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Non-Metal Based Reagents Notes | Study Organic Chemistry - Chemistry

Document Description: Non-Metal Based Reagents for Chemistry 2022 is part of Organic Chemistry preparation. The notes and questions for Non-Metal Based Reagents have been prepared according to the Chemistry exam syllabus. Information about Non-Metal Based Reagents covers topics like Non-Metal Based Reagents, 1. SeO2 [Selenium Dioxide] , 2. DCC, 3. Dichloro Dicyano Quinone [DDQ], 4. Peroxy Acid [RCO3H] , 5. Ozone [O3], 6. HIO4 [periodic acid], 7. N-Bromo Succinimide [NBS]  and Non-Metal Based Reagents Example, for Chemistry 2022 Exam. Find important definitions, questions, notes, meanings, examples, exercises and tests below for Non-Metal Based Reagents.

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Table of contents
Non-Metal Based Reagents
1. SeO2 [Selenium Dioxide] 
2. DCC
3. Dichloro Dicyano Quinone [DDQ]
4. Peroxy Acid [RCO3H] 
5. Ozone [O3]
6. HIO4 [periodic acid]
7. N-Bromo Succinimide [NBS] 
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Non-Metal Based Reagents

Some Non- Metal Based Reagents are: 

  1. SeO2
  2. DCC
  3. DDQ
  4. Peroxyacid
  5. Ozone
  6. Periodic Acid
  7. NBS

1. SeO2 [Selenium Dioxide] 

It is a very important oxidizing reagent which is used for the oxidation of  allyl or allylic hydrogen into alcohol.  

Non-Metal Based Reagents Notes | Study Organic Chemistry - Chemistry

There are some important points that must be followed during the oxidation process be SeO2.

  • Rule 1: Oxidation always occurs at the more substituted site of double bond.
  • Rule 2: If there is the possibility of two different types of allylic position out of which one present in the ring & other present outside the ring then SeO2 performs oxidation at the allylic position present in the ring.

Non-Metal Based Reagents Notes | Study Organic Chemistry - Chemistry

  • Rule 3: Oxidation does not occur at bridgehead position.  
  • Rule 4: Order of reactivity of allyl and allylic carbon:Non-Metal Based Reagents Notes | Study Organic Chemistry - Chemistry
  • Examples of formation of 1,2 dicarbonyl compounds:

Non-Metal Based Reagents Notes | Study Organic Chemistry - Chemistry

Non-Metal Based Reagents Notes | Study Organic Chemistry - Chemistry

Mechanism & Examples of Oxidation by SeO2 

Allylic Oxidation Mechanism

During the process of oxidation with SeO2 first of all ene reaction will occur which will followed by [2, 3] sigmatropic rearrangement and after the hydrolysis process, allylic alcohol is formed.

Allylic Oxidation MechanismAllylic Oxidation Mechanism

Examples:

Non-Metal Based Reagents Notes | Study Organic Chemistry - Chemistry

Non-Metal Based Reagents Notes | Study Organic Chemistry - Chemistry

Non-Metal Based Reagents Notes | Study Organic Chemistry - Chemistry  Non-Metal Based Reagents Notes | Study Organic Chemistry - Chemistry

Non-Metal Based Reagents Notes | Study Organic Chemistry - Chemistry

  • If the alkene possesses a methyl substituent, oxidative cleavage of C (elimination of a selenium atom and water) furnishes the corresponding, β-unsaturated aldehyde. Lower yields of products were obtained when using stoichiometric amounts of SeO2. t-Butyl hydroperoxide is used to reoxidize selenium. 
  • Besides giving the desired carbonyl compounds, the reaction may also produce allylic alcohols as side products.
  • In some cases, SeO2 can also behave as a dehydrogenating reagent.

Non-Metal Based Reagents Notes | Study Organic Chemistry - ChemistryNon-Metal Based Reagents Notes | Study Organic Chemistry - Chemistry

  • In the case of alkynes & alkenes, it can also give hydroxylation & the mechanism of hydroxylation is anti-hydroxylation or trans-hydroxylation.

Non-Metal Based Reagents Notes | Study Organic Chemistry - Chemistry

2. DCC

DCC (acronym for N,N'- dicyclohexylcarbodiimide) was one of the first carbodiimides developed as a reagent. It is widely used for amide and ester formation.

  • It will behave as a coupling reagent by condensation.  
  • It acts as a dehydrating reagent; hence it is stored under an anhydrous condition.

DCCDCC

Application of DCC in Organic Chemistry 

(i) Esterification or lactonization
(ii) Amide formation of Lactan formation peptide synthesis
(iii) Anhydride formation
(iv) Peroxide formation

Synthesis of DCC

  • Preparation from Urea

Non-Metal Based Reagents Notes | Study Organic Chemistry - Chemistry

  •  Esterification or lactonization: Fisher Esterification

Non-Metal Based Reagents Notes | Study Organic Chemistry - Chemistry

Non-Metal Based Reagents Notes | Study Organic Chemistry - Chemistry

Non-Metal Based Reagents Notes | Study Organic Chemistry - Chemistry

                                           Non-Metal Based Reagents Notes | Study Organic Chemistry - Chemistry

After the reaction, DCC is converted into cyclohexyl urea DCU.

Non-Metal Based Reagents Notes | Study Organic Chemistry - Chemistry

3. Dichloro Dicyano Quinone [DDQ]

It is the special reagent which helps in the removal of hydrogen from the substrate means it will behave as dehydrogenating agent.
  • In many cases, if the cyclic ring is present then it will aromatize that ring and gain aromaticity.

DDQDDQ

Mechanism of DDQ

In the mechanism of DDQ, first of all, it will abstract the hydride from the substrate and form the cation and then followed by the removal of proton produce the double bond.

Non-Metal Based Reagents Notes | Study Organic Chemistry - ChemistryNon-Metal Based Reagents Notes | Study Organic Chemistry - Chemistry

Problem:

Non-Metal Based Reagents Notes | Study Organic Chemistry - Chemistry

4. Peroxy Acid [RCO3H] 

The reaction of alkenes with peroxy acids provides for convenient and selective oxidation of double bonds.

General Formula: Peroxy AcidGeneral Formula: Peroxy Acid

  • The peroxy acids most commonly used in the laboratory are m-chloroperoxybenzoic acid (mCPBA), monoperoxyphthalic acid magnesium salt (MMPP), peroxybenzoic acid, peroxyformic acid, peroxyacetic acid, trifluoroperoxyacetic acid, and t-butyl hydroperoxide [(CH3)3COOH]. 

mCPBAmCPBA

  • The order of reactivity for epoxidation of alkenes follows the trend CF3CO3H > mCPBA ~ HCO3H > CH3CO3H >> H2O2 >

Non-Metal Based Reagents Notes | Study Organic Chemistry - Chemistry

Stereoselectivity

Epoxidation involves an electrophilic syn-addition of the oxygen moiety of the peroxy acid to the double bond. The concerted formation of two new C—O bonds ensures that the reaction is stereospecific: cis alkenes furnish the corresponding cis-epoxides and trans-alkenes the corresponding trans-isomers (racemic).

Non-Metal Based Reagents Notes | Study Organic Chemistry - ChemistryNon-Metal Based Reagents Notes | Study Organic Chemistry - Chemistry

With conformationally rigid cyclic alkenes, the reagent preferentially approaches the double bond from the less hindered side.

Non-Metal Based Reagents Notes | Study Organic Chemistry - Chemistry

Chemoselectivity

The rate of epoxidation increases with the number of electron-donating substituents on the double bond. The order of alkene reactivity with peroxy acids is as follows

Non-Metal Based Reagents Notes | Study Organic Chemistry - Chemistry  Non-Metal Based Reagents Notes | Study Organic Chemistry - Chemistry

Hence, the more nucleophilic (more substituted) double bonds of the diene and triene depicted below undergo preferential epoxidation when treated with an equimolar amount of the peroxy acid.

Non-Metal Based Reagents Notes | Study Organic Chemistry - Chemistry

Conjugation of the alkene double bond with an electron-withdrawing group reduces the rate of epoxidation. Thus, α, β-unsaturated carboxylic acids, and esters require a stronger oxidant, such as trifluoroperoxyacetic acid, for oxidation.

Non-Metal Based Reagents Notes | Study Organic Chemistry - Chemistry

Hydroxyl-Directed Epoxidation with Peroxy Acids 

Although epoxidation of cyclic alkenes occurs preferentially from the less hindered side, the presence of a polar substituent near the double bond may reverse the facial direction of attack by the peroxide. 

  • For example, the introduction of an allylic OH group onto the substrate provides a means for interaction with the peroxy acid by hydrogen bonding and thereby promotes stereoselective epoxidation. 
  • Cycloalkenes with an OH group at the allylic position are attacked by peroxy acids syn to the double bond to give the corresponding cis-epoxy alcohol diastereomers, even if that face is sterically more hindered. 
  • It is assumed that the directive effect of the hydroxyl group is due to hydrogen bonding between the hydroxyl group and the peroxy acid, stabilizing the transition state for epoxidation. 
  • If the OH group is replaced by an acetate group, a nearly equimolar mixture of diastereomeric epoxides is observed.

Non-Metal Based Reagents Notes | Study Organic Chemistry - ChemistryNon-Metal Based Reagents Notes | Study Organic Chemistry - Chemistry

Non-Metal Based Reagents Notes | Study Organic Chemistry - Chemistry

  • The hydroxyl-directed epoxidation of alkenes using peroxy acids is sensitive to the nature of the solvent. 
  • In the example below,  alkene epoxidation occurs on the more accessible a-face when Et2O is used as a solvent, whereas the use of CH2C12 promotes hydroxyl-directed epoxidation on the b-face of the alkene.

Non-Metal Based Reagents Notes | Study Organic Chemistry - ChemistryNon-Metal Based Reagents Notes | Study Organic Chemistry - Chemistry

5. Ozone [O3]

Ozonolysis is widely used both in degradation work to locate the position of double bonds and in the preparation of aldehydes, ketones, and carboxylic acids.

Non-Metal Based Reagents Notes | Study Organic Chemistry - Chemistry

Chemoselective Cleavage

The electrophilic ozone preferentially attacks a more nucleophilic double bond it is possible to achieve chemoselective cleavage in compounds containing two or more double bonds by limiting the amount of ozone. The relative reactivity of double bonds toward ozone decreases in the following order:

Non-Metal Based Reagents Notes | Study Organic Chemistry - ChemistryNon-Metal Based Reagents Notes | Study Organic Chemistry - Chemistry

The example depicted below indicates preferential cleavage of the electron-rich enol ether double bond over the trisubstituted one by the electrophilic ozone. Thus, Birch reduction of methoxy-substituted benzenes followed by ozonolysis of the resultant enol ethers provides a powerful route to functionally substituted (Z) alkenes.

Non-Metal Based Reagents Notes | Study Organic Chemistry - ChemistryNon-Metal Based Reagents Notes | Study Organic Chemistry - Chemistry

In the competition with a more nucleophilic trisubstituted double bond with a terminal disubstituted one, ozone exhibits a high preference for the former.

Non-Metal Based Reagents Notes | Study Organic Chemistry - Chemistry

Ozone is a l,3-dipolar species and undergoes 1,3-dipolar cycloadditions with alkenes.

Non-Metal Based Reagents Notes | Study Organic Chemistry - Chemistry

Non-Metal Based Reagents Notes | Study Organic Chemistry - Chemistry

There are two modes used to decompose ozonides:

1. Reductive: Me2S, Zn—AcOH, H2/Pd, KI-CH3OH, metal hydrides

2. Oxidative: H2O2

Non-Metal Based Reagents Notes | Study Organic Chemistry - Chemistry

6. HIO4 [periodic acid]

Cleavage of glycols derived from OsO4 or KMnO4 oxidation of alkenes by periodates or by periodic acid (HIO4·2H2O) results in two carbonyl fragments whose structures depend on the degree of substitution of the double bond.

Mechanism

The mechanism for periodate cleavage of glycols involves a cyclic periodate ester, which undergoes decomposition into two carbonyl fragments. Both cis- and trans- l,2-dihydroxycyclo-hexanes are cleaved but the trans-isomer at slower rate.

Non-Metal Based Reagents Notes | Study Organic Chemistry - ChemistryNon-Metal Based Reagents Notes | Study Organic Chemistry - Chemistry

Below are summarized some cleavage possibilities of 1,2-diols, an a-hydroxy ketone, and a 1,2-ketone with sodium periodate in EtOH—H2O.

Non-Metal Based Reagents Notes | Study Organic Chemistry - ChemistryNon-Metal Based Reagents Notes | Study Organic Chemistry - Chemistry

Non-Metal Based Reagents Notes | Study Organic Chemistry - ChemistryNon-Metal Based Reagents Notes | Study Organic Chemistry - Chemistry


7. N-Bromo Succinimide [NBS] 

Most important use of N-bromo succinimide is bromination at allylic, benzylic or propargylic position bromination.

Non-Metal Based Reagents Notes | Study Organic Chemistry - Chemistry

The mechanism of this bromination is free radical substitution there are two ways for this free radical substitution.

(1) NBS + hv or NBS/CCl4/Δ                             Non-Metal Based Reagents Notes | Study Organic Chemistry - Chemistry

The rate of bromination at propargylic position is more than that of allylic position.
The free radical substitution at bridgehead position can possible with NBS.  The free radical substitution at chiral carbon proceeds with retention in configuration.

Non-Metal Based Reagents Notes | Study Organic Chemistry - ChemistryNon-Metal Based Reagents Notes | Study Organic Chemistry - Chemistry

Non-Metal Based Reagents Notes | Study Organic Chemistry - ChemistryNon-Metal Based Reagents Notes | Study Organic Chemistry - Chemistry

Non-Metal Based Reagents Notes | Study Organic Chemistry - Chemistry

Non-Metal Based Reagents Notes | Study Organic Chemistry - Chemistry

If NBS is used in the presence of any nucleophilic source or any nucleophile then it will give the electrophilic addition reaction across the double bond through ionic mechanism via formation of Brominium ion.

Non-Metal Based Reagents Notes | Study Organic Chemistry - Chemistry

Non-Metal Based Reagents Notes | Study Organic Chemistry - ChemistryNon-Metal Based Reagents Notes | Study Organic Chemistry - Chemistry

Non-Metal Based Reagents Notes | Study Organic Chemistry - Chemistry

The addition of NBS with a double bond is according to the Markovnikov rule.

The document Non-Metal Based Reagents Notes | Study Organic Chemistry - Chemistry is a part of the Chemistry Course Organic Chemistry.
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