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Carbon is an essential element of organic compounds, it has four electrons in its outer most shell. According to the ground state electronic configuration of carbon, it is divalent. Tetravalency of carbon can be explained by promoting one of the 2 s^2 - electrons to the unocupied 2 pz 1 atomic orbital.
The four valencies of carbon atom are similar and they are symmetrically arranged around the carbon atom. According to Le Bell and Van’t Hoff the four valencies of carbon do not lie in one plane. They are directed towards the corners of a regular tetrahedron with carbon atom at the centre and the angle between any two valencies is 109o 28 .
Hybridisation in Organic Compounds
(1) The process of mixing atomic orbitals to form a set of new equivalent orbitals is termed as hybridisation. There are three types of hybridisation, (i) sp^3 hybridisation (involved in saturated organic compounds containing only single covalent bonds), (ii) sp^2 hybridisation (involved in organic compounds having carbon atoms linked by double bonds) and (iii) sp hybridisation (involved in organic compounds having carbon atoms linked by a triple bonds). Table : 23. Type of hybridisation
sp^3 sp^2 sp Number of orbitals used
1 s and 3 p 1 s and 2 p 1 s and 1 p Number of unused p - orbitals
Nil One Two
Bond Four - Three - One -
Two - Two - Bond angle 109.5 120 180 Geometry Tetrahedra l
Trigonal planar
Linear
% s - character 25 or 1/4 33.33 or 1/
50 or 1/
(2) Determination of hybridisation at different carbon atoms : It can be done by two methods, (i) First method : In this method hybridisation can be know by the number of bonds present on that particular atom. Number of – bond/s 0 1 2 Type of hybridisation sp^3 sp^2 sp Examples :
(i)
(^3)
3 (^3222)
3
| |
sp
CH
O
sp
C sp
CH sp
CH
sp
CH
(ii)
(^2)
2 2
2 sp
CH sp
C
sp
CH
(iii)
(^) sp
N
sp
C
sp
CH
sp
CH
sp
CH
sp
CH
3
2 3 2 2
3
(iv)
2
2 (^2) sp
CH
sp
CH
sp
C
sp
HC
In diamond carbon is sp^3 hybridised and in graphite carbon is sp^2 hybridised.
General Organic Chemistry
Chapter
(ii) Second method : (Electron pair method) ep = bp + lp; where ep = electron pair present in hybrid orbitals , bp = bond pair present in hybrid orbitals
Number of bp = Number of atoms attached to the central atom of the species
Number of lp ’s can be determined as follows, (a) If carbon has - bonds or positive charge or odd electron, than lp on carbon will be zero. (b) If carbon has negative charge, then lp will be equal to one. Number of electron pairs ( ep ) tells us the type of hybridisation as follows,
ep 2 3 4 5 6 Type of hybridisation
sp sp (^2) sp (^3) sp (^3) d sp (^3) d 2
Example : (i)
ep sp
lp
bp
CH CH
2
(ii)
2
2
3 ,
1
2
ep sp
lp
bp
CH CH
(iii)
2
3
2 3
ep sp
lp
bp
CH
CH C CH
(iv)
ep sp
lp
bp
CH C
2 ,
1
1
(v)
3
3 3
4 ,
1
3
ep sp
lp
bp
CH CH CH
(3) Applications of hybridisation (i) Size of the hybrid orbitals : Since s - orbitals are closer to the nucleus than p - orbitals, it is reasonable to expect that greater the s character of an orbital the smaller it is. Thus the decreasing order of the size of the three hybrid orbitals is opposite to that
of the decreasing order of s orbital character in the three hybrid orbitals. sp^3 sp^2 sp (ii) Electronegativity of different orbitals (a) Electronegativity of s - orbital is maximum. (b) Electronegativity of hybrid orbital % s- character in hybrid orbitals
- characterindecreasingorderandelectronegativityindecreasingorder
% - character 50 33. 33 25
Orbital 2 3 s
sp sp sp s ^
Thus sp - hybrid carbon is always electronegative in character and sp^3 - hybrid carbon is electropositive in character. sp^2 - hybrid carbon can behave as electropositive (in carbocation) as well as electronegative (in carbanion) in character. CH 3 CH 2
CH (^) 2 CH
(c) Electronegativities of different hybrid and unhybrid orbitals in decreasing order is as follows
%s-characterandelectronegativityindecreasingorder.
2 3 s ^ sp sp sp p
(iii) Bond length variation in hydrocarbons % of s orbital character
bondlength
bondlength
C C C H
Table : 23. Bond type ( C – H )
Bond length
Bond type (C – C)
Bond length sp^3 s (alkanes)
1.112Å (^) sp^3 sp^3 (al kanes)
1.54 Å
sp^2 s (alkenes)
1.103Å (^) sp^2 sp^2 (al kenes)
1.34Å
sp s (alkynes)
1.08Å sp sp (alk ynes)
1.20Å
(iv) Bond strength in hydrocarbons : The shorter is the bond length, the greater is the compression between atomic nuclei and hence greater is the strength of that bond. Table : 23. Bond type ( C –
Bond energy
Bond type ( C – C )
Bond energy
2 H
Central atom
bp = 3
1 2
bp
H C C H
H
H
Third atom
Central atom
C = C
H
H
First atom
Second atom bp = 3
sp^2
Electropositive carbon
Electronegative carbon having positive charge
sp
Inductive effect or Transmission effect
(1) When an electron withdrawing ( X ) or electron-releasing ( Y ) group is attached to a carbon chain, polarity is induced on the carbon atom and on the substituent attached to it. This permanent polarity is due to displacement of shared electron of a covalent bond towards a more electronegative atom. This is called inductive effect or simply as I – effect.
C C C C Non polar C C C C X C C C C Y (2) Carbon-hydrogen bond is taken as a standard of inductive effect. Zero effect is assumed for this bond. Atoms or groups which have a greater electron withdrawing capacity than hydrogen are said to have–I effect whereas atoms or groups which have a greater electron releasing power are said to have +I effect.
N H NO CN SOH CHO CO COOH COCl COOR
3 2 3
CONH (^) 2 F Cl Br I OH OR NH 2 C 6 H 5 H
- I power of groups in decreasing order with respect to the reference H
ter. alkyl > sec. alkyl > pri. alkyl > CH (^) 3 H
- I power in decreasing order with respect to the reference H
- I power number of carbon in the same type of alkyl groups
CH 3 CH 2 CH 2 CH 2 CH 3 CH 2 CH 2 CH 3 CH 2
- I power in decreasing order in same type of alkyl groups
(3) Applications of Inductive effect (i) Magnitude of positive and negative charges : Magnitude of + ve charge on cations and magnitude of
- ve charge on anions can be compared by + I or – I groups present in it.
Magnitude of ve charge I
Ipowerof thegroup
(^1) power of the group.
Magnitude of ve charge (^) Ipowerof the^1 group I^ power of the group.
(ii) Reactivity of alkyl halide : + I effect of methyl group enhances – I effect of the halogen atom by repelling the electron towards tertiary carbon atom.
C X
CH
CH
HC
3
3
3 X
CH
H C CH
3 3
CH 3 CH 2 X CH 3 X
Tertiary > Secondary > Primary > Methyl (iii) Relative strength of the acids : (a) Any group or atom showing + I effect decreases the acid strength as it increases the negative charge on the carboxylate ion which holds the hydrogen firmly. Alkyl groups have + I effect. Thus, acidic nature is, HCOOH CH 3 COOH C 2 H 5 COOH C 3 H 7 COOH C 4 H 9 COOH
- I effect increases, so acid strength decreases Formic acid, having no alkyl group, is the most acidic among these acids. (b) The group or atom having – I effect increases the acid strength as it decreases the negative charge on the carboxylate ion. Greater is the number of such atoms or groups (having – I effect), greater is the acid strength. Thus, acidic nature is,
Acetic^3 acid aceticacid
Monochloro^2 aceticacid
Dichloro^2 aceticacid
Trichloro^3
CCl COOH CHClCOOH CHClCOOH CHCOOH
(– Inductive effect increases, so acid strength increases) (c) Strength of aliphatic carboxylic acids and benzoic acid COOH
Igroup
R
COOH Igroup
CH
6 5
Hence benzoic acid is stronger acid than aliphatic carboxylic acids but exception is formic acid. Thus, HCOOH > C (^) 6 H 5 COOH > RCOOH Acid strength in decreasing order Decreasing order of acids : NO (^) 2 CH 2 COOH FCH 2 COOH ClCH 2 COOH BrCH 2 COOH. F 3 C COOH Cl 3 C COOH Br 3 C COOH I 3 C COOH.
alcohol
Tert butyl alcohol
Isopropyl Alcohol
Ethyl alcohol
Methyl
CH OH CH CH OH CH CHOH CH COH
3 ^32 ( 3 ) 2 ( 3 ) 3
As compared to water, phenol is more acidic (– I effect) but methyl alcohol is less acidic (+ I effect).
Phenol
OH H Water OH >
Methylalcohol
CH 3 OH
(vi) Relative strength of the bases (Basic nature of (^) NH 2 )
The difference in base strength in various amines can be explained on the basis of inductive effect. The + I effect increases the electron density while – I effect decreases it. The amines are stronger bases than NH 3
as the alkyl groups increase electron density on nitrogen due to + I effect while ClNH 2 is less basic due
to – I effect. “So more is the tendency to donate electron pair for coordination with proton, the more is basic nature, i.e., more is the negative charge on nitrogen atom (due to + I effect of alkyl group), the more is basic nature”.
Thus, the basic nature decreases in the order;
amineChloro Ammonia^32 amineMethy l^
3 2 amineEthy l^
3 2 2 amineDiethy l
( C (^) 2 H 5 ) 2 NH CHCHNH CHNH NH ClNH
The order of basicity is as given below; Alkyl groups ( R – )
Relative base strength
CH 3 R 2 NH RNH 2 R 3 N NH 3
C 2 H 5 R 2 NH RNH 2 NH 3 R 3 N
( CH 3 ) 2 CH RNH 2 NH 3 R 2 NH R 3 N
( CH 3 ) 3 C NH 3 RNH 2 R 2 NH R 3 N
The relative basic character of amines is not in total accordance with inductive effect ( t s p )but it is in the following order: Secondary > Primary > Tertiary. The reason is the steric hindrance existing in the t - amines.
In gas phase or in aqueous solvents such as chlorobenzene etc, the solvation effect, i.e. , the stabilization of the conjugate acid due to H - bonding are absent and hence in these media the basicity of amines depends only on the + I effect of the alkyl group thus the basicity of amines follows the order : 3 o^ 2 o 1 o NH 3. (vii) Basicity of alcohols : The decreasing order of base strength in alcohols is due to + I effect of alkyl groups.
CH COH CH CHOH CHCHOH CHOH (^3) ( 3 3 o (^) ) (^3) ( (^22) o ) (^3) ( 1 o ) 2 3
(viii) Stability of carbonium ion : + I effect tends to decrease the (+ ve ) charge and – I effect tends to increases the + ve charge on carbocation. ( CH 3 ) 3 C ^ ( CH 3 ) 2 CH CH 3 CH 2 CH 3
(ix) Stability of carbanion : Stability of carbanion increases with increasing – I effect. CH 3 CH 3 CH 2 ( CH 3 ) 2 CH ( CH 3 ) 3 C
Resonance effect or mesomeric effect
(1) The effect in which electrons are transferred from a multiple bond to an atom, or from a multiple bond to a single covalent bond or lone pair ( s ) of electrons from an atom to the adjacent single covalent bond is called mesomeric effect or simply as M-effect. In case of the compound with conjugated system of double bonds, the mesomeric effect is transmitted through whole of the conjugated system and thus the effect may better be known as conjugative effect. (2) Groups which have the capacity to increase the electron density of the rest of the molecule are said to have M effect. Such groups possess lone pairs of electrons. Groups which decrease the electron density of the rest of the molecule by withdrawing electron pairs are said to have M effect, e.g., (a) The groups which donate electrons to the double bond or to a conjugated system are said to have M effect or Reffect. M effect groups :
Cl Br I NH NR OH OR SH OCH SR
.. ..
, , , , , ,
.. , ,, 2 2 3
(b) The groups which withdraw electrons from the double bond or from a conjugated system towards itself due to resonance are said to have M effect or R effect. M effect groups :
CHO COOH SO H
O NO (^) 2 C N C , , , 3
|| , ,
(3) The inductive and mesomeric effects, when present together, may act in the same direction or oppose each other. The mesomeric effect is more powerful than the former. For example, in vinyl chloride due to – I effect the chlorine atom should develop a negative charge but on account of mesomeric effect it has positive charge.
ortho-para directing group and activating group for electrophilic aromatic substitution reaction because of the hyperconjugation.
The electron donating power of alkyl group will depends on the number of resonating structures, this depends on the number of hydrogens present on - carbon. The electron releasing power of some groups are as follows,
3 3
3 3 3 2 CH
CH
CH CH C
CH
CH
CH CH CH
Increasing inductive effect Electron donating power in decreasing order due to the hyperconjugation.
(6) Heat of hydrogenation : Hyperconjugation decreases the heat of hydrogenation.
(7) Dipole moment : Since hyperconjugation causes the development of charges, it also affects the dipole moment in the molecule.
The increase in dipole moment, when hydrogen of formaldehyde ( 2. 27 D )is replaced by methyl group, i.e ., acetaldehyde ( 2. 72 D ) can be referred to hyperconjugation, which leads to development of charges.
( 2. 27 )
D
O
H
H C
( 2. 72 )
|
|
D
CH O H
H H C
CH O
H
H
H C
(8) Orienting influence of alkyl group in o , p -
positions and of CCl 3 group in m - position : Ortho-
para directing property of methyl group in toluene is partly due to I effect and partly due to hyperconjugation.
Reverse Hyperconjugation : The phenomenon of hyperconjugation is also observed in the system given below,
C C
X
C
; where X halogen
In such system the effect operates in the reverse direction. Hence the hyperconjugation in such system is known as reverse hyperconjugation.
2
| |
CH CH
Cl
Cl
Cl C 2
CH CH
Cl
Cl
Cl C
2
CH CH
Cl
Cl
Cl C
CH C H
Cl
Cl
Cl C
The meta directing influence and the deactivating effect of (^) CX 3 group in electrophilic aromatic substitution reaction can be explained by this effect.
X
X
X C
X
X
X C
X
X
X C
X
X X C ||
|
Inductomeric effect
Inductomeric effect is the temporary effect which enhances the inductive effect and it accounts only in the presence of an attacking reagent. Example,
C Cl H
H
H
HO Cl
H H
C
H
HO ............ ............
Cl H
H
H
HO C
In methyl chloride the – I effect of Cl group is further increased temporarily by the approach of hydroxyl ion.
Electromeric effect
(1) The phenomenon of movement of electrons from one atom to another in multibonded atoms at the demand of attacking reagent is called electromeric effect. It is denoted as E-effect and represented by a curved arrow ( ) showing the shifting of electron pair.
Reagent :
A B E^ A B
(2) (i)When the transfer of electrons take place towards the attacking reagent, the effect is called E effect. The addition of acids to alkenes.
H
C C H C C |
CH (^) (^3) Propene CH CH 2 H CH 3 CH CH 3
Since, CH 3 group is electron donating, the
electrons are transferred in the direction shown. The attacking reagent is attached to that atom on which electrons have been transferred.
(ii) When the transfer of electrons takes place away from the attacking reagent, the effect is called E effect. Example, The addition of cyanide ion to carbonyl compounds.
(^) O
CN
C O CN C |
The attacking reagent is not attached to that atom on which electrons have been transferred.
(3) Direction of the shift of electron pair : The direction of the shift of electron pair can be decided on the basis of following points.
(i) When the groups linked to a multiple bond are similar, the shift can occur in either direction.
(ii) When the dissimilar groups are linked on the two ends of the double bond, the shift is decided by the direction of inductive effect.
In the case of carbonyl group, the shift is always towards oxygen, i.e., more electronegative atom.
:
C O C O
In cases where inductive effect and electromeric effect simultaneously operate, usually electrometric effect predominates.
Cleavage (fission or breaking) of covalent bonds
Breaking of covalent bond of the compound is known as bond fission. A bond can be broken by two ways,
(1) Homolytic bond fission or Homolysis (i) In homolysis, the covalent bond is broken in such a way that each resulting species gets its own electron. This leads to the formation of odd electron species known as free radical.
Freeradical
A : B A B
(ii) The factor which favours homolysis is that the difference in electronegativity between (^) A and (^) B is less or zero. (iii) Homolysis takes place in gaseous phase or in the presence of non polar solvents (^) ( CCl 4 , CS 2 ), peroxide, UV light, heat ( 500 oC ), electricity and free radical. (iv) Mechanism of the reaction in which homolysis takes place is known as homolytic mechanism or free radical mechanism. (2) Heterolytic bond fission or heterolysis (i) In heterolysis, the covalent bond is broken in such a way that one species ( i.e. , less electronegative) is deprived of its own electron, while the other species gains both the electrons.
: (^) carbanion : carbocation
A B A B
Thus formation of opposite charged species takes place. In case of organic compounds, if positive charge is present on the carbon then cation is termed as carbocation. If negative charge is present on the carbon then anion is termed as carbanion. (ii) The factor which favours heterolysis is greater difference of electronegativities between A and (^) B. (iii) Mechanism of the reaction in which heterolysis takes place is known as heterolytic mechanism or ionic mechanism. (iv) The energy required for heterolysis is always greater than that for homolysis due to electrostatic forces of attraction between ions.
Reaction Intermediates
Short lived fragments called reaction intermediates result from homolytic and heterolytic bond fission. The important reaction intermediates are free radicals, carbocations, carbanions, carbenes, benzyne and nitrenes.
Table : 23. Characteristi c
Free radical Carbocation Carbanion Carbene
(iii) Unsubstituted nitrene :)
( H N can be obtained by photolysis of (or by passing electric discharge through) NH 3 , N 2 H 4 or N 3 H.
Attacking reagents
The fission of the substrate molecule to create centres of high or low electron density is influenced by attacking reagents. Most of the attacking reagents can be classified into two main groups. Electrophiles or electrophilic reagents and Nucleophiles or nucleophilic reagents. (1) Electrophiles : Electron deficient species or electron acceptor is an electrophile. It can be classified into two categories : (i) Charged electrophiles : Positively charged species in which central atom has incomplete octet is called charged electrophile.
N OSOH O
O
H X R N , 3
All cations are charged electrophiles except
cations of IA, IIA group elements,^ Al and
NH 4 (ii) Neutral electrophiles : It can be classified into three categories, (a) Neutral covalent compound in which central atom has incomplete octet is neutral electrophile, .. ,
BeCl 2 , BH 3 , ZnCl 2 , AlX 3 , FeX 3 , CH 3 CH 2 CX 2 (b) Neutral covalent compound in which central atom has complete or expended octet and central atom has unfilled – d- sub-shell is neutral electrophile, SnCl 4 , SiCl 4 , PCl 5 , SF 6 , IF 7 (c) Neutral covalent compound in which central atom is bonded only with two or more than two electronegative atoms is called neutral electrophile. BeCl 2 , BX 3 , AlX 3 , FeX 3 , SnCl 4 , PCl 3 ;
, , , ,
PCl 5 , NF 3 , CX 2 CO 2 SO 3 CS 2 Cl 2 , Br 2 and I 2 also behave as neutral electrophiles. Electrophiles are Lewis acids. (2) Nucleophiles : Electron rich species or electron donors are called nucleophiles. Nucleophiles can be classified into three categories :
(i) Charged nucleophiles : Negatively charged species are called charged nucleophiles. H , OH , R O , CH 3 , X , SH , R S
(ii) Neutral nucleophiles : It can be classified into two categories : (a) Neutral covalent compound, in which central atom has complete octet, has at least one lone pair of electrons and all atoms present on central atom should not be electronegative, is neutral nucleophile.
3 , 2 2 3 2 2
.... ,
.. ,
.. ,
.. .. N H R NH R NHR NNH N H (Nitrogen nucleophile)
H O HR O HR O R
.. ..
,
.. ..
,
.. ..
(Oxygen
nucleophiles)
H S HR S HR S R
.. , ..
.. , ..
.. ..^ (Sulphur nucleophiles) .. ,
.. ,
.. ,
.. PH 3 RPH 2 R 2 PHR 3 P (Phosphorus nucleophiles) (b) Organic compound containing carbon, carbon multiple bond/ bonds behaves as nucleophile. Alkenes, Alkynes, Benzene, CH (^) 2 CH CH CH 2 , CH 2 CH C CH (iii) Ambident nucleophiles : Species having two nucleophilic centres out of which, one is neutral (complete octet and has at least one lone pair of electrons) and the other is charged (negative charge) behaves as ambident nucleophile
OH
O
O
C NO N OO S
,
Organometallic compounds are nucleophiles. Nucleophiles are Lewis bases. Organic compounds which behave as an electrophile as well as a nucleophile : Organic compound in which carbon is bonded with electronegative atom ( O , N , S ) by multiple bond/bonds behaves as electrophile as well as nucleophile :
Cl
O OHR C
O RR C
O HR C
O R C
|| || || || , , , ,
NH R C NR N C
O ORR C
O R C
, 2 , ,
|| ||
During the course of chemical reaction electrophile reacts with nucleophile. Strong Lewis acid is stronger electrophile CO (^) 2 NO 2 SO 3 H
. Stronger is an acid, weaker is its conjugated base or weaker is the nucleophile.
Examples : HF H 2 O NH 3 CH 4 F ^ OH NH 2 CH 3 Increasing order of nucleophilicity.
Types of organic reactions
It is convenient to classify the numerous reactions of the various classes of organic compound into four types, Substitution reactions, Addition reaction, Elimination reactions, Rearrangement reactions,
Substitution reactions
Replacement of an atom or group of the substrate by any other atom or group is known as substitution reactions.
Examples :
CH CH Br NaOH CH CHOH NaBr Ethylbromide Ethylalcohol
3 ^2 ^ 3 2
(Bromine atom is replaced by hydroxyl group) Types of substitution reactions : On the basis of the nature of attacking species substitution reactions are classified into following three categories,
(1) Nucleophilic substitution reactions (2) Electrophilic substitution reactions (3) Free radical substitution reactions (1) Nucleophilic substitution reactions (i) Many substitution reactions, especially at the saturated carbon atom in aliphatic compounds such as alkyl halides, are brought about by nucleophilic reagents or nucleophiles.
SubstrateR^ X NucleophilOH e R OH LeavingXgroup
^
Such substitution reactions are called nucleophilic substitution reactions, i.e., SN reactions ( S
stands for substitution and N for nucleophile).
(ii) The weaker the basicity of a group of the substrate, the better is its leaving ability.
Leaving power of the group Basicityof^1 thegroup
Example : Decreasingacidity
HI ^ HBr HCl HF
Decreasingleavingability
Increasingbasicity
I Br Cl F
(iii) The leaving power of some nucleophilic groups are given below in decreasing order,
O Br
O
O
CF S
|| (^3) || 3
|| || O CH
O
O
S
O
O
O
S
|| ||
O
O
O
O CH S
O
O
C H S
| | (^3) ||
|| (^65) ||
O
O O HO Cl F CH C
O I Br CF C
|| 3
|| 3 (iv) In these reactions leaving group of the substrate is replaced by another nucleophile. If reagent is neutral then leaving group is replaced by negative part of the reagent. Negative part of the reagent is always nucleophilic in character. R L E^ Nu R Nu L
;
R L Nu R Nu L (v) In (^) SN reactions basicity of leaving group should be less than the basicity of incoming nucleophilic group. Thus strongly basic nucleophilic group replaces weakly basic nucleophilic group of the substrate. Example : R Cl ( NaOH OH^ ) R OH Cl .....(A)
Basicity of OH is more than
(^) Cl hence
(^) OH replaces Cl as
Cl.
R OH ( HClCl ) R Cl OH ......(B)
Basicity of
Cl is less than
OH , hence
Cl will not replace OH as
OH hence reaction (B) will not occur. (vi) Unlike aliphatic compounds having nucleophilic group as leaving group, aromatic compounds having same group bonded directly with aromatic ring do not undergo nucleophilic substitution reaction under ordinary conditions. The reason for this unusual reactivity is the presence of lone pair of electron or bond on the key atom of the functional group. Another factor for the low reactivity is nucleophilic character of aromatic ring. (vii) The SN reactions are divided into two classes,^ S N 2 and^ S N 1 reactions. Table : 23.5 Distinction between SN 2 and SN 1 reactions
Leaving group
Substituting or attacking group
R H D ⇋
R D H R H T ⇋
R T H
ring to form carbonium ion (or arenium ion) which is stabilized by resonance.
Step 3. Carbonium ion loses the proton to form substitution product.
The bromination of benzene in the presence of FeBr 3 is a example of electrophilic substitution reaction. Similarly, Nitration, sulphonation and Friedel-Crafts reaction…..etc., in benzene nucleus are the other examples of electrophilic substitution reactions.
(3) Free radical substitution reactions : Free radical substitution reactions involves the attack by a free radical. These reactions occurs by free radical mechanism which involves Initiation, Propagation and Termination steps. Examples,
(i) Chlorination of methane : The chlorination of methane in the presence of ultraviolet light is an examples of free radical substitution.
CH Cl lightUV CHCl HCl Methane
(^42) Methyl chloride 3
(ii) Arylation of aromatic compounds (Gomberg reaction) : The reaction of benzene diazonium halide with benzene gives diphenyl by a free radical substitution reaction.
C H H CH NX CH CH N 2 HX (^6) Diphenyl 5 6 5 Alkali (^65) Benzene (^6) diazonium (^52) halide
(iii) Wurtz reaction : Ethyl bromide on treatment with metallic sodium forms butane, ethane and ethylene by involving free radical mechanism.
(iv) Allylic bromination by NBS ( N - Bromosuccinimide) : NBS is a selective brominating agent and it normally brominates the ethylenic compounds in the allylic ( CH 2 CH CH 2 ) position.
This type of reaction involving substitution at the alpha carbon atom with respect to the double bond is termed Allylic substitution. It is also used for benzylic bromination. Some examples are:
^4
2
2 (^3) Propene 2
CCl
NBS
N Br CH CO
CH CO
CH CH CH
Succinimid^2 e
2 Ally l^2 bromide (^2) CH CO
CH CO
Br CH CH CH
NH
Addition reactions
These reactions are given by those compounds which have at least one bond,
i.e., ( , , , ).
|| C N
O
C C C C C In such reaction there is loss of one bond and gain of two bonds. Thus product of the reaction is generally more stable than the reactant. The reaction is a spontaneous reaction. Types of addition reactions : Addition reactions can be classified into three categories on the basis of the nature of initiating species. (1) Electrophilic additions (2) Nucleophilic additions (3) Free radical additions (1) Electrophilic addition reactions (i) Such reactions are mainly given by alkenes and alkynes. (ii) Electrophilic addition reactions of alkenes and alkynes are generally two step reactions.
(iii) Alkenes and alkynes give electrophilic addition with those reagents which on dissociation gives electrophile as well as nucleophile. (iv) If the reagent is a weak acid then electrophilic addition is catalysed by strong acids (Generally H 2 SO 4 ).
(v) Unsymmetrical alkenes and alkynes give addition reactions with unsymmetrical reagents according to Markownikoff’s rule.
The negative part of the addendum adds on that doubly bonded carbon of the alkene which has least number of hydrogen atom. This rule can be used only in those alkenes which fulfil the following conditions:
(a) Alkene should be unsymmetrical. (b) Substituent/substituents present on doubly bonded carbon/( s ) should only be + I group. (c) If phenyl group is present on doubly bonded carbon, then both doubly bonded carbons should be substituted by phenyl groups.
For example, the following alkenes will give addition according to the Markownikoff’s rule.
CH 3 CH CH 2 , ,
3
2
3
CH
C CH
CH
6 5
6 5 ,
6 5
C H
C CH CH
CH
C 6 H 5 CH CH 2
Following alkenes will not give addition reaction according to Markownikoff’s rule.
CH 2 CH 2 , R^ CH CH R ,
R R
C C
R R
6 5 6 5
6 5 6 5
CH C H
C C
CH CH
(vi) Unsymmetrical alkenes having the following general structure give addition according to anti Markownikoff’s rule.
CH (^) 2 CH G , where G is a strong – I group such
as
Z
O CX NO CN CHO COR COOH C
|| 3 , 2 , , , , , ( Z Cl , OH , OR , NH 2 ) Example:
CH CHO
Cl CH CH CHO HCl CH 2 2 Anti-Markownikoff'saddition | 2 (vii) Mechanism of electrophilic addition reactions is as follows,
C Olefin C Carboniumion
| |
Slow (^) Electrophi E (^) le C C E
(^) Fast Nucleophile
| |
C C E X
Additionproduct
| | C |^ E X
C
(2) Nucleophilic addition reactions : When the addition reaction occurs on account of the initial attack of nucleophile, the reaction is said to be a nucleophilic addition reaction. Due to presence of strongly electronegative oxygen atom, the - electrons of the carbon-oxygen double bond in carbonyl group ( (^) C O ) get shifted towards the oxygen atom and thereby such bond is highly polarised. This makes carbon atom of the carbonyl group electron deficient.
^ C O C O C O
Example : The addition of HCN to acetone is an example of nucleophilic addition.
Acetone^3 cy anohy drin
3
(^3) Acetone
3
CN
OH C CH
CH C O HCN CH
CH
The mechanism of the reaction involves the following steps: Step 1. HCN gives a proton( )
H and a nucleophile, cyanide ion( )
CN. HCN H CN Step 2. The nucleophile ( CN ) attacks the positively charged carbon so as to form an anion [ H does not initiate the negatively charged oxygen as anion is more stable than cation].
C O CH
CN
CH
C O
CH
CH
CN
3
3 3
3
or CN
O C CH
CH
3
3
Step 3. The proton ( H )combines with anion to form the addition product.
3
3
3
3
|
CH
NC C OH
CH
C O H
CH
CN
CH
or CN
OH
C
CH
CH
3
3
()
(^3) | 3 Slowstep
( )^3
|
| 3
3
3
|
| 3
3
1 3
TS RI
Cl CH
Cl CH C CH CH
CH C
CH
CH
CH C Cl
CH
^
Step 2.
2
(^2) | (^32) | 3 3 3
..
TS
CH CH
CH B H CH C CH
B H CH C
3
3 2 CH
CH
fast (^) BH CH C
(ii) E 2 (Elimination bimolecular) reaction : Consider the following reaction,
CH CH CH Br Base^ B CH CH CH H Br
3 ^2 2 ^ () 3 2
(a) Reaction velocity depends only on the concentration of the substrate and the base used; thus reaction is bimolecular reaction. Rate [Substrate] [Base]
(b) Since the reaction is a bimolecular reaction, the product formation will take place by formation of transition state (TS).
(c) Rearrangement does not take place in E 2 reaction but in case of allylic compound rearrangement is possible.
(d) Reaction is carried out in the presence of polar aprotic solvent.
(e) The E 2 reaction occurs in one step,
TS
H
Br
H
C
B H
H
H CH C
H
Br
C
H
H
C
B
CH
| (^3) |
| Slowstep |
| (^3) |
fast ^ CH 3 CH CH 2 BH Br (2) Orientation in - elimination reactions : If substrate is unsymmetrical, then this will give more than one product. Major product of the reaction can be known by two emperical rules.
(i) Saytzeff rule : According to this rule, major product is the most substituted alkene i.e., major
product is obtained by elimination of
^ H from that - carbon which has the least number of hydrogen. Product of the reaction in this case is known as Saytzeff product.
Saytzeff product
3 3
(^3) |
| 3 Alc. /
3
(^3) | 1
2 CH CH
CH CH C CH
Cl CH CH
CH CH
HCl
KOH ^
(ii) Hofmann rule : According to this rule, major product is always least substituted alkene i.e. , major product is formed from - carbon which has maximum number of hydrogen. Product of the reaction in this case is known as Hofmann product.
Hofmannproduct
2 3
|^2
| 3 3 Alc. / 3
|
3
|^2
| 3
3
2 1
CH CH CH
C CH
CH CH CH
Br CH CH
CH C CH
CH ^ KOH ^
In E 1 reactions, product formation always takes place by Saytzeff rule. In E 1cb reactions, product formation always takes place by Hofmann rule. In E 2 reactions, product formation takes place by Saytzeff as well as Hofmann rule. In almost all E 2 reactions product formation take place by Saytzeff rule.
(3) Examples of - elimination reactions (i) Dehydrohalogenation is removal of HX from alkyl halides with alcoholic KOH or KNH 2 or ter- BuO K (Potassium tertiary butoxide) and an example of - elimination, e.g., ( )^2 Ethene 2 CH (^) 3 CH 2 X Alc. HC CH HX KOH ^ (^) ;
( )^3 Propene 2 3 Alc. (^3) | CH CHCH CH X
CH CH (^) HXKOH ^
( )^32 - Butene(Major)^3 3 Alc. (^3 2) | CH CH CH CH CH X
CH CH CH HXKOH
(^3 1) - Butene (^2) (Minor) 2
CH CH CH CH
(ii) Dehydration of alcohol is another example of elimination reaction. When acids like conc. H 2 SO 4 or H 3 PO 4 are used as dehydrating agents, the mechanism is E 1. The proton given by acid is taken up by alcohol. Dehydration is removal of (^) H 2 O from alcohols,
e.g., CH (^) 3 CH 2 OH Conc. H (^^2 HSO 2 O^4 ),^170 C H 2 C CH 2
( )^3 Propene 2
Conc. , 170 (^3) Propan (^2) - 1 - ol (^22)
CH CH CH OH^2 4 CH CH CH
HO
H ^ SO C
Dehydration of alcohols is in the order:
(3) (2) (1)
Tertiary Secondary Primary
2° and 3° alcohol by E 1 process and 1° alcohol by E 2 process. Alcohols leading to conjugated alkenes are more easily dehydrated than the alcohols leading to non-conjugated alkenes. (^2) | CH 3
OH
CH^ CH CH is easily
dehydrated than
(^3 2) | CH 3 OH
CH CH CH and so
(iii) Dehalogenation : It is removal of halogens,
e.g., (-ZnBr)^2 Ethy lene 2
inCH ,heat
Ethy lenebromide
| 2 | 2 2
Zn dust 3 HC CH Br
CH
Br
CH OH ^
(iv) Dehydrogenation : It is removal of hydrogen,
e.g.,
Acetone
(,^300 )^3 ||^3 Isopropy lalcohol
(^3) | 3 2^ CH O
CH CH C OH
CH CH Cu ^ (^) H C
Rearrangement reactions
The reactions, which involve the migration of an atom or group from one site to another within the molecule (nothing is added from outside and nothing is eliminated) resulting in a new molecular structure, are known as rearrangement reactions. The new compound is actually the structural isomer of the original one.
It is convenient to divide rearrangement reactions into following types:
(1) Rearrangement or migration to electron deficient atoms (Nucleophilic rearrangement) : Those rearrangement reactions in which migrating group is nucleophilic and thus migrates to electron deficient centre which may be carbon, nitrogen and oxygen.
X
C
B
Y C C C
Y
C
X
C
| |
| |
| | | |
| | :
Bridged or non-classical carbocation X = Nucleophilic species, Y = Electronegative group, B = Another nucleophile. (2) Rearrangement or migration to electron rich atoms (Electrophilic rearrangement) : Those rearrangement reactions in which migrating group is electrophile and thus migrates to electron rich centre. (3) Rearrangement or migration to free radical species (Free radical rearrangement) : Those rearrangement reactions in which the migrating group moves to a free radical centre. Free radical rearrangements are comparatively rare. (4) Aromatic rearrangement : Those rearrangement reactions in which the migrating group moves to aromatic nucleus. Aromatic compounds of the type ( I ) undergo rearrangements in the manner mentioned below,
The element X from which group Y migrates may be nitrogen or oxygen.
Isomerism
Organic compounds having same molecular formula but differing from each other at least in some physical or chemical properties or both are known as isomers (Berzelius) and the phenomenon is known as isomerism. The difference in properties of isomers is due to the difference in the relative arrangements of various atoms or groups present in their molecules. Isomerism can be classified as follows: B:
X
X–H
Y
X–Y X–H
Y
( I )
Isomerism
Configurational or stereo isomerism The isomerism arises due to different arrangement of atoms or groups in space. It deals with the structure of molecules in three dimensions. Thus stereoisomers have:
Geometric al isomerism
Optical isomeris m
Conformation al isomerism
Chain isomeris m
Position isomeris m
Ring chain isomeris m
Functiona l isomeris m
Meta merism
Tauto - merism
Constitutional or structural isomerism Without referring to space, the isomers differ in the arrangement of atoms within the molecule is called structural isomerism. Thus structural isomers have:
(^) >
OH OH (^) OH
C 3 H 9 N :
(^3) Propan (^2) - 1 - amine 2 2
CH CH CH NH ;
Methy lethanamine 3
3 2
N
CH
H CH CH N
Propan-2-amine
3 2
3 | CH
NH
CH CH ;
, methy lmethanamine^3
3 3 2 NN Di
CH
CH CH CH N
(vii) Alcohols and phenols
(viii) Oximes and amides
C 2 H 5 NO :
(^3) Acetaldoxime
CH CH NOH ;
Acetamide 2
|| 3 NH
O
CH C
(4) Ring-chain isomerism : This type of isomerism is due to different modes of linking of carbon atoms, i.e., the isomers possess either open chain or closed chain sturctures.
C 6 H 12 :
Cy clohexane
2
2
2 2
2 2
H
H
C
C
CH
CH
HC
HC
Methy lcy clopentane
3
2 2 2 2
| CHCH
CH CH
CH CH
(^3 21) Hexene 2 2 2
CHCHCHCHCH CH
Ring – chain isomers are always functional isomers.
(5) Metamerism : This type of isomerism is due to the difference in the nature of alkyl groups attached to the polyvalent atoms or functional group. Metamers always belong to the same homologous series. Compounds like ethers, thio-ethers ketones, secondary amines, etc. show metamerism.
(i) C 5 H 13 N : Dimethy l propy l amine^3
3 (^3 7) CH
C H N CH ;
Diethy l methy lamine^3
2 5 (^2 5) CH
C H N CH
(ii) C 6 H 15 N : (^3 7) Dipropy l amine 3 7
C H NH CH ;
(^2) Buty l (^5) ethy lamine 4 9
C H NH CH
If same polyvalent functional group is there in two or more organic compounds, then chain or position
isomerism is not possible, there will be metamerism e.g., (a) (Pentan 2 one)
(^3) || 2 2 3
CH CH CH
O
CH C ;
(Pentan 3 one)
(^32) || 2 3
CHCH
O
CHCH C are metamers and not position
isomers. (b)
(Pentan 2 one)
CHCHCH
O
CH C ;
(3- Methy lbutan- 23 - one)
3 3 || | CH
CH CH
O
CH C are metamers and not chain isomers.
Alkenes does not show metamerism. (6) Tautomerism (i) The type of isomerism in which a substance exist in two readily interconvertible different structures leading to dynamic equilibrium is known as tautomerism and the different forms are called tautomers (or tautomerides). The term tautomerism (Greek: tauto = same; meros = parts) was used by Laar in 1885 to describe the phenomenon of a substance reacting chemically according to two possible structures. (ii) It is caused by the wandering of hydrogen atom between two polyvalent atoms. It is also known as Desmotropism (Desmos = bond and tropos = turn). If the hydrogen atom oscillates between two polyvalent atoms linked together, the system is a dyad and if the hydrogen atom travels from first to third atom in a chain, the system is a triad. (a) Dyad system : Hydrocyanic acid is an example of dyad system in which hydrogen atom oscillates between carbon and nitrogen atoms. H C N ⇌ C ^ N H (b) Triad system Keto-enol system : Polyvalent atoms are oxygen and two carbon atoms. Examples :
(Keto)
H
C
O
C ^ ⇌
(Enol)
C
OH
C
Acetoacetic ester (Ethyl acetoacetate) :
Keto form(92.1%)^225
|| 3 CHCOOCH
O CH C ⇌ Enol form(7.9%)^25
| 3 CHCOOCH
OH
CH C
Acetoacetic ester gives certain reactions showing the presence of keto group (Reactions with HCN ,
CH 2 OH OH
CH 3
Benzyl alcohol o - Cresol
C 7 H 8 O ;
H 2 NOH , H 2 NNHC 6 H 5 , etc. ) and certain reactions
showing the presence of enolic group (Reactions with Na , CH 3 COCl , NH 3 , PCl 5 , Br 2 water and colour with neutral FeCl 3 , etc. ).
Enolisation is in order CH (^) 3 COCH 3 CH 3 COCH 2 COOC 2 H 5 C 6 H 5 COCH 2 COOC 2 H 5 CH 3 COCH 2 COCH 3 CH (^) 3 COCH 2 CHO
Acid catalysed conversion Keto 2
|| 3 CH R
O
CH C
(Enolform)
| 3
(Intermediate)
|
| 3 CH R
OH R CH C H
CH
OH CH C H^
Base catalysed conversion
Keto form^2
|| 3 CH R
O
CH C CH R
O
CH C
|| 3
(Enol)
| 3
| 3 2 CH R
OH
CH R CH C
O
CH C
OH
H^ O
(c) Triad system containing nitrogen : Examples Nitrous acid exists in 2 forms
H nitrite O form N O Nitroform
O H N O
Nitro acinitro system
nitroform(i)
3 2 O
CH CH N O
Aciform(ii)
3 OH
CH CH N O
(iii) Characteristics of tautomerism (a) Tautomerism (cationotropy) is caused by the oscillation of hydrogen atom between two polyvalent atoms present in the molecule. The change is accompanied by the necessary rearrangement of single and double bonds. (b) It is a reversible intramolecular change. (c) The tautomeric forms remain in dynamic equilibrium. Hence, their separation is a bit difficult. Although their separation can be done by special methods, yet they form a separate series of stable derivatives. (d) The two tautomeric forms differ in their stability. The less stable form is called the labile form. The relative proportion of two forms varies from compound to compound and also with temperature, solvent etc. The change of one form into another is also catalysed by acids and bases. (e) Tautomers are in dynamic equilibrium with each other and interconvertible (⇌). (f) Two tautomers have different functional groups.
(g) Tautomerism has no effect on bond length. (h) Tautomerism has no contribution in stabilising the molecule and does not lower its energy. (i) Tautomerism may occur in planar or nonplanar molecules. Keto=enol tautomerism is exhibited only by such aldehydes and ketones which contain at least one - hydrogen. For example CH 3 CHO , CH 3 CH 2 CHO , CH 3 COCH 2 COCH 3 etc,. Tautomerism is not possible in benzaldehyde ( C 6 H 5 CHO ) , benzophenone ( C 6 H 5 COC 6 H 5 ), tri methyl acetaldehyde, ( CH (^) 3 ) 3 C CHO and chloral CCl (^) 3 CHO as they do not have H. Number of structural isomers Molecular formula Number of isomers Alkanes C 4 H 10 Two C 5 H 12 Three C 6 H 14 Five C 7 H 16 Nine C 8 H 18 Eighteen C 9 H 20 Thirty five C 10 H 22 Seventy five Alkenes and cycloalkanes C 3 H 6 Two (One alkene + one cycloalkane) C 4 H 8 Six (Four alkene + 2 - cycloalkane) C 5 H 10 Nine (Five alkenes + 4 – cycloalkanes) Alkynes C 3 H 4 Two C 4 H 6 Six Monohalides C (^) 3 H 7 X Two C (^) 4 H 9 X Four C (^) 5 H 11 X Eight Dihalides C 2 H 4 X 2 Two C 3 H 6 X 2 Four C 4 H 8 X 2 Nine C 5 H 10 X 2 Twenty one
H
OH
