Preparation of 2-chloro-2-methylpropane
The simplest organic molecules contain only the elements carbon and hydrogen. As these elements have similar electronegativities, the resultant compounds are essentially non-polar. Further, if all the carbon atoms are sp3 hybridised, i.e. there are no p-bonds), the molecule will be quite unreactive.
Carbon forms strong bonds with a large number of other elements (called hetero-atoms). In the majority of cases, there is a large difference between the electronegativities of carbon and the hetero-atom, and the resultant bond is therefore polarised. This means that many organic molecules have relatively slightly negatively charged and slightly positively charged centres. These centres are very important in a vast number of chemical reactions and have been given special names - viz. nucleophile and electrophile .
A nucleophile (from the Greek for "nucleus loving") has an electron-rich site and forms a bond by donating an electron pair to an electrophile. Conversely, an electrophile (from the Greek for "electron loving") has an electron-poor site and will react with the electron pair of a nucleophile to form a bond.
The most common reaction of the functional groups in saturated alkyl compounds is nucleophilic substitution. The nucleophile, or electron-rich species, attacks the electrophilic carbon of the alkyl group to give the substituted product. A different nucleophile is generated as a byproduct of the reaction.
Not all nucleophiles react with all electrophiles. For a nucleophilic substitution reaction to occur, it is necessary for the bond to be broken at an appreciable rate. This is highly dependent on the nature of the leaving group (displaced nucleophile). In general, the most stable nucleophiles are the best leaving groups, and can be readily identified - they are the conjugate bases of strong acids. The ease with which a reaction proceeds, or indeed whether a reaction will "go" at all, is dependent on the exact mechanism of the reaction, the nature of the attacking nucleophile, the stability of any carbocation generated, temperature, the relative concentrations of reagents, etc.
In this experiment, 2-chloro-2-methylpropane is prepared from 2-methyl-2-propanol by the action of concentrated hydrochloric acid as shown below. This reaction takes place rapidly at room temperature and its ease is typical of tertiary alcohols. By contrast, secondary alcohols require more drastic conditions to undergo a reaction with the same reagent, and primary alcohols more forcing conditions still (e.g. heating at high temperature for many hours).
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A liquid b.p. 51 oC |
The starting material and product of this reaction have markedly different physical properties which makes it a relatively simple matter to isolate the desired product in a pure state.