Syllabus: Second Part Honours
Third Paper
Group-A
1. Distribution of velocity of gases
2. Reaction Rate Process and Catalyst
3. Electrochemical cell
4. Ionic Equilibria
Group-B
1. Nature of a Chemical Bonds
2. Chemistry of a 3d Block Elements
3. Coordination Chemistry
4. Chemistry of Individual Elements
5. Volumetric Estimation
Fourth Paper
Group-A
1. Thermodynamics
2. Ehemical Equilibrium
3. Colloids
4. Distribution Law
Group-B
1. Review of Structural Isomerism
a. Geometrical Isomerism
b. Optical Isiomerism
2. Classification ofReactions and Mechanism
3. Active Methylene Compounds
4. Hydroxy Acids
5. Carbohydrates
6. Aromatic Chemistry
Physical Chemistry (Paper: 3rd & 4th)
Paper:3rd / Distribution of Velocity of Gases
Root Mean Square Velocity(Crms):
It is defined as the square root of the mean value of the squares of the velocities of all the molecules at a particular temperature.
example: If a container contains a gas molecules having n molecules and at a particular moment the velocities of the molecules are C1, C2, C3 ... Cn
then-
From kinetic gas equation-
C2 = 3PV/M = 3RT/M
or, Crms = √3RT/M
where M is the molecular weight of the gas.
Average Velocity (Cavg):
It is defined as the mean of the velocities of all molecules at a particular temperature.
Cavg = √8RT/πM
Most Probable Velocity (Cmpv):
The velocity possessed by the largest number of gas molecules at a particular temperature is called most probable velocity.
Cmpv = √2RT/M
where M is the molecular weight of the gas.
Relationshipamong the RMSV, AV and MPV:
From the equation
Cmpv : Cavg : Crms = 1 : 1.128 : 1.225
Average Velocity = 0.921 X Crms
Cmpv = 0.816 X Crms
Mean Free Path:
The gas molecules are always in state of rapid motion colliding with each other. The distance travelled by a gas molecules before colliding with another molecules is called free path and the average length of a large number of such paths is called mean free path. It is denoted by l and is directly proportional to temperature and inversely proportional to pressure.
Collision Number:
Collision number is defined as the "number of collisions" per unit time.
A collision is an interaction between two or more movable bodies.
The collision number is indicated by the symbol of Z. When one molecule involved, the average number of collisions per unit time nearly one second per moles of reactant between reacting molecules is called collision number.
Collision Frequecny:
Number of collisions suffered by gas per second per cubic meter of a gas is called collision frequency. It is equal to the ratio of the rms velocity and mean free path of a gas. i.e.
Collision frequency = Number of collision per second
or, Collision frequency = Crms / mean free path
Equipartition Energy:
Law of equipartition energy states that for a dynamical system in thermal equilibrium the total energy of the system is shared equally by all the degrees of freedom.
The RMS velocity of gas molecules can be resolved into its components along x, y and z axis as -
c2 = cx2 + cy2 + cz2
multiplying by 1/2m, we get-
1/2mc2 = 1/2mcx2 + 1/2mcy2 + 1/2mcz2
or, E = Ex + Ey + Ez -----(1) (as K.E.(E) = 1/2mc2)
We know that gas molecules do not have any preferential direction. So, velocities along all three axes are equally probable-
cx = cy = cz
or, Ex = Ey = Ez
or, E = 3 Ex -----(2)
Each energy term in component contributes equally to the total energy. This is called the law of equipartition of energy.
We know that-
PV = 1/3 mnc2
PV = 2/3 X 1/2 mnc2 = 2/3 K.E.
or, K.E. = 3 X 1/2 PV
or, E = 3 X 1/2 RT (as PV = RT for one mole)
From equation (2), we have-
3 Ex = 3 X 1/2 RT
so, Ex = Ey = Ez = 1/2 RT
Hence, the average kinetic energy possed by molecules in each component per degree of freedom is 1/2 RT per mole.
Paper:3rd / Reaction Rate Process and Catalyst
Catalyst:
Those substances which change the rate as well as mechanism of the reaction are called catalyst. Most of the time a catalyst is used to increase the rate of the reaction. Although it participates in the reaction but it gets regenerated at the end of the reaction. A catalyst can be either solid, liquid or gasses.
Positive Catalysts:
Catalysts which increase the rate of a chemical reaction are positive catalysts. It increases the rate of reaction by lowering the activation energy barriers such that a large number of reactant molecules are converted into products.
Examples:
Manganese dioxide as a catalyst accelerates the decomposition of potassium chlorate to liberate oxygen.
2 KClO3 ---MnO2---> 2 KCl +3 O2
Platinum as a catalyst accelerates the decomposition of hydrogen peroxide.
2 H2O2 ---Pt---> 2 H2O + O2
Negative Catalysts:
Catalysts which decrease the rate of reaction are negative catalyst. It decreases the rate of reaction by increasing the activation energy barrier which decreases the number of reactant molecules to converted into products.
Example:
The decomposition of hydrogen peroxide is suppressed by adding glycerol or Acetanilide to the solution of hydrogen peroxide. Here glycerol or Acetanilide acts as negative catalyst.
2 H2O2 ---glycerol or Acetanilide---> 2 H2O + O2
Promoter or Accelerators:
A substance which increases the activity of catalyst are known as Promoters or accelerators.
Example: In Haber’s process (manufacture of ammonia) molybdenum act as Promoters.
Catalyst Poisons or Inhibitors:
Substances which decrease the activity of catalyst are known as catalyst poisons or inhibitors.
Example: In the hydrogenation of alkyne to an alkene, catalyst Pd is poisoned with BaSO4 in quinolone solution and the reaction is stopped at alkene level.
Autocatalysis:
The phenonenon in which one of the products of reaction itself acts as a catalyst is called autocatalysis.
Example: The hydrolysis of ester is catalysed by H+ ions which is one of the products of the reaction.
CH3COOC2H5 + H2O ⇌ CH3COO− + H+ + C2H5OH
Autocatalysed reactions proceed slowly at the start because there is little catalyst present, the rate of reaction increases progressively as the reaction proceeds as the amount of catalyst increases and then it again slows down as the reactant concentration decreases.
The graph for autocatalytic reactions is a sigmoid curve.
Paper:3rd / Electrochemical Cell
Paper:3rd / Ionic Equilibria
Ostwald Dilution Law:
According to Arrhenius electrolytic dissociation theory, ions and unionized electrolytic molecules are in dynamic equilibrium. Ostwald applied the law of mass action to this equilibrium as follows:
Let a weak electrolyte AB in aqueous solution has the degree of dissociation α , then-
This equation is called Ostwald dilution law. Lower the K value, lower the α value i.e. weaker the electrolyte.
Ionic Product of Water:
Pure water is a very weak electrolyte and is ionises as,
H2O ⇌ H+ + OH−
Applying law of mass action at equilibrium, the value of dissociation constant-
K = [H+] [OH−] / [H2O]
or, K [H2O] = [H+] [OH−]
Concentration of undissociated water molecules [H2O], may be regarded as constant as dissociation takes place to a very small extent. Thus, the product K [H2O] gives another constant which is written as Kw called ionic product of water.
So, Kw = [H+] [OH−]
The product of concentrations of H+ and OH− ions in water at a particular temperature is known as ionic product of water. The value of Kw increases with the increase of temperature, i.e., the concentration of H+ and OH− ions increases with increase in temperature.
Buffer Solution:
A buffer solution is an aqueous solution consisting of a mixture of a weak acid and its conjugate base, or vice versa. Its pH does not change or changes very little when a small amount of strong acid or base is added to it.
It is of three types, i.e. Acidic Buffer, Basic Buffer and Neutral buffer.
Acidic Buffer: Mixture of the solution of weak acid and salt of its conjugate base in equal amount.( e.g. acetic acid and sodiumacetate). pH of acidic buffer is less than seven.
Basic Buffer: Mixture of the solution of weak base and salt of its conjugate acid in equal amount.(e.g. ammoniumhydroxide and ammoniumchloride).pH of basic buffer is more than seven.
Neutral Buffer: It is single substance showing properties of buffers. (e.g. ammonium acetate).
Buffer capacity:
The property of buffer solution to resist the change its pH value is known as buffer capacity. It has been found that if the ratio [Salt] / [Acid] or [Salt] / [Base] is unity, the pH of a particular buffer does not change at all.
Quantitatively, buffer capacity is defined as number of moles of acid or base added in one litre of solution to change the pH by unity.
Buffer Capacity = No. of moles of acid or base added to one liter of buffer / Change in pH.
or, β = n / ΔpH
where: β is buffer capacity, n is the number of moles of an acid or a base added one liter of buffer solution and ΔpH is the change in pH.
ΔpH = final pH - initial pH
A buffer capacity always has a positive value.
Buffer capacity is maximum when:
[Salt] = [Acid], i.e., pH = pKa for acid buffer
[Salt] = [Base], i.e., pOH = pKb for base buffer under above conditions, the buffer is called efficient.
Buffer Range:
The buffer range is the pH range where a buffer effectively neutralizes added acids and bases, while maintaining a relatively constant pH.
Paper:4th / Thermodynamics
Spontaneous Process and Nonspontaneous Process
The process which proceeds of its own accord without any outside assistance or help is called Spontaneous Process or Natural Process and the reverse process which does not proceed on its own accord is called nonspontaneous process.
Spontaneous process is unidirectional, for reverse change work has to be done. For spontaneous process time is no factor. It may takes palce rapidly or very slowly. If a system is not in equilibrium state, a spontaneous change is inevitable. The change will continue untill equilibrium exist. Once the system attains the equilibrium it does not undergo further spontaneous change. Spontaneous process or change is accompanied by decrease of internal energy of enthalpy.
The tendency of a process to occur naturally is called Spontaniety.
Examples
A rolling ball down the hill spontaneously but it will not rall uphill unless work is done on it.
When two metal ball one hot and one cold are connected to each other, the heat flow from hotter to colder ball spontaneously but can never from colder to hotter ball spontaneously.
Entropy (S)
Entropy Change in mixing of ideal gases
Gibbs-Helmholtz Equation
Helmholtz Free Energy(F)
Gibbs free energy(G or F)
Variation of Free Energy with Temperature and Pressure
Free Energy Change of Expansion of an Ideal Gas
It is a thermodynamic state function that is a measure of the randomness and disorderness of the molecules of the system. Greater the randomness, greater would be entropy. It is denoted by 'S'. As it is a state function so depends upon initial and final satate only. So change in entropy
ΔS = Sfinal − Sinitial
For a reversible process at constant temperature,the change in entropy is equal to energy absorbed or evolved divided by the temperature.
ΔS = q/T
If heat is absorbed, ΔS is positive and entropy increases while If heat is evolved, ΔS is negative and entropy decreases.
If heat change takes place at different temperatures, then-
dS = dq1/T1 + dq2/T2 + dq3/T3 + ... = Σ(dqrev/dT)
or, ∫ds = ∫dqrev/T
ΔS = ΔS
at equilibrium, ΔS = 0
→→→→→→→
→→→→→→→
→→→→→→→
well orderd
Low Entropy
→←↓→↑←↓→
→↓←↓→↑←↓
←↑→↓←↓→↑
Random
High Entropy
Cal K-1
or, J K-1
When two ideal gases are brought in contact , they mix with each other and incerase in their entropy.
Let n1 moles of gas A whose volume is V1 mixed with n2 moles of other gas B whose volume is V2 and form a new volume V1 + V2 of mixed gas. So the entropy change in gas A due to mixing is-
ΔS1 = n1 Rln(V1 + V2/V1)
Similarly, for gas B, entropy change is-
ΔS2 = n2 Rln(V1 + V2/V2)
So, the total entropy change-
ΔSmix = ΔS1 + ΔS2
or, ΔSmix = n1 Rln(V1 + V2/V1) + n2 Rln(V1 + V2/V2)
or, ΔSmix = −R(n1 ln(V1/V1 + V2) + n2 ln(V2/V1 + V2)
If X1 and X2 br the mole fraction of gas A and B respectively, then-
X1 = (n1/n1+n2) = (V1/V1+V2)
and X2 = (n2/n1+n2) = (V2/V1+V2) at NTP
So, ΔSmix = −R(n1 lnX1 + n2 lnX2)
or, ΔSmix = −RΣni lnXi
Again from this equation
ΔSmix = −R(n1 lnX1 + n2 lnX2)
ΔSmix = −R[n1/(n1+n2) lnX1 + n2/(n1+n2) lnX2](n1+n2)
Now entropy change for one mole-
n1+n2 = 1
ΔSmix = −R(X1 lnX1 + X2 lnX2)
or, ΔSmix = −RΣXi lnXi
Since all Xi is less than one so, entropy of mixing of ideal gas is always positive.
When a system undergoes a reversible change, the change in free energy with temperature and pressure is given by-
dF = VdP − SdT -----(equation-1)
at constant pressure, dP = 0
so, (equation-1) becomes-
[dF/dT]p = −S
∴ −ΔS = −(S1 − S2)
or, −ΔS = (dF2/dT)p −(dF1/dT)p
or, −ΔS = [(F2 − F1)/dT]p
or, −ΔS = [d(ΔF)/dT]p -----(equation-2)
we know that ΔF = ΔH − TΔS -----(equation-3)
so putting the value of ΔS from equation-2 in equation-3 we get-
ΔF = ΔH + T[d(ΔF)/dT]p -----(equation-4)
equation-4 is called Gibbs-Helmholtz Equation. and is applicable to all process which takes place at constant pressure.
Mathematically, Helmholtz free energy is expressed as-
F = U - TS
on differentiation we get-
dF = dU − TdS − SdT
From 1st law of thermodynamics-
dQrev = dU + Wmax
or, TdS = dU + Wmax
So, dF = −Wmax − SdT
at constant temperature-
or, (−dF)T = Wmax
So, Helmholtz free energy is the function decrease of which measures maximum amount of work which is obtainable from the system isothermally and reversibly.It is a state function and extensive property.
Gibbs free energy is the property of the system whose decrease is the measure of the maximum external work available during the transformation of system reversibly at constant pressure and temperature and is state function. So depend only on initial and final state of the system.
If S is the entropy of a system at ToK and H is its enthalpy then, Gibbs free energy is mathematically expressed as-
G = H - TS
On differentiation we get,
ΔG = ΔH − TΔS − SΔT
we know that- ΔH = ΔE + PΔV + VΔP
so, ΔG = ΔE + PΔV + VΔP − TΔS − SΔT
or, ΔG = ΔA + PΔV + VΔP − SΔT (as ΔE − TΔS = ΔA (work function))
at constant pressure and temperature-
or, ΔG = ΔA + PΔV
or, ΔG = −Wmax + PΔV (as W = −ΔA from 1st law of thermodynamics)
or, −ΔG = Wmax − PΔV
PΔV is the work done due to expansion against a constant pressure. So, decrease in free energy accompanying a process which occurs at constant pressure and temperature is the maximum work obtained from the system other than PΔV. Hence it is also called Net Work.
So, Net Work = −ΔG
We know that-
F = H − TS -----(equation-1)
or, F = E + PV − TS (as H = E + PV)
differentiating this equation we get-
dF = dE + PdV + VdP − TdS − SdT
or, dF = dq + VdP − TdS − SdT (as dq = dE + PdV)
or, dF = TdS + VdP − TdS − SdT (as dqrev/T = dS)
or, dF = VdP − SdT -----(equation-2)
at constant Temperature-
or, dF = VdP
or, (dF/dP)T = V -----(equation-3)
and at constant Pressure-
or, dF = − SdT
or, (dF/dT)P = − S -----(equation-4)
When a system undergo a reversible change, the change in free energy with temperature and pressure is given as-
dF = VdP − SdT
at constant temperature-
dF = VdP
or, dF = nRT.(dP/p) (as PV = nRT)
on integrating the above equation we get-
∫dF = nRT∫dP/P
or, ΔF = nRT lnP2/P1
or, ΔF = nRT lnV1/V2 (as P1V1 = P2V2 )
Paper:4th / Chemical Equilibria
Paper:4th / Colloids
Paper:4th / Distribution Law
Nernst's Distribution Law:
When a solute distribute itself between two immicible solvents in equilibrium, then the ratio of its concentration in two solvents remains constant. This is called Distribution law.
Example: The solute iodine distributes itself in CCl4 and water solvents. Let C1 and C2 be the concentration of I2 in CCl4 and water respectively. Then from Distribution law-
C1 / C2 = KD
where KD is partition coefficient.
The value of KD depends upon the Temperature, Nature of solute, Nature of two solvents and Manner in which the constant is expressed
i.e. C1 / C2 or C2 / C1
Essential Conditions for the Distribution Law:
Followings are the essential conditions for that-
1. Temperature must be constant.
2. Solutions are must be dilute.
3. Two solutions should be insoluble or very sparingly soluble and their solubility should not be affected by the presence of solute.
4. Solute must not be undergo dissociation or association or interaction with any of the solvents.
Thermodynamic Derivation of the Distribution Law
The thermodynamic derivation of the distribution law is based upon the principle that if there are two phases in equilibrium (i.e. two immiscible solvents containing the same solute dissolved in them), the chemical potential(μ) of a substance present in them must be same in both the phases.
From thermodynamics, we know that the chemical potential (μ) of a substance is a solution given by-
μ = μo + RT lna
Where μo is the standard chemical potential and 'a' is the activity of the solute in the solution.
Thus for the solute in liquid A, we have-
μA = μoA + RT lnaA
Similarly for the solute in liquid B we have-
μB = μoB + RT lnaB
But as already stated, since the liquids A and B are in equilibrium,
μA = μB
or, μoA + RT lnaA = μoB + RT lnaB
or, RT lnaA − RT lnaB = μoB − μoA
or, ln(aA/aB) = (μoB − μoA)/R -----(equation-1)
At a given temperature, μoAand μoB are constant for given substance in the particular solvents. Hence at constant temperature, equation (1) becomes-
ln(aA/aB) = Constant
or, aA/aB = Constant -----(equation-2)
This is the exact expression of the distribution law. However, if the solutions are dilute, the activates are equal to the concentrations so that the (equation-2) is modified as-
CA/CB = Constant -----(equation-3)
(equation-3) is the original form of the distribution law.
Modified form of distribution law when association of solute occurs in one of the solvent
nA → An
Number of particles decreases after association.
If a solute A present in solvent-I where its concentration is CI and in solvent-II, n molecules of solute A associates to form An and a few molecules of solute A are also present in solvent-II. If the concentration of A and An be CA and CII in solvent-II, respectively, then from distribution law-
CI/CA = KD -----equation(1)
now equilibrium constant for the reaction nA ⇌ An is-
KC = [An]/[A]n
or, KC = CII/CnA
Taking nth root , we get-
n√CII/CA = n√KC -----equation(2)
Now, dividing equation (1) by equation (2), we get-
(CI/CA) x (CA/n√CII) = (KD/n√KC)
CI/n√CII = KD/n√KC = K -----equation (3)
Equation (3) is a modified form of distribution law when association of solute occurs in one of the solvent.
Modified form of distribution law when dissociation of solute occurs in one of the solvent
Let a solute molecule A which does not dissociate in solvent-I has concentration CI. When it dissociates into x and y in solvent-II having total concentration CII.
If α be the degree of dissociation of solute A in solvent-II, then-
A ⇌ x + y
CII(1 − α) CIIα CIIα
so, the concentration of undissociated molecules of solute A in solvent-II will be CII(1 − α)
Hence, the modified form of distribution law when dissociation of solute occurs in one of the solvent will be-
CI/CII(1 − α) = K
Solvent extraction
The most important application of the distribution law is in the process of extraction, in the laboratory as well as in industry.
In the laboratory, it is frequently used for the removal of a dissolved organic substance from aqueous solution with solvents such as benzene, ether, chloroform, carbon tetrachloride, etc. the advantage is taken of the fact that the partition coefficient of most of the organic compounds is very largely in favour of organic solvents.
The process of extraction is more efficient if the solvent is used in a number of small portions than in one whole. This is called Multiple Extraction.
Determination of Equilibrium Constant from Distribution Coefficient
Distribution law helps in the determination of equilibrium constant of a reaction when one of the reactants is soluble in two immiscible solvents.
Example: When KI reacts with I2 to form KI3
KI + I2 ⇌ KI3
This reaction can be carried out in water, while I2 is soluble in both water and benzene.
Procedure
To find Distribution Coefficient of I2
I2 is shaken with water and benzene in a bottol. The concentration of I2 in the two layers is then determined by titration against standard thiosulphate solution.
So, (Conc. I2 in water/Conc. I2 in benzene) = K
To find Equilibrium Constant using the value of K
A solution of KI of concentration 'a' is shaken with I2 in a bottol, some benzene is also added and shaken. On standing, the mixture seperates into two layers.
The concentration of I2 is determined in the two layers by titration against standard thiosulphate solution.
Let 'b' the concentration of I2 in benzene layer, 'c' be the concentration of I2 in water layer which is really the total of the concentration of free I2 and KI3.
K is the value of Distribution coefficient of I2 between water and I2.
So, (Conc. I2 in water/Conc. I2 in benzene) = K
∴ Concentration of free I2 in water layer = K X b
Hence, Concentration of KI3 in water layer = c-Kb
∴ Concentration of KI in water layer = a-(c-Kb) = a-(c+Kb)
Now, we can write the equilibrium constant of the reaction-
KI + I2 ⇌ KI3
so, Kc = [KI3]/[KI ] [I2]
or, Kc = (c-Kb)/(a-(c+Kb))Kb
Other example of complex formation
CuSO4 + NH3 ⇌ CuSO4.4NH3 or [Cu(NH3)4]+2SO4-2
Inorganic Chemistry Paper: 3rd
Nature of Chemical Bond
Chemistry of 3d Elements
Coordination Chemistry
Chemistry of Individual Elements
Volumetric Estimation
Organic Chemistry Paper: 4th
Stereochemistry of Organic Compounds
Elements of Symmetry
Elements of symmetry are a simple tool to identify whether a molecule is chiral or not. The essential conditions for optically active molecule to be chiral is that, the molecule should not possess any kind of symmetry elements. The elements of symmetry are given below:
a. Simple axis of symmetry (Cn)
b. Plane of symmetry (σ)
c. Centre of symmetry (Ci)
d. Alternating axis of symmetry (Sn)
Simple axis of symmetry (Cn)
When a rotation of 360°/n (where n is any integer like 1,2,3...etc.) around the axis of a molecule is applied, and the rotated form thus obtained is
indistinguishable from the original molecule, then the molecule is known to have a simple axis of symmetry. It is represented by Cn.
Plane of symmetry (σ)
It is defined as when a plane that devide a molecule into two equal halves which are related to object and mirror image is known as plane of symmetry. It is represented by σ.
Centre of symmetry (Ci)
It is also called centre of inversion. A molecule has a centre of symmetry when, for any atom in the molecule, an identical atom exists diametrically (diagonally) opposite to this centre and at equal distance from it.
Alternating axis of symmetry (Sn)
An alternate axis of symmetry is defined as, when a molecule is rotated by 360°/n degrees about its axis and then a reflection plane is placed exactly at perpendicular to the axis, and the reflection of the molecule thus obtained is identical to the original. It is represented by Sn.
Enantiomers
Optically active chiral molecules which are non-super imposable on its mirror
images are called enantiomers and the phenomenon is known as enantiomerism. For a molecule to be an optical isomerism the molecule must have at least one asymmetric carbon atom.
We can easily understand the chirality by comparing our hands (left hand and right hand). Our left hand and right hand are the best example of non-superimposable mirror image of each other. Each hand is therefore considered as chiral.
The main Properties of enantiomers are given below-
The main properties of enantiomers are given below-
a. Enantiomers always exist in pair.
b. Enantiomers are non-super imposable on its mirror image.
c. Enantiomers have same physical properties (like boiling point, melting point, solubility, density, viscosity, refractive index etc.)and chemical properties in achiral environment.
d. Each enantiomers have opposite behavior with respect to plane polarized light, if one of them will rotate the plane polarized light towards right hand direction then definitely the other will rotate the plane polarized light towards left hand direction.
e. Each enantiomers shows the same chemical reactivity with achiral reagent; however they have different reactivity with chiral reagent.
Diastereomers
Diastereomers are simply not enantiomers. They are compounds which have the same molecular formula and sequence of bonded elements but are not mirror image and nonsuperimposable. The must essential conditions for a molecule to be diastereomer there must be at least two asymmetric carbon.
The main properties of. Diastereomers are given below-
a. Diastereomers have different physical properties such as melting points, boiling points, densities, solubilities, refractive indices, dielectric constants and specific rotations.
b. Diastereomers other than geometrical isomers may or maynot be optically active.
c. Diastereomers show similar, but not identical chemical properties. The rates of reactions of the two diastereomers with a given reagent provided tha reagent is not rapidly active.
d. Diastereomers can be separated from one another through techniques like fractional crystallization, fractional distillation, chromatography etc.
Chiral and Achiral Molecules with Two Stereogenic Centres:
Chiral molecules are those in which the central carbon atom is bonded directly through four different atoms or group of atoms and do not have any type of symmetry element present in it and the molecule has non-super imposable mirror image.
Achiral molecules are those molecules in which central carbon atom is directly bonded through four different atoms of group of atoms and it satisfied any type of symmetry elements are called achiral molecule. Achiral molecules have super imposable mirror images.
Let us consider the stereoisomers of Tartaric acid which has two stereocenters with identical atoms or group of atoms attached to both the stereocenters (chiral carbon).
The tartaric acid have two stereocenters and
can have four stereoisomers (i.e. 22) out of which two stereoisomers are non-super imposable mirror image of each other called enantiomers and chiral, and the rest two are identical to each other and also have plane of symmetry hence it can be divided in to two equal halves. So, they are achiral.
Erythro (Syn) and Threo (Anti) Diastereomers
The threo and erythro naming is given only to those diastereomers having two adjacent stereocentres. Erythro and Threo are generally applied only to those molecules which do not have symmetric ends.
If the similar atoms or group of atoms on adjacent stereocentres of diastereomer are on same (syn) side it is designated as Erythro, whereas if the similar atoms or group of atoms on adjacent stereocentres of diastereomer are on opposite (anti) side the diastereomer is designated as threo.
Each erythro and threo stereoisomer can have their non-super imposable mirror image (enantiomers).
Meso Compounds
A compound with two or more carbon stereo centre (i.e. chiral centres) but also having a plane of symmetry (i.e. mirror plane) and we can superimpose it on its own mirror image is called meso compounds.
To identify a chiral compound, find out two or more chiral centres and an internal plane of symmetry. Then check the stereochemistry of the compound i.e. determine whether it is R or S. If there are two chiral centres and RS = SR, then it is a meso compound. Meso compound is optically inactive.
Cyclohexane-1,2-diol (cis- isomer) is a meso compound because It has two chiral centres and an internal plane of symmetry but its trans- isomer is not a meso compound because It lacks the internal mirror plane.
Racemic Mixture or Racemate
Racemic mixture is also called racemate, a mixture of equal amount of two enantiomers (50% d- and 50% l-) that have dissymmetric molecular structures that are mirror images of one another. Each enantiomer rotates the plane-polarized light through a characteristic angle, but, because the rotatory effect of each component exactly cancels that of the other, the racemic mixture is optically inactive. Racemic mixture is not a meso compound because it is an equimolar mixture of two enantiomers
Racemic mixture in liquid and vapor phase shows physical properties (like boiling points, density, refractive index etc.) identical to those of pure enantiomers. However, the solid phase enantiomeric mixtures have some properties different from the pure enantiomers.
The process by which an optically active substance is transformed into the corresponding racemic modification is known as Racemization and the reverse process, by which a racemic modification is separated into the two enantiomers, is known as Resolution.
Since enantiomers have identical physical properties(like solubility, boiling point, melting point, density, refractive index etc.), therefore, they can not be separated by common physical processes such as direct crystallization, distillation or basic chromatography. There are four general methods that are extensively being used for the resolution of racemic mixtures.
a. Mechanical separation (crystallization method) method
b. Diastereomer formation method
c. Chromatographic method
d. Biochemical/enzymatic methods
Classification of Organic Reaction
Active Methylene Compounds
Hydroxy Acids
Carbohydrates
Carbohydrates
Carbohydrate are composed of mainly carbon, hydrogen and oxygen. Carbohydrates are technically hydrates of carbon in which hydrogen and oxygen are in 2:1 ratio. The empirical formula of carbohydrate is Cn(H2O)m, n and m may or may not be same. All carbohydrates does not follow this emperical formula like Rhamnose (C6H12O5). Many molecules which follow this emperical formula but they are not carbohydrate(HCHO & CH3COOH etc.).
Structurally, carbohydrates are polyfunctional compounds containing two types of functional group i.e. hydroxyl group and carbonyl group. They are polyhydroxy aldehydes or ketones or compounds which are converted to these on hydrolysis.
Classification
Carbohydrates are classified into two main classes, sugars and polysaccharides. Sugars are sweet crystalline substances that are soluble in water. These are further classified on the basis of their behavior on hydrolysis.
The simplest form of carbohydrates is the monosaccharide. 'Mono' means 'one' and 'saccharide' means 'sugar'.
Monosaccharides
Monosaccharides are polyhydroxy aldehyde or ketone that
cannot be hydrolyzed further to give simpler sugar. They may again be classified on the basis of the nature of carbonyl group.
a. Polyhydroxy aldehydes are called aldoses. e.g. Glucose
b. Polyhydroxy ketones are called ketoses. e.g. Fructose
The aldoses and ketoses are further divided on the basis of the number of carbons present in their molecules, as trioses, tetroses, pentoses, hexoses etc. They are referred to as aldotrioses, aldotetroses, aldopentoses, aldohexoses, ketohexoses etc.
Oligosaccharides
Carbohydrates that produce two to ten monosaccharide units during the hydrolysis are called oligosaccharides. They can be further classified based on the number of monosaccharide units formed on hydrolysis.
If two monosaccharide units are obtained on hydrolysis it is called disaccharide. Similarly, for three, four, five... monosaccharide units, it is called tri, tetra, penta... saccharide respectively.
Polysaccharides
Carbohydrates that produce a large number of monosaccharide units on hydrolysis. These monosaccharide units are joined together by oxide bridges. These linkages are called glycosidic linkages. The common and widely distributed polysaccharides are not sweet in taste, so they are called non-sugars. Some common examples are starch, cellulose, glycogen, etc.
D and L Notations
The notations D and L are used to describe the configurations of carbohydrates and amino acids. Glyceraldehyde has been taken as arbitrary standard for the D and L notation because, it has an asymmetric carbon and can exist as a pair of enantiomers.
In a Fischer projection, the carbonyl group is always placed on the top position for monosaccharide. In this structure, if the '–OH' group attached to the first asymmetric center from the bottom is on the right side, then, the compound is a D-sugar. If the '–OH' group is on the left side, then, the compound is a L-sugar. Almost all sugars found in nature are D-sugar.
They do not indicate whether the compound rotates polarized light to the right or to the left. For example, D-glyceraldehyde is dextrorotatory, whereas D-lactic acid is levorotatory.
Mutarotation
Normally D-(+)-glucose has a melting point of 146°C. When D-(+)-glucose is crystallized by evaporating an aqueous solution above 98°C, a second form of D-(+)-glucose with a melting point of 150°C can be obtained. When the optical rotations of these two forms are measured, they are found to be significantly different, but when an aqueous solution of either form is allowed to stand, its rotation changes. The specific rotation of one form decreases while the other form increases, until both solutions show the same value of rotation.
For example, a solution of α-D-(+)-glucose (mp 146°C) specific rotation
gradually decreases from an initial value of +112.2° to +52.7°, while The β-D-(+)- glucose (mp 150°C) specific rotation gradually increases from an initial value of + 18.7° to + 52.7°. These forms of glucose reach equilibrium with the specific rotation of +52.7. This change or mutation in the specific rotation toward equilibrium is called mutarotation.
Osazone Formation
Aldose and ketose react with one equivalent of phenylhydrazine to produce phenylhydrazones.
When three equivalent of phenylhydrazine reacts with Aldose and ketose it forms bis-hydrazone known as an osazone. C-1 and C-2 both reacts with phenylhydrazine and forms osazone along with aniline, ammonia and water.
D-Glucose and D-fructose form the same osazone as their structure are same except at C-1 and C-2 position.
H-C=N-NH-Ph
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C=N-NH-Ph
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Structure of glucose
1. On the basis of elemental analysis and molecular weight determination the molecular formula of glucose is C6H12O6.
2. The reduction of glucose with red phosphorus and HI gives n-hexane confirm the six carbon atoms in glucose are in a straight chain.
3. It forms penta acetate on treatment with acetic anhydride which indicates the presence of five hydroxyl groups in the molecule. Since glucose is a stable molecule so, all the five hydroxyl groups are on seperate carbon atoms.
4. Glucose reacts with hydroxyl amine to form an oxime and with hydrogen cyanide to form cyanohydrins. It indicates the presence of a carbonyl group.
5. The mild oxidation of glucose with bromine water or sodium hypobromide yields a monocarboxylic acid (gluconic acid) containing same number of carbon atoms as in glucose. This confirms the carbonyl group must be an aldehyde group.
6. The catalytic reduction of glucose gives a hexahydric alcohol (sorbitol) which gives hexaacetate on treatment with acetic anhydride. The sixth hydroxyl group must be obtained by the reduction of aldehyde group, thus further confirming the presence of an aldehyde group and five hydroxyl groups in glucose.
7. Oxidation of gluconic acid with nitric acid yields a dicarboxylic acid (glucaric acid) with the same number of carbon atoms as in glucose. Thus besides aldehyde group, glucose must contain a primary alcoholic group also, which generates the second carboxylic group on oxidation.
On the basis of above reactions, Fisher assigned an open chain structure of glucose shown below-
-----structure of glucose-----
Aromatic Chemistry
Aromaticity:
Structure of benzene :
Elemental analysis and molecular weight determine the molecular formula of benzene is C6H6
Open Chain Structure:
Based upon observable facts given above and the tetravalency of carbon, the following open chain structures were proposed for benzene.
Drawbacks of open chain structure:
The open chain structure for benzene was rejected due to the following reasons:
1. Addition reactions usually given by alkenes and alkynes are not given by benzene.
2. Benzene forms only one kind of mono-substituted product.
3. An open chain structure however, can form more than one kind of monosubstituted product.
4. The open chain compounds do, not give reactions such as FriedelCraft reaction, nitration, sulphonation.
5. On reduction with hydrogen in the presence of Ni at 200°C, actually a cyclic compound cyclohexane is obtained.
Ring Structure:
In 1858, August Kekule had proposed that carbon atoms can join to one another to form chains and in 1865, he gave the structure of benzene as:
1. All the carbon-to-carbon bonds in benzene are equivalent.
2. The molecule is unusually stable.
3. Chemists often represent benzene as a hexagon with an inscribed circle
The inner circle indicates that the valence electrons are shared equally by all six carbon atoms (that is, the electrons are delocalized, or spread out, over all the carbon atoms).
4. Each corner of the hexagon is occupied by one carbon atom, and each carbon atom has one hydrogen atom attached to it.
5. Any other atom or groups of atoms substituted for a hydrogen atom must be shown bonded to a particular corner of the hexagon.
6. The six bond lengths are identical and they are one-and-a half bonds and their length, 1.39 A or 139 picometer, is intermediate between the lengths of single and double bonds (is shorter than typical single-bond lengths, yet longer than typical double-bond lengths).
Electrophilic Substitution Reactions in Benzene:
Directive Influence of Functional Groups and Orientation :
Benzene undergoes typical electrophillic substitution reactions forms mono-substituted benzene as the product. When this mono-substituted benzene is subjected to further electrophillic substitution, it forms three possible disubstituted products. The major product depends on the reactivity of the mono-substituted benzene.
When mono substituted benzene undergoes an electrophilic attack, the rate of reaction and the site of attack vary with the functional group already attached to it.
Electron donating groups increase the reactivity of benzene ring and are known as activating groups while electron withdrawing group decrease the reactivity of benzene ring are known as deactivating groups.
The electron donating groups direct the incoming group to ortho and para positions are called ortho and para directing groups, as the electron density is more on ortho-and para-positions. Hence the electrophilic substitution takes place mainly at these two positions.
The electron withdrawing groups direct the incoming group to meta- positions are called meta directing groups, as the electron density is more on meta-positions. Hence the electrophilic substitution takes place mainly at this position.
Methods of Preparation of Phenol
Methods of Preparation of Aniline
Important Reactions of Aniline
Methods of Preparation of Benzaldehyde
Important Reactions of Benzaldehyde
Methods of Preparation of Acetophenone
Important Reactions of Acetophenone
Methods of Preparation of Benzoic Acid
Important Reactions of Benzoic Acid
Important Questions For Examination 2020
Physical Chemistry Paper: Three
Physical Chemistry Paper: Four
Inorganic Chemistry
Organic Chemistry
Previous Year Questions
Physical Chemistry Paper: Three
Physical Chemistry Paper: Four
Inorganic Chemistry
Organic Chemistry
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