Hydrolysis reaction of alkyl halides

(former SN1 nucleophilic substitution mechanism)

Background and actual explanation

In a first step, there is a cleavage of the C-Cl bond, which allows the loss of the good leaving group, a halide ion, to give a carbocation intermediate. This is the rate determining step (bond breaking is endothermic):

In a second step there is an attack of the nucleophile, the lone pairs on the O atom of the water molecule, on the electrophilic carbocation, which creates an oxonium species:

In a third step, a deprotonation by a base, yields the alcohol as the final product of reaction. 

From long time this mechanism is repeated in any chemistry book or scientific texts. A little curios and strange for actual theories can be considered the article of Dale, Johannes entitled Inadequacies of the SN1 Mechanism published in Journal of Chemical Education, vol. 75, nr 11/1998. The article will be detailed discussed in the book. For the moment, only the abstract, available in the internet, worth to be reminded here: Although it has been recognized for half a century that alkyl carbocations are extremely reactive and can exist only in the vacuum of the mass spectrometer or in nonnucleophilic solvents like the superacids, textbooks of organic chemistry still present them as well-defined intermediates in solvolytic displacement reactions. A planar free carbocation intermediate is used as a simple model to explain racemization in solvolysis; but the problem is that racemization is never complete, the enantiomer in excess having the inverted configuration. Also, the unimolecularity has never been demonstrated, but is an assumption only. Racemization is commonly observed for reactions that are reversible, as in the hydrolysis of an alkyl chloride. It is also necessary that either the substrate, the product, or both can undergo a degenerate reaction. This is equivalent to saying that both the attacking nucleophile and the leaving nucleofuge can play both roles. Such exchanges were demonstrated by isotopic labeling 40 years ago. A number of examples are given to show that in irreversible solvolysis reactions of stereoisomeric pairs, which by an SN1 mechanism should have a common carbocation intermediate, produce instead different compounds which are those that can be predicted from an S_N2-like inversion.

PROPOSED MECHANISM

The actual quantum explanation supposes a charge formation during SN1 and E1 reaction, based on charged intermediate.

In these conditions, a substance able to give a SN1 or E1 reaction should have a strong polarity of C-X bond and the disponibility for a heterolitical cleavage. Such compound should be soluble in polar solvents.

Let’s take as example triphenilchlormethane (C₆H₅)₃CCl the ideal candidate of SN1 reactions and let’s look a little bit at its properties.

Triphenilchlormethane is insoluble in water but easy soluble in organic solvents. Its solutions in non polar solvents (benzene, chloroform, ethyl acetate etc) are uncolored and bad conductor of electricity. Its solutions in SO₂ liquid are yellow and fairly good conductor of electricity.

Not only the properties of triphenilchlormethane, but also the conditions of SN1 are not favorable to an ionic mechanism.

Triphenilchlormethane undergo a hydrolysis at simple reaction with water at normal temperature. Benzyl chloride gives the same reaction with water at boiling. Because the hydrolysis reaction is reversible a large quantity of water is normally used.

In these condition is hard to explain, in actual quantum mechanic, how electric charge appears at simple boiling or mixing.

In proposed explanation at least the mechanism of actual SN1 and E1 reactions are radicalic.

There are possible two variants for an intermediate radical mechanism.

Variant 1 – (replacing the old SN1 mechanism)

According to this mechanism, in a first step there is a homolitical cleavage of the C-X bound from the halogenated compound and two radicals are formed.

In a second step the alkyl radical combine with a water molecule giving an alcohol and a hydrogen radical (atom). The hydrogen atom combine with chlorine radical and hydrochloric acid is formed.

Variant 2 – possible

Even in pure water there are traces of hydrogen radicals (see the acidity new interpretation on the site!) and these can act as promoter of the reaction after next mechanism:

The formed hydrochloric acid, being a strong acid is completely dissociated in water, and practically formed HCl act as a catalyst for the reaction.

It is necessary a new complex and complete study about hydrolysis reaction to decide between variant 1 and 2. The technological advance can detect small quantity of Cl radical or small quantity of HCl able to differentiate between these variants.

The stereochemistry of hydrolysis reaction will be discussed in the book.

A similar radicalic mechanism for E1 reaction will be posted soon.