Synthesis of an Alkyl Halide

Topics: Nucleophilic substitution, Alcohol, Ethanol Pages: 6 (1646 words) Published: July 1, 2012




Alkyl halides are molecules which have a carbon atom attached to a halogen atom (e.g., chlorine, iodine, or bromine.) Alkyl halides are very important since they are used in many of the products used today. They can be synthesized by means of adding alcohols to hydrogen halides (such as HCl, HI, or HBr) resulting in nucleophilic substitution (specifically the SN1 reaction.) The experiment was done with the goal of synthesizing an alkyl halide from alcohol and a hydrogen halide. In order to do this, tert-butyl alcohol was mixed with HCl to form crude tert-butyl chloride. Solid NaHCO3 and CaCl2 were used to prevent possible hydrolysis of the product. It was then purified through distillation. Results confirm the successful production of 5.01 g (51.5% yield) of the alkyl halide, tert-butyl choride. -------------------------------------------------


The importance of alkyl halides is great as it can be used in making pharmaceuticals, Teflon pans, flame retardants, fire extinguishers, solvents, and propellers [1][2] and other things used on a daily basis. However, they are pollutants that cause serious damage to the ozone layer. Nevertheless, they are invaluable since they can also be used as intermediates to produce other functional groups (e.g., amines, esters, and alkenes).

Alkyl halides have the general formula RX (where R= alkyl/substituted alkyl group and X = F, Cl, Br, or I.) They are insoluble in water and sulfuric acid and are soluble in organic solvents.[3] They are synthesized for their many uses.

There are several ways by which alkyl halides can be synthesized. Some of those are halogenation of alkanes with Cl2 or Br2, addition of HX to alkenes, addition of X2 to alkenes, and halogen exchange. Among the various methods, the major method used is turning alcohols into alkyl halides by using hydrogen halides (HX, X = Cl, Br, or I)[4] with the exception of HF.[5] The alcohol used may be primary, secondary, or tertiary (determined by the number of C atoms bound to the C atom attached to the hydroxyl group.) Tertiary alcohols are preferred though, since they can be readily converted to alkyl halides with the use of HX while primary and secondary alcohols require the presence of a lewis acid in order for them to react. The alkyl halide forms via nucleophilic substitution. There are two kinds of nucleophilic substitution reactions: SN1 and SN2. SN1 is a first-order reaction and unimolecular while SN2 is a second-order reaction and bimolecular. The combination of a tertiary alcohol and HX undergoes SN1 reaction where elimination and rearrangement occur to produce the alkyl halide.[6]

In the experiment, 10 mL tert-butyl alcohol (a tertiary alcohol) and 20 mL cold concentrated HCl were swirled inside a separatory funnel. The resulting solution was left undisturbed. 3 mL NaCl was added then the organic layer (which was collected in an Erlenmeyer flask) was separated from the aqueous layer (which was discarded.) A small amount of solid NaHCO3 was then added to the organic layer and was swirled gently then decanted into another flask. A small amount of anhydrous CaCl2 was added to the filtrate and was again decanted into a round bottom flask. The product was then purified through distillation using the set-up found in figure 1.

The sample inside the flask was heated to a gentle boil. Once the temperature remained constant, the boiling point was recorded and the heat was adjusted so that there is only 2 drops of distillate per second. The first 1 mL of distillate was discarded and the fraction that distilled at a constant temperature was collected and the temperature was recorded as the boiling point of the...
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