ORGANIC CHEMISTRY 2
AROMATIC COMPOUNDS (ARENES)
These are organic compounds with benzene ring as functional group.
These are organic compounds with benzene ring as functional group.
Molecular formula of benzene is C6H6.
– It is a highly unsaturated molecule but it does not undergo reaction readily and it tends to undergo substitution reaction.
STRUCTURES OF BENZENE
Structure of benzene can be expressed (shown) by using;
i. Kekule structure
ii. Resonance structure
I. KEKULE STRUCTURE (1865)
According to Kekule;
– Structure of benzene is hexagonal (It is cyclic structure with six carbon atoms).
– In structure of benzene carbon-carbon double bond alternate carbon–carbon single bond.
– The structure of benzene is interconvertible.
STRENGTH OF KEKULE STRUCTURE
– It gives correct molecular formula of benzene which is C6H6.
– It is true that C-H bonds in benzene are all alike (This can be seen through x-ray diffraction).
WEAKNESS OF KEKULE STRUCTURE
– It fails to explain why benzene does not undergo addition reaction readily and it tends to undergo substitution reaction steadily.
– Through x-ray diffraction it can be seen that carbon–carbon bonds are equal throughout the benzene, a fact which cannot be explained by Kekule structure (According to Kekule structure there is C=C and C-C so it was expected that bond length of C=C to be shorter than that of C-C).
EXAMPLES OF ELECTROPHILIC SUBSTITUTION REACTIONS IN BENZENE
a) HALOGENATION
MECHANISM
i. Formation of an electrophile.
ii. Formation of intermediate carbonium ion.
iii. Formation of product and regeneration of catalyst.
Thus, overall reaction is
(b) ALKYLATION (FRIEDEL CRAFT ALKYLATION)
Friedel-Craft alkylation is the electrophilic substitution reaction between benzene and haloalkane under presence of Lewis acid catalyst to give alkylbenzene.
Generally;
Example.
MECHANISM
- i. Formation of an electrophile.
ii. Formation of intermediate carbonium ion.
iii. Formation of product and regeneration of catalyst.
Hence, overall reaction.
(c) ACYLATION (FRIEDEL CRAFT ACYLATION)
Friedel-Craft acylation is the electrophilic substitution reaction between benzene and acyl compounds under presence of Lewis acid catalyst to give aromatic ketone.
Generally;
MECHANISM
i. Formation of an electrophile.
ii. Formation of intermediate carbonium ion.
iii. Formation of product and regeneration of catalyst.
iv.
Thus, overall reaction is
(d) CUMENE FORMATION
Benzene reacts with propene under presence of acid medium to give isopropyl benzene (cumene).
MECHANISM
i. Formation of an electrophile.
ii. Formation of intermediate carbonium ion.
iii. Formation of product and regeneration of catalyst.
Hence, overall reaction.
(e) NITRATION
Benzene reacts with nitric acid under presence of sulphuric acid yielding nitrobenzene.
i.e
MECHANISM
i. Formation of an electrophile.
ii. Formation of intermediate carbonium ion.
iii. Formation of product and generation of catalyst.
Hence, overall reaction is
Also.
Above reaction sulphuric acid itself is a good Lewis acid (There is no need of another Lewis acid catalyst).
DIRECT EFFECT IN MONOSUBSTITUTED BENZENE
ACTIVATOR AND DEACTIVATOR
- Reactivity of benzene towards electrophile (in electrophilic substitution reaction of benzene) depends on the electron density in benzene ring.
- If the electron density is high then benzene will be more reactive towards electrophile and if it is low then benzene will be less reactive toward an electrophile.
- When substituents in benzene increase electron density in benzene ring, then the substituents are said to increase reactivity of benzene towards an electrophile.
- So any factor which affects the electron density in benzene ring is said to affect reactivity of benzene towards an electrophile.
- When substituents in benzene increase electron density in benzene ring, then the substituents are said to increase reactivity of benzene towards an electrophile, i.e., it is said to activate electrophilic substitution reaction of benzene and hence the substituent is known as ACTIVATOR.
· On the other hand, if the substituents decrease electron density in benzene ring, then the substituents are said to decrease reactivity of benzene towards an electrophile, i.e., it is said to deactivate electrophilic substitution reaction of benzene and hence the substituent is known as DEACTIVATOR.
Qn. How can we recognize if the substituents are activator or deactivator?
ANS
Before studying recognition of activators and deactivators, it is better to first study effects which cause activation and deactivation in benzene.
There are two effects which cause activation in benzene.
i. Positive Inductive effect (+I).
ii. Positive mesomeric effect (+M).
i. POSITIVE INDUCTIVE EFFECT (+I)
This is the effect which arises in organic compounds as a result of partial movement of electron pair towards the functional group (in this case benzene ring).
ii. POSITIVE MESOMERIC EFFECT (+M)
This is the effect which arises in organic compounds as a result of total movement of an electron pair towards the functional group (in case benzene ring) and move back again to its original position within the same molecule. Thus +M causes activation in benzene. Substituents which cause +M (in benzene) are those with atoms possessing lone pair or negatively charged atom and itself directly bonded to another atom by sigma (σ) bond.
Example
OH–, NH2–, RO–, X.
On the other hand, there are two effects which cause deactivation in benzene.
i. Negative Inductive effect (-I).
ii. Negative mesomeric effect (-M).
i. NEGATIVE INDUCTIVE EFFECT (-I)
This is the effect which arises in organic compounds as a result of partial withdrawal of an electron pair from functional group (in this case benzene ring).
Inductive effect deactivates the benzene by partial withdrawal of electron pair from benzene ring.
Substituents which cause (-I) are strongly electronegative atoms or electron attracting radicals.
Examples.
OH–, X, etc.
ii. NEGATIVE MESOMERIC EFFECT (-M)
This is the effect which arises in organic compounds as a result of partial withdrawal of an electron pair from functional group (in this case benzene ring) and then moving back again to the original position within the same molecule.
- So -M causes deactivation in benzene by withdrawal of an electron pair from benzene ring.
- Substituents which cause –M are those with atoms possessing lone pair or negatively charged electron and itself is bonded to atom by π-bond.
Example:
· There are cases where there is competition between mesomeric effect and inductive effect, i.e., the same substituent causes negative inductive and positive mesomeric effect (+M).
· When this occurs, in most cases mesomeric effects tend to outweigh inductive effects, i.e., when the same species causes –I and +M, the effect which will be considered is +M and these will be ACTIVATOR (not deactivator).
- Halogens are an exception to the above explanations, i.e., in halogens inductive effects tend to outweigh mesomeric effects. Why?
REASONS
- Halogens are the most electronegative elements among all substituents of benzene as a result of their smallest atomic size. This makes halogens exert the strongest negative inductive effect.
- On the other hand, halogens have the maximum number of lone pair electrons, thus making them less available for participation in mesomerism, thus halogens exert the weakest mesomeric effect among all substituents.
- So while halogens exert the strongest negative inductive effect, they also exert the weakest (-M) effect; hence in halogens inductive effect outweighs mesomeric effect.
- Generally, we can conclude that all substituents which cause positive inductive effect and those which cause positive mesomeric effect, except halogens, are ACTIVATORS. And all substituents which cause negative mesomeric effect with addition of halogens (which cause –I) are DEACTIVATORS.
DIRECTING EFFECT
Carbons in benzene with only one substituent group can be formed as follows;
If the substituent is activated, then it tends to direct incoming electrophile substituent at ortho and para positions, i.e., all activators are ortho–para directors.
This can be explained considering;
i. Position of carbonium ion.
ii. Stability of intermediate carbonium ion.
I. POSITION OF CARBONIUM ION
Understand this considering mesomerism of phenol in which OH– activator is directly attached to benzene ring.
Above mesomerism (+M). It can be seen that despite the fact OH– (activator) increases electron density throughout the benzene, ortho and para positions are more affected and hence ortho and para carbons become better sites for incoming electrophile.
II. STABILITY OF INTERMEDIATE CARBONIUM ION
1st CASE
Incoming electrophile attaches at ortho position. Consider electrophilic substitution reaction in aniline.
Mesomerism clearly shows that intermediate carbonium ion is stabilized by lone pair electrons in nitrogen of amino group (-NH2) and hence it is more stable.
2nd CASE
If incoming electrophile attaches (substitutes) at meta position, consider the same reaction in aniline.
- In this case intermediate carbonium ion is not stabilized by lone electrons of nitrogen in amino group and hence it is less stable.
3rd CASE
If incoming electrophile substitutes at para position, consider the same reaction in aniline.
· In this case carbonium ion is stabilized by lone pair of nitrogen in amino group hence it is more stable.
CONCLUSION
Since carbonium ions formed in 1st and 3rd cases are more stable than that formed in 2nd case, ortho and para positions are preferred sites for incoming electrophile.
NOTE:
- Alkyl groups act as ortho-para directors by partially neutralizing the positive charge formed on the adjacent carbon.
(The partial neutralization is done by positive inductive effect exerted by alkyl groups). Hence ortho-para directing of alkyl groups is simply explained by considering stability of intermediate carbonium ion like in (ii) above.
Are ortho-para directors due to stability of intermediate carbonium ion. This is simply because despite the fact that lone pairs in halogens have not good participation in mesomerism for reasons which have been explained, but in presence of positive charge on adjacent carbon lone pair electrons participate in neutralizing positive charge on the carbon.
- Among the two products (ortho product and para product), in most cases para product is the major product. Why?
Reason
Due to steric hindrance exerted by the substituent originally present in benzene, ortho carbons which are closer to the substituents experience the effect strongly and hence incoming electrophile is more favored to substitute at para carbon which is far from the substituent.
But if the substituent is halogen, ortho product becomes the major product. Why?
Reason
Halogens like Cl have very small atomic size, thus they exert very small steric hindrance, thus making incoming electrophile substitute first at ortho carbons (for every two ortho carbons there is only one para carbon).
Deactivators with exception of halogens direct incoming electrophile at meta position, i.e., deactivators (with exception of halogens) are meta directors.
This can be explained by considering
i. Position of carbonium ion
ii. Stability of intermediate carbonium ion.
From the above shown mesomerism it can be seen that despite the fact that carboxylic group (-COOH) deactivates the whole benzene ring, ortho and para positions are more affected and hence meta carbon somehow becomes preferred position for incoming electrophile.
II. STABILITY OF INTERMEDIATE CARBONIUM ION
Consider the electrophilic substitution reactions in benzoic acid.
1st CASE
If incoming electrophile substitutes at ortho position.
- Intermediate carbonium ion is not stable as a result of very large repulsion force between closer positively charged ions in adjacent carbons.
2nd CASE
If incoming electrophile substitutes at meta position.
- Carbonium ion formed in this case is somehow more stable as a result of comparatively small repulsion force between positively charged carbons which are not adjacent.
3rd CASE
If incoming electrophile substitutes at para position. Also intermediate carbonium ion formed is not stable as a result of very large repulsion force between closer positively charged ions in adjacent carbons.
CONCLUSION
Intermediate carbonium ion formed in second case is more stable than in 1st case and 3rd case and hence meta position is better site for incoming electrophile.
SUMMARY ON DIRECTING EFFECT
SOLVED PROBLEMS
QN 1. Arrange the following compounds in order of reactivity towards:
i. Nucleophile.
Qn. 2. Explain why alkylation of nitrobenzene is much slower than that of methylbenzene?
ANS
Alkylation in given compounds is electrophilic substitution reaction so presence of nitro group which is an electron withdrawing (deactivator) in nitrobenzene deactivates its reaction towards electrophile while presence of methyl group which is electron donating group (activator) in methylbenzene activates its reaction towards electrophile and hence alkylation of nitrobenzene is less than that of methylbenzene.
Qn. 03. Complete the following organic reactions.
Qn. 04. NECTA 1994
Write structural formula of main substitutional product in the following organic reactions.
Qn 05. NECTA 1993
Which substituent entered first in the following organic compounds giving reasons.
ANS
i. Either of the two substituents entered first.
Reason
In given compound OH and CH3 are para related and OH– and CH3 are ortho–para directors forming para product as a major product and hence either of the two entered first so as to direct incoming substituent at para position.
ii. NO2 entered first
Reason
In given compound CH3 and NO2 are meta related so being meta director it must be entered first so as to direct the incoming CH3 group at meta position.
iii. Cl entered first
Reason
In given compound OH and Cl are ortho related and OH and Cl are ortho-para directors, OH– forming para product as major product (as result of its large steric hindrance) while Cl forms ortho product as major product (as result of low steric hindrance) and Cl– must be entered first so direct at ortho position.
Reason
In given compound CH3 and COOH are para related, CH3 being ortho–para director forming para product as major product must be entered first so as to direct incoming –COOH at para position.
Qn 6.
Show how the following conversions can be achieved.
FURTHER CHEMICAL REACTIONS OF BENZENE
Apart from electrophilic substitution reactions benzene can undergo the following reactions.
i. ADDITION REACTIONS
- Under vigorous conditions benzene can undergo addition reaction.
eg.
(a) HYDROGENATION
Benzene can react with hydrogen under presence of nickel or platinum catalyst yielding cyclohexane.
i.e.
(b) CHLORINATION
- Under presence of U.V and very high temperature benzene reacts with chlorine to give 1,2,3,4,5,6-hexachlorocyclohexane.
TOLUENE (METHYL BENZENE)
- Toluene is the aromatic compound which is formed when one halogen atom of benzene is replaced by methyl group.
i.e. Structure of toluene is –
PREPARATION OF TOLUENE
(a) METHYLATION OF BENZENE
Generally;
Example:
(b) Reaction between halobenzene and halomethane under;
Presence of sodium and dry ether.
Generally.
Example
PHYSICAL PROPERTIES OF TOLUENE
- It is denser than water.
- It is soluble in non-polar solvents like organic solvents (Toluene itself is a good organic solvent).
- It melts at temperature of -95°C and boils at 111°C.
- Its vapor density is larger than that of air.
- It is a colorless liquid at room temperature.
In most cases toluene is used as organic solvent instead of benzene because it is less toxic.
CHEMICAL REACTION OF TOLUENE
Commonly toluene undergoes the following chemical reactions:
i. Side chain chemical reactions
ii. Electrophilic substitutions in benzene ring
I. SIDE CHAIN CHEMICAL REACTIONS
Under this heading toluene undergoes the following:
a) Oxidation
b) Free radical substitution reactions.
A) OXIDATION
With milder oxidizing agent like MnO2, benzaldehyde is formed.
i.e
But with strong oxidizing agent like KMnO4 and K2Cr2O7, acid is formed.
eg.
B) FREE RADICAL SUBSTITUTION REACTION
· With halogens under presence of U.V or very high temperature, toluene tends to undergo side chain radical substitution reactions.
II. ELECTROPHILIC SUBSTITUTIONS IN BENZENE RING
Consider this heading toluene undergoes similar chemical reactions as those of benzene. The only difference is that methyl group in toluene acts as ortho director forming para product as major product.
Example of electrophilic substitution reactions of Toluene
i. HALOGENATION
Generally
Example
ii. ALKYLATION
Generally
Example
iii. ACYLATION
Generally
SOME STRUCTURE OF AROMATIC COMPOUNDS & THEIR COMMON NAME
II. SECONDARY (20) HALOALKANE
These are haloalkanes where a carbon with halogen is directly bonded to two alkyl groups.
Thus for haloalkane with only one halogen the carbon with halogen is also directly bonded to only one hydrogen atom.
III. TERTIARY (30) HALOALKANES
These are haloalkanes where a carbon containing halogen is directly bonded to three alkyl groups.
Thus in tertiary haloalkanes there is no hydrogen atom which is directly bonded to carbon with halogen.
Haloalkanes are named by naming halogen as substituent of alkanes.
PREPARATIONS OF HALOALKANES
a). FREE RADICAL SUBSTITUTION REACTION OF ALKANES
Generally.
Example.
Ø The main reaction under this heading is halogenations of alcohol.
Examples
Involved in the above reaction produce denser white fumes with ammonia so that reaction is used as test of presence of halogen group in the compound.
B. REACTION WHICH INVOLVE OH– GROUP AND β-HYDROGEN.
Under this heading alcohol undergo elimination reaction to form alkene.
Example
The production of ether in above reaction is more favoured when H2SO4 is dilute and cold.
C. OXIDATION
Ø With weak oxidizing agent like
primary alcohol tends to form aldehyde.
Example
With strong oxidizing agent like
carboxylic acid is formed.
Example
– With either weak oxidizing or strong oxidizing agent secondary alcohol forms ketone.
NOTE
Tertiary alcohols resist oxidation since there is no hydrogen to be removed.
IODOFORM TEST
Ø This is the chemical test for presence of terminal methyl group which is directly bonded to carbon with OH– group or (to carbonyl group in the case of carbonyl compounds) by giving yellow ppt of trichloromethane (Iodoform) CHI3.
Ø When compounds with terminal methyl group bonded to carbon with OH group is heated with I2 in presence of NaOH they give yellow ppt of Iodoform.
Example
CHEMICAL TEST TO DISTINGUISH BETWEEN PRIMARY, SECONDARY AND TERTIARY ALCOHOLS.
– Three classes of alcohol can be distinguished by using Lucas reagent.
– Lucas reagent is the mixture of concentrated HCl and ZnCl2 mixed at equal proportion.
– With Lucas reagent at room temperature, primary alcohols do not form cloudiness or turbidity (insoluble substance) at all.
– Secondary alcohols tend to form turbidity (insoluble substance which is chloroalkane within 5 minutes).
– Tertiary alcohols tend to form turbidity immediately.
Example
Qn. Give chemical test to distinguish between
i. Ethanol and propanol (I test)
ii. Butan-1-ol and butan-2-ol (2 test)
iii. Butan-2-ol and 2-methylpropan-2-ol (2 tests apart from using Lucas reagent)
ANS
i. Ethanol gives positive Iodoform test while propanol gives negative Iodoform test.
ii.
1st TEST
With Lucas reagent at room temperature butan-2-ol gives turbidity (cloudiness) within 5 minutes while butan-1-ol does not give turbidity at all.
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