Download Organic Chemistry: Electron Displacement Effects, Acidity, and Basicity and more Lecture notes Chemistry in PDF only on Docsity!
1.1 Introduction:
Organic reaction involve the breaking and making of covalent bonds. The breaking and making of covalent bonds usually occurs in several descrete steps before transformation into product. The detailed sequential description of all steps of the transformation into products is called the mechanism of a reaction. Complete information regarding all the steps is seldom obtained. However, a good deal of data can be gathered from the following (a) study of kinetics of the reactions (b) isolation of intermediate, if isolable. (c) study of reactions in the presence of other similar substrate. (d) study of the isotopically labelled atom in the reactants. (e) trapping of free radicals (f) crossover experiments (g) stereochemical aspects etc.
Reaction mechanism containing following tools
solvent Reactant temperature/lightpressure
Intermediate
Transition state
OR Product
(i) Reactant : Reactant are classified into substrate and reagent (A) substrate (^) species at which reagent attack. (B) Reagent (^) attacking species (more reactive species)
Case-I: If reaction occurs between organic and inorganic species, organic species act as substrate and inor- ganic species act as reagent.
substrate reagent
Organic species Inorganic species Product
Case-II: If reaction takes place between organic species then higher charge species act as reagent, other species act as substrate.
less charge excess charge substrate reagent
Organic species Organic species Product
General Organic Chemistry
CHAPTER
Example: H 3 C CH 2 CH 2 Br (^) + KOH H 3 C CH 2 CH 2 OH (^) + KBr substrate reagent (organic) (inorganic)
product
H 3 C C H 2
C H
O
(substrate)
+ H 2 C
C CH 2
O
CH 3
H 3 O+
(reagent)
H 2 C
H CH 3 C
OH
CH 2
C
O
CH 2
CH 3
1.2. Electrophiles :
(Electron loving species) electrophiles are electrons-deficient species and tend to attack the substrate at a site of high electron density. They may be neutral species as examplified by Lewis acid (such as BF 3 , AlCl 3 , ZnCl 2 ), carbene and carbocations.
H Proton as electrophiles
+ OH
hydroxide ion as nucleophile
H
O H water Classification of electrophiles: (a) Species having positive charge Example : H , R , Cl , NO , NO , C H^ ^ ^ ^2 6 5 N , CH 2 3 C Oetc. (b) Neutral species having vacant p-orbitals Example : BF 3 , BCl 3 , BBr 3 , carbene, etc. (c) Species having vacant d-orbital Example : FeCl 3 , ZnCl 2 , FeCl 2 etc. (d) Species having low lying -antibonding molecular orbital. Example : Br 2 , Cl 2 , I 2 etc. (e) -bonding molecule:
Example : SO 3 , CO 2 ,
O , (^) R C N , ,
O
etc.
(f) Element in their atomic state : Example : O, S etc.
1.3. Nucleophiles:
(Nucleus-loving species) Nucleophiles are electron donar species. Nucleophilic reagents tend to attack the electron deficient species (electrophiles). Classification of Nucleophiles: (a) Negative charge species: OH , OR ,SH ,SR , R ,Cl , Br , I^ ^ ^ ^ ^ ^ ^ etc (b) Organometallic reagent: R MgX, R Li, R CuLi, R Cd, R Zn etc 2 2 2 (c) Lone pair containing species:
H 2 O ,^ R O H ,^ NH 3 , H 2 S^ ,
N
etc
Other Effect: (a) Steric inhibition of resonance (b) Ortho effect.
1.4. Inductive effects:
In a covalent bond between two different atoms, the electrons in the^ ^ ^ bondare not shared equally. The
electrons are attracted towards the most electronegative atom. An arrow drawn above the line representing the covalently bonded electrons shifts towards higher electronegative atom can show this. Electrons are pulled in the direction of the arrow. When the atom (X) is more electronegative than carbon electrons attracted to X
C X
negative inductive effect (–I effect)
When the atom (Z) is less electronegative than carbon electrons attracted to carbon
C Z
positive inductive effect (+I effect) –I groups +I groups X=Br, Cl, NO 2 , OH, OR, SH, SR, NH 2 , NHR, NR 2 , CN, CO 2 H, CHO, COR
Z=R(alkyl or aryl), metals (e.g. Li or Mg)
The more electronegative the atom(X), the stronger the –I effect
The more electropositive the atom (Z), the stronger the +I effect.
Pauling electronegativity scale K = 0. C = 2. N = 3. O = 3.
I = 2. Br = 2. Cl = 3. F = 4. Higher the value, more electronegative will be atom
The inductive effect of the atom rapidly diminishes as the chain length increases
H 3 C CH 2 CH 2 CH 2 Cl
experiences a negligible –I effect
experiences a strong –I effect
The overall polarity of a molecule is determined by the individual bond polarities, formal charges and lone pair contributions, and this can be measured by the dipole moment (μ). Higher the dipole moment (measured in debyes (D)), more polar will be compound.
1.5. Hyperconjugation:
A bondcan stabilise a neighbouring carbocation (or positively charged carbon) by donating electrons to
the vacant p-orbital. The positive charge is delocalised or ‘spread out’, and this stabilising effect is known as “ no-bond resonance ”.
C–H -bond
vacant p-orbital
H The electrons in the -bond spend time in the vacant p-orbital
Points to Remember :
Number of hydrogen ^ number of hyperconjugating structure ^ stability
(^1) Polarity dipole moment 1 Heat of hydrogenation bond length
Problem : The correct order for the stability among following compound is
CH 3 CH 2 CH 3 CH 2 CH 2
(II) (III)
CH CH 2
H 3 C
H 3 C (II) (^) (IV)
C CH 2
H 3 C H 3 C H 3 C
(a) I > II > III > IV (b) I > III > IV > II (c) IV > III > II > I (d) IV > III > I > II
Soln. Number of hydrogen stability..
Thus,
CH 3 CH 2 CH 3 CH 2 CH 2
(II) (III)
CH CH 2
H 3 C
H 3 C
(I) (IV)
C CH 2
H 3 C H 3 C H 3 C Number of hydrogen 3 2 1 0 Stability, I > II > III > IV Hence, option (a) is correct.
Problem : Which is more stable methyl -D glucopyranoside or methyl -D glucopyranoside.
O
OCH 3
H
O
H
O CH 3
Methyl -D glucopyranoside Methyl^ -D glucopyranoside
orbital
axial lone pair
(axial-axial interaction is more feasible)
There is stabilising interaction i.e. hyperconjugation between the unshared pair on the hetero atom and *
orbital for the axial C–X bond in the case of anomer. thus it is more stable as compared to its analogue
in which there is no such interaction. Note : If oxygen is replaced by carbon, there is no such stability interaction as like as above. Thus, stability can only be decided on steric ground. Thus stability order for such species will be
OMe >>
OMe
(Stability)
1.6. Mesomeric effects:
Whilst inductive effects pull electrons through the bondframework, electrons can also move through the
bondnetwork. A^ ^ ^ bondcan stabilise a negative charge, a positive charge, a lone pair of electrons or
an adjacent bond by resonance (i.e. delocalisation or ‘spreading out’ of the electrons). Curly arrows are used to represent the movement of or non-bondingelectrons to give different resonance forms. It is only the electrons, not the nuclei, that move in the resonance forms and a double-headed arrow is used to show their relationship.
Mesomeric effects can be effective over much longer distances than inductive effects, provided that conjuga- tion is present (i.e. alternating single and double bonds). Whereas inductive effects are determined by distance, mesomeric effects are determined by the relative positions of +M and –M groups in a molecule.
1.7. Resonance:
The different structure of a compound divised by different methods of pairing electrons in a fixed atomic skeleton are called resonance of canonical structure. The actual structure of the compound is then a combina- tion of these structure and hence the compound is called a resonance hybrid. A hybrid is more stable than any one of the contributing structures. The contributing resonance structure are shown by double-headed arrows ^ indicating that the real structures involves both way of pairing electrons. Key point about resonance: (i) Resonating structure/contributing structure/canonical structure are imaginary hypothetical, while resonance hybrid is the true strucure.
(ii) Resonance involve the delocalization of lone pair and -electrons.
(iii) Resonance is the intramolecular process. (iv) Resonance must follow the Lewis octate rule, i.e. C-atom, N-atom never pentavalent and O-atom never tetravalent. (v) In the resonating structure arrangement of atoms remain same, there should differ only w.r.to arrangement of electrons. (vi) Resonance stabilisation is greatest, when there are equivalent resonating structure.
C
O
R O
O
R O carboxylic ion
O O O O O
Phenoxide ion Resonance hybrid (vii) The energy difference in between resonance hybrid and most stable resonating structure is called reso- nance energy (viii) Resonance work only at ortho and para position with equal intensity, it never work at meta position.
(ix) Resonance proceeds in the system via -electrons.
(x) Hyper conjugation works just like resonance. X
X
–Y
–X
CH 3
Nu–E
CH 3 Nu
E
Classification of Resonance: (A) + R effect (B) –R effect (A) +R effect (Electron releasing effect): Lone pair of electron density containing compound attached with conjugation.
HO C H
CH 2 HO^ CH CH 2
G
, G^
G = Electron Releasing Group
–OH, – OR, NH , NR 2 2 , O^ C
O
CH 3 , NH
C
CH 3
O
, –F, –Cl, –Br, –I, –SH, etc
(B) –R effect (Electron withdrawing effect): Electronegative atom attached with multiple bond containing compound (a) Electron withdrawing resonance effect Example :
N
O
O
, C N , C
O , (^) C
O
H
, C
O
OH
H 2 C CH N
O
O
H 2 C CH NH
O
H O (b) Species having Vacant d-orbitals
PMe 3 PMe 3
, (^) PMe 3 , (^) –AsMe 3 , –SbMe 3 , –SR 2
(c) Species having vacant - p - orbitals: BR 2 BR 2
1.8. Application of inductive effect, hyperconjugation and mesomeric effect:
Acidity and basicity: Acids: An acid is a substance that donates a proton (Bronsted-Lowry). Acidic compounds have low pKa- values and are good proton donors, as the anions (or conjugate bases), formed on the deprotonation, are relatively stable. In water:
HA (^) + H 2 O
Ka (^) H 3 O^ +^ A (^) is acidity constant Acid (^) Base Conjugate Acid^ Conjugate Base
where, Ka
(b) Inductive and mesomeric effects and phenols: Mesomeric effects can also stabilise positive and negative charges.
arg – M arg M
The negative ch e needs to be on adjacent carbon atom for a group to estabilise it The poisitive ch e needs to be on adjacent carbon atom for a group to stabilise it
On deprotonation of phenol the phenoxide anion is formed. This stabilised by delocalisation of the negative charge at the 2-, 4-and 6-positions of the benzene ring.
OH O O^ O
Base
4
6 2
O
Temporary Effect :
(a) Electromeric Effect :
- Temporary effect.
- Takes place between two atoms joined by a multiple bond
- Occurs at requirement of attacking reagent.
C O + Z
Instantaneous shift of electron pair of carbonyl group towards oxygen. It is of two types. (1) + E effect : Transition of electron towards the attacking reagent.
C C + H C C
H (2) –E effect : Transition of electron away from attacking reagent.
C O + CN
O
(b) Inductomeric Effect ;
- Temporary effect.
- Takes place in sigma bonded system
- In presence of attacking reagent, transition of a electron cloud takes place more readily. Example :
R O H + (^) B In presence of base B, movement of sigma electron takes place faster.
(3) Other Effect : (a) Effect of inertia/steric inhibition of resonance: Resonance ability of an atom is lost if it looses planarity with the other part of the system due to steric crowding by bulky group in adjacent positions.
H 3 C CH 3
NO 2
NH 2
NO 2
NH 2
(A) (B)
H 3 C CH 3
The above two compounds A and B have everything identical except position of the two methyl group. It is expected that A should be stronger base than B due to closeness of two electron donating methyl group to – NH 2. The fact is opposite to this. In compound B –NO 2 is surrounded by two bulky methyl group and they sterically repel the –NO 2 group. In order to minimize the steric repulsion by the two adjacent methyl group, the nitro group loses planarity with the benzene ring. So, now –NO 2 due to lack of planarity weigh ring, not able to resonate. This is known as steric inhibition of resonance. Thus in B , –NO 2 is not decreasing basic strength by resonance. In A –NO 2 lies in the plane of the ring, it is in resonance with the ring, decreases basic strength of –NH 2 by resonance, hence weaker base. Similarly we can explain the acidic strength of C and D
H 3 C CH 3
NO 2
COOH
NO 2
COOH
(C) (D)
H 3 C CH 3
C is stronger acid inspite of closeness of two electron donating methyl group to –COOH. (b) Ortho effect: If any electron withdrawing group present on ortho position of the benzoic acid. It always increases acidic nature of acid because this group increases outer resonance of the ring toward acidic nature. Similarly if any group present on ortho position of aniline, it decreases basic nature. This effect is known as ortho effect. Problem : The correct order of acidity among the following compound I-IV is
COOH COOH COOH COOH
NO 2
NO 2
NO 2 (I) (II) (III) (IV)
(a) II > III > IV > I (b) IV > II > III > I (c) II > IV > III > I (d) IV > III > II > I Soln. Because of ortho effect o-nitro benzoic acid is most acidic followed by para and meta. Thus order will be II > IV > III > I. Hence, option (c) is correct.
Keynotes in Organic Chemistry:
- If –M groups are introduced at 2-, 4- and/or 6-positions, the anion can be further stabilised by delocalisation through the system, as the negative charge can be spread onto the –M group. We can use double- headed curly arrows to show this process.
C
O
H
O
O H (3)
C
O
H
O
O (4)
–H +
Intramolecular hydrogen bonding can, of course, operate in the undissociated acid as well as in the anion, but it is likely to be considerably more effective in the latter than in the former - with consequent relative stabilisation - because the negative charge on oxygen in the anion will lead to stronger hydrogen bonding. The effect is even more pronounced where hydrogen bonding can occur with hydroxyl groups in both o - position, and 2, 6-dihydroxybenzoic acid is found to have pK (^) a = 1.30.
Dicarboxylic Acids:
HCO H 2
CH CO H 3 2
CH CH CO H 3 2 2
C H CO H 6 5 2
HO CCO H 2 2
HO CCH CO H 2 2 2
HO CCH CH CO H 2 2 2 2
HO CC H CO H 2 6 4 2
o - 2.98; m - 3.46; p - 3.
Acid pKa Value Acid pKa Value
Bases: A base is a substance that accepts a proton (Bronsted-Lowry). Basic compounds have high pKa-
values and are good proton acceptors, as the cations (or conjugate acids), formed on protonation, are relatively stable.
In water :
B (^) + H 2 O
Kb BH + (^) HO where, Kb is basicity constant Base (^) Acid ConjugateAcid ConjugateBase
The strength of bases are usually described by the Ka-and pKa-values of the conjugate acid.
BH + H 2 O
Ka B (^) + H 3 O^ where Ka^ is acidity constant.
2
3 a As H O is in excess
B H O
K
BH
- If B is a strong base, then BH will be relatively stable and not easily deprotonated. (^) BH will therefore have a high pKa-value
- If B is a weak base, then (^) BH will be relatively unstable and easily deprotonated. BH will therefore have a low pKa-value.
The cation can be stabilised by +I and +M groups, which can delocalise the positive charge. (The more ‘spread out’ the positive charge, the more stable it is).
(c) Inductive effects and aliphatic (or alkyl) amines: On protonation of amines, ammonium salts are formed.
R NH 2 + H X R NH 3 X
The greater the +I effect of the R group, the greater the electron density at nitrogen and the more basic the amine. The greater the +I effect, the more stable the ammonium cation and the more basic the amine.
N H
H
H
H N H
Et
H
Et N H
Et
H
Et N H
Et
H
Et
pKa 9.3^ 10.7^ 10.9^ 10.
The pKa-values should increase steadily as more +I alkyl groups are introduced on nitrogen. However, the pKa-values are determined in water, and the more hydrogen atoms on the positively charged nitrogen, the greater the extent of hydrogen-bonding between water and the cation. This solvation leads to the stabilisation of the cations containing N–H bonds. In organic solvents (which can not solvate the cation), the order of pKas is expected to be as follows:
3 2 2 3 ^ most basic least basic
R N R NH RNH NH R I alkyl group
The presence of –I and /or –M groups on nitrogen reduces the basicity, and hence, for example, amides are poor bases. Ethanamide has a pKa of –0.5.
C
O
H 3 C NH 2 –M, –I
C
O
H 3 C NH 2
The C=O group stabilises the lone pair on nitrogen by resonance. This reduces the electron density on nitrogen
(d) Mesomeric effects and aryl (or aromatic) amines: The lone pair of electrons on the nitrogen atom of aminobenzene (or aniline) can be stabilised by the delocalisation of the electrons onto the 2-, 4-and 6-positions of the benzene ring. Aromatic amines are therefore less basic than aliphatic amines.
NH 2
6 2
4
NH 2 NH 2 NH 2
- If –M groups are introduced at the 2-, 4-and/or 6-positions (but not at the 3- or 5-position), the anion can be further stabilised by delocalisation, as the negative charge can be spread onto the –M group. This reduces the basicity of the amine.
- If –I groups are introduced on the benzene ring, the order of –I stabilisation is 2-position > 3-position > 4- position. This reduces the basicity of the amine.
