NITROGEN

Symbol --- N

Abundance --- 0.002% in earth’s crust, 78% by volume and 75% by weight in air.

Allotropes --- dinitrogen (N≡N)

Physical state --- gas

Colour --- colourless gas

Discovery --- Daniel Rutherford

Atomic no --- 7

Atomic weight --- 14.007

Period --- 2

Group --- 15 or VA

Block --- p block

Known isotopes --- ₇N¹¹, ₇N¹², ₇N¹³, ₇N¹⁴, ₇N¹⁵, ₇N¹⁶, ₇N¹⁷, ₇N¹⁸, ₇N¹⁹, ₇N²⁰, ₇N²¹, ₇N²²

Main isotopes --- ₇N¹⁴, ₇N¹⁵

Isotopic abundance --- ₇N¹⁴ (99.58%), ₇N¹⁵ (0.34%)

Melting Point --- - 210 ⁰C (63 K)

Boiling Point --- - 196 ⁰C (77 K)

Triple Point --- 63 K, 12.5 kPa

Critical Temperature --- 126 K

Critical Pressure --- 3.38 MPa

Heat of Fusion --- 0.71 KJ/mol (dinitrogen)

Heat of Vaporisation --- 5.56 KJ/mol (dinitrogen)

ΔH⁰_{(atomization)} --- 473 KJ/mol

Molar heat capacity --- 29.12 J/(mol-K) (dinitrogen)

Density --- 1.25 g/L (at 0 ⁰C)

Molar volume --- 0.0111

Electron configuration --- [He] 2s² 2p³

Electrons per shell --- 2 (1 st shell), 5 (2 nd shell)

Oxidation state --- -3, -2, -1, 0, +1, +2, +3, +4, +5

Valance --- 3

Electronegativity --- 3.04 (Pauling scale)

Electron affinity --- 7 KJ/mol

Ionisation energy --- 1403 KJ/mol (1 st), 2855 KJ/mol (2 nd), 4577 KJ/mol (3 rd), 7475.5 KJ/mol (4 th)

Covalent radius --- 71 pm

Van der Waals radius --- 155 pm

Atomic radius --- 56 pm

Natural occurrence --- primordial

Crystal structure --- hexagonal

Specific heat --- 1039 J/ (Kg K) (gas)

Thermal conductivity --- 0.025 W/ (m K)

Speed of sound --- 353 m/s (gas)

Magnetic type --- diamagnetic

Mass magnetic susceptibility --- -5.3 x 10^-9 m³/Kg

Molar magnetic susceptibility --- -1.5 x 10^-10 m³/mol

Volume magnetic susceptibility --- -6.7 x 10^-9

Lattice angles --- π/2, π/2, 2π/3

Lattice constants --- 385.9 pm, 385.9 pm, 626.3 pm

Quantum numbers --- ⁴S_3/2

Neutron cross section --- 1.9

Neutron mass absorption --- 4.8 x 10^-3

Refractive index --- 1.0002

 Carbocation

When an organic species containing a carbon atom and that carbon atom bearing only three electron pairs, means six electrons is called a carbocation. Carbocation containing a positive charge and the carbon atom of the carbocation is sp² hybridized. The carbon atom of the carbocation uses its three hybrid orbitals for forming three sigma bonds with three substituents and the remaining p orbital remains vacant. The structure of carbocation is flat structure, having all the three covalent bonds in one plane. The bond angles between these covalent bonds are 120⁰.

Formation of Carbocation

Carbocation can be formed in a number of ways. Some of these reactions by which carbocations are generated are summarized below---

(1)         Carbocation is formed by the solvolysis of C―X bond (where X = halogen such as F, Cl, Br, I or OBs etc) ---

R―X -----> R⁺ + X^-

(2)         When alkyl halide (R―X) reacts with superacids such as SbF₅, HSO₃F etc, carbocation is formed --

R―F + SbF₅ -----> R⁺ + SbF₆^-

(3)         When acetyl chloride (R―CO―Cl) reacts with aluminium chloride (AlCl₃) carbocation is generated---

R―CO―Cl + AlCl₃ ----> R―C⁺=O + AlCl₄^-

(4)         When nitrous acid (HNO₂) reacts with amine (R―NH₂), carbocation is produced--

R―NH₂ + HNO₂ ------> R⁺ + N₂ + H₂O + OH^-

(5)         Protonation of alcohol (R―OH) by acid, followed by dehydration produced carbocation-

R―OH + H⁺ -----> R―OH₂⁺

R―OH₂⁺ -----> R⁺ + H₂O

(6)         When carbonyl compound reacts with H⁺, carbocation is formed----

R―CO―R + H⁺ -----> R₂C⁺―OH

(7)         When alkene reacts with H⁺, carbocation is formed----

R―CH=CH₂ + H⁺ -----> R―CH⁺―CH₃

(8)         When allyl halide reacts with Ag⁺, allyl carbocation is formed----

CH₂=CH-CH₂-Br + Ag⁺ ---> CH₂=CH-CH₂⁺ + AgBr

(9)         When benzyl halide reacts with Ag⁺, benzyl carbocation is formed-----

Ph―CH₂―Br + Ag⁺ ----> Ph―CH₂⁺ + AgBr

Classification of Carbocation

Carbocations are classified into three categories----

Primary Carbocation

When a positively charged carbon ion linked with another carbon atom, then this carbocation is called primary carbocation.

CH₃―CH₂⁺

Secondary Carbocation

When a positively charged carbon ion linked with other two carbon atoms, then this carbocation is called secondary carbocation.

CH₃―CH⁺―CH₃

Tertiary Carbocation

When a positively charged carbon ion linked with other three carbon atoms, then this carbocation is called tertiary carbocation.

Stability of Carbocation

The relative stabilities of the alkyl substituted carbocations are----

R₃C⁺ > R₂CH⁺ > RCH₂⁺ > CH₃⁺

The stabilities of the alkyl substituted carbocations can be explained by various factors, which are---

Inductive Effect

The alkyl groups have +I effect, so they tend to release electrons and partially compensate for the electron deficiency of the positively charged carbon. When the number of alkyl groups attached to the positively charged carbon atom increases, electrons release by this alkyl groups increases and partially compensate for the electron deficiency of the positively charged carbon also increases. So, the stability of tertiary carbocation (containing three alkyl groups) is greater than secondary carbocation (containing two alkyl groups). The stability of secondary carbocation is greater than primary carbocation (containing only one alkyl group). The stability of primary carbocation is greater than methyl carbocation (containing no alkyl group).

Hyperconjugation

The sigma (σ) electrons of an alpha (α) C―H bond can be delocalized into the unfilled ‘p’ orbital of the positively charged carbon atom. Thus, spreading the charge over all such bonds. So, several hyper conjugative resonance structure can be drawn for alkyl substituted carbocation and each of these resonance structure containing same number of covalent bonds as the first structure.

For primary carbocation three structure are drawn, for secondary carbocation six structure are drawn and for tertiary carbocation nine structure are drawn. With increase the number of hyper conjugative resonance structure, the stabilities of the carbocation increases.

Mesomeric Effect or Resonance Effect

The stabilities of conjugated carbocations can be explained by mesomeric effect or resonance effect. Resonance enhances the stability of a carbocation by delocalization of its charge in conjugated system like allyl or benzyl carbocation. More number of resonating structures of a carbocation, more will be its stability. The order of stability of allyl and benzyl carbocations are----

CH₂=CH-CH₂⁺<Ph-CH₂⁺<(Ph)₂CH⁺<(Ph)₃C⁺

Number of resonating structures of allyl carbocation is 2, whereas number of resonating structures of Ph―CH₂⁺ is 5, (Ph)₂CH⁺ is 9 and (Ph)₃C⁺ is 13.

Steric Effect

When carbocations derived from highly substituted substrate, steric effects play an important role in the formation and stability of carbocations. Here steric relief is the key factor for the formation and stability of carbocations. Tri-isopropyl chloride is a highly substituted substrate. In tri-isopropyl chloride three bulky isopropyl groups are pushed together due to sp³ angle of 109⁰28'. So due to this pushing together, there arises a strain called B-strain. When the substrate tri-isopropyl chloride ionizes, the angle expands from 109⁰28' to 120⁰. So, a relief in strain arises due to this angle expands and space between alkyl groups (isopropyl groups) increases.

Nylon 6.6

Nylon 6.6 is a diamine-dibasic acid polymer. Nylon 6.6 is generally prepared by the reaction of hexamethylene diamine [H₂N―(CH₂)₆―NH₂] and adipic acid [HOOC―(CH₂)₄―COOH]. In nylon 6.6 first 6 indicates the number of carbon atoms in the hexamethylene diamine part and second 6 indicates the number of carbon atoms in the adipic acid part. Since this polyamide contain 6 carbon atoms in the first part and 6 carbon atoms in the second part, so, it is nylon 6.6.

Manufacture of Adipic Acid

Adipic acid is prepared by various synthetic methods. Among these various synthetic methods, two such methods are----

Synthetic Method – 1

Adipic Acid from Phenol

Reaction of phenol with hydrogen in presence of powdered Ni catalyst at 160 – 170 ⁰C formed cyclohexanol. Oxidation of cyclohexanol in presence of Cu at 250 ⁰C produced cyclohexanone. When cyclohexanone treated with 6% HNO₃ at 85 – 90 ⁰C adipic acid is formed.

Synthetic Method – 2

Adipic Acid from Benzene

Reaction of benzene with hydrogen in presence of powdered Raney Ni catalyst at 180 – 250 ⁰C and at 10 – 60 atm formed cyclohexane. Oxidation of cyclohexane in presence of air at 125 ⁰C and in presence of Mn, Co catalyst produces cyclohexanol and cyclohexanone. When this mixture of products reacted with 40% HNO₃ at 80 – 85 ⁰C adipic acid is formed.

Manufacture of Hexamethylene diamine

Hexamethylene diamine is prepared by various synthetic methods. Among these various synthetic methods, three such methods are----

Synthetic Method – 1

Hexamethylene diamine from Butadiene

Reaction of butadiene with chlorine produced 1,4-dichlorobut-2-ene. When this 1,4-dichlorobut-2-ene treated with KCN in presence of CuCl at 80 – 85 ⁰C 1,4-dicyanobut-2-ene is formed. Reaction of 1,4-dicyanobut-2-ene with hydrogen in presence of Pd-C produced adiponitrile. When adiponitrile treated with hydrogen at high pressure in presence of NH₃ hexamethylene diamine is formed.

Synthetic Method – 2

Hexamethylene diamine from Adipic Acid

Reaction of adipic acid with NH₃ at 350 – 400 ⁰C in silica gel produced adipamide. This adipamide heated to formed adiponitrile. Reaction of adiponitrile with hydrogen in presence of NH₃ at high pressure produced hexamethylene diamine.

Synthetic Method – 3

Hexamethylene diamine from Acrylonitrile

Reaction of acrylonitrile with hydrogen at law pressure produced adiponitrile. When adiponitrile treated with hydrogen in presence of Co-Cu at 100 – 135 ⁰C produced hexamethylene diamine.

Manufacture of nylon 6.6

Adipic acid and hexamethylene diamine both are soluble in water. So, aqueous solution of both reactants is mixed in 1:1 molar ratio, and maintain 7.8 P^H of the mixture. The solution is evaporated under vaccum. 0.5 – 1 % mole of CH₃COOH as viscosity and molecular weight stabilizer is added to the liquid residue, and the liquid residue charged to an autoclave under 250 poise pressure and heated at about 270 – 280 ⁰C. Vaccum is applied after some progress of the reaction to facilitate to release and removal of the rest water from the reaction zone. After 3 – 4 hours, the process has been completed. By a nitrogen pressure of 40 -50 poise, the molten polymer is extruded through the bottom of the autoclave as a ribbon with molecular weight ranging between 12000 – 16000, which after cooling by water and air in the particular stages, is cut into cubes of regular sizes.

Properties of nylon 6.6

Nylon 6.6 is tough, strong, and elastic in nature. Nylon 6.6 possess high tensile strength and abrasion resistance. Its moisture resistance is very good and it maintain its elasticity upto 150 ⁰C. Melting point of nylon 6.6 is 263 ⁰C in nitrogen and in air is 250 ⁰C. Specific gravity of nylon 6.6 is 1.14. Strength and abrasion resistance of nylon 6.6 is high and it maintain its toughness upto 150 ⁰C and its solvent resistance is good. Nylon 6.6 attacked by air or water at elevated temperature and it is opaque to sunlight. In sunlight nylon 6.6 undergoes fading in colours.

 Carbon

Symbol --- C

Abundance --- 0.8% in earth’s crust

Allotropes --- diamond, graphite, fullerenes, graphene, carbon nanotube, carbon nanobud, amorphous carbon, lonsdaleite (hexagonal diamond), diamane, AA’-graphite

Physical state --- solid

Colour --- diamond (transparent), graphite (black),

Discovery --- Egyptians and Sumerians

Atomic no --- 6

Atomic weight --- 12.011

Period --- 2

Group --- 14 or IVA

Block --- p block

Known isotopes --- ₆C¹⁰, ₆C¹¹, ₆C¹², ₆C¹³, ₆C¹⁴, ₆C¹⁵, ₆C¹⁶, ₆C¹⁷, ₆C¹⁸, ₆C¹⁹, ₆C²⁰

Main isotopes --- ₆C¹², ₆C¹³, ₆C¹⁴

Isotopic abundance --- ₆C¹² (98.8%), ₆C¹³ (1.1%), ₆C¹⁴ (trace) (t_1/2 = 5570 years)

Melting Point --- 4000 K (diamond at 125 kbar), 4000 K (graphite at 9 kbar)

Density --- diamond (3.51 g/cm³), graphite (2.26 g/cm³), amorphous carbon (1.9 g/cm³)

Electron configuration --- [He] 2s² 2p²

Electrons per shell --- 2 (1 st shell), 4 (2 nd shell)

Oxidation state --- +4

Valance --- 4

Electronegativity --- 2.5 (Pauling scale)

Electron affinity --- 152.3 KJ/mol

Ionisation energy --- 1086 KJ/mol (1 st), 2352 KJ/mol (2 nd), 4619 KJ/mol (3 rd), 6221 KJ/mol (4 th)

Covalent radius --- 77 pm (sp³), 72.5 pm (sp²), 69 (sp)

Van der Waals radius --- 169.8 pm

Crystal structure --- diamond (transparent) face centered cubic, graphite (black) hexagonal

Molar heat capacity --- diamond [6.15 J/(mol-K)], graphite [8.51 J/(mol-K)]

Specific heat --- graphite 710 J/ (Kg K)

Thermal expansion --- diamond [0.8 μm/ (m K)]

Thermal conductivity --- diamond [900-2300 W/ (m K)]

Speed of sound --- diamond (18345 m/s)

Magnetic type --- diamagnetic

Lattice angles --- π/2, π/2, 2π/3

Lattice constants --- 246.3 pm, 246.3 pm, 670.9 pm

Quantum numbers --- ³P₀

Neutron cross section --- 0.0035

Neutron mass absorption --- 1.5 x 10^-5

Electrical resistivity --- graphite (7.8 μΩ.m)

Mohs hardness --- diamond (10)

graphite (1-2)

Young’s modulus --- diamond (1050 GPa)

Shear modulus --- diamond (475 GPa)

Bulk modulus --- diamond (440 GPa)

Poisson ratio --- diamond (0.1)

Refractive index (546 nm) --- diamond (2.41), graphite (2.15)

 Urea

Urea is the most important nitrogenous fertilizer contain about 45 – 47% of nitrogen.

Raw Materials

The raw materials required for the manufacture process of urea are carbon di oxide (CO₂) and ammonia (NH₃).

Manufacture Process

The following steps are used for the manufactured of urea---

Through a silver lined special autoclave, liquid carbon di oxide (CO₂) and liquid ammonia (NH₃) are passed. So, carbon di oxide (CO₂) and ammonia (NH₃) are compressed and reacted at about 100 – 200 atm and at 170 – 190 ⁰C to form ammonium carbamate (NH₄COONH₂).

CO₂ + 2NH₃ -------> NH₄COONH₂    ΔH⁰ = -37.4 Kcal

In the next step urea is formed by dehydration in a low-pressure stripping operation.

NH₄COONH₂ -------> NH₂CONH₂ + H₂O    ΔH⁰ = 6.3 Kcal

Process modifications occurs in recycle of unreacted ammonia (NH₃). Undesirable side reaction is---

2NH₂CONH₂ -------> NH₂CONHCONH₂.H₂O

Effect of Temperature

According to Le-chatelier principle an increase in temperature favoured the overall reaction because the rate determining step is endothermic. It has been observed that, at 170 ⁰C, the reaction rate reaches the maximum limit and the reaction rate falls below this temperature.

Effect of Pressure

Increase of pressure has a favourable effect on the yield of urea, so the reaction occurs at high pressure such as 100 – 200 atm.

[Ammonia/Carbon di oxide] Ratio

Used of ammonia in slight excess (at about 10% excess) shift the equilibrium to the product side, so more and more urea is produced.

Phase of Reactant

Carbon di oxide (CO₂) and ammonia (NH₃) are present in liquid state.

Percentage of Conversion

At about 40% urea is produced from ammonium carbamate in this reaction.

Types of the Manufacturing Process

There are three major manufacturing process for the manufactured of urea depending upon the utilization of unreacted ammonia (NH₃) and carbon di oxide (CO₂).

Method – 1

By passing liquid carbon di oxide (CO₂) and liquid ammonia (NH₃) through a silver lined special autoclave in 1:2 molar ratio, ammonium carbamate is formed. Then ammonium carbamate is heated at about 130 – 140 ⁰C and under about 35 atm pressure urea is formed. By this method yield of urea is 40%.

Method – 2

At 160 – 180 ⁰C and at 150 – 200 atm pressure urea can be manufactured by reacting of one part of solid carbon di oxide (CO₂) by weight and two or more parts of ammonia (NH₃) by weight. By this method yield of urea is 65 – 70%.

Method – 3

By this method urea can be manufactured by reacting 3 – 5 moles of liquid ammonia (NH₃) with liquid carbon di oxide (CO₂) at about 400 atm pressure and at about 200 – 210 ⁰C. By this method yield of urea is 75%.

Uses of Urea

Since urea contain about 45 – 47 % of nitrogen, so urea widely used as most important nitrogenous fertilizer. The P^H of the soil does not change by urea, so urea widely used as fertilizer for all types of crops in all kind of soil.

Urea should be applied in combination with earth or sand because urea is highly concentrated.

When the soil contains free water, urea should not be applied.

Diborane Structure and Bonding

 Structure and Bonding of Diborane (B₂H₆)

Boron is an electron deficient element. Due to its electron deficient character, the trivalent boron leads to dimerization to form diborane (B₂H₆). If we assumed that, the structure of diborane (B₂H₆) is similar to single bond structure of ethane (C₂H₆), then this assumption is rejected because single bond structure of ethane (C₂H₆) contains 14 electrons [(4+4 = 8 electrons from two carbon atoms) and (1x6 = 6 electrons from six hydrogen atoms)] but in diborane there are only 12 electrons [(3+3 = 6 electrons from two boron atoms) and (1x6 = 6 electrons from six hydrogen atoms)].

If we further assumed that, diborane (B₂H₆) molecule contains two one-electron bonds in its structure, to account for 12 electrons then diborane (B₂H₆) must be paramagnetic but actual fact that, the diborane (B₂H₆) molecule is diamagnetic. So, this assumption is also rejected.

Again, if we further assumed that, the diborane (B₂H₆) molecule contain two-electron three-centred bond (2e-3c bond), then the problem has been solved.

Electron diffraction study and other physical studies indicate the below structure for the molecule of diborane (B₂H₆).

Two B―H―B bridges joined the two B atoms. The B―H bridge bond length (1.33 A⁰) are longer than B―H terminal bond length (1.19 A⁰). The B―B distance is 1.77 A⁰. Specific heat measurement data confirm that the two bridging H-atoms are in a plane perpendicular to the rest of the molecule and prevent rotation between the two B-atoms.

Raman spectroscopy confirm that four terminal H-atoms are in a different environment from two bridging H-atoms. It is also confirmed by the fact that, methylation of diborane (B₂H₆) with B(CH₃)₃ produced tetramethyl diborane [(CH₃)₄B₂H₂], further attempt for methylation breaking the molecule into B(CH₃)₃ fragments. Four terminal H-atoms which are linked to B-atoms by normal covalent bonds, means two-electron two-centred bond (2e-2c) are replaced on methylation. The two bridging H-atoms, which are not methylated are connected through peculiar two-electron three-centred (2e-3c) bonds. The whole structure of diborane (B₂H₆) molecule can be explained as follows.

Electron configuration of boron is----

1s² 2s² 2p³

So, boron contains three electrons in its outermost shell. Two boron atoms of diborane (B₂H₆) molecule undergoes sp³ hybridisation. Two sp³ hybrid orbitals of each boron atoms overlap with the 1s orbitals of two terminal H-atoms.

The other two sp³ hybrid orbitals of one B atom overlap with two other sp³ hybrid orbitals of another B atom through the 1s orbitals of the two bridging H-atoms. As a result of these overlap, there formed total two B―H―B two-electron three-centred (2e-3c) bonds.

Such type of bond or molecular orbital has banana shape, so these bent bonds are called banana bonds.

Molecular orbital approach of diborane (B₂H₆) molecule---

Molecular orbital approach of diborane molecule begin with four roughly sp³ hybrid orbitals of each boron atoms and the 1s atomic orbitals of six hydrogen atoms. Two sp³ hybrid orbitals of each boron overlap with two 1s atomic orbitals of two hydrogen atoms and formed four terminal B―H bonds (covalent bonds, 2e-2c bonds). Due to this overlap four bonding and four anti bonding molecular orbitals are formed by four sp³ hybrid orbitals of two boron (two sp³ hybrid orbitals of each boron) and four 1s atomic orbitals of four hydrogen atoms. The four bonding molecular orbitals are occupied by eight electrons (four electrons from two boron atoms and four electrons from four hydrogen atoms). Remaining four sp³ hybrid orbital of two boron (two sp³ hybrid orbitals from each boron) and two 1s atomic orbitals of remaining two hydrogen atoms forms two sets of B―H―B bridge bonds (2e-3c bonds). Due to this overlap three sets of molecular orbitals are formed. One set of bonding molecular orbitals, one set of nonbonding molecular orbitals and one set of antibonding molecular orbitals. Remaining four electrons (two from two boron atoms and two from two hydrogen atoms) occupied the lowest energy bonding molecular orbitals.

The molecular orbital diagram of diborane (B₂H₆) molecule----