Nucleophilic Substitution at a Saturated Carbon Atom
so here again ready polarisability is an advantage. Thus ease of
expulsion decreases in the series I^0 > Br® > CI® > F®; this, of course,
exemplifies the well-known decrease in reactivity seen as we go from
alkyl iodide to alkyl fluoride. The fact that I® can both attack and be
displaced so readily means that it is often used as a catalyst in nucleo
philic reactions, the desired reaction being facilitated via successive
attacks on and displacements from the centre under attack:
slow
RC1+H,0—>ROH+H® + CI®RCI + Ie—•Cte + Rl •
•f fast I
fast H.O
l© + H® + R-OHThe overall effect is thus facilitation of the hydrolysis of RC1, which
does not readily take place directly, via the easy formation of RI
(I® as an effective attacking agent) followed by its ready hydrolysis
(I® as an effective leaving agent).
In general terms, it can be said that the more basic the leaving group
the less easily can it be displaced by an attacking nucleophile; thus
strongly basic grouos such as RO®, HO®, H 2 N® and F^0 , bound to
carbon by small atoms that may not readily be polarised, cannot
norrrfSTly be displaced Tinder ordinary conditions. They can, however,
be displaced in acid solution due to initial protonation providing a
positively charged species (rather than a neutral molecule) for the
nucleophile to attack, and resulting in readier displacement of the
much less basic YH rather than Y®:
H® S Br®
R • OH > R • OH —• RBr + HsO
©
Thus even the extremely tightly held fluorine in alkyl fluorides may
be displaced by nucleophilic reagents in concentrated sulphuric acid
solution. The use of hydrogen iodide to cleave ethers
PhOR t-Ph-O-R s-PhOH+RI
©
is due to the fact that I® is the most powerful nucleophile that can be
obtained in the strongly acid solution that is necessary to make
reaction possible.