1) - Predict the order of reactivity of the following compounds in $S_{N} 1 and S_{N} 2$ reactions:

A) - The four isomeric bromobutanes

2) - Predict the order of reactivity of the following compounds in $S_{N} 1 and S_{N} 2$ reactions:

B) - $C_{6} H_{5} C H_{2} B r, C_{6} H_{5} C H (C_{6} H_{5}) B r, C_{6} H_{5} C H (C H_{3}) B r, C_{6} H_{5} C (C H_{3}) (C_{6} H_{5}) B r$

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Solution

1.Order of reactivity for $S_{N} 1$ reaction
In $S_{N} 1$ reaction, the intermediate carbocation is formed. So, as the stability of carbocation increases, reactivity towards $S_{N} 1$ also increases.

Stability of carbocation $\propto + R > + H > + I \propto \frac{1}{- R > - H > - 1}$

Order of stability of carbocation follow as:

$I V > I I I > I I > I$

Therefore, order of reactivity towards $S_{N} 1$ reaction follows as:

$C H_{3} C H_{2} C H_{2} C H_{2} B r < (C H_{3})_{2} C H C H_{2} B r < C H_{3} C H_{2} C H (B r) C H_{3} < (C H_{3})_{3} C B r$

Order of reactivity for $S_{N} 2$ reaction
The reactivity of $S_{N} 2$ reactions follow the reverse order as the sterric hinderance around the electrophilic carbon increases in that order.

As the number of alkyl or aryl group increases steric hinderance also increases and reactivity towards $S_{N} 2$ decreases.

Hence, order of reactivity towards $S_{N} 2$ reaction follows as:

$C H_{3} C H_{2} C H_{2} C H_{2} B r > (C H_{3})_{2} C H C H_{2} B r > C H_{3} C H_{2} C H (B r) C H_{3} > (C H_{3})_{3} C B r$

2)- Order of reactivity for $S_{N} 1$ reaction
In $S_{N} 1$ reaction, the intermediate carbocation is formed. So, as the stability of carbocation increases, reactivity towards $S_{N} 1$ also increases.

Stability of carbocation decreases as:

Resonance effect > Hyperconjugation effect also, $3^{\circ} > 2^{\circ} > 1^{\circ}$

Therefore, order of reactivity towards $§_{N} 1$ reaction follows as:

$C_{6} H_{5} C (C H_{3}) (C_{6} H_{5}) B r > C_{6} H_{5} C H (C_{6} H_{5}) B r > C_{6} H_{5} C H (C H_{3}) B r > C_{6} H_{5} C H_{2} B r$

Order of reactivity for $S_{N} 2$ reaction

The reactivity of $S_{N} 2$ reactions follwo the reverse order as the steric hindercance around the electrophilic carbon increases in that order.

Hence, order of reactivity towards $S_{N} 2$
reaction follows as:

$C_{6} H_{5} C (C H_{3}) (C_{6} H_{5}) B r < C_{6} H_{5} C H (C_{6} H_{5}) B r < C_{6} H_{5} C H (C H_{3}) B r < C_{6} H_{5} C H_{2} B r$

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S_{N} 1

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