Other Substitution And Elimination Reactions



Certain soil systems contain reactant species other than H2O which can react with organic chemicals and yield products through analogous substitution reactions. A good exampleis the reaction of halogenated alkanes with H2S which is found in groundwater in certain localities. The reaction proceeds through a SN2 mechanism.
H2S/HS­R'CH2 SH

R ­ CH2 Br Æ R ­ CH2 SH + Br ­ Æ R ­ CH2 ­ S ­ S ­ CH2 ­ R'
Alkyl SN2OxidationAlkyl disulfide
bromide
R'CH Br
R ­ CH2 SH \|O(Æ,¨) CHS­ Æ R ­ CH2 ­ S ­ CH2 ­ R'
SN2
The activation energies for alkyl halide reactions with H2S, HS­ are lower than those of hydrolysis, thus these reacts will occur preferentially over alkyl halide reactions with OH­ and H2O.
Elimination Reaction
Elimination reactions involve the loss of two leaving groups from adjacent atoms within a molecule. The loss results in the formation of a new double (or triple) bond.
ߦ
R ­ CH ­ CH2 Æ R ­ CH = CH2
||
X1X2
The above reaction is commonly known as the beta elimination because one of the leaving group (X2) is lost from the a carbon and the other (X1) from the adjacent b carbon.
As in the case with the substitution reactions, there are two general types of elimination reactions, E1 and E2. The E1 (elimination, monomolecular) is a two-step process in which the rate limiting step is the ionization of the organic chemical to produce a carbo cation which rapidly loses a b proton to a base, usually the solvent.
R2R3R2R3
||slow||
R1 ­ C ­ C ­ X ¤ R1 ­ C ­ C+ + X­
ba||
||HR4
HR4carbo cation

R2R3strongR3
||base|
R1 ­ C ­ C+ ¤ R1 ­ C = C + H+ solvent
|||
HR4R4

The first reaction is the same as in the case of SN1. The second reaction is different because the solvent pulls a proton from the b carbon.
The E2 mechanism (elimination, bimolecular) is a one-step process in which two leaving groups depart simultaneously with the proton being pulled off by a base. This mechanism is similar to the SN2 mechanism and often competes with it.
R2XR1R3
||||
R1 ­ C ­ C ­ R3 Æ C = C + X­ + BH
||||
HR4R2R4
For alkyl halides the elimination reactions are terms dehydrohalogenation.
If an alkyl halide has more than one type of b carbon with leaving hydrogen, then it can yield more than one alkene.
b2 CH2 CH3HCH2 CH3HCH3
|H2O||||
b2 CH3 ­ C ­ cl Æ C = C + C = C
|||||
b1 CH3HCH3CHCH3
Under the right circumstances (e.g., presence of strong base), elimination reactions can compete with the substitution reactions. Tertiary alkyl halides with hydrogen attached to a b carbon undergo elimination reactions with ease. For example, even during the hydrolysis of t-butyl chloride in absence of bases, isobutylene is formed as a minor product.
CH3CH3HCH3
||||
CH3 ­ C ­ cl + H2O Æ CH3 ­ C ­ OH + C = C
||||
CH3CH3HCH3
83%17%
The significance of the reactions in the soil systems is highly dependent on soil surface. However, the very diverse nature of soil surface makes exact predictions difficult.

OXIDATION
Oxidation is removal of electrons from chemicals. In general, it will occur for organic chemicals by two different pathways:

1)heterolytic
In these reactions, an electrophilic agent attacks an organic molecule and abstracts an electron pair (polar oxidation).
2)homolytic
In this pathway, an agent abstracts only an electron (this is also referred to as the free radical oxidation). The activation energy for these reactions is less than the activation energy for heterolytic reactions.
The reactions involve two or more steps. The first step involves removal of a single electron through thermal exposure, radiant exposure, and irradiation with high energy particles. The addition of the radical to another molecule or radical is the second step. Its occurrence will depend upon the nature of reactants and reaction conditions. Most free radicals are highly reactive and will react with the first available molecule or free radical they contact. Since concentration of free radicals relative to the molecules is low, free radicals will react with a molecule instead of another radical. The resulting product may be another free radical. Let us take the case of benzene:


In this case, phenyl radical attacks the ring as an electrophile. The intermediate is stable due to resonance and can react further to form larger molecules. The final termination of free radicals involve terminations and can occur through:
A. Simple coupling where the two radicals combine to form a stable molecule.

B. Disproportionation where a hydrogen is transfered to one of the products.

C. Abstraction where a H€ is removed by a reactant radical.


The first two reactions will yield hydrobiphenyls that do not readily oxidize to biphenyls. Similar reactions can also occur in cases where the initiating species is a radical cation rather than a neutral radical. Since medical cations species are electron deficient, the presence of electron donating moieties (OR, NR2 or R) on the aromatic ring make the ring more susceptible to attack by radical cations.
Many substituted aromatic chemicals can undergo free radical oxidation. These include benzene, toluene, benzidene, ethyl benzene, naphthalene and phenol.
Photochemical Transformation
Many organic chemicals introduced into the environment absorb sunlight and are transformed. The process is referred to as photo transformation, photolysis or photo degradation. As already mentioned, due to poor penetration, photochemical transformation can occur at soil surfaces or in the aquatic systems and the atmosphere.
In order to make an assessment of a given contamination scenario, we as environmental scientists or engineers must be able to measure and/or predict the rates and products of these transformations.
Let us review the absorption of light:
The Beer-Lambert Law for light absorption relates the incident light (Io) on a solution to the emergent light (I) according to the following expression:

log \|F(Io,I) = S € C € l = absorbence
whereS isextinction coefficient (an absorption constant characteristic of the chemical
Cismolar concentration of the solution
lispath length of the cell
The expression can be written in the exponential form:
I = Io 10­SCl
The amount of light absorbed at a given wavelength Il can then be written as:
Il = Io ­ I = Io (1 ­ 10­SCl)
The excitation energy of the molecules is determined by the wavelength l of absorbed light:
E = hn = \|F(hc,l) 2.86 x 104
whereE=energy in ergs
h=Planck's constant (6.62 x 10-27 erg/sec)
n=frequency of light (cm-1)
c=velocity of light (3.00 x 10m/sec­)
l=wavelength in nm
The unit of light on the molecular level (photon) can be related on a molar basis as einstein. In molar terms, the previous expression can be given the following form:
E = Nhn = \|F(Nhc,l)
where N =Avogadro's number 6.02 x 1023
Quantum Yield
The Grotthus-Draper Law states that "only light absorbed by a molecule can bring about a photochemical change" or, a molecule which undergoes a photochemical change does so as a result of the absorption of a single quantum of energy. However, often less than one mole of chemical transforms for each einstein of absorbed light because some of the energy is lost in radiationless processes or re-emitted as phosphorescence or fluorescence.
F = \|F(number of moles chemical reacted,number of einsteins absorbed)
In the environment, phototransformation reactions are generally unimolecular and follow a first order rate kinetics. The rate constant (k) can be expressed by the first order kinetics equation given earlier for hydrolysis:
K = \|F(2.303,t) log \|F(co,ct)
wherek=rate constant
t=irradiation time (min/hrs or days)
co = initial concentration
ct = concentration after time t