed as a side product. This occurs by way of Sn1 reaction and is shown below:

Reaction Mechanism:
As previously state, there are two types of nucleophilic substitution reactions, Sn1 and Sn2. In Sn2 reactions, the formation of carbon-nucleophile bond and the breaking of carbon-leaving group bond occur simultaneously through a single transition state. Because the transition state involves both the nucleophile and substate, a second order reaction kinetics can be observed. It is also important to note that the nucleophile approaches the molecule from the back side, the side opposite to the leaving group, in this mechanism. Steric effects are important in determining whether an Sn2 reaction will occur. If the molecule undergoing substitution has too many alkyl substituents, the reaction cannot occur since the nucleophile will be unable to get close enough to molecule to do a backside attack. Thereby, primary and secondary structures are able react via Sn2 mechanisms, but tertiary structures cannot because of this steric hindrance. Tertiary structures react via Sn1. One of the factors governing Sn1 reactivity is the ability to form a stable carbocation.

Alkyl substituents increase the stability of a carbocation, so increasing the alkyl substitution of the carbon atom increase the probability of an Sn1 reaction occurring. Therefore, the rate of Sn1 reactivity are as follows: tertiary > secondary > primary. Sn1 reactions occur in two steps: first being the breakdown of the alkyl halide into an alkyl carbocation and a leaving group anion, followed by bond formation between the nucleophile and the alkyl carbocation. Formation of the carbocation is the slowest step, and determines the overall rate of the reaction. Because the rate determining step depends only on the concentration of the halide, it is said to be a unimolecular nucleophilic substitution reaction. Because t-butyl alcohol is a tertiary structure and able to form a stable carbocation intermediate, preparation of t-butyl chloride proceeds via Sn1 reaction. The reaction occurs in three steps: the protonation of the alcohol; formation of tertiary carbocation by dehydration; and the capture of carbocation by halide. The first step of the reaction occurs quickly when the acid protonates the alcohol, leading to an oxonium ion. The hydroxyl group is considered the leaving group. This step is shown below:

In the second step, the hydroxyl group binds together with the H+ to form water. As a result, the oxonium ion losses a water molecule and forms a carbocation intermediate in the rate-determining step. In the third step, the carbocation acts as electrophile. Because it lacks electrons, it attracts the nucleophile chloride ion. Wanting to achieve a stable molecule, the chloride ion attacks the carbocation. As a result, the carbocation acts accepts electron from the chloride ion, forming t-butyl chloride. The two steps are shown in the reaction below:

Other Methods of Preparation:
T-butyl-chloride can also be prepared by other methods. One method of preparation involves the free radical chlorination of isobutane. The reaction can be promoted by heat or light, which breaks the chlorine molecule into free-radical fragments. Because the reaction tends to yield a complex mixture of products, this method of preparation is not used when wanting to synthesize a specific alkyl halide. In addition, t-butyl-chloride can be prepared through the hydrohalogenation of an alkene, specifically by adding hydrochloric acid to isobutene. Procedure:

1. Fourteen mL of t-butyl-alcohol was added to a hundred and twenty-five mL separatory funnel. 2. Fifty mL of concentrated hydrochloric acid was then added to the funnel. The mixture was shaken for ten to fifteen minutes and relieved of any pressure by opening the stopcock. 3. Mixture was allowed to stand until a distinct separation between layers formed. Lower layer was drained and discarded. 4. Fifteen mL of water was added to the funnel and shaken. Two layers were allowed to separate, and bottom layer was drained and discarded. 5. Fifteen mL of ten percent aqueous sodium carbonate was added to funnel. Mixture was shaken, funnel was vented, and the two layers were allowed to separate. Lower layer was discarded. 6. Fifteen mL of water was added to the funnel and shaken. Two layers were allowed to separate. Lower layer was discarded. 7. The organic layer was transferred into a dry flask collected.

Half a gram of anhydrous calcium chloride was added to flask. The filtrate was dried for fifteen minutes to remove traces of water left in the crude product. 8. Simple distillation apparatus was assembled and a sample bottle was weighed and recorded. 9. Crude t-butyl-chloride product was decanted into a fifty mL round bottom flask, boiling chips were added, and the flask was assembled into simple distillation apparatus. 10. Product was distilled, and portion of distillate that boiled between forty-eight and fifty-two degrees Celsius was collected in sample to ensure purification by removing low and high boiling impurities. 11. Collected distillate was transferred to pre-weighed small mouth bottle. 12. The lid was placed on the bottle, and the bottle containing the final product was weighed. 13. The observed yield was calculated and recorded.

Observations:
One of the first observations made was the cold temperature of the hydrochloric acid and how the strong acid was added in excess. The excess amount and temperature prevent alkene formation and make sure an alkyl halide is synthesized. It is added in excess to drive the reaction forward and ensure a reasonable about of alkyl halides will be able to form. Also, upon adding hydrochloric acid to t-butyl-alcohol, the formation of a cloudy solution was observed. Cloudiness of the solution could have been an indication that a reaction between hydrochloric acid and t-butyl alcohol was occurring and by-products were forming. In addition, the importance of ensuring the crude t-butyl-chloride product was completely dry before moving to next purification step was observed. The importance of this step was noted because water could interfere with the distillation process and even hydrolyze the product back to an alcohol. Lastly, a low percent yield of t-butyl chloride was observed. This could be a result of the formation of by-products. The reaction proceeds via a Sn1 mechanism, a pathway that has a tertiary carbocation intermediate. This carbocation can react in two ways either by Sn1 or E1 pathways to yield different products, tert-butyl chloride or isobutylene. The low yield could also be due to mistakes made while using the separatory funnel. T-butyl chloride could have accidently been removed with the aqueous layer being discarded if careful technique was not used. Data and Calculations:

Compound| Molecular Weight( g/ mol )| Density( g/ ml )| Weight Used( g )| Moles Used| t-butyl-alcohol| 74.12| 0.775| 10.85| 0.147|

Weight used: Density * Volume = (0.775 g/ml)(14ml) = 10.85g
Theoretical Yield Based on Moles:
Weight used = 10.85g = 0.147 moles Molecular
Weight 74.12 (g/mol)

Compound| Molecular Weight ( g/ mol )| Moles Produced| t-butyl-chloride| 92.5| 0.063|

Weight Yielded:(Mass of Sample Bottle + Product)–(Mass of Sample Bottle)= 31.6 – 25.8= 5.8g
Experimental Yield Based on Moles:
(5.8g) * 1 mol t-butyl-alcohol = 0.063 moles
74.12g t-butyl-alcohol
Percent Yield of t-Butyl-Chloride
Percent Yield = Experimental Yield * 100 = 0.063 = 0.429 * 100 = 43% Theoretical Yield 0.147 Safety Considerations:
Caution should be taken when handling concentrated hydrochloric acid. Hydrochloric acid is extremely corrosive and gloves should be worn when handling the acid. In addition, the chemical should only be used and handled in the fumehood.

Also, safety should be used when handling mixtures in the separatory funnel. It’s important to remember to vent the separatory funnel by opening the stopcock from time to time. This relieves pressure within the funnel and prevents the stopcock or top of funnel from bursting or becoming loose, thus coming in contact with solution. Venting the funnel is especially important quickly when doing the sodium carbonate wash because of the buildup of carbon dioxide. In addition, it’s important to point the funnel towards the back of the fumehood when releasing excess pressure to prevent bodily exposure to contents being released.

Bibliography

Macroscale and Microscale Organic Experiments. 6th ed. Williamson, Minard, and Masters, Houghton Miffin Co, 2007. Organic Chemistry, 10th edition, Solomon and Fryhle. John Wiley & Sons, Inc., 2011