https://docs.google.com/document/d/1W7ms3UMM8kwZNzPprNAmp07P9kucjVOj/edit?usp=sharing&ouid=109474854956598892099&rtpof=true&sd=true
MODULE – III
📷
Contents:
1. General Organic Chemistry
2. Hydrocarbons
3. Electrophilic Aromatic Substitution (General Mechanisms Only)
4. Alkyl Halides, Alcohols and Ethers
5. Qualitative Analysis
📷
P L A N N I N G
In general organic chemistry, we would emphasise on the basics of organic chemistry, which are very important to you to understand organic chemistry in a more lucid way.
In hydrocarbons, we will see about the various preparation methods and properties of hydrocarbons and last but not the least we will discuss about stereo specific reactions.
In Electrophilic aromatic substitutions, we will see the general mechanisms for the reactions, and also about orientation of electrophiles in a monosubstituted benzene ring
In alkyl halides, alcohols and ethers our main emphasis will be on substitution, elimination and we will also see the other properties of these compounds.
In qualitative analysis, we will basically see the reagents that can separate cations and anions and also about the reactions of cations and anions.

GENERAL ORGANIC CHEMISTRY

The chemistry dealing with study of carbon and its compounds can be termed as organic chemistry.
If we observe carefully, we find that in organic chemistry, carbon is involved in covalent bonding with other elements, and concepts of covalent bonding are expressed in terms of hybridisation.
Hybridisation:
The first question which arises in our mind is how to find the hybridisation by looking at structure of molecule, then the answer is quite simple i.e.,
(i) If a carbon is bonded to four atoms then it involves sp3 hybridised orbitals for formation of that compound.
(ii) If a carbon is bonded to three atoms then hybridisation is sp2
(iii) If a carbon is bonded only to two atoms then hybridisation is sp
Illustration 1: Identify the hybridisation of each carbon atom in the following compounds
(a) 📷 (b) 📷
(c) Allene (carbon) (d) Allylene (carbon)
(e) Acetyl Acetone (f) Acetonyl Acetone
(a) 📷, here carbon is attached to 3 other atoms i.e., 1 oxygen and 2 hydrogens ∴ it is sp2 hybridised
(b) 📷, sp3 hybridised
(c) Allene is propadiene i.e., H2C = C = CH2, here central carbon is sp hybridised while terminal carbons are sp2 hybridised.
(d) Allylene is propyne i.e., 📷
CH3 carbon is sp3 hybridised C – 2, C – 3 carbons are sp hybridised
(e) 📷 1,3,5 carbons are sp3 hybridised
2,4 are sp2 hybridised
(f) 📷
C –1, C – 3, C – 4, C – 6 are sp3 hybridised whereas C –2, C –4 are sp2 hybridised
Fission of Covalent Bond
After knowing the concept of hybridisation, let us now try to know how many ways, various organic compounds undergo reactions. If we consider an organic reaction A + R – X ⎯⎯→ A – R + X, in this reaction, the bond between R – X is broken and a new bond is formed between A – R, the mechanism of reaction depends on how the bonds were broken, the breaking of bonds can take place in two ways.
(a) 📷 ————→📷
If the bond is broken such that each atom is taking one electron, then this cleavage is called as homolytic fission.
(b) R – X ⎯⎯→ R+ + :X–
R — X⎯⎯→:R– + X+
If the bond breaks in a way such that both the electrons of bond are taken by the same atom, then this is called heterolytic fission.
Depending on the above two processes, organic reactions are mainly classified as
📷
Apart from the above the two other important types of organic reactions are molecular rearrangement and pericylic reactions.
Reactions which involve homolytic fission, proceed via free radical intermediates, some examples of this type will be seen in free radical substitution, free radical addition reactions. On the other hand, reactions involving heterolytic fission may lead to formation of either an electron deficient carbon or electron excess carbon, which are usually called as carbocations and carbanions respectively.
In 📷 if A is more electronegative than carbon, then a carbocation 📷—A → 📷 or carbonium ion is formed, so now any negatively charged species or species containing lone pair of electrons can attack this resulting in the formation of products. Such type of reactions 📷 ⎯⎯→ 📷 are said to be involving carbocation intermediates In 📷——A if A is less electronegative than carbon then a carbanion can be generated 📷——— A ⎯⎯→ 📷 + A+
If any positively charged species or electron deficient species can attack and result in the formation of products. This reaction involves a carbanion intermediate
📷 + B+ ⎯⎯→ 📷
Depending upon their nature the attacking species in organic reactions are classified as

1. Free radicals
   
2. Electrophiles
   
3. Nucleophiles
   

Free radicals are the species containing only one unpaired electron. They are very reactive and their life times are extremely short in solution state. Free radicals are usually obtained by thermal or photochemical cleavage of molecules.
eg: 📷 📷
📷 📷
Electrophiles are species which contain positive charge or electron deficient compounds without any charge on them
📷
Ex: H+, Br+, Cl+, 📷, 📷, 📷 BF3,AlCl3,ZnCl2 carbenes, FeCl3
📷, 📷
Nucleophiles are the species which contain negative charge or the molecules where one of the atom contains lone pair of electrons.
📷
X–, 📷 📷
CH ≡ C– etc. 📷
After knowing about the types of species that can attack a carbon, now let us see the mechanistic part of reactions.
Mechanism is the pathway of the conversion of reactants to products. For studying the organic reactions we have to go through the field effects which are responsible for these reactions, depending on these effects we can explain why certain substrates undergo reactions by only one particular type of mechanism.

Effect of structure on reactivity of organic compounds

The effect of structure on reactivity can be divided into three major types: field, resonance and steric. First let us see about the field effects.
Inductive effect: The effect because of which there is permanent displacement of electrons towards the more electronegative atom is termed as inductive effect.
Inductive effect is always transmitted along a chain of carbon atoms
📷
Now since chlorine has greater electronegativity than carbon, the electron pair between C1 and Cl will be displaced more towards the chlorine atom because of which C1 acquires a small positive charge which will be less than that on C1 and in the same way C3 also acquires a small positive charge, as length of carbon chain goes in increasing, this effect starts diminishing, i.e., after C3 carbon inductive effect will be negligible. It is represented by symbol →⎯, where the arrow points towards more electronegative atom
📷 →⎯ 📷 →⎯ 📷→⎯ 📷→⎯ 📷
Any atom or group which attracts electrons more strongly than hydrogen in R3 C-H is said to have negative inductive effect. In the groups are also said to be electron attracting or electron withdrawing groups.
Any atom or group which attracts electrons less strongly than hydrogen in R3 C-H is said to have positive inductive effect. The groups are also said to be electron repelling or electron releasing.
Some of the examples of +I and –I effect causing groups
– I effect
+ I effect
NO2, -CN, -COOH, F,Cl, Br, OAr, COOR,OR,OH,Ph2, = CH2, 📷, 📷
-CR3, -CHR2, -CH2R,-CH3, 📷, COO–
Illustration 2: Assign the following groups to be either exhibiting +I or –I effect in the following molecules ?
📷
Solution : (i) 📷 is an –I effect causing group
(ii) 📷 -s - I effect causing group
(iii) –OC6H5 is –I effect causing group
(iv) 📷 is +I effect causing group
Applications of Inductive effect
Increase in –I effect decreases the bond length
C-F, C – Cl, C – Br, C – I
C-F bond has shortest bond length
Stability of Carbocations:
Positive inductive effect causing group increases the stability of carbocations
📷 , 📷, 📷, 📷
As number of positive inductive effect causing groups increases, they will release electrons and hence the electron deficiency on the carbon atom containing positive charge will decrease and hence as an uncharged or less charged structure is more stable than the charged ones, its stability will be greatly increased.
Negative inductive effect increases the acidity of carboxylic acids. As the distance between negative inductive effect causing group and –COOH group increases, the acidic character will decrease.
Ex: 📷, 📷, 📷, 📷
📷 📷 📷
In the above example the decreasing order of acidity is as follows
III > IV > V > VI > I > II
Basic Character of amines:
The basic character of amines is due to the presence of unshared electron pair on nitrogen atom which accepts proton.Due to +I effect of alkyl groups, nitrogen atom becomes richer in electron because of which the lone pair of electrons on nitrogen atom is more easily available in amines than in ammonia. Trifluoro amine has no basic property due to negative inductive effect of fluorine.
Therefore the relative basic character of amines on the basis of inductive effect alone should be 3° amine > 2° amine > 1°amine
However the above order was not found to be true instead the order is 2° amine > 1° amine > 3° amine.
We say that steric hindrance is the only reason why 3°amine is least basic, because is amines, the three bonding pairs and the lone pair occupy sp3 orbitals, and a lone pair causes crowding of bonding pairs, protonation will relieve crowding by transforming the lone pair to a bond pair. In going from Me2NH to Me3N, the ion becomes less solvated and so the stabilisation is lost and as decrease in solvation is more pronounced than the lone pair availability due to inductive effect, hence the basic character decreases.
Mesomeric Effect:
The phenomenon in which electrons are displaced from a double bond to adjacent single covalent bond, or from a double bond to an atom or from an atom containing unshared pair of electrons to the adjacent single covalent bond, is termed as mesomeric effect or resonance effect.
📷
📷
In mesomeric effect or resonance effect if electrons are transferred away from an atom or group then it is said to be causing –M effect.
📷
Here Y is said to be –M effect causing group
If the electron pair is transferred towards the aotm, then it is said to be causing +M effect.
📷
+M effect causing groups
-M effect causing groups
-📷, 📷 etc.
📷
📷N, -SO3H
-COOH, -COOR etc
Applications of Mesomeric effect:
Basic character of amines:
Aromatic amines are weakly basic when compared aliphatic amines because in aromatic amines the lone pair of electrons are not readily available due to resonance.
📷
Bond length in Allylic system
In the case of CH2 = CH – CH2Cl, there should be two bond lengths i.e, C – C and C = C but only one bond length is found out whose value is intermediate value of C = C and C – C because due to resonance C – C be changed to C = C.
Here, the atoms covered the localised electron by delocalised electrons must lie in a plane. In picryl iodide, bondlengths, for ortho and para nitro groups are different. Oxygens of p-introgroup are in the plane of the ring and b has partial double character, while the oxygen of o – nitrogroups are forced out of plane by large iodine atom
📷
Hyper conjugation : Delocalisation which takes place involving ‘σ’ electrons is normally termed as hyperconjugation. When a carbon attached to atleast one hydrogen is attacked to an unsaturated atom or one with an unshared orbital, canonical structures can be written which are termed as hyper conjugative structures.
📷 ←⎯→ 📷
In the above example, overlap of the σ orbital of C – H bond and π orbital of –C bond is resulting in hyperconjugation.
Till now we have gone through all the fundamental aspects required for knowing about an organic reaction, now we shall look into some structural features of organic compounds.
Isomerism: Isomers are the compounds containing same molecular formula but different properties. It is divided into following types.
📷
Of all the above types we will discuss about functional group; tautomerism and stereo isomerism.
Functional Group isomerism:
Class of organic compound
Gen. Formula
Functional Gp isomer
Alkanes
CnH2n+2
-
Alkenes
CnH2n
Cycloalkanes
Alkynes
CnH2n-2
Cycloalkenes, alkadienes
Alkyl halides
CnH2n+1X
-
Alcohols
CnH2n+1 OH
Ethers
Aldehydes
CnH2nO
Ketones, Cyclic ethers
Carboxylic acids
CnH2nO2
Esters, hydroxyaldehydes
Tautomerism:
This is a phenomenon which involves rapid shifts back and forth among the molecules
Keto – enol Tautomerism 📷 📷 📷
R′ = H, alkyl , if then the equilibrium lies more towards the left, keto form has C – H, C – C, C = O bonds where as enol form has C – O, C = C, O – H,. The bond energies prove that keto form is thermodynamically more stable than enol by about 12 Kcal / mol.
Stereoisomerism
The isomers which differ only in the orientation of atoms in space are known as stereoisomers.
Optical activity:
Any material that rotates the plane polarised light is said to be optically active. The property of non superimposability of an object on its mirror image is called as chirality. One of the main preliminary tests for knowing whether a compound is optically active or inactive is presence of symmetry.
The symmetry elements can be further classified into (I) Axis of symmetry (2) Plane of symmetry (3) Centre of symmetry.
If a molecule is rotated through an axis 📷, where ‘n’ is even no after one followed by reflection in a plane perpendicular to the original axis and results in an identical molecule, then it is said to posses n – fold alternating axis of symmetry.
Example :
📷
Althoguh the molecule is resulting in an identical by rotation about 360, but because of plane of symmetry’s presence it is a chiral.
📷
I and III are identical hence here C2 axis of symmetry is present
Meso tartaric acid
📷
Different arrangement of atoms (groups) in space in a molecule that can be readily converted into one another is known as conformation. For this we have to know a bit about various projection formulae.
C2H6 (ethane)
📷
----- Indicates that atoms are behind the plane (α)
Projecting atoms in front of the plane (β)
__ In the plane
Conformations of ethane : - X3C – CX3 (X = H)
In case of ethane two conformations are possible I indicates eclipsed conformation where the energy is high and the other one is the more stable staggered conformation.
📷
Eclipsed is less stable than staggered by 3Kcal / mole due to presence of bond eclipsing (or) bond opposing or Torsional or Pfitzer’s strain.
📷
Comparison of stability of most preferred Meso and Active compounds
📷
This is known by finding out the number of interactions in the system
Meso
Active
2L-s
2L-s
2L-M
2L-M
2M-S
1M-M
​
1S-S
AS (EM-M + ES-S) > E2 (m-s)
The most preferred meso is more stable than most preferred active when gauche interactions are repulsive.
Illustration 3: Meso 2,3 butane diol reacts slowly with a carboxyl compound than active 2,3 butane diol, explain the reason.
Solution:
📷
📷
After seeing the conformational isomers, let us now begin our exploration into compounds containing asymmetric carbons.
An organic compound containing ‘n’ number of asymmetric carbons should have 2n number of optical isomers. Let us consider the example of tartaric acid.
COOH —CHOH — CHOH — COOH
The molecule can be represented in plane formula as below.
📷
Formula (IV) on rotation through 180° in plane of paper becomes identical with III. Now the number of isomer is 3. If the force which rotates the plane of polarised light is from H to OH then.
(i) Will rotate plane of polarised light to the right
(ii) Will rotate plane of polarised light to the left
(iii) Upper half will rotate to the right while the lower half will rotate to the left and hence the compound will be optically inactive
(I) and (ii) are enantiomers
HYDROCARBONS
Hydrocarbons are the compounds containing carbon and hydrogen. They are divided into two types.
📷
Hydrocarbons containing only single bonds are known as saturated hydrocarbons, alkanes are examples of this type of compounds. Let us discuss first about alkanes and then we will continue our discussion with the other hydrocarbons.
Alkanes: These are the compounds containing C – H, C – C bonds. All the bonds present are ‘σ’ bonds. Their general formula is CnH2n+2.
Methods of Preparation:
(i) From Alkyl halides
(i) Reduction : - Alkyl halides are reduced in presence of LiAlH4, LiEt3BH, NaBH4 etc. R – X 📷
LiAlH4 can reduces Alkyl halides of all types i.e., vinylic, bridgehead and cyclopropyl halides also.
LiEt3BH reduces primary, secondary, allylic, benzylic and neopentyl substrates but not tertiary or aryl halides.
NaBH4 can be used for reduction in the presence of –COOH, -COOR, -COO groups without affecting them.
One halogen of a gem dihalide can only be reduced by using BH4SnH.
(b) Wurtz reaction: Two moles of Alkyl halide undergo reaction with sodium metal to give an alkane.
R – X + Na + Na + X – R ⎯⎯→ R – R
The basic point which we have to rember here is that an alkane containing double the number of carbon atoms as that of alkyl halide is obtained.
Mechanism: Two types of mechanism have been proposed for this reaction
(a) C2H5 – Br + 2Na ⎯⎯→ C2H5–Na+ + NaBr
C2H5– – Na+ + C2H5Br ⎯⎯→ C2H5- C2H5 + NaBr
(b) C2H5 – Br ⎯⎯→ 📷 + 📷
📷⎯⎯→ C2H5 – C2H5
Best yields of alkanes are obtained by using single alkyl halide i.e, if we take one mole of an alkyl halide R – X and second mole of other halide R′ - X mixture of products is obtained.
📷
(2) Reaction with organometallic compounds
Alkyl bromides, chlorides and iodides react with dialkyl copper in ether or THF to give alkanes
R – X + R′2CuLi ⎯⎯→ R - R′
R May be Primary alkyl , allylic, benylic, aryl, vinylic, or allenic and may contain Keto, COOH, COOR and CONR2 groups. R′ in R′2 CuLi may be primary allyl, vinylic allylic or aryl. R′2 does not react with ketones.
📷
(3) From Carboxylic acids
(a) Sodium salt of a carboxylic acid on treatment with soda lime gives alkane containing one carbon less than the original carboxylic acid
R — 📷— ONa + NaOH 📷 R — H + Na2CO3
(v) Kolbe’s electrolysis: -
When sodium salt of a carboxylic acid is subjected to electrolysis an alkane is obtained.
2CH3COONa ⎯⎯→ CH3 — CH3 + NaOH + CO2
Properties of alkanes
Halogenation: Alkanes undergo halogenation by free radical mechanism
📷
Mechanism
X2 📷 📷 ⎯⎯→ (1) Chain initiating
📷+ RH ⎯⎯→ R – X + 📷 ⎯⎯→ (2) Chain Propagating
📷📷 + 📷 ⎯⎯→ R – R } Chain terminating
The various halogenation agents are sulfuryl chloride (SO2Cl2), mixture of Br2 and HgO, NBS, CCl4, Cl2O, COCl2. In all these cases chain initiating catalyst is required.
A tertiary hydrogen is easily abstracted when compared to secondary hydrogen which is easily abstracted when compared to primary hydrogen. The ratio of their reactivity towards chlorination is 5.0 : 3-8:1.0. In bromination ratios are 1600:82:1 respectively.
Nitration: They undergo vapour phase nitration to yield nitroalkanes
CH3 – CH3 📷 CH3 – CH2 NO2
Pyrolysis: Alkanes undergo decomposition by the action of heat to yield smaller alkanes and alkenes.
Alkenes
Compounds containing C = C functional group are termed as alkenes. Their general formula is CnH2n
Alkenes can exhibit geometrical isomerism. Alkenes having structure Cab = Ccd, Cab = Cab can exhibit geometrical isomerism
Preparation
(1) Dehydrohalogenation of alkyl halides
📷+ KOH 📷 📷
Dehydrohalogenation is an example of β - elimination reaction. It was found that the dehydrohalogenation reaction when carried out in presence of a strong base follows second order kinetics.
Rate = K (Rx] [ B]
E2 Mechanism
(1) This reaction involves single step
(2) Favoured in polar aprotic solvent
📷
Base pulls a proton away from carbon and simultaneously a halide ion departs and a double bond is formed.
📷 📷 + 📷
📷 📷 + 📷
Saytzeff’s rule: In dehydrohalogenation the preferred product is the alkene that has greater number of alkyl groups
Reacting of R – X towards E2 = 3° > 2° > 1°
Illustraion 4:
📷
Although R4N+ undergo this reaction why 📷 do not undergo this reaction
Solution: Above reaction involves a base promoted elimination where a moderately basic R3N is the leaving group. But when RNH3 is used, once the abstraction of proton takes place we have an amine but not ion, and amines do not undergo this type of reaction.
2-methyl –3- pentyl tosylate was heated in n-butanol with no added base, what are the products obtained.
📷
As there is not strong base and reaction conditions favour unimolecular reaction.
(a) Dehydration of alcohols:-
Dehydration of alcohols is usually carried out by heating alcohol with sulfuric or phosphoric acid or by passing the alcohol vapour over a catalyst
Ease of dehydration of alcohols : 3° > 2° > 1°
Illustration 5: Neohexyl alcohol 📷 A + B + C + D
Identify products A, B,C and D
📷
Steroelectronic factors for elminations
Two leaving groups must have anti conformation
📷
Transition state has less energy
📷
Properties
Reaction of Alkenes

1. Addition of Hydrogen

📷 📷
Quantity of heat evolved when one mole of an unsaturated compound is hydrogenated is called as heat of hydrogenation. Based upon heats of hydrogenation we can know stability of alkenes. Based on heats of hydrogenation stability order is as follows:
R2C = CR2 > R2C = CHR > R2C = CH2, RCH = CHR > RCH = CH2 > CH2 = CH2
Addition of hydrogen halides
Addition involves two steps
📷 ⎯⎯→ 📷:X– (slow)
📷 ⎯⎯→ 📷 (Fast )
In the slow step attack on loosely held Pie electron of alkene is taking place and the attacking reagent is an electron deficient species, hence such reactions are classified as electrophilic additions.
Since this reaction is involving the formation of a carbocation intermediates, rearrangement are bound to occur and rearrranged products are usuallly obtained. Generally in electrophilic addition Markonkoff’s rule is followed. The rule says that negative part of addendum is attached to carbon atom containing least number of hydrogens.
📷
Addition of Halogens
📷
In the addition of bromine, the intermediate involved is cyclic bromonium ion.
Mechanism:
📷
📷
Addition of water
(a) Oxymercuration – demercuration
📷
Oxymercuration demercuration gives markonikov’s product without any rearranged product.
📷
(b) Hydroboration – oxidation
With diborane alkenes undergo hydroboration to yield alkyl boranes which on oxidation gives alcohols.
📷
We get Antimarkonikov’s product by following Markonikov’s rule.
Addition of HBr in the presence of Peroxide: -
Presence of peroxide initiates a free radical mechanism
(1) H — O — O — H —→ 2 📷
(2) 📷 + HBr —→ 📷 + H2O
(3).
📷
(4).
📷
As 2° free radical is more stable is formed as major product
Hydroxylation:-
Alkenes are hydroxylated by using certain reagents like KMnO4, OsO4 and peroxy acids to give glycols.
Addition of alkenes with KMnO4 is cis addition while with Peroxyacids it is a trans addition
eg.
📷
Ozonolysis: It is one of the important reactions by which structural elucidation of alkenes is performed.
📷
Alkynes
Hydrocarbon containing C ≡ C bond are termed as alkynes
Preparation
(1) From Alkyldihalides 📷 📷—C≡C—
(2) 📷 ⎯⎯→ —C≡ C—R + LiX
Reactions of Alkynes
(1) Reduction
📷
📷
(2) Hydration: Alkynes undergo addition of H2O in presence of H2SO4, HgSO4 to give carboxyl compounds.
📷
Except acetylene all alkynes will give ketones as major products

1. Formation of acetylides: -

Hydrogen attached to triply bonded carbon at end of chain shows appreciable acidity, they react with sodium metal to give acetylides
HC ≡ CH + Na ⎯⎯→ HC ≡ C– Na+ 📷H2
Because of this property terminal alkynes can be distinguished from non terminal alkynes as well as from alkenes
ELECTROPHILIC AROMATIC SUBSTITUTION
Electrophilic Aromatic Substitutions proceed by only one mechanism in the substrate.
📷
Above mechanism is known as arenium ion mechanism. The attacking species, Electrophile may be positive ion or a dipole. In these cations the intermediate formed is not aromatic, but by loss of proton in the last step it is reverting back to its aromatic character.
📷
The important reactions of this type are

1. Nitration 2. Sulfonation
   
2. Halogenation 4. Friedel – Craft’s alkylation
   
3. Friedel – Craft’s acylation
   

Nitration: - It is usually carried out by using a mixture of concentrated nitric acid and sulphuric acid. The other nitrating agents which can be used are.
N2O5 in CCl4 is used in presence of P2O5 when anhydrous conditions are required
(b) Ethylnitrate is used to carry out nitration in alkaline medium
Mechanism:
HO – NO2 + H2SO4 ⎯⎯→ 📷 + HSO4–
📷 ⎯⎯→ 📷 + H2O
H2O + H2SO4 ⎯⎯→ H3O+ + HSO4–
——————————————————
HNO3 + 2H2SO4 ⎯⎯→ NO2+ + 2HSO4– + H3O+
Once an electrophile is generated put NO2+ instead of E in general mechanism and it gives mechanism of nitration.
Sulfonation: It is usually affected by using Con H2SO4 or with fuming H2SO4 or chlorosuforic acid.
Mechanism:
2H2SO4 📷 SO3 + H3O+ + HSO4–
📷
📷
📷
Halogenation:
Aromatic compounds can be brominated or chlorinated by treatment with bromine or chlorine in the presence of a catalyst. When thallium (III) acetate is catalyst many substrates react with high regioselectivity at para to an O-P directing group. Iodine is least reactive among halogens. Except for active substrates, an oxidizing agent should be normally present to oxidise I2 to a better electrophile.
Ex: HNO3, HIO3, SO3, CH3CO3H, H2O2.
Mechanism:
X – X + FeX3 ⎯⎯→ X+ + X– - FeX3
📷
Fridel – Craft’s alkylation:
📷
The electrophile involved in this reaction is R+ (Alkyl carbo cation)
It is usually generated from Alkyl halides and even from alcohols.
Ex: RCl + AlCl3 ⎯⎯→ R+ + AlCl4–
ROH + AlCl3 ⎯⎯→ ROACl2 ⎯⎯→R+ + –OAlCl2
ROH + H+ ⎯⎯→ RO+H2 ⎯⎯→ R+ + H2O
In the case of olefins proton donating acid is definitely required without it reaction does not take place.
Reactivity of Catalysts:
AlBr3 > AlCl3 > GaCl3 > FeCl3 > SbCl5 > BCl3. As the reaction involves formation of a carbocation intermediate, rearrangements are bound to occur and this is serious limitation of this method.
Illustration 6:
📷
📷
📷
Now 3° butyl group (cation) is electrophile end product obtained is 3° butyl benzene.
📷
Identify X
Solution:
In the above example ring closure takes place, the product obtained will be
📷
Friedel – Crafts Acylation:
The electrophile involved in this reaction is 📷 (Acyl carbocation). Apart from acylhalides carboxylic acid, anhydrides and ketenes can also be used
📷
Formylation cannot be done
Because formyl chloride or formic anhydride are not stable at ordinary temperatures. When non polar solvents a free ion is not the attacking species but the attack is mainly by complex.
📷
Orientation in mono substituted benzenes:
When a group is attached to bezene ring and increases the rate of electrophilic substation reactions, it is termed as an activating gp, whereas the gp which decreases the rate of electrophilic substitution reactions is a deactivating group.
Activating groups: –R, 📷 📷 etc.
Activating groups: -NO2, -SO3H, 📷 etc.
The rate of electrophilic aromatic substitution depends on tendency of substituent to release or withdraw electrons. The classification of substituents into activating and deactivating can be perfectly understood by knowing the resonance structures.
Resonance structures for electrophilic substitution:-
📷
If X is an electron withdrawing group, it destabilises the resonance structure III and hence it acts as a deactivating group by making the carbon still more positive on the other hand if X is electron releasing then it stabilises the carbocation by donating electrons and hence increases the rate of such reactions.
If a group already present on the benzene ring orients the incoming group to ortho, para positions it is called as an o,p directing group while if it directs to meta it is termed as a meta directing group.
Resonance structures for ortho para and meta attacks
📷
📷
📷
If x is an electron releasing group, the structures III, V are stabilised hence ortho para attack is more favoured than meta attack.
If X is an electron withdrawing group then structures III, V will be highly destabilised and as a result of this ortho para attack is not possible but in meta attack, all the structures will be more stabilised when compared to that of ortho para attack hence m – attack will take place.
In the case of halogens their reactivity is governed by –I effect but orientation is governed by +m effect hence although they are deactivating groups they are o,p directors.

ALKYL HALIDE, ALCOHOLS & ETHERS