Aromaticity

Short Revision for Aromaticity, suitable for JEE and IAT! Answer mode: Answer with Definition. Question mode: just flashcards. I'll add more and make it more comprehensive in the future maybe!

What is the general formula for aromatic compounds?

$$C_nH_{2n-6y}$$

where:

• $n$ is the number of carbons

• $y$ is the number of rings

What does naphthalene look like?

What is Huckel’s rule?

if a compound has $4n+2$ $\pi$ electrons (where $n$ is whole number), the compound is aromatic.

if a compound has $4n$ $\pi$ electrons (where $n$ is whole number), the compound is anti-aromatic.

if the number of $\pi$ electrons does not fit those two rules, the compound is nonaromatic.

What should be the hybridisation of carbons in aromatic and antiaromatic compounds?

$sp²$ 

it can be $sp$ in very big rings

What is the stability order of aromatic, nonaromatic, and anti-aromatic compounds?

aromatic > nonaromatic > anti-aromatic

What is the aromaticity of this compound?

aromatic

What is the aromaticity of this compound?

anti-aromatic

What is the aromaticity of this compound?

aromatic

What is the aromaticity of this compound?

anti-aromatic

What is the aromaticity of this compound?

non-aromatic

What is the aromaticity of this compound?

anti-aromatic

What is the aromaticity of this compound?

aromatic

What is the aromaticity of this compound?

aromatic

What is the aromaticity of this compound?

aromatic

What is the aromaticity of this compound?

aromatic

What is the aromaticity of this compound?

nonaromatic

What is the aromaticity of this compound?

it’s nonaromatic because it’s not planar

What is the aromaticity of this compound?

aromatic

What is the aromaticity of this compound?

aromatic

When preparing arenes from alkanes, what reagent do you use? (there are 2 choices)

$Cr_2O_3$ or $Al_2O_3$ and heat

What are the byproducts when forming arenes from alkanes?

$4H_2$ 

What is the reagent when forming arenes from alkynes?

red hot iron tube

What is the reagent when forming arenes from phenol?

$Zn$ and heat

(it is a reduction reaction)

What is the reagent when forming arenes when there is an aldehyde present?

(i) $NH_2-NH_2$

(ii) $OH^-/\Delta$ 

What is the reagent when forming arenes when there is a ketone present?

$Zn-Hg$ and $HCl$

What is the reagent when forming arenes from benzoic acid?

Soda Lime! ($NaOH+CaO$) and heat

What is the byproduct formed when forming arenes from benzoic acid?

$$CO_2$$

What is the reagent when forming arenes from sulphonic acid?

$H_3O^+/\Delta$
yes it is hydrolysis

What is the reagent when forming arenes from a diazonium salt?

$$H_3PO_2/C_2H_5OH$$

What is the reagent when forming arenes from Grignard’s Reagent?

$H_2O$ or $C_2H_5OH$

What is the byproduct when forming arenes from Grignard’s Reagent

$$Mg(OH)Br$$

How do you form arenes from Wurtz’s fitting reaction?

You take two chlorobenzenes and react them with $Na/\text{dry ether}$ and heat to form this thang

What is the substrate when forming arene from Wurtz’s fitting reaction?

chlorobenzene

What is the reagent when forming arene from Wurtz’s fitting reaction?

$$Na/\text{dry ether}$$

What is the characteristic reaction of benzene, or benzene derivatives?

Electrophilic Substitution Reaction

What is the mechanism of Electrophilic Substitution Reactions for arenes?

An electrophile attacks one of the carbons on benzene, which breaks one of its pi bonds. This leaves a positive charge on one of the adjacent carbons, which resonates through ortho and para positions. This intermediate has lost its aromaticity. It’s called arenium ion.

A nucleophile comes along and abstracts the hydrogen proton that is attached to the same carbon as the electrophile, leaving the extra electron to form a bond with the adjacent carbon with positive charge, thus gaining back aromaticity.

How does the electron density on the benzene ring affect the reactivity of Electrophilic Substitution Reactions on the benzene ring?

Reactivity increases with increase in electron density.

How does the stability of the Arenium ion affect the rate of the Electrophilic Substitution Reaction?

the stability of the Arenium ion increases the rate of reaction

How does the electron-donating-nature of the electron donating group affect the rate of Electrophilic Substitution Reaction?

the more the electron-donating-nature, the greater the rate of the reaction.

What does the Potential Energy graph look like for Electrophilic Substitution Reactions of Arenes?

What is the characteristic of Ortho/Para directing groups on Arenes?

they provide $+M/+I/+H$ effects

They increase the electron density on benzene ring at ortho and para positions.

Is $-NH_2$ an activating group or not?

yeah

Is $-NHR$ an activating group or not?

yeah

Is $-NR_2$ an activating group or not?

yeah

Is $-OH$ an activating group or not?

yeah

Is $-OR$ an activating group or not?

yes

Is $-NHCOR$ an activating group or not?

yes

Is $-SH$ an activating group or not?

yes

Is $-OCOR$ an activating group or not?

yes

Is $-CH_3$ an activating group or not?

yes

What is the characteristic of Meta directing groups on Arenes?

they provide $-M/-H/-I$ effect

they decrease the electron density at ortho and para positions on benzene ring

Are $+M$ groups on benzene activating or deactivating groups?

activating

Are $-M$ groups on benzene activating or deactivating groups?

deactivating

Is $-CHO$ an activating group or not?

nope

Is $-COOH$ an activating group or not?

nope

Is $-COOR$ an activating group or not?

nope

Is $-COR$ an activating group or not?

nope

Is $-CN$ an activating group or not?

nope

Is $-NO_2$ an activating group or not?

nope

Is $-SO_3H$ an activating group or not?

nope

Is $-CCl_3$ an activating group or not?

nope

Why is $FeCl_3$ a Lewis Acid?

Because $Fe$ accepts a Lone Pair into its vacant d-orbital.

Why is $AlCl_3$ a Lewis Acid?

Because $Al$ has an incomplete octet and can accept 2 more electrons.

If benzene and chlorobenzene are present in a solution together, what would an electrophile prefer to react with?

benzene.

$-I$ of chlorobenzene wins over $+M$ 

Is $-Cl$ ortho/para directing or meta directing?

Ortho/Para directing

$+M$ wins over $-I$.

In Halogenation of benzene, what is the reagent?

$Cl_2$ or $Br_2$ with $FeCl_3$

In Halogenation of benzene, what could be the byproducts?

$$HCl/HBr$$

In Halogenation of Toluene, if you want to add the halogen to the benzene ring, what reagent do you use?

$Cl_2/Br_2$ with $FeCl_3/FeBr_3$ 

In Halogenation of Toluene, if you want to add the halogen to the alkyl group, what reagent do you use?

$Cl_2/Br_2$ with sunlight ($h\nu$)

In Nitration of toluene, what reagent do you use?

concentrated $HNO_3\ +$ concentrated $H_2SO_4$

In Nitration of toluene, what is the electrophile?

$$NO_2^+$$

In Sulphonation of Toluene, what reagent do you use?

concentrated $H_2SO_4$ 

In Sulphonation of Toluene, what is the electrophile?

$$HS^+O_3$$

What is alkylation of benzene?

substitution of alkyl group ($R$) to benzene ring

What is acylation benzene?

substitution of acyl group ($COR$) to benzene ring

What is acetylation benzene?

substitution of acetyl group ($COCH_3$) to benzene ring

During Alkylation of benzene, what are the possible reagents used? (three of them)

1. $R-Cl$ and a Lewis acid

2. $R-OH$ and $H^+$

3. alkene and either $H^+$or a Lewis Acid

Lewis Acids can be $FeCl_3/AlCl_3/ZnCl_2$

What is the electrophile in Alkylation of arenes?

$$R^+$$

Why can’t you use hydrous $AlCl_3$ during Alkylation of arenes?

the $Al$ atom can accept lone pair from the oxygen from water as well, leading to no formation of $R^+$.

What is Friedel-Craft Reaction?

Alkylation or Acylation of Benzene and Benzene Derivatives

In Acylation of Benzene, what are the possible reagents? (2 possible)

1. Acid Halide ($RCOX$) and a Lewis Acid

2. Acid Anhydride ($RCOCR$) and a Lewis Acid

Lewis Acids can be $FeCl_3/AlCl_3/ZnCl_2$