ELECTROPHILIC SUBSTITUTION OF BENZENE

Benzene is an aromatic compound that shows exceptional but explainable behavior. Being a highly saturated compound,it ought to undergo addition reactions but instead undergoes substitution.
The reason for this is the conjugated system of pi bonds in benzene that is further explained in "Modern Concept Of Benzene".

Electrophilic substitution reactions involve an electron-poor species(atom ,ion or molecule)which maybe a positive ion or partial positive end of a polar molecule and is called the ELECTROPHILE and an electron-rich species.The electrophile replaces a hydrogen atom in the ring.

TYPES OF ELECTROPHILIC REACTIONS IN BENZENE ON THE BASIS OF ELECTROPHILE:

• HALOGENATION:

The process in which a ring hydrogen is replaced by a halogen in the presence of Lewis Acid catalyst is called halogenation.

The electrophillic reaction in which a halogen takes the place of a hydrogen atom in the ring in the 

presence of a Lewis acid catalyst.

CATALYST:

Iron reacts with some of the chlorine in the following manner:

REAGENT:
Taking CHLORINE as an example of a halogen,it is difficult to form a positive ion since that requires too much energy,hence it is merely polarized.

FORMATION OF ELECTROPHILE:
As the chlorine molecule approaches a benzene ring, the delocalised electrons in the ring repel the electrons in the chlorine-chlorine bond. That induces a dipole in the chlorine.

Also nearby is the FERRIC chloride, and this encourages the polarization of the chlorine.The FERRIC is strongly attracted to the slightly negative end of the chlorine molecule, and pulls electrons even more towards that end.

The Electrophilic substitution mechanism:

STAGE 1:
Formation of intermediate arenium ion:
Two electrons from the delocalization form a new bond with the partial positive end of the chlorine molecule.This breaks the delocalization partly and the ring attains a positive charge.It is now called arenium or benzene ion.

The breaking of the chlorine molecule transfers 1 atom to the benzene and one to the ferric chloride forming AlCl4

STAGE 2:
Formation of HCl and regeneration of catalyst:
One of the aluminium-chlorine bonds breaks and both electrons from it are used to join to the hydrogen. Removing the hydrogen from the ring forms the HCl which is also produced in the reaction, and the ferric chloride catalyst is re-generated. The electrons which originally joined the hydrogen to the ring are now used to re-establish the delocalised system.(3)

• SULPHONATION:

The process in which a sulphonic acid group replaces a ring hydrogen atom.

The formation of the electrophile:
The electrophile is sulphur trioxide, and this arises in one of two ways depending on which sort of acid you are using.
1-DISSOCIATION OF SULPHURIC ACID:
Due to dissociation of concentrated Sulphuric acid ,it contains traces of SO3.(1)

2-FUMING SULPHURIC ACID:
It is a solution of SO3 in H2SO4. Hence, contains more SO3.

Sulphur trioxide is highly polar with all the oxygen atoms pulling the electrons towards themselves leaving a partially positive sulphur atom which attacks the benzene.

The electrophilic substitution mechanism:

STAGE 1:
Formation of intermediate arenium ion:
Two electrons from the delocalization form a new bond with the partial positive end of the chlorine molecule.This breaks the delocalization partly and the ring attains a positive charge.It is now called arenium or benzene ion.

To make room for the new bond between the ring and benzene, two of the electrons joining the oxygen to the sulphur(in the double bond) are transferred entirely to the oxygen making it a negative ion.

STAGE 2:
formation of Benzene Sulphonic acid:
The removal of hydrogen from the benzene ring is affected by the oxygen with the lone pairs.It forms a bond with the hydrogen and releases the electrons previously holding hydrogen into the ring to re-establish the delocalization. The benzene sulphonic acid is formed.

• NITRATION:

The process in which a nitro group replaces a hydrogen atom in benzene is called Electrophilic nitration.

Formation of electrophile:
NO2+!  or nitronium or nitryl cation is the electrophile.  It is formed by the reaction of sulphuric acid and nitric acid.

The electrophilic substitution mechanism:

STAGE 1:
Formation of intermediate arenium ion:
As the nitronium ion approaches the benzene ring ,the delocalized electrons are attracted to the positive ion and two of them form a new bond with the nitronium ion thus breaking the delocalization.(2)

STAGE 2:

Formation of nitro benzene and Regeneration of catalyst:

The second stage involves the negative hydrogen sulphate ion, formed along with the nitronium ion.
The lone pairs of one of its oxygen atoms forms a bond with the hydrogen of the ring thus regenerating the catalyst by forming sulphuric acid and at the same time releasing the bonding electrons of hydrogen to re-establish the delocalization.

• FRIEDEL-CRAFTS ACYLATION OF BENZENE:

The reaction in which hydrogen of the benzene ring is replaced by an acyl group.

FORMATION OF ELECTROPHILE:
The electrophile is CH3CO+.It is formed by reaction with catalyst aluminium chloride.(1)Alongside negative ion AlCl4-1 is also formed.

The electrophilic substitution mechanism:

STAGE 1:Formation of intermediate arenium ion:

Two electrons from the delocalised system are used to form a new bond with the CH3CO+ ion affectively breaking part of the delocalization.(2)

STAGE 2:
Formation of ketone and regeneration of catalyst:
The second stage involves the AlCl4 -1 formed alongside the positive ion.One of the aluminium chlorine bonds break and the chlorine with both electrons forms a bond with hydrogen from the ring.This results in the formation of phenyl ethanone and regeneration of AlCl3 catalyst.

• FRIEDEL-CRAFTS Alkylation OF BENZENE

The reaction involving replacement of a hydrogen of the benzene ring with an alkyl radical.

The formation of the electrophile:
The electrophile is CH3+. It is formed by reaction between the chloromethane and the aluminium chloride catalyst.

The electrophilic substitution mechanism:

STAGE 1:
Formation of intermediate arenium ion:
Two electrons from the delocalized system are used to form a new bond with the ion CH3+.The delocalization is partly broken.Arenium ion is formed.(2)

STAGE 2:
Formation of alkyl benzene and regeneration of catalyst:
The second stage involves the AlCl4 -1 formed alongside the positive ion.One of the aluminium chlorine bonds break and the chlorine with both electrons forms a bond with hydrogen from the ring.This results in the formation of methyl benzene and regeneration of AlCl3 catalyst.

oh LORD! the stories we spin...............

CELL DIVISION

MEIOSIS

It is a reduced type of cell division taking place in germ cells that results in the formation of 4 haploid daughter cells or gametes.

• ANIMATED VERSION

STAGES OF MEIOSIS:

• MEIOSIS I : RESULTS IN THE FORMATION OF 2 HAPLOID CELLS
• MEIOSIS II :RESULTS IN THE FORMATION OF 4 DAUGHTER CELLS WITH HAPLOID CHROMOSOMAL NUMBER MAINTAINED.

MEIOSIS I:

Consists of 4 phases:

1. PROPHASE I
2. METAPHASE I 
3. ANAPHASE I
4. TELOPHASE I

Figure 7-18. Early primary spermatocytes of the mouse. Left to right: leptotene,
zygotene, early pachytene, midpachytene. Sudan Black squash (x1200).

PROPHASE 1:

It is the longest phase of the cycle and consists of the following substages.

• Leptotene (lepto-Greek thin , -tene -Greek ribbon)

In this first substage of prophase I, the chromosomes have appeared within the nuclear envelope but are not yet fully condensed.  Each is a thin thread of DNA along which clearly defined beads of local coiling (chromomeres) can be seen. The chromosomes, while they have this threadlike form, are called chromatonemata .The chromosomes appear single because the sister chromatids are still so tightly bound to each other that they cannot be separately seen.

• Zygotene (zygonema) (zygo- is Greek for union, fusing)

During zygotene homologs begin to unite (synapse) by coming into approximate alignment .Synapsis ,the process of fusion that occurs between homologs begins at various points along the chromosome and extends outward, zipper-fashion, until complete. When synapsis is finished, the fused homologs look like single chromosome under the light microscope, but that are actually double.
 The interface where two homologs unite is called a synaptonemal complex, which can be seen under an electron microcope.

In the  early zygotene , some  regions of paternal and maternal homologs have fused while In the  late zygotene, both homolog pairs have fused over their entire lengths .

Tetrads: Once the homolog pairs synapse they are called tetrads (each has four chromatids; tetra is Greek for four) or bivalents.Bivalent is the preferred term.

• Pachytene (pachynema). (pachy- is Greek for thick). 

During pachytene the two sister chromatids of each chromosome separate from each other. This makes the chromosomes look thicker .Homologs are still paired at this point.

Each chromosome splits into two chromatids and thus each pair will have four chromatids two paternal and two maternal. They are now called tetrads.

CROSSING OVER:
The non-sister chromatids of the paternal and maternal chromosomes overlap each other. They appear to be joined at several regions along their length. These points are called Chiasmata. Each chiasma is the site of an exchange of genetic material between the two chromatids.

A kind of localized breakage of the DNA occurs, which is followed by exchange of DNA between them. This process is called crossing over or genetic recombination .Crossing over produces "cross-over chromatids," each composed of distinct blocks of DNA, some blocks derived from the mother, others from the father.

• Diplotene OR Diplonema:

At the beginning of this stage each chromatid of each chromosome is still fused to a chromatid of that chromosome's homolog (recall that sister chromatids are already separate at this point).

As diplotene progresses, these initially fused non-sister chromatids begin to separate from each other. However, they cannot separate completely because they are still connected by two strands of DNA at each of the points where exchanges took place.At each such cross-over site, the two strands form an x-shaped structure called a chiasma (pl. chiasmata).

TERMINALIZATION:
Chiasmata then begin moving toward the ends of the chromatids. This process of sliding toward the ends is known as terminalization. 

DICTYOTENE IN OOCYTES:
In oocytes, a special, extremely prolonged form of diplotene occurs called dictyotene. The primary oocyte undergoes the first three of the substages of prophase I (leptotene, zygotene, and pachytene) during late fetal life. The process is then suspended during diplotene until puberty or thereafter. Therefore, in dictyotene (and consequently prophase I) can last months or even years, depending on the type of organism in question.

• Diakinesis

During this, the last stage of Prophase I, the nucleolus disappears, terminalization reaches completion, and the chromosomes coil tightly, and so become shorter and thicker. The nuclear envelope begins to disappear. The centrosomes reach the poles.

• QUIZ ON MEIOSIS

LASER

LASERS--NO LONGER RESTRICTED TO CRIME-THRILLERS


A laser is a device that emits light (electromagnetic radiation) through a process of optical amplification based on the stimulated emission of photons.
The term "laser" originated as an acronym for "Light Amplification by Stimulated Emission of Radiation".
The emitted laser light is notable for its high degree of spatial and temporal coherence.

CHARACTERISTICS OF LASER LIGHT:
Laser light is very different from normal light.

• The light released is monochromatic.
•  It contains one specific wavelength of light (one specific color).
•  The light released is coherent. It is “organized” -- each photon moves in step with the others.
•  The light is very directional. A laser light has a very tight beam and is very strong and concentrated.

BOHR'S ATOMIC THEORY:

• When an electron absorbs energy either from light (photons) or heat (phonons), it receives that incident quantum of energy. But transitions are only allowed in between discrete energy levels such as the two shown above. This leads to emission lines and absorption lines.

• When an electron is excited from a lower to a higher energy level, it will not stay that way forever. An electron in an excited state may decay to a lower energy state which is not occupied, according to a particular time constant characterizing that transition. 

When such an electron decays without external influence, emitting a photon, that is called "spontaneous emission".

• These excited state electrons can return to their resting state by emitting energy in the form of photons.The photon emitted has a very specific wavelength (color) that depends on the state of the electron's energy when the photon is released. Two identical atoms with electrons in identical states will release photons with identical wavelengths.

Working of a laser:
In order to produce the light rays associated with a laser thrre principles must be followed in the following order.

METASTABILITY:

STATE 1 IS METASTABLE  WHILE STATE 3 IS THE MOST STABLE

 Upon receiving certain quanta of energy ,electrons transit to another energy state (say E1 to E3).Since the stability of E3 is of the order 10ex-8 sec hence the electron soon decays to a lower energy level (say E2).However E2 is a metastable state since its life time is of the order 30ex-3 sec.

POPULATION INVERSION:

A number of problems limit the effectiveness of this approach. The central problem occurs because the lower laser level is the ground level, which is the normal state for most atoms or molecules. In order to produce the population inversion, a majority of ground state electrons must be promoted to the highly excited energy level, requiring a significant input of external energy. In addition, the population inversion is difficult to sustain for an appreciable time, and therefore, three-level lasers must be operated in pulsed mode rather than continuously.

As a result of metastability of E2 energy level,there are far more electrons in the E2 than in the E1 level.This is called population inversion as the rate of flow of electrons from E3 to E1 is greater than from E2 to E1.
This is achieved by very intense flashes of light or electrical discharges.

STIMULATED EMISSION:
Now that the electrons are in high energy levels the  stimulated emission must occur.In the process,an incoming photon stimulates an excited atom to give up its stored energy in the form of a photon that is identical in wavelength, direction, polarization, and phase to the stimulus photon.
If the excited atom is unable to produce a photon that matches the incoming photon, then stimulated emission cannot take place.
                                                                                         
As a photon passes through the collection of excited atoms, it can stimulate the generation of many trillions of photons, or more, creating an avalanche of light.

SUSTAINING STIMULATED EMISSION:
Two mirrors at either end of the lasing medium reflect these photons facilitating the light gain.The active medium can thus be regarded as an amplifier that takes in a small signal (one photon, say) and delivers a large signal (many photons, all identical to the first) at the output.

ASSEMBLY OF MIRRORS .ONE IS PARTIALLY AND THE OTHER FULLY SILVERED.

Monochromatic, single-phase, columnated light leaves the laser through the half-silvered mirror -- laser light!

PONT PHYSIQUE 

bridge circuit is a type of electrical circuit in which two circuit branches (usually in parallel with each other) are "bridged" by a third branch connected between the first two branches at some intermediate point along them

Wheatstone Bridge
The Wheatstone bridge is an electrical circuit for the precise comparison of resistances.It was invented by Samuel Hunter Christie in 1833 and improved and popularized by Sir Charles Wheatstone

CONSTRUCTION:

The Wheatstone bridge is an electrical bridge circuit used to measure resistance. It consists of

1. a common source of electrical current (such as a battery) 
2. a galvanometer that connects two parallel branches, containing four resistors,of which two are known R_1and R_3.One of the parallel branches contains one adjustable resistor R_2 and an unknown R_X.

 WORKING:

BALANCING THE BRIDGE:

If the bridge is unbalanced, the direction of the current indicates whether R_2 is too high or too low. R_2 is varied until there is no current through the galvanometer, which then reads zero.

If the ratio of the two resistances in the known leg (R_2 / R_1) is equal to the ratio of the two in the unknown leg (R_x / R_3), then the voltage between the two midpoints (B and D) will be zero and no current will flow through the galvanometer V_g.Such a bridge is called BALANCED.

Detecting zero current with a galvanometer can be done to extremely high accuracy. Therefore, if R_1, R_2 and R_3 are known to high precision, then R_x can be measured to high precision. Very small changes in R_x disrupt the balance and are readily detected.

At the point of balance, the ratio of R1 to R2 is equal to R3 and R4.

METER BRIDGE
It consists of a meter long wire of high resistance and low temperature co-efficient.

CONSTRUCTION:

• It consists of a wire AB of 1 meter length and uniform cross section.
• A battery of emf ' e ' , a plug key 'K' are connected between the two terminals A and B.
• A graduated meter scale S is fixed by the side of the wire for taking the lengths of the wire from the +ve terminal i.e A. 
• Three strips C1 ,C2 ,C3 of copper or bronze with negligible resistances are also stretched on the board with gaps in between them. 

• A resistance box 'RB' is connected in the gap 'G1' . Resistance in RB = P 
• The unknown resistance 'X' is connected in gap 'G2' .
• In between the centre C of strip C2 , a galvanometer G and a high resistance 'H.R' are connected in series . 

The other end of the galvanometer is connected to a ‘jockey’ which is essentially a metallic rod whose one end has a knife-edge which can slide over the wire to make electrical connection.

• When the jockey is at a point 'D' on the wire , it divides the wire into two parts AD and DB of lengths L1 and L2.
• Resistance of AD length of wire =R = L1 [sigma] 
• Resistance of DB length of wire = S = L2 [sigma]  

where [sigma] is the resistance per unit length of the wire.

Now , the circuit is exactly similar to a Wheatstone bridge.

WORKING:

The jockey is now pressed at various points one the wire from 'A' to towards 'B' , until we get near null deflection in the galvanometer . 

At this stage , the high resistance is shunted and the exact balance point giving null deflection at D is obtained . 

CALCULATIONS:

The length l1 from A to D is noted . The unknown resistance X can be calculated from the following equation.
When the bridge is balanced , we have

     [(P)/(X)]   =  [(R)/(S)]   =  [(l1)/(l2)]

But in a meter bridge l2 = ( 100 - l1 ) . So the balanced condition of a meter bridge is

[(P)/(X)]   =  [(l1)/(100-l1)]

USES:

The meter bridge can be conveniently used to

•  determine an unknown resistance
•     compare two resistance
•     determine the specific resistance of the material of a wire . 

THE SHORT AND ONLY LIFE OF A CELL

Interphase:
It is the phase of the cell cycle in which the cell spends the majority of its time and performs the majority of its purposes(obtaining nutrients, growing, and conducting other "normal" cell functions) including preparation for cell division.

Interphase generally lasts at least 12 to 24 hours in mammalian tissue.

STAGES OF INTERPHASE:

There are three stages of interphase, with each phase ending when a cellular checkpoint checks the accuracy of the stage's completion before proceeding to the next. The stages of interphase are:

• G1:GAP 1:

During this stage new organelles are being synthesized, so the cell requires both structural proteins and enzymes, resulting in a great amount of protein synthesis and a high rate of metabolism in the cell.

CHECKPOINT:
The G1/S transition is a major checkpoint in the regulation of the cell cycle. Depending on levels of nutrients, energy and external factors, cells must decide to enter the cell cycle or move into a non-dividing state known as G0 phase.

• G0:GAP ZERO:

A cell may pause in the G1 phase before entering the S phase and enter a state of dormancy called the G0 phase.

• S Phase: 

To produce two similar daughter cells, the complete DNA instructions in the cell must be duplicated. DNA replication occurs during this S (synthesis) phase.

• G2: GAP 2: 

It is a period of rapid cell growth and protein synthesis during which the cell readies itself for mitosis. Curiously, G2 phase is not a necessary part of the cell cycle, as some cell types (particularly young Xenopus embryos and some cancers) proceed directly from DNA replication to mitosis.

X-RAYS
X-rays are generated when free electrons give up some of their energy when they interact with the orbital electrons (k-shell)or nucleus of an atom (Brehmsstrahlung).

REQUIREMENTS FOR PRODUCTION:

To generate X-rays, we must have three things. We need to have a source of electrons, a means of accelerating the electrons at high speeds, and a target material to receive the impact of the electrons and interact with them.

So what elements emit X-ray lines?
The more protons an element has, more energetic its lines can be. Carbon atoms (6 protons each) can emit X rays. But carbon lines are at the low end of X rays. Many X-ray instruments cannot detect these photons. Of the common elements in the universe, iron (26 protons) and oxygen (8 protons) usually are the two most prominent sources of X-ray lines along with tungsten and copper.

X-RAY PRODUCTION:

Bremsstrahlung X-rays

In an X-ray tube the electrons emitted from the cathode are accelerated towards the metal target anode by an accelerating voltage of typically 50 kV.

In the Bremsstrahlung process, a high speed electron traveling in a material is slowed or completely stopped by the forces of any atom it encounters.

If the electron is slowed down, it will exit the material with lesser energy.( The law of conservation of energy tells us that this energy cannot be lost and must be absorbed by the atom or converted to another form of energy). The energy used to slow the electron is excessive to the atom and the energy will be radiated as x-radiation of equal energy.

If the electron is completely stopped by the strong positive force of the nucleus, the radiated x-ray energy will have an energy equal to the total kinetic energy of the electron. This type of action occurs with very large and heavy nuclei materials.

K-SHELL EMISSION

The K-shell is the lowest energy state of an atom. An incoming electron can give a K-shell electron enough energy to knock it out of its energy state. About 0.1% of the electrons produce K-shell vacancies; most produce heat. Then, a tungsten electron of higher energy (from an outer shell) can fall into the K-shell. The energy lost by the falling electron shows up in an emitted x-ray photon. Meanwhile, higher energy electrons fall into the vacated energy state in the outer shell, and so on.

When outer-shell electrons drop into inner shells, they emit a quantized photon "characteristic" of the element. The energies of the characteristic X-rays produced are only very weakly dependent on the chemical structure in which the atom is bound, indicating that the non-bonding shells of atoms are the X-ray source.

K-shell emission produces higher-intensity x-rays than Bremsstrahlung, and the x-ray photon comes out at a single wavelength.

The X-ray photons produced by electrons that move from higher energy levels to k-shells, form the
K-SERIES.
If the electron moves from L-SHELL to K-SHELL,it gives rise to the first line of the K-SERIES which is "kα".
If the electron moves from M-SHELL to K-SHELL,it gives rise to the second line of the K-SERIES which is "kβ".
If the electron moves from N-SHELL to K-SHELL,it gives rise to the third line of the K-SERIES which is "kγ".
Similarly X-ray photons produced by movement of electrons from higher energy levels to L-shells constitute the L-SERIES and those into M-shells make up the M-SERIES.

DISTINGUISHING TESTS

FOR ALKENE AND ALKANE:

Bayer's Test:

It is a quantitative test for the presence of unsaturation (double or triple bonds).
a sample is treated with small amount of elemental bromine either

• as aqueous solution
• as a solution in CCl4
• as solution in CH2Cl2

REACTION:
Deep brown colouration of bromine rapidly disappears as it is consumed by reaction with sample containing alkenes.

Potassium permanganate test:

It is the quantitative test for the presence of unsaturation.
Dilute KMnO4 is purple and the test depends on its ability to act as an oxidizing agent.

REACTION:
When alkenes are oxidized with cold, alkaline KMnO4, dihydroxy compounds (diols or glycols) are formed. The KMnO4 gets decolorized and in its place brown ppt of MnO2 is seen.

A more compressed form in which to express the reaction in is:

ALKANE AND ALKYNE:

Potassium permanganate test:

At room temperature:
Alkynes react with KMnO4 to give "dioics" and decolorize the purple hue of  the oxidizing agent.

At low temperature:
Alkynes may react to give an 'oic' at low temperatures.

ALKENE AND ALKYNE:

The formation of acetylides is used as mark of identification of alkynes.
ACETYLIDES are the dianions with the formula C2−2.They are formed by deprotonation of alkynes and subsequent addition of metals to the carbon atom.

Alkenes do not react as such with metals.

                                                                                         DI SODIUM ACETYLIDE

The above forms DISILVER ACETYLIDE which cools to form a white solid and DICOPPER ACETYLIDE which cools to form a red solid.

ALKANE AND ALKYL HALIDES:

The most efficient way of distinguishing alkyl halides is by a substitution reaction which would convert the halogen into halide ions.The ions can then be tested by Silver Nitrate solution.

REACTION:

Haloalkanes are warmed with sodium hydroxide solution in a mixture of ethanol and water.Everything will    dissolve.

The nitric acid is added to remove other ions that give a confusing precipitate.
Silver Nitrate solution is added.
It may form white ,pale yellow or yellow ppt depending on the halogen.

ALDEHYDE AND KETONE:

Aldehydes are easily oxidized by all sorts of different oxidizing agents: Ketones aren't.
For Aldehydes ,Oxidation under :
Acidic conditions---gain of oxygen
Alkaline conditions---lose of hydrogen and gain of oxygen

Fehling's Test:

Fehling's solution contains copper (II) ions complexed with tartrate ions in sodium hydroxide solution.

Aldehydes reduce the complexed copper (II) ion to copper (I) oxide. 

Because the solution is alkaline, the aldehyde itself is oxidized to a salt of the corresponding carboxylic acid.

ketone         :No change in the colourless solution.  

Aldehyde     :Red ppt of Cu2O.

Tollen's Test:

Tollens' reagent contains the diamminesilver (I) ion, [Ag (NH3)2] +.

Aldehydes reduce the diamminesilver(I) ion to metallic silver. 

Because the solution is alkaline, the aldehyde itself is oxidised to a salt of the corresponding carboxylic acid.

• ketone     : No change in the colourless solution.  
• aldehyde  : The colourless solution produces a Grey precipitate of silver, or a silver mirror on the test tube.