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Sodium Hydroxide (aq) is very effective in plucking off an acidic hydrogen. How effective will it be in substituting chlorine of an alkyl hallide (let us assume that the alkyl hallide is primary) and why ?

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Even if, The reagents you are using are the same for both substitution or elimination - the halogenoalkane and either sodium or potassium hydroxide solution. In all cases, you will get a mixture of both reactions happening - some substitution and some elimination. What you get most of depends on a number of factor.

Nucleophilic substitution reaction has $\ce{SN2}$ reaction mechanism.

For $\ce{SN2}$ favors the reaction with primary substituted alkyl halide more than secondary than tertiary, because its effected by steric hinderance. The more crowded it is, the more difficult it is for a nucleophile to attack.

So, whatever you do with tertiary halogenoalkanes, you will tend to get mainly the elimination reaction, whereas with primary ones you will tend to get mainly substitution.

For a given halogenoalkane, to favor elimination rather than substitution, use:

higher temperatures a concentrated solution of sodium or potassium hydroxide pure ethanol as the solvent

To favor substitution rather than elimination, use:

lower temperatures more dilute solutions of sodium or potassium hydroxide more water in the solvent mixture

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