EFFECTS OF LIGHT ON CHLORINE DISSOCIATION, AN SPECTROSCOPIC VIEW

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EFFECTS OF LIGHT ON CHLORINE DISSOCIATION, AN SPECTROSCOPIC VIEW
EFFECTS OF LIGHT ON CHLORINE DISSOCIATION, AN SPECTROSCOPIC VIEW
Focus is on the Chain Initiating Step in the SIDE CHAIN CHLORINATION

The electromagnetic spectrum includes all wavelengths of electromagnetic radiation.
Toluene chlorination proceeds by a chain mechanism or chain reaction. In a chain reaction, an intermediate chlorine atom is produced in a chain-initiating step.
In this mechanism, the first step is the light-induced dissociation or photodissociation of chlorine into reactive chlorine atoms.The weak chlorine-chlorine bond is broken homolytically to give chlorine atoms. This reaction is driven by light energy, and the frequency of light that promotes the overall reaction is that frequency that is absorbed by the chlorine molecule. This is a free radical chain reaction which is initiated photochemically by the homolytic cleavage of chlorine molecules to give chlorine atoms,
Cl₂ + hv ---------------> 2 Cl· (chain-initiating step)
Cl₂ + 245.5 kJ --------------> 2 Cl·
The reaction is initiated by the input of energy (heat or light). The weak chlorine-chlorine bond is broken homolytically to give chlorine atoms.
This reaction is driven by light energy, and the frequency of light that promotes the overall reaction is that frequency that is absorbed by the chlorine molecule
In all of the reactions bonds are broken and new bonds are made. We can describe a reaction buy how bonds break and form. This description of how a reaction occurs is called the mechanism of the reaction. There are two ways a covalent two-electron bond can break. A bond can break symmetrically so that one electron remains with each of the product parts. This bond breaking (and the reaction description) is said to be homolytic.

Most radicals are elctronically neutral (have neither a positive nor a negative charge), but they are highly reactive because electrons do not like to be unpaired.
A radical can abstract an atom from another molecule, leaving behind a new radical species. This is a kind of radical substitution reaction.
This is called "homolytic" bond cleavage since in the products the distribution of the electron pair is quite even. We use the "fishhook" (curved arrow with only one "barb" on the arrowhead) to show the motion of one electron. Such homolytic bond cleavages occur when the bond involved is not polar and there is no electrophile or nucleophile at hand to promote heterolytic patterns.

Photochemical Reaction or Photodissociation occurs when the breaking of a bond results from the absorption of a photon by a molecule. For a reaction induced by radiation to occur, the photons must have sufficient energy to break the required bonds, and the molecules must absorb the photons. Photodissociation is the rupture of a chemical bond induced by absorption of a photon.
The dissociation or excitation of a molecule caused by the absorption of a photon. How a molecule will react to absorbing a photon depends on the photon's energy and the molecule absorbing the photon. Excitation of a molecule can cause the molecule to react with other atoms or molecules more readily. Dissociation of a molecule can cause the dissociated parts of the molecule to react more readily with other atoms or molecules.
Photolysisis the dissociation of a molecule caused by the absorption of a photon. And photolyze is the action of a molecule breaking apart after absorbing a photon. Chlorine is photolyzed by ultraviolet light and Blue light.
The reaction is initiated by the input of energy (heat or light). The weak chlorine-chlorine bond is broken homolytically to give chlorine atoms. Since reactions of organic compounds involve the making and breaking of bonds, the strength of bonds, or their resistance to breaking, becomes an important consideration. The chlorine dissociation is induced by breaking a relatively weak Cl-Cl covalent bond.
Bond energy is the energy required to break a covalent bond homolytically (into neutral fragments). Bond energies are commonly given in units of kcal/mol or kJ/mol, and are generally called bond dissociation energies when given for specific bonds, or average bond energies when summarized for a given type of bond over many kinds of compounds. Tables of bond energies may be found in most text books and handbooks.
A TABLE here is a collection of average bond energies for a variety of common bonds. Such average values are often referred to as standard bond energies, and are given here in units of kcal/mole.

This is the Chain initiating step in the SIDE CHAIN CHLORINATION
The cleavage of one mole of chlorine by photodissociation requires 1 mole of light quanta (hv). According to the Quantum theory of radiation Energy is radiated in the form of discrete packets called "Quanta". Quantum of light is called Photon. Energy of each quanta is directly propotional to the frequency of radiation with which the quanta is associated. This theory is developed by Max Plank in 1900.
Consider light from a particles point of view, Photons are packets of light or particles of light energy.
The energy of a photon depends of frequency. And frequency is also related to the wavelength of light.
The energy of a photon can be given by the equation
E = hv
Where h = Planck's constant (6.626 x 10^-34 Js)
v = frequency of light = c /l
Note that E = hv = hc /l
l = wavelength of light
Speed of light in vacuum, c = 3 x 10⁸m/s

The energy of of one mole of photons =6.022x10²³ h c / l
A mole contains 6.023x 10²³ molecules.
The higher the frequency, v, the shorter the wavelength, l, and the higher the energy of the radiation E.

The minimum energy required to induce this reaction depends on the dissociation energy of Cl (245.5 kJ/mol).
Dissociation Energy = 245.5 kJ / mol for the breaking of the Chlorine double bond.
Two conditions are necessary:
1. The photon must have sufficient energy
2. The light must be absorbed. The photons must collide with the Cl₂ effectively.
Cl₂ -----hv----> Cl·

This is the maximum wavelength for this particular dissociation reaction. If light has an energy which is greater than 245.5 kJ, it will also carry out this reaction.
488 nm is maximum wavelength which would cause a reaction to occur.

A wavelength of above 488 nm would not cause this reaction to occur because the wavelength is too high and energy of photon would be less than the dissociation energy of the bond between chlorine and chlorine atoms in the chlorine molecule.
488 nm is the maximum wavelength for this particular dissociation reaction. If light has an energy which is greater than 245.5 kJ, it will also carry out this reaction.

Note that this is in the BLUE or Ultraviolet region. UV radiation, which falls between x-rays and visible light on the electromagnetic spectrum, is divided into three types, according to wavelength. These are: UVA (320-400nm), UVB (290-320), and UVC (200-290nm).

The higher the frequency, v, the shorter the wavelength, l , and the higher the energy of the radiation. For a chemical reaction induced by radiation to occur, the photons must have sufficient energy to break the required bonds, and the molecules must absorb the photons. Photodissociation is the rupture of a chemical bond Cl----Cl induced by absorption of a photon hv . The minimum energy required to induce this reaction depends on the dissociation energy of Cl₂ (245.5 kJ/mol). The longest wavelength light that causes the formation of chlorine atoms is 488 nm. The minimum energy required to induce this reaction depends on the dissociation energy of Chlorine Cl (245.5 kJ/mol). A molecule absorbs energy, causing the loss of an electron. Thus the photon must have sufficient energy to remove an electron when it is absorbed by a molecule.
Since an amount of energy equal to 245.5 kJ corresponds to a wavelength of 488 nm Therefore only blue light or light of even shorter wavelengths like UV can be used, but not longer-wavelength yellow or red light. Thus the chlorination of toluene reaction is initiated by blue light but not red light. Apart from a visual/audible effect it also gives an example of a chain reaction and also can be used to demonstrate that energy is often required for a reaction to begin.

COMPARISON OF BLUE AND ULTRAVIOLET LIGHT
If I say that BLUE light would give higher rate of chlorination than ULTRA VIOLET light , will you accept this ? , but yes this is a fact although surprising at first sight and you would appreciate this only if you understand BEER's LAW from in and out and you go through till the bottom of this page.

The Beer-Lambert law, also known as Beer's law or the Beer-Lambert-Bouguer law is an empirical equation relating the absorption of light to the properties of the material the light is travelling through. It was independently discovered (in various forms) by Pierre Bouguer in 1729, Johann Heinrich Lambert in 1760 and August Beer in 1852.
It is very important law and whole spectroscopy revolves around this law because it links light absorption and concentration and is the basis behind the use of spectroscopy to identify substances.

Where A = Absorbance and has no units.
( for a given compound at a given l)
If you reduce concentration by half than Absorbance would also get reduced by half.

I₀ = Intensity of the incident radiation
I   = Intensity of the incident radiation trasmitted
e   = Absorptivity coefficient= Extinction coefficient. This is wavelength dependent property of absorbing material. The value of e varies from material to material and is constant for a given material at a given wavelength. It shows how intense is the absorption of a material (at a given l ) and varies with the l as the absorption varies with the l.
We can say this the amount of light absorbed per unit concentration.
e = A / c l
A compound with a higher e would absorb more light than a lower e compund (at the same l ) for same concentration.
l = Path of light (distance that light travels through the material)
C = Concentration of absorbing material
(For liquids if mole fraction is used then the units of e would be cm ^-1)
(For liquids if molarity is used then the units of e would be Mole ^-1 cm² And we can say that e here molar absorptivity is the Absorbance of a 1 cm layer of a 1 mole per litre solution.)
(For gases if density is used to express concentration then the units of e would be Mole ^-1 cm² And we can say that e here molar absorptivity is the Absorbance of a 1 cm layer of a 1 mole per litre solution.)
For example a compound has molar absorptivity of 2 mole ^-1 lit cm^-1 at maximum at 480 nm.

For example if the 99% of the intensity I₀ of the incident radiation of either BLUE or UV light is absorbed.

e for Blue Light (at 436 nm ) = 4
e for UV Light (at 350 nm ) = 125
lblue light = 4.605/4 = 11.5125 cm
lUV light = 4.605/125 = 0.3684 cm

The rate of chlorination of toluene would depend on the chlorine dissociation (Chain initiating Step) that would further depend on the absorbance of light by chlorine.
No doubt e is higher in UV light than Blue light but Blue light would give higher rate of chlorination because Blue light would penetrate a larger depth in chlorine (pure chlorine.), For 99% absorption of UV light we need to place the source of UV light in every 3.6 mm of reaction mixture approximately, making the design of reactor very difficult. (Definitely the effective high degree of mixing would affect the process). Or else, after 3.6 mm in the reaction mixture there would be no UV light and therefore no dissociation of chlorine, after 3.6 mm from the start of reaction mixture. If the reactor glass thickness is 5 mm and no loss of light takes place in glass wall of reactor, then the rate of chlorination would be much higher in blue than UV light for a given reactor setup. Also the Absrorbance of UV light by chlorine would depend on the concentration of chlorine gas in that particular 3.6 mm length. We can not increase the concentration of chlorine in reactor to increase the Absorbance, the reason for that is we need to keep the concentration of chlorine related to the degree of chlorination selected for the process.We can not increase the concentration of chlorine in reactor more than the degree of chlorination selected for the process.

The Table below showing the rate constants for the side chain chlorination of toluene with different light sources at two temperature levels verifies that the BLUE LIGHT gives Higher rates of chlorination than the ULTRAVIOLET LIGHT. And the main reason of carrying out reaction at 100^oC than the 40^oC even though the k value at 40^oC is higher than that of the k value at 100^oC, is the wt% of the nuclear chloroproducts formed, these are nearly 10 times at the 40^oC than that of the nuclear chloroproducts formed at 100^oC, therefore the 100^oC is selected because at 40^oC ring chlorination of toluene is accelarated.

100 ⁰C			Source of light	Energy maximum nm	40 ⁰C
k₁	k₂	k₃			k₁	k₂	k₃
0.71	0.035	0.006	Darkness	-	-	-	-
0.73	0.12	0.02	Yellow	590 nm	0.9	0.11	0.013
-	-	-	Green	520	2.4	0.29	0.034
5.2	0.85	0.15	Blue	425	10.7	1.3	0.15
3.2	0.53	0.093	Ultraviolet	370	6.0	0.71	0.083
-	-	-	Bactericide	253.7	3.9	0.47	0.055