Chemistry Test - 16

Explanation:

Clemmensen reduction

→ The Clemmensen reduction is a reaction that is used to reduce aldehydes or ketones to alkanes using hydrochloric acid and zinc amalgam. The Clemmensen reduction is named after a Danish chemist, Erik Christian Clemmensen.

Zn(Hg) - conc. HCl is used as a reagent in this reaction.

This reaction is particularly effective in aryl-alkyl ketones reduction formed in Friedel-Crafts acylation. The reaction is more effective in the reduction of cyclic ketones or aliphatic and zinc metals.

Wolff - Kishner reduction-

→ Wolff Kishner's reduction mechanism begins with the formation of a hydrazone anion which then releases the nitrogen atom to form a carbanion.

→ This carbanion then reacts with the water in the system to give a hydrocarbon. Typically, diethylene glycol is used as a solvent for this method.

→ This reduction is an organic reaction where aldehydes and ketones are reduced to alkanes. Some carbonyl compounds are stable in strongly basic conditions, Hence they can be easily reduced to alkanes (The carbon-oxygen double bond becomes two carbon-hydrogen single bonds).

Explanation-

Silver mirror test-

→ The silver mirror test is used to distinguish aldehydes from ketones. Aldehydes give a positive silver mirror test. Ketones do not give this test.

→ In this test, Tollen's reagent is used which is ammoniacal silver nitrate solution. It acts as a mild oxidizing agent and oxidizes aldehyde to acetate ion.

→ Silver ion is reduced to metallic silver. This forms a bright silver mirror on the inner side of the container or it forms a grayish-black precipitate.

→ R-CHO + 2[Ag(NH₃)₂]⁺ + 3OH^- \(\xrightarrow{\Delta}\)  R - COO^- + 2Ag (ppt) + 2H₂O + 4NH₃

Acetone (a ketone) will not give the silver mirror test. 

→ The silver mirror test is given by aldehydes (such as Benzaldehyde and Acetaldehyde) or those alcohols (such as methanol) that can be easily oxidized to aldehydes. 

Explanation-

Claisen condensation-

The Claisen condensation reaction is an organic coupling reaction that results in the formation of a C-C bond between either a single ester and one carbonyl compound or between two esters.

The reaction proceeds when a strong base is present and the product of the reaction is a beta-keto ester or a beta-diketone.

The mechanism of the Claisen condensation reaction proceeds with the removal of an alpha proton through the action of a strong base to result in the formation of an enolate ion.

Given reaction and analysis-

The compound (A) can form oxime on reaction with NH₂​OH.

Since the molecule can involve simple proton transfer in an intramolecular fashion, it can undergo tautomerism, as it has alpha hydrogen present in it.

Iodoform test is used to prove the presence of methyl ketone.  Since there is a methyl ketone it shows the iodoform test.  

So all the statements are true.

Explanation-

Aldol Condensation-

→ Aldol condensation occurs in aldehydes having α-hydrogen with a dilute base to give β-hydroxy aldehydes called aldols. This reaction is most commonly known as aldol condensation.

→ If the condensation reaction occurs between two different carbonyl compounds it is called crossed aldol condensation.

→ Aldol Condensation can be defined as an organic reaction in which an enolate ion reacts with a carbonyl compound to form β-hydroxy ketone or β-hydroxy aldehyde, followed by dehydration to give a conjugated enone. Aldol Condensation plays a vital role in organic synthesis, creating a path to form carbon-carbon bonds.

The general reaction of aldol condensation is

→ C₆​H_5​CHO has no H and α− carbon. Thus, no aldol condensation reaction is given by this.

 Important Points

Crossed Aldol Condensation-

→ The condensation reaction between two different molecules of an aldehyde or ketone in a protic solvent such as water or alcohol constitutes the crossed aldol reaction. When condensation is between two different carbonyl compounds, it is called crossed aldol condensation.

→ When both aldehydes have alpha hydrogens, both can form carbanions and can also act as carbanion acceptors. Hence a mixture of four products is formed which has little synthetic value.

Explanation-

Formaldehyde-

Formaldehyde is the simplest aldehyde made of hydrogen, carbon, and oxygen with the formula CH₂O.

• Formaldehyde is one of a large family of chemicals known as volatile organic compounds which evaporates and becomes gaseous at room temperature.
• Formaldehyde is a reactive molecule and the first in the series of aliphatic aldehydes. It is one of the most important industrial chemicals. Formaldehyde is usually described as a gas, but it also exists dissolved in water or other solvents.

Hence At room temperature-, formaldehyde changes to trioxane which does not reduce Tollen's reagent.

Additional Information

•  Tollens Reagent refers to the chemical reagent which is used in the detection of an aldehyde functional group, an aromatic aldehyde functional group, or an alpha-hydroxy ketone functional group in a given test substance.

Explanation:

Explanation-

Enol-

Enol is “an organic compound that contains a hydroxyl group bonded to a carbon atom having a double bond and that is usually characterized by the grouping C=C(OH)”.

Resonance form-

Resonance structures are sets of Lewis structures that describe the delocalization of electrons in a polyatomic ion or a molecule

⇒ In Resonance form, the position of atoms remains unchanged, Only electrons shift

Additional InformationEnols are alkenes with a hydroxyl group attached to one of the double bonds’ carbon atoms. Enedials are alkenes containing hydroxyl groups on both sides of the double bond.

The enolic form of acetone contains 9-sigma, 1 pi-bond, and 2 lone pairs.

Acetone shows tautomerism with keto and enol form. Acetone is a carbon double bonded to oxygen. Then that carbon is further bonded to 2 more carbons which each have 3 hydrogens.

So when it goes to the enolic form, the oxygen becomes protonated and the double bond moves between the central carbon and one of the other carbons.

After this, if the number of bonds is counted, then-

 Central carbon has 1 pi and 3 sigmas. The double-bonded carbon has 2 sigmas. The other carbon has 3 sigmas. The oxygen has 1 sigma and 2 lone pairs. 

3+2+3+1=9 sigma and 1 pi and 2 lone pair.

The resonance form of the enolate ion will be .

Explanation-

Nucleophilic addition-

A nucleophilic addition reaction is a chemical addition reaction in which a nucleophile forms a sigma bond with an electron-deficient species.

These reactions are considered very important in organic chemistry since they enable the conversion of carbonyl groups into a variety of functional groups. Generally, nucleophilic addition reactions of carbonyl compounds can be broken down into the following three steps.

• The electrophilic carbonyl carbon forms a sigma bond with the nucleophile.
• The carbon-oxygen pi bond is now broken, forming an alkoxide intermediate (the bond pair of electrons are transferred to the oxygen atom).
• The subsequent protonation of the alkoxide yields the alcohol derivative.

The carbon-oxygen double bond is directly attacked by strong nucleophiles to give rise to the alkoxide. However, when weak nucleophiles are used, the carbonyl group must be activated with the help of an acid catalyst for the nucleophilic addition reaction to proceed.

The reaction of carbonyl compounds with LiAlH4​ and NaBH4​ is written as:

Explanation-

Hydration formation-

→ Hydrate Formation is a formation that occurs due to the reaction of water with hydrocarbons present in the reservoirs. These hydrocarbons include methane, ethane, propane or hydrogen sulfide, nitrogen, and carbon dioxide.

→ Hydrates are solid-shaped particles that can be compared to ice and can cause problems in the well operations.

→ These hydrates are solids that are formed when natural gas and water combine at low temperature and high pressure. This may happen in both oil and gas wells.

→ The rate of hydration formation depends upon the rate of nucleophilic addition.

Nucleophilic addition-

→ A nucleophilic addition reaction is a chemical addition reaction in which a nucleophile forms a sigma bond with an electron-deficient species. These reactions are considered very important in organic chemistry since they enable the conversion of carbonyl groups into a variety of functional groups.

→ More the e⁻−withdrawing group, more is the NA reactivity.

→ So electron-withdrawing group increases the reactivity.

So the correct order will be III < IV < I < II.

Concept:

Ozonolysis reaction-

• When ozone is passed into a solution of an alkene in presence of an inert solvent such as CCl4, ozonolysis takes place.
• The reaction occurs at very low temperatures such as -78°C.
• The mechanism is concerted cycloaddition.
• The first step is the addition of ozone to the alkene to form an ozonide.
• Ozonides are very unstable and hence cleaved directly with reductive or oxidative conditions to produce carbonyl compounds.
• The general mechanism is:

Explanation:

• When but-2ene is subjected to ozonolysis and then reduced in presence of dimethyl sulfide or Zn dust, it yields acetone and acetaldehyde.
• Cycloaddition of a molecule of ozone to form ozonides.
• The ozonide is then cleaved reductively by Zn to form acetaldehyde.
• The net reaction is:

Hence, the intermediate in the following reaction

CH3 - CH = CH - CH3 + O3 → CH3 - CHO is Ozonide.

Mistake Points

• Oxidative ozonolysis of an alkene yields a ketone and a carboxylic acid.