E1cB elimination reaction

The E1cB elimination reaction is a type of elimination reaction which occurs under basic conditions, where a particularly poor leaving group (such as -OH or -OR) and an acidic hydrogen eliminate to form an additional bond.

 E1cB is a three-step process. First, a base abstracts the most acidic proton to generate a stabilized anion. The lone pair of electrons on the anion then moves to the neighbouring atom, thus expelling the leaving group and forming double or triple bond. 

The name of the mechanism - E1cB - stands for Elimination Unimolecular conjugate Base. Elimination refers to the fact that the mechanism is an elimination reaction and will lose two substituents. Unimolecular refers to the fact that the rate-determining step of this reaction only involves one molecular entity. Finally, conjugate base refers to the formation of the carbanion intermediate, which is the conjugate base of the starting material.

An example of the E1cB reaction mechanism in the degradation of a hemiacetal under basic conditions.

There are two main requirements to have a reaction proceed down an E1cB mechanistic pathway. The compound must have an acidic hydrogen on its β-carbon and a relatively poor leaving group on the α- carbon.

The first step of an E1cB mechanism is the deprotonation of the β-carbon, resulting in the formation of an anionic transition state, such as a carbanion. The greater the stability of this transition state, the more the mechanism will favor an E1cB mechanism. This transition state can be stabilized through induction or delocalization of the electron lone pair through resonance. An example of an E1cB mechanism that has a stable transition state can be seen in the degradation of ethiofencarb - a carbamate insecticide that has a relatively short half-life in earth's atmosphere. Upon deprotonation of the amine, the resulting amide is relatively stable because it is conjugated with the neighboring carbonyl. In addition to containing an acidic hydrogen on the β-carbon, a relatively poor leaving group is also necessary. A bad leaving group is necessary because a good leaving group will leave before the ionization of the molecule. As a result, the compound will likely proceed through an E2 pathway. Some examples of compounds that contain poor leaving groups and can undergo the E1cB mechanism are alcohols and fluoroalkanes. It has also been suggested that the E1cB mechanism is more common among alkenes eliminating to alkynes than from an alkane to alkene.^[

One possible explanation for this is that the sp² hybridization creates slightly more acidic protons. Although it should be noted that this mechanism is not limited to carbon-based eliminations.

It has been observed with other heteroatoms, such as nitrogen in the elimination of a phenol derivative from ethiofencarb.

Degradation of ethiofencarb illustrating the presence of a stable anion due to resonance between the amide functional group and the carbonyl group.

Distinguishing E1cB-elimination reactions from E1- and E2-elimination reactions

E1	E2	E1CB
Stepwise reaction	Concerted reaction	Stepwise reaction
Carbocation Intermediate	removal of proton, formation of double bond, and loss of leaving group	Carbanion intermediate
no kind of conclusion	No preference	kind of conclusion
Good leaving	groupsLeaving group	Poor leaving groups
Less acidic B-H	Acidic B-H	More acidic B-H

When trying to determine whether or not a reaction follows the E1cB mechanism, chemical kinetics are essential. The best way to identify the E1cB mechanism involves the use of rate laws and the kinetic isotope effect. These techniques can also help further differentiate between E1cB, E1, and E2-elimination reactions.

Comparing the E1 and E2 Reactions M.Sc 1 sem

Now the we’ve at gone through the mechanisms of the E1 and E2 reactions, let’s take a moment to look at them side by side and compare them.

Here’s how each of them work:

Here’s what each of these two reactions has in common:

• in both cases, we form a new C-C π bond, and break a C-H bond and a C–(leaving group) bond
• in both reactions, a species acts as a base to remove a proton, forming the new π bond
• both reactions follow Zaitsev’s rule (where possible)
• both reactions are favored by heat.

Now, let’s also look at how these two mechanisms are different. Let’s look at this chart:

The rate of the E1 reaction depends only on the substrate, since the rate limiting step is the formation of a carbocation. Hence, the more stable that carbocation is, the faster the reaction will be. Forming the carbocation is the “slow step”; a strong base is not required to form the alkene, since there is no leaving group that will need to be displaced (more on that in a second). Finally there is no requirement for the stereochemistry of the starting material; the hydrogen can be at any orientation to the leaving group in the starting material [although we’ll see in a sec that we do require that the C-H bond be able to rotate so that it’s in the same plane as the empty p orbital on the carbocation when the new π bond is formed].

The rate of the E2 reaction depends on both substrate and base, since the rate-determining step is bimolecular (concerted). A strong base is generally required, one that will allow for displacement of a polar leaving group. The stereochemistry of the hydrogen to be removed must be anti to that of the leaving group; the pair of electrons from the breaking C-H bond donate into the antibonding orbital of the C-(leaving group) bond, leading to its loss as a leaving group.

Now we’re in a position to answer a puzzle that came up when we first looked at elimination reactions. Remember this reaction – where one elimination gave the “Zaitsev” product, whereas the other one did not. Can you see why now?

So what’s going on here?

• The first case is an E2 reaction. The leaving group must be anti to the hydrogen that is removed.

• The second case is an E1 reaction.

• In our cyclohexane ring here, the hydrogen has to be axial. That’s the only way we can form a π bond between these two carbons; we need the p orbital of the carbocation to line up with the pair of electrons from the C-H bond that we’re breaking in the deprotonation step. We can always do a ring flip to make this H axial, so we can form the Zaitsev product.
• Here’s that deprotonation step:

As you can see, cyclohexane rings can cause some interesting complications with elimination reactions! In the next post we’ll take a detour and talk specifically about E2 reactions in cyclohexane rings.

E1 and E2 mechanism Sem 1 M.Sc

E1 Mechanism Overview: The general form of the E1 mechanism is as follows: B: = base X = leaving group (usually halide or tosylate) In the E1 mechanism, the the first step is the loss of the leaving group, which leaves in a very slow step, resulting in the formation of a carbocation. The base then attacks a neighboring hydrogen, forcing the electrons from the hydrogen-carbon bond to make the double bond. Since this mechanism involves the formation of a carbocation, rearangements can occur. An example of the E1 reaction: Base Strength: A strong base not required, since it is not involved in the rate-determining step Leaving groups: A good leaving group is required, such as a halide or a tosylate, since it is involved in the rate-determining step.  Rearangements: Since the mechanism goes through a carbocation intermediate, rearangements can occur.

E2 Mechanism Overview: The general form of the E2 mechanism is as follows: B: = base X = leaving group (usually halide or tosylate) In the E2 mechanism, a base abstracts a proton neighboring the leaving group, forcing the electrons down to make a double bond, and, in so doing, forcing off the leaving group. When numerous things happen simultaneously in a mechanism, such as the E2 reaction, it is called a concerted step. An example of the E2 reaction: Base Strength: A strong base is required since the base is involved in the rate-determining step. Leaving groups: A good leaving group is required, such as a halide or a tosylate, since it is involved in the rate-determining step. Stereochemistry requirements:  Must occur with antiperiplanar stereochemistry.

Object: Preparation of Benzilic acid in Benzil.

Object: Preparation of benzilic acid in benzil.

(I) Preparation of Benzil step I

Chemical Required:

Benzoin      -        10 gm

Acetic acid  -        50ml

Conc. HNO₃         -        25ml

Procedure:

A mixture of benzoin (10gm, 0.047 mole), acetic acid (50 ml) and concentrated HNO₃ (25ml) is heated in a round bottom flask boiling water bath for 2hrs. The flask is connected to a trap containing NaOH solution to absorb oxides of nitrogen. Alternatively, the reaction is carried out in a fume cupboard. The reaction mixture is cooled, poured with stirring into an ice cold water. The mixture is stirred well and the separated product filtered and wash with water. It is crystallized  from rectified spirit as yellow prisms.

yield – 8.7

          mpt  - 95°C

          

Second step-

Preparation of benzilic acid from benzil

Chemical Required-

Benzil                                      –        8 gm

Potassium Hydroxide pellets   –        10 gm (in 20ml H₂O)

Rectified spirit                          –        25 ml

Conc. HCl                                –         35 ml

A mixture of benzil (8 gm 0.038 mole), KOH solution (10 gm dissolved is 20ml water) and rectified spirit (25ml) is refluxed in water bath for 12 minutes. The hot reaction mixture is poured into water (150 ml) contained in a beaker. In case the reaction does not go to completion a colloidal solution is obtained due to separation of unreacted benzil.

Pharmacokinetics Semester 3 Medicinal Chemistry UNIT 3

Pharmacokinetics

   Objectives:

            The aim of this unit is to study the drug movement inside the outside the body. We also get knowledge about Pharmacokinetics; the quantitative study of drug movement inside, through and outside of the body is done. We also studied drug absorption by different ways, Distribution & disposition of drugs, excretion and elimination of drugs & Pharmacokinetics of elimination and different Pharmacokinetics in drugs development process has also studied.

            We will get knowledge of pharmacodynamics that deals enzyme stimulation, enzyme inhibition, sulphonamides, membrane active drug metabolism, xenobioties & significances of drug metabolism in medicinal chemistry.

   Introduction

        Pharmacokinetics is the quantitative study of drug movement inside, through and outside of the body. Therefore, pharmacokinetic considerations determine the routes of drug administration does, time of peak action duration of action and frequency of administration of drug. drug movement occurs through the membranes. Biological membrane is a bi layer, 100 A in thickness made up of phospholipid, cholesterol, polymeric sugars, amino sugars and sialic acids are attached on the surface and formed glycoprotein or glycolipids. The protein molecules are able to freely float through the membrane and some proteins are fixed and present in the full thickness of the biological membrane, which are also surrounded by fine aqueous pores. The movement of drugs across the membranes occur by following processes

 (A)     Passive diffusion

(B)       filtration

(C)       Specialized transport

The drug diffuses the biological membrane in the direction of its concentration gradient. The drugs which are soluble in lipids, diffused by dissolving in the lipoidal matrix of the Biological membrane. Highly lipid soluble drug attains higher concentration in the  membrane and diffuses quickly.

          Generally most of the drugs are weak electrolytes and their rate of ionization depend on pH     of drugs. They in the following manner :

(1) Acidic drugs, e.g., aspirin, whose pka value is 3.5, are pH of gastric juices. These drugs are absorbed by stomach.

(2) Basic drugs, e.g., atropine, whose pka value is 10, are largely ionized. These are absorbed only when they reach in the intestine.

(3) Unionized form of acidic drugs cross the surface membrane of gastric mucosal cell. These drugs also revert to the ionized  form within the cell (pH 7.0) than slowly pass to the extracellular fluid. This is called ion trapping of drug.

(4) Basic drugs attain higher concentration intracellular.

(5) Acidic drugs are rapid ionized in alkaline urine. They do not diffuse back in the kidney tubules and are excreted quickly.

(6) If urine is acidified, basic drugs are excreted faster.

(B)    Filtration : Drugs filtration occur through aqueous pores of the membrane or through para cellular spaces. It is faster when osmotic pressure gradient is available. Most of the cells, e.g., RBC, intestinal mucosa etc. consist of very small pore size, i.e., 4 A, and drugs with molecular weight more then 100 or 200 are not able to penetrate them. Drugs of larger molecular wt. e.g. albumin can filter through capillaries depend on rate of blood flow.

(C)     Specialized transport : This of two types:

          (a)     Carrier transport

          (b)     Pinacytosis

(a)     Carrier transport :

          In the carrier transport system, a drug combines with a carrier which is present in the biological membrane and forms a complex.

(i)                Active transport : This transport system need energy and occurs against the concentration gradient. It gets inhibited by metabolic  poisons.

DRUG ABSORPTION

          Absorption of drug is the movement from its site of administration into the circulation. When given intravenously, the drug has to cross the biological membrane. The absorption of drugs is governed by the above described principles. The factors affecting the absorption are as follows:

(a)  Aqueous solubility :  If drug is solid, it is necessary to dissolve it in aqueous bio phase before absorption. A drug given as watery solution is absorption are as follows :

(b) Concentration: Drug given as concentrated solution is absorbed faster than from dilute solution.

(c)  Surface area: If surface area of drug absorption is larger, it means faster absorption.

(d) Vascularity of the absorbing surface: circulation of blood removes the drug from the site of absorption and maintains concentration gradient across the biological membrane.  

Route of Administration : Route of administration affects the absorption of drug.

(a)  Oral route : Drug Which are taken  orally, absorb in the following manner :

a.     Non ionized lipid-soluble drugs : are quickly absorbed by stomach and intestine.

b.     Acidic drugs : salicylates and barbiturates etc. are acidic drugs which are unionized in gastric juices. These drugs are readily absorbed by stomach.

c.      Basic drugs: Quinine, morphine etc. are highly ionized drugs. These are absorbed only in duodenum.

d.     Solid drugs : Drugs which are given in the form of solid dosage are governed by rate of dissolution and rate of abortion,

e.      Absorption in presence of food: Presence of food reduces the absorption of drugs. Some drugs form an complex compound with food constituents, e.g., an antibiotic “tetracycline”., form a complex with a calcium which is present in milk. These most of the drugs absorbed better when taken in empty stomach condition.

f.       Ionized drugs: Drugs e.g. streptomycin etc. are highly ionized in nature and are poorly absorbed when given orally.

g.     Degradation of drugs by gastric juices: Insulin is a drug which is degraded by peptidase enzyme of gastrointestinal tract, if taken orally. Therefore, it is administered intramuscular, similarly penicillin G is degraded by acid, and is also ineffective orally.

h.     Luminal effect of drugs: If two drugs are taken together, they may form an insoluble complex, this known as Luminal effect of drugs. Hence, to minimize this effect, two drugs must be taken at 2-3 hour intervals. Example of such drugs is phenytoin with sucralfate and tetracycline with antacid and iron preparations.

i.       Gut _ flora changing drugs.

j.       Gut wall effects

(b) Subcutaneous and Intramuscular :

Absorption of drugs from subcutaneous site is slower than from intramuscular site, but routes are generally faster and consistent than oral absorption n. Application of heat and muscular exercise accelerate drug absorption by increasing blood flow.

     By these routes the drug deposited directly in the capillaries. The capillaries are highly porous and they do not obstruct absorption of large lipid-insoluble molecules or ions. Extremely large molecules are absorbed through lymphatics. )

(c)  Topical sites administration:

This types of drug administration occurs though skin, cornea and mucous membranes and depends on lipid-solubility of drugs. Only few drugs such as hyoscine and nitroglycerine can significantly penetrate intact skin.

Cornea is pcrmeable to lipid soluble, unionized physostigmine but not to highly jonized neostigmine.

     Abraided surface of skin quickly absorbs durgs. If tannic acid is applied over burnt skin surface, causes a side effect, “hepatic necrosis.” Similarly corticosteroids applied over skin can produce systemic effects and pituitary – adrenal suppression. Organophsophate insecticides coming in contact of skin can cause systemic toxicity effect.

   Distribution And Disposition of Drugs

          After administering the drug in drug in the blood stream, it is ready to distribute to the tissues. Distribution of drugs depend upon its (a) solubility in lipid; (b)differences in regional blood flow (c) binding to plasma and tissue proteins (d) ionization at physiological pH.

          The distribution of drug continues till an  equilibrium occurs between unbound drug.

          Apparent volume of distribution

          V =   Intravenous dose administered / Plasma concentration

          “V is the volume which would accommodate all the drug in the body if its concentration is same as is plasma."

Penetration of drug into brain and cerebro spinal fluid : In the brain blood capillaries do not contain large inter cellular pores and have tight junctions. Neural tissues cover the capillaries of endothelial cells in brain. Thus they form a “blood-brain barrier’. Choroidal epithelium tissues also line the capillaries and form a similar “ blood-cerebro spinal fluid barrier”.

          Efflux carrier such as P-glycoprotein present in brain capillary endothelial cells, extrude several drugs which enter in brain by other processes. Dopamine doses not enter into brain but its precursor levodopa can penetrate. In the capillary walls or cells liming of brain monoamine oxidase, cholinesterage and some other  enzymes are present, they also form an “enzymatic blood brain barrier, this barrier does not permit acetylcholine, catecholamines, 5- hydrxy tryptamine to enter into brain in active form.

          Passage across placenta : Plancental membranes are lipoidal and permit free passage only to lipid – periods by mother, it is gained by foetus. Thus it is an incomplete barrier and almost any drug administered by the mother can affect the new born.

          Plasma protein binding : Acidic drugs bind to plasma albumin and basic drugs bind to 1 acid glycoprotcins. Extent of binding depends on the individual compound e.g.,  sulphamethaxine binds 30% sulphadiazine, 50% Sulphamethoxazole 60% and sulfisoxazole binds 90%

          The comical importance of plasma proteins binding are as follows:

(a)  The bound fraction of drugs is not available for action. This fraction is in equilibrium with free drug in plasma nad dissociates whin the concentration of the latter is decreased due to elimination.

(b) If protein and drug binding is very high, then it makes the drug long acting,

(c)  One drug can bind to many sited of the protein albumin Opposite to it more then one drug can bind to the same sit.

Tissue storage :         Drugs may also accumulate in specific  organs or get bound to specific tissue constituents. Some drugs may also bind to specific intracellular organelle, e.g., tetracycline to mitochondria and chloroquine to nucleus. Certain drugs possess high toxicity Because of chloroquine on retina, emetine on heart and skeletal muscle, and tetracycline on bone and teeth.

   Excretion and elimination

              After absorption of drugs, they undergo the process of biotransformation, i.e., altered chemically in the body, and thus form their metabolites which are excreted in the following way :

(a)  Urine:

(b) Faeces :

(c)  Exhaled air : Gases and volatile liquids such as alcohol, general anaesthetics and paraldehyde etc . are eliminated by lungs.

(d) Saliva sweat: In the excretion of drugs, the importance of sweat and saliva is negligible. However, potassium iodide, lithium, rifampin, heavy metals and thiocyanates are excreted through this way.

(e)  Milk: most of the drugs enter in breast milk by passive diffusion, such as more lipid soluble and less protein bound the drugs.

Renal excretion: All water soluble drugs are excreted by kidney.

Glomerular filtration: In the capillaries of glomerular, larger pores are found which are able to filter all non protein bound drugs. In renal failure of after the age of 50 glomerular filtration rate decreases progressively.

Tubular reabsorption: Lipid soluble drugs filtered at the glomerulus diffuse back in the tubules, because 99% of glomerular filtrate is reabsorbed, but non lipid soluble and highly ionized drugs are not able to do so. This occur by following way.

(a)  Weak acids ionize more and are less reabsorbed in alkaline urine.

(b) Weak bases inonize more and are less reabsorbed in acidic urine.

This principle is important to utilze for elimination of organic acids and bases. Tubular transport mechanisms are not well developed at birth. Duration of action in many drugs, e.g. penicillin, aspirin cephaloprrins etc. is longer neonates. These systems mature during infancy.

2.2.2 Pharmacokinetics of Elimination

Drug elimination is sum total of metabolic inactivation and excretion the pharmacokinetics of of drug gives an idea to devise ratronal dosage reginefisand to modify them according to individaalneeds. There are three pharmacokinetic parameters, such as:

(a)  Clearance

(b) Bio availability

(c)  Volume of distribution

(a)            Clearance: The clearance of a drug is the theoretical volume of plasma from which the drug removed completely in unit time. Clearance (CL) can be expressed as

CL=      Rate of elimination of drug/  Plasma concentration (C)

(1)  First order kinetics: The rate of elimination of drug is directly proportional to drug concentration while clearance remains constant, or a constant fraction of the drug present in the body is eliminated in unit time.

(2) Kinetics of Drugs alter from first order to zero order at higher doses.

Plasma half-life: The plasma half-life of a drug is the time taken for its plasma concentration to be reduced to half of its original value.

A drug which has one compartment distribution and first order of elimination is plot is drawn between plasma concentration and time, which shows two slopes:

(a)  Due to distribution, initially declining an-phase.

Half-life, of the drugs. Plasma concentration: Time plot of a drug eliminated by first order kinetics after intravenous injection elimination ½ is:

Where in2= natural logarithm of or 0.693

K= elimination rate constant of the drug i.e the fractions of total amount of drug in the body which are removed in per unity of time

K= Clearance (CL)/ Volume of distribution (V)

                  

t1/2  = 0.693x K

                     CL      

Hence

For example, if 2g of a drug present in the body and 0.1 g of it is eliminated every hour then,

K=     0.1=0.05

                                       2

                        The drugs can be eliminated from the body as:

                   1-50% drug is eliminated from body

                   2-75% (50+25) drug is eliminated

                   3- 87% (50+25+12.5) drug is eliminated

                   4-93.75(50+25+12.5+6.25) drug is eliminated

                 

  Repeated drug administration:

If the therapeutic  plasma concentration of the drug has been worked out and its clearance

Is know the dose rate can be obtained as:

Does rate= target Cpss x clearance

Dose rate= target Cpss x clearance

                                                       Fraction

When drugs eliminated by first order kinetics, the does rate cpss relationship is linear,

Some drugs follow Michaelis Menten Kinetics, the climination occurs by zero order kineties over

The therapeutic range.

          In this case, the rate of drug elimination is expressed as:

Rate of drug elimination =  (V max (c)

                                             K m+C

Target level strategy.   

          Loading dose: Loading is a single or few rapidly repeated doses which are given in the beginning to attain target concentration, it may be expressed as:

Loading dose = Target plasma conecentration x Volume

                             Fraction of drug (F)

Maintenace dose rate = target plasmaconcentration  x clearamnce (CL)

                                            Fraction of dose (F)

Bioavailability of drug is 100% which is administered intravenously, but is low when taken orally because

(1) The drug may be absorbed partially.

(2) The absorbed drug may undergo first pass metabolism in intestinal wall, liver of to be excreted in bile.

Oral formulation of a drug from different manufactures of different batches from the same blood level. i.e. biologically in equivalent.

When a drug is taken in solid form, it must break into particles of the active drug, before its absorption.

Tablets and capsules consist of a number of different materials such as binders, lubricants, diluent, stabilizing agents etc. The nature of these and details of the manufacture process, The rate of dissolution is governed by the particle size. Solubility crystal from etc. of the physical properties of drug. Difference in bioavailability may be due the change in dissolution and disintegration rate.

 Pharmacokinetics In Drug Development Process

     

To use a drug for longer time, it is generally advantageous to modify a drug by following manner:

           (1)  By prolonging absorption from site of administration:

(a)  Oral: Drug particles are coated with resins, plastic materials etc. which disperse release of the active ingredients in gastrointestinal tract. A semipermeable membrane is used to control the release of the active ingredients in gastrointestinal tract. A semipermeable membrane is used to control the release of drug from the bable of capsule.

(b) Parenteral: The subcutaneous and implantation and biodegradable implants may from or as oily solution pallet implantation and biodegradable implants may develop a drug action.

(c)  Transdernal drug delivery: The drug which is used as ointement, in adhesive patches, or strips applied on skin is becoming popular.

(2) By increasing plasma protein binding: Development of drugs have been made by increasing plama proteins binding which may be slowly released in the free active form e.g. sulphadoxine.

(3) By retarding renal excretion. The tubular secretion of drug an active process which can be reduced by a competing substance, for instance, probenecid prolongs time of action of penicillin and ampicillin.