Chemistry Test - 12

Explanation:

Reaction with Alkyl halides -

Alkyl halides goes for two reactions - Substitution and elimination reaction.

In substitution reaction the X group in R-X is replaced by different group i.e.

R-X → R-OH + X⁺

In elimination reaction, the elements of H-X are eliminated from R-X; the product is very often alkene.

Hence the correct option is Substitution.

Additional Information

Addition Reaction:

• It occurs when two reactants combine to form a single new product with no atoms left over.
• In addition reaction, new group X and Y are added to the starting material.
• A π bond is broken and two σ bond are formed.

Concept:

Nucleophilic substitution reactions-

Substitution reactions are the types of reactions where a nucleophile is the attacking reagent.

• There are three types of substitution reactions depending on the nature of the substrate.
  • Nucleophilic substitution at saturated carbon. 
  • Nucleophilic acyl substitution
  • Nucleophilic aromatic substitution.
• SN1 - Unimolecular nucleophilic substitution:
  • Depends upon the concentration of the substrate.
  • Is independent of the concentration of the nucleophile.
  • Follows first-order kinetics.
• SN2- bimolecular nucleophilic substitution.
  • the rate depends on the concentration of both the reactant and the substrate.
  • It follows second-order kinetics

Elimination reactions:

• Benzene and other aromatic compounds show characteristic electrophilic substitution reactions.
• In this reaction, a hydrogen atom of the aromatic ring is substituted by an electrophile.
• The substitution takes place through an addition-elimination mechanism.

Explanation:

Dow's Process: 

Dow’s process is a process by which phenol is prepared by reacting it with molten NaOH at very high-temperature conditions. The phenol formation mechanism by this method is called the benzyne mechanism since it involves the formation of the reactive intermediate Benzyne. 

Chlorobenzene is fused with NaOH at 623K and 300 atmospheric pressure giving sodium phenoxide (C6H5O-Na+).

Phenol is obtained by the acidification of sodium phenoxide so produced.

The given reaction is well known as Dows process, for the preparation of phenols.

As we can see that the OH-ion is the attacking reagent in the process, so it is a nucleophilic substitution reaction.

Correct Answer: 3)

Explanation:

• On heating with an ethanolic solution of silver nitrite, alkyl halides yield nitroalkanes.
• Some of the alkyl nitrites are also formed because nitrite is an ambidentate ligand.
• It has a lone pair of electrons on nitrogen as well as oxygen.
• Thus it can be bonded to an alkyl group of alkyl halide through oxygen to form alkyl nitrite and via Nitrogen to form nitroalkane.
• The reactions take place as follows:

• Hence, the treatment of ethyl bromide with alcoholic silver nitrite gives Nitroethane.

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• Alkyl halides when treated with sodium or potassium salt of nitrite give alkyl nitrite as the major product.

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Explanation:-

The reaction of alcoholic KOH:-

• Alcoholic KOH dissociates in water to give RO- ions.
• RO- ions are strong bases of chemical nature.
• It is abstracted to give hydrogen and is further accompanied by an elimination reaction.
• Alcoholic KOH is used for dehydrohalogenation reactions.
• It is also used to form alkyl halides but aqueous KOH is used to form alcohols from alkyl halides.

Additional InformationBeta- Elimination Reaction:-

A chemical reaction in which atoms or the groups of atoms are detached from the adjacent atoms results in a pi-bond formation between the product.

Concept:

• Alkenes react with a solution of halogen and water to form halohydrins.
• Halohydrins are compounds having a hydroxyl group and a halogen group adjacent to each other.
• When a mixture of Cl₂ + H₂O is taken, it forms hypohalous acids with general form X-OH, where X is the halogen (here, Cl).
• The halogen part gets a positive charge and behaves as an electrophile whereas the hydroxyl group OH^- behaves as a nucleophile.
• The reaction takes place following Markownikoff's rule which states that: 'In an electrophilic addition reaction of an unsymmetrical alkene with an unsymmetrical addendum (reagent), the positive part of the addendum adds to the less substituted carbon and the negative part adds to the more substituted carbon of the alkene.
• The general reaction is:

Explanation:

1. However, the reaction  \(\rm CCl_3CH=CH_2 \xrightarrow{Cl_2 \ + \ H_2O} X\) does not take place by Markownikoff's rule because of the presence of electron-withdrawing group Cl.

• The reaction proceeds via:

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• The positive part cl gets attached to the more substituted carbon and OH gets attached to the less substituted part.
• Hence, the product is:
• Additional Information
• Kharash discovered that the addition of HBr to unsymmetrical alkenes in presence of organic peroxides takes place against Markownikoff's rule.
• The more saturated carbon gets the positive part of the addendum whereas the less saturated carbon atom gets the negative part of the addendum when an addition reaction of an unsymmetrical alkane takes place in presence of peroxide. this is known as Anti - Markownikoff's rule.

Explanation:-

​Reaction:-

C3H7Cl + H2 \(\rm \xrightarrow{A\ , \ H^+} \) C3H8 + HCl

Here, A =  Cu 

Copper as a catalyst:-

• In the chemical industry, copper-based catalysts are commonly used to convert water and carbon monoxide to hydrogen, carbon dioxide, and methanol.
• Although theoretical models have been developed to explain this response, a thorough understanding of the process has remained elusive.
• Copper, with its varied oxidation states Cu2+ and Cu³⁺, is an excellent example of a transition metal.
• The metal's imperfect d-orbital allows it to permit electron exchange.
• Transition metals are excellent catalysts because they can easily give and accept electrons.

Additional Information

1-Chloropropane:-

• 1-chloropropane is a colorless, transparent liquid. 46.6°C is the boiling point.
• Below 0°F, there is a risk of a flashback.
• It has a lower density than water and is only marginally soluble in it.
• These are van der Waals dispersion forces and dipole-dipole attractions in 1-chloropropane.
• The formula of 1-chloropropane is C3H7Cl.

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Explanation-

Wurtz reaction-

• Wurtz’s reaction is an organic chemical coupling reaction wherein sodium metal is reacted with two alkyl halides in the environment provided by a solution of dry ether in order to form a higher alkane along with a compound containing sodium and the halogen.

The general form of the Wurtz reaction equation can be written as follows:

2R-X + 2Na → R-R + 2Na⁺ X^–

• It can be observed from this equation that the two R groups are joined, yielding an alkane with a longer chain along with NaX, where X is a Halogen.

Given data and Analysis-

\(2\rm C_2H_5Br +Na\xrightarrow{Na/Dry\ ether}C_2H_5-C_2H_5 +2NaBr\)

→ It can be observed from this equation that the two R groups are joined, yielding an alkane with a longer chain along with NaBr, where Br is a Halogen.

So it is Wurtz's reaction.

 Important Points

Difference between Wurtz and Ullmann reaction-

• The Wurtz–Fittig reaction is the chemical reaction of aryl halides with alkyl halides and sodium metal in the presence of dry ether to give substituted aromatic compounds as their products.
• The Ullmann reaction or Ullmann coupling is a coupling reaction between aryl halides and copper.

Additional Information

Sandmeyer's reaction-

Sandmeyer reaction is a type of substitution reaction that is widely used in the production of aryl halides from aryl diazonium salts. Copper salts like chloride, bromide, or iodide ions are used as catalysts in this reaction.

Notably, the Sandmeyer reaction can be used to perform unique transformations on benzene. The transformations include hydroxylation, trifluoromethylation, cyanation, and halogenation.

Williamson's synthesis

• The general method for the synthesis of ether is Williamson ether synthesis, which involves nucleophilic displacement of a halide ion or other good leaving group by an alkoxide ion.
• The basic mechanism of the reaction is: R-O- + R'-X → R-O-R + X–

Ullmann reaction

The Ullmann reaction (also known as Ullmann coupling) is an organic named reaction that involves the coupling of two aryl halides in the presence of copper to yield a biaryl as the product.

The general format of the Ullmann reaction is illustrated below.

Concept:

• Electrophilic aromatic substitution (EAS) is where benzene acts as a nucleophile to replace a substituent with a new electrophile. That is, benzene needs to donate electrons from inside the ring. An electrophile attacks the region of high electron density.
• Activating group is that which increases the rate of an electrophilic aromatic substitution reaction, relative to hydrogen.CH₃ is a perfect example of an activating group; when we substitute a hydrogen on benzene for CH₃, the rate of nitration is increased.
• A deactivating group, on the other hand, decreases the rate of an electrophilic aromatic substitution reaction, relative to hydrogen. The trifluoromethyl group, F₃, drastically decreases the rate of nitration when substituted for a hydrogen on benzene.
• This definition is ultimately based on experimental reaction rate data.  It doesn’t tell us why each group accelerates or decreases the rate. “Activating” and “deactivating” just refers to the effect of each substituent on the rate, relative to H.
• CH₃ is an activating group because of its +I effect, −Cl and −NO₂ are deactivating due to their −I and −M effect. −CH₃ group, having +I effect, increases the electron density in the benzene ring whereas −Cl group having −I effect decreases the electron density the benzene ring.

Calculation:

→ Order of strength for electrophilic substitution

+ M > + I > - I > - M

→ SE ∝ EDG

\(SE \propto \frac{1}{{EWG}}\)

Where, EDG is Electron Drawing Group,

EWG is Electron Withdrawing Group

→ Thus, the increasing order of reactivity of the given compounds towards aromatic electrophilic substitution reaction is

D (-M) < A (-I) < C (+I) < B (+M)

Explanation:

We know DDT is dichloro-diphenyl-trichloroethane.

From the name, it is significant that for an ethane group there will be 3 chlorine, and two phenyl group has chlorine attached to it.

So the above description matches with option 2

Explanation:

⇒ On refluxing alkaline solution of phenol with CHCI₃ at 330 K, and on subsequent acidification, o-hydroxy benzaldehyde (salicylaldehyde) is obtained.

⇒  It is called the Reimer-Tiemann reaction, o-isomer predominates but is separated from p-isomer by steam distillation.

If the "o-position" is already occupied, the aldehyde group goes to the "p-position"