# Order of selectivity

Have you ever found the benzene rings attractive yet sophisticated?? Here are a few reactions with benzene, different other compounds when they have different types of groups attached to it and their mechanism explained in a very easy way for you to grab easily. Hope you find it much helpful for your preparation process.

Order of selectivity:

If an e⁻ withdrawing group is attached to the Benzene ring, then electrophile will get substituted at the meta position

If an e⁻ donating group is attached to benzene ring, then electrophile will be substituted on ortho or para position depending on the reactant

Example:Due to Resonance of CO₂H it withdraw from ortho & para position. Comparatively e⁻ density is more on meta position.

∴ NO₂⁺ attacks on meta position.

If in the above, substitute is already present on one meta position, then substitution occur on remaining meta position i.e. ifHaloform reaction:

This reaction is possible for aldehyde, ketone, alcohol & alkyl halide & is carried out in basic medium. It is the diagnostic test of keto-methyl group.

This reaction is one of the best method for oxidation of ketone into carboxylic acid CHI₃ forms yellow ppt

$$C{{H}_{3}}-\overset{OH}{\mathop{\overset{|}{\mathop{CH}}\,}}\,-C{{H}_{3}}\xrightarrow{{{X}_{2}}/NaOH}C{{H}_{3}}-\overset{O}{\mathop{\overset{||}{\mathop{C}}\,}}\,-OH+CH{{X}_{3}}$$.

$$C{{H}_{3}}-C{{H}_{2}}-OH\xrightarrow{-//-}+ve\,Haloformtest$$.

$$C{{H}_{3}}-\underset{C{{H}_{3}}}{\overset{C{{H}_{3}}}{\mathop{\underset{|}{\overset{|}{\mathop{C}}}\,}}}\,-OH\xrightarrow{-//-}-ve\,Haloformtest$$.

$$C{{H}_{3}}C{{H}_{2}}X\xrightarrow{-//-}+ve\,Haloform$$.

$$C{{H}_{3}}-\underset{X}{\mathop{\underset{|}{\mathop{CH}}\,}}\,-C{{H}_{3}}\xrightarrow{-//-}+ve\,Haloform$$.

$$C{{H}_{3}}-\overset{C{{H}_{3}}}{\mathop{\overset{|}{\mathop{\underset{C{{H}_{3}}}{\mathop{\underset{|}{\mathop{CH}}\,}}\,}}\,}}\,-X\xrightarrow{-//-}-ve\,Haloform$$.

$$C{{H}_{3}}-\overset{O}{\mathop{\overset{||}{\mathop{C}}\,}}\,-OH\to -ve\,Haloform$$.

$$C{{H}_{3}}-\overset{O}{\mathop{\overset{||}{\mathop{C}}\,}}\,-N{{H}_{2}}\to -ve\,Haloform\,test$$.

$$C{{H}_{3}}-\overset{O}{\mathop{\overset{||}{\mathop{C}}\,}}\,-Cl\to -ve\,Haloform\,test$$.

Condensation reactions:

This is the most important part of aldehydes & ketones. Condensation means formation of new carbon-carbon bonds by forming some smaller products from original reactant

Condensation reactions are examples of nucleophilic addition

Reactions r.d.s (rate determining step) is the conversion of SP² to SP³ hybridized atom

Aldol condensation:

It is possible for aldehydes of ketones having at least one α – hydrogen.

Aldol is β – hydroxyaldehyde or β –hydroxyketone

$$2C{{H}_{3}}-\overset{O}{\mathop{\overset{||}{\mathop{C}}\,}}\,H\xrightarrow{NaOH/{{H}_{2}}O}\underset{(\beta -hydroxy\,\,aldehyde)}{\mathop{C{{H}_{3}}-\overset{OH}{\mathop{\overset{|}{\mathop{C}}\,}}\,\underset{\beta }{\mathop{H}}\,-\underset{\alpha }{\mathop{C{{H}_{2}}}}\,-\overset{O}{\mathop{\overset{||}{\mathop{C}}\,}}\,H}}\,$$.

Mechanism:

$$C{{H}_{3}}-\overset{O}{\mathop{\overset{||}{\mathop{CH}}\,}}\,\xrightarrow{NaOH/{{H}_{2}}O}C{{H}_{2}}^{-}-\overset{O}{\mathop{\overset{||}{\mathop{CH}}\,}}\,$$.

$$C{{H}_{3}}-\underset{H}{\overset{\sigma }{\mathop{\underset{|}{\overset{|}{\mathop{C}}}\,}}}\,-C{{H}_{2}}-\overset{O}{\mathop{\overset{||}{\mathop{C}}\,}}\,H\xrightarrow{{{H}_{2}}O}\,\,C{{H}_{3}}-\overset{O}{\mathop{\overset{||}{\mathop{C}}\,}}\,H-C{{H}_{2}}-\overset{O}{\mathop{\overset{||}{\mathop{C}}\,}}\,H$$.

$$\underset{Final\,product}{\mathop{C{{H}_{3}}-CH=CH-\overset{O}{\mathop{\overset{||}{\mathop{C}}\,}}\,H}}\,$$.

For ketone:

$$2C{{H}_{3}}-\overset{O}{\mathop{\overset{||}{\mathop{C}}\,}}\,-C{{H}_{3}}\xrightarrow{NaOH/{{H}_{2}}O}C{{H}_{2}}-\overset{O}{\mathop{\overset{||}{\mathop{C}}\,}}\,-C{{H}_{3}}+C{{H}_{3}}-\overset{{{O}^{-}}}{\mathop{\overset{||}{\mathop{{{C}_{+}}}}\,}}\,-C{{H}_{3}}$$.

$$C{{H}_{3}}-\underset{C{{H}_{3}}}{\overset{{{O}^{-}}}{\mathop{\underset{|}{\overset{|}{\mathop{C}}}\,}}}\,-C{{H}_{2}}-\overset{O}{\mathop{\overset{||}{\mathop{C}}\,}}\,-C{{H}_{3}}\xrightarrow{{{H}_{2}}O}\underset{(\beta -hydroxy\,ketone)}{\mathop{C{{H}_{3}}-\underset{C{{H}_{3}}}{\overset{OH}{\mathop{\underset{|}{\overset{|}{\mathop{C}}}\,}}}\,-C{{H}_{2}}-\overset{O}{\mathop{\overset{||}{\mathop{C}}\,}}\,-C{{H}_{3}}}}\,$$.

$$\underset{Final\Pr oduct}{\mathop{C{{H}_{3}}-\overset{C{{H}_{3}}}{\mathop{\overset{|}{\mathop{C}}\,}}\,==CH-\overset{O}{\mathop{\overset{||}{\mathop{C}}\,}}\,-C{{H}_{3}}}}\,$$.

Note: Formaldehyde doesn’t undergo aldol condensation

Ex:$$Ph-\overset{O}{\mathop{\overset{||}{\mathop{C}}\,}}\,H+CH-\overset{O}{\mathop{\overset{||}{\mathop{C}}\,}}\,H$$→ will this reaction be called aldol condensation reaction?