Halogenoalkanes hydroxide

We have seen that primary and secondary halogenoalkanes when attacked by a hydroxide anion normally undergo inversion at the carbon centre at which substitution occurs, because they proceed via an SN2 pathway. In the case of l-chloro-2-hydroxyalkanes, it is observed that when treated with a hydroxide... 

Reaction between halogenoalkanes and hydroxide or cyanide ions... 

A carbon-halogen bond is polar (C " -X ) and the carbon can be attacked by groups that carry an unshared pair of electrons. The result is a substitution reaction, in which one atom, ion or group is substituted for another. If some halogenoalkanes are heated with aqueous sodium hydroxide, the halogen is substituted by an -OH group, producing an alcohol ... 

The substitution reactions of halogenoalkanes with sodium hydroxide... 

Towards the end of Chapter 10 we introduced a classic example of a nucleophilic substitution reaction, namely the hydrolysis of a halogenoalkane by a warm aqueous solution of sodium hydroxide. As a result of the polarization of the carbon-halogen bond, the carbon atom is an electron-deficient centre susceptible to attack by a nucleophile such as the hydroxide ion (OH ). Primary halogenoalkanes are thought to undergo a substitution mechanism that involves a single reaction step. This one-stage reaction involves the simultaneous attack of the nucleophile and departure of the halide ion. We will use as an example the reaction between bromomethane and sodium hydroxide solution ... 

Tertiary halogenoalkanes also undergo a substitution reaction, but kinetic studies show that the mechanism is different from that occurring with primary halogenoalkanes. For example, consider the reaction between 2-bromo-2-methylpropane and hydroxide ions (Figure 20.7). 

Bromo-2-metbylpropane is a tertiary halogenoalkane and tbe hydrolysis of this compound proceeds by an S l mechanism (Figure 20.7). The hydrolysis is a first-order reaction, which means that the rate doubles if we double the concentration of the halogenoalkane, for instance. However, if we double the concentration of hydroxide ions, OH , the rate does not change at all. The rate depends only on the concentration of the bromoalkane and is independent of the hydroxide ion concentration. Kinetically this means that hydroxide ions cannot be involved in the rate-determining step. The mechanism is shown in Figure 20.9. 

The first step of the reaction involves only the heterolysis of the C—Br bond, forming the carbocation intermediate and a bromide ion. The original halogenoalkane is tetrahedral in shape around the target carbon atom. This is the slowest (rate-determining) step in the reaction, and the hydroxide ions do not participate in it. If the concentration of hydroxide ions were increased, the rate of the second step would also increase. But this second step is already faster than the first one, so the rate of the overall reaction is unaffected. 

When an aqueous solution of sodium hydroxide is added to a halogenoalkane, a nucleophilic substitution reaction takes place. The halogen atom in the halogenoalkane is replaced by an —OH, hydroxyl group, so the organic product formed is an alcohol ... 

The aqueous hydroxide ion behaves as a nucleophile here, because it is donating a pair of electrons to the carbon atom bonded to the halogen in the halogenoalkane. This is why the reaction is called a nucleophilic substitution. 

This mechanism is called an Sj 2 mechanism. The S stands for substitution and the N stands for nucleophilic. The 2 tells us that the rate of the reaction, which is determined by the slow step in the mechanism (see page 141), involves two reacting species. Experiments show us that the rate depends on both the concentration of the halogenoalkane and the concentration of the hydroxide ions present. 

A tertiary halogenoalkane reacts with a hydroxide ion by a two-step mechanism. The first step in the mechanism... 

Halogenoalkanes will also undergo elimination reactions when heated with ethanolic sodium hydroxide, forming alkenes. 

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