1. Hydrolyses of Alkyl Halides
Prof . Sanaa Taher Arab
By: Maha Saud AL-subyi . Chemistry Department , Faculty of Sciences for girls , King Abdul-Aziz University , Jeddah - KSA

2. Hydrolyses of Alkyl Halides
The hydrolyses of alkyl halides-RX- represent the simplest group of organic substitution reactions .
Hydrolyses may occur by reaction with a water molecule or
a hydroxide ion , both of which are nucleophilic reagants ; there is therefor the possibility of both SN1 and SN2 reactions .

3. Hydrolyses of Alkyl Halides (contd.)
The SN1 reactions occur by the mechanism …
R — X slow R X
R + OH fast ROH
or R + H2O fast ROH + H
The ion R produced in the initial step is known as a carbonium ion .

4. Hydrolyses of Alkyl Halides (contd.)
The rates of such reactions will be increased by introduction of an electron releasing ( + I ) substituent , such as CH3 , which will favour the initial ionization .
Moreover , the rate will be independent of whether the subsequent reaction involves OH or H2O , and will be the same for other reactants such as Cl .

5. Hydrolyses of Alkyl Halides (contd.)
If the mechanism is of the SN2 type ..
on the other hand , the rate is determinded by the ease with which the RX can accept electrons from the nucleophilic reagent ; the rate will be therefore incerase with the nucleophilicity , and will decrease if an electron releasing substituent is introduced into R .
It is quite possible for the mechanism to change in
a series in which substituents are progressively introduced .

6. Hydrolyses of Alkyl Halides (contd.)
An interesting case of this , first pointed out by Gleave , Hughes and Ingold , is to be found in the hydrolysis of the simple alkyl iodides .
For the series …

7. Hydrolyses of Alkyl Halides (contd.)
In which methyl groups are progressively added to the carbon atom , the rates are as in the Fig (16) .
The addition of a methyl group decreases the rate of SN2 reaction and increases the rate of SN1 reaction . The reaction will occur mainly by the mechanism having the higher rate .

9. Hydrolyses of Alkyl Halides (contd.)
It therefore appears that the reaction occurs largely by
an SN2 mechanism for the methyl and ethyl iodides, but largely by the SN1 mechanism for the iso-propyl and
tert-butyl iodides .
The SN1 mechanism becomes gradually more important than the SN2 one as the methyl group are substitued .

10. Hydrolyses of Alkyl Halides (contd.)
If the nucleophilic reagent is varied ,becoming gradually less nucleophilic as shown in Fig (17) , the SN2 mechanism becomes gradually less important , until finally the SN1 mechanism takes over when the nucleophilic reagant is too weak to contribute much to reaction .

12. Hydrolyses of Alkyl Halides (contd.)
The vinyl and aryl halides are only hydrolyses with difficulty .
The reason is that the resonance of the type …
— C=C—X — C—C=X
hinders the ionization of the halide ion and hence makes reaction more difficult by either the SN1 or the SN2 mechanism .

13. Comparison of SN1 and SN2 Mechanisms of Nucleophilic Substitution in Alkyl Halides

16. Thank you for your attention