Chemistry Test - 13

Explanation-

The reaction R−OH + HX → RX + H₂​O is a nucleophilic substitution reaction where the reactivity of R−OH will depend upon the stability of carbocation that is formed during the reaction via the S_N​1 mechanism.

SN1 reaction-

• SN1 reaction mechanism follows a step-by-step process wherein first, the carbocation is formed from the removal of the leaving group.
• Then the carbocation is attacked by the nucleophile. Finally, the deprotonation of the protonated nucleophile takes place to give the required product.
• The rate-determining step of this reaction depends purely on the electrophilicity of the leaving group and is not impacted at all by the nucleophile.
• The carbocation intermediate formed in step 1 of the SN1 reaction mechanism is an sp2 hybridized carbon. Its molecular geometry is trigonal planar, therefore allowing for two different points of nucleophilic attack, left and right.

SN1 reaction involves the formation of a carbocation intermediate. More the stability of carbocation more is the reactivity towards SN1 reaction.

Tertiary carbocations are more stable due to the positive Inductive effect of alkyl groups then secondary and the least stable is primary carbocation.

The reactivity of alcohols will follow the order Tertiary alcohol > secondary alcohol > primary alcohol.

Explanation:

→ Alkyl fluorides are prepared by heating alkyl bromide or chloride in presence of metallic fluoride-like AgF, SbF_3, or Hg₂F₂. This reaction is known as the Swartz reaction.

→ CH3Br + AgF → CH3F + AgBr is an example of the Swartz reaction.

In Swartz's reaction:

• Higher alkanes are formed from lower alkyl halide.
• Alkanes with an even number of carbon atoms are formed.
• The reaction takes place in the presence of dry ether because sodium is a very reactive metal if it forms oxide with water.
• It is a simple nucleophilic substitution reaction.​

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Additional Information→ Alkyl chlorides or bromides when treated with NaI in presence of dry acetone give alkyl iodide. This reaction is known as the Finkelstein reaction.

→ 2 CH3Cl + 2 Na \(\xrightarrow{Dryether}\)CH3.CH3 + 2 NaCl, During this reaction Ethane is formed from methyl chloride. This is the Wurtz reaction.

Explanation-

SN1 reaction-

• S_N1 reaction mechanism follows a step-by-step process wherein first, the carbocation is formed from the removal of the leaving group.
• Then the carbocation is attacked by the nucleophile.
• Finally, the deprotonation of the protonated nucleophile takes place to give the required product.
• The rate-determining step of this reaction depends purely on the electrophilicity of the leaving group and is not impacted at all by the nucleophile.
• The carbocation intermediate formed in step 1 of the S_N1 reaction mechanism is an sp² hybridized carbon. Its molecular geometry is trigonal planar, therefore allowing for two different points of nucleophilic attack, left and right.
• If the reaction takes place at a stereocenter and if neither avenue for the nucleophilic attack is preferred, the carbocation is then attacked equally from both sides, yielding an equal ratio of left and right-handed enantiomers as shown below.

• Thus, the tertiary/secondary alkyl halides can react with tertiary/secondary alcohols to undergo a nucleophilic substitution reaction. The halide is replaced with the nucleophile in the product.
• S_N1 reaction involves the formation of a carbocation intermediate. More the stability of carbocation more is the reactivity of alkyl/aryl halides towards S_N1 reaction.
• C6H5C(CH3)(C6H5)Br → C6H5C⁺(CH3) + Br^-
• C6H5C+(CH3) has more stability so it is the most reactive towards S_N1 reaction.

Explanation-

Option 1- Reaction with alcoholic KOH

Here when it reacts with alcoholic KOH β-elimination reaction will take place and the final product will be formed as shown below.

Br-H₂C-CH₂-Br → H₂C=CH-Br

Option 2- Alcoholic KOH followed by NaNH2

Here when it reacts with alcoholic KOH β-elimination reaction will take place and the final product will be formed as shown below.

Br-H2C-CH2-Br → H2C=CH-Br

When it reacts with NaNH₂, it will extract hydrogen as NaNH₂ is a strong base. so the final product formed is shown below.

Explanation:

The allylic carbon atom is the sp³ hybridized carbon atom in the allylic group RCH₂-CH=CH₂ that is bonded with the -CH=CH₂ group.

For example in propene, the highlighted carbon atom is the allylic carbon atom (CH₃-CH=CH₂). Similarly, in cyclohexene, the carbon atoms next to the double bond are allylic carbon atoms.

Unlike the vinyl group, the allylic carbon atom is sp³ hybridized as it bonded with CH = CH₂ through a single covalent bond.

Both allyl and vinyl groups have slightly similar structures with a small variation.

The key difference between these two structural components is the number of carbon and hydrogen atoms.

Here Br is bonded with allylic carbon as explained above.

So it can be classified as Allyl.

Explanation-

Electrophilic aromatic substitution reactions-

Electrophilic aromatic substitution reactions are organic reactions wherein an electrophile replaces an atom that is attached to an aromatic ring.

Commonly, these reactions involve the replacement of a hydrogen atom belonging to a benzene ring with an electrophile.

The aromaticity of the aromatic system is preserved in an electrophilic aromatic substitution reaction.

For example, when bromobenzene is formed from the reaction between benzene and bromine, the stability of the aromatic ring is not lost.

Given reaction-

m-xylene on bromination with bromine in presence of ferric bromide gives 1-bromo-2,4-dimethylbenzene.

This is an electrophilic aromatic substitution reaction and since the methyl group is ortho para directing, the substitution occurs at a position that is ortho to one methyl group and para to other methyl groups.

This is represented by option 1 which shows a pair of identical compounds.

Explanation-

Elimination reaction-

An elimination reaction is a type of reaction that is mainly used to transform saturated compounds (organic compounds which contain single carbon-carbon bonds) to unsaturated compounds (compounds that feature double or triple carbon-carbon bonds).

The elimination reaction consists of three fundamental events, and they are;

• Proton removal.
• C-C pi bond is formed.
• There is a breakage in the bond of the leaving group.

Depending on the reaction kinetics, elimination reactions can occur mostly by two mechanisms namely E1 or E2 where E is referred to as elimination and the number represent the molecularity.

Given reaction-

C6H6Cl6 - Benzene Hexachloride

This will follow elimination reaction as explained above,and the reaction will be-

So the final product will be TrichloroBenzene.(C₆H₃Cl₃)

Explanation-

Haloalkanes-

• Haloalkanes are generally colorless and odorless compounds. 

• Boiling point –
  • The boiling point of haloalkanes is higher than alkanes if the number of carbon atoms is the same in both.
  • The boiling point of haloalkanes al so increases with the increasing number of halogens in haloalkanes. It means 1-Bromo-2-chloroethane will have a higher boiling point than chloroethane.
  • Boiling points of haloalkanes scale with the atomic weight of halides. Although fluoroalkanes are exceptions. They show a lower boiling point than their analogous alkanes. It is due to the lower polarizability of fluorine. The boiling point of 2- methylpropane is -11.7℃ while 2- fluoropropane is -10℃.
  • As the branching increases, boiling points of isomeric haloalkanes decrease. For example, the boiling point of 1-bromobutane is 375K while 2-bromopropane is 346K
  • The boiling point increases with an increase in the size of the hydrocarbon part for the same haloalkanes.

Given data and Analysis-

All the given haloalkanes and haloarenes contain the same halogen atom i.e., bromine but the number of carbon atoms in the hydrocarbon part of the molecule is increasing from ethane to benzene.

So, the boiling point is minimum for 1-bromoethane and maximum for 1-bromobenzene.

So the correct order will be 1-bromoethane < 1-bromopropane < 1-bromobutane < Bromobenzene.

Explanation-

Substitution reaction-

The substitution reaction is defined as a reaction in which the functional group of one chemical compound is substituted by another group or it is a reaction which involves the replacement of one atom or a molecule of a compound with another atom or molecule.

Substitution Reactions are of two types naming nucleophilic reaction and electrophilic reactions.

These two types of reactions mainly differ in the kind of atom which is attached to its original molecule. In the nucleophilic reactions the atom is said to be electron-rich species, whereas, in the electrophilic reaction, the atom is an electron-deficient species.

Given reaction-

It is a substitution reaction.

Here, −Cl is replaced because phenyl carbocation is highly unstable while benzyl is stable.

So the product formed is shown above.

Explanation-

Steric hindrance-

It is a consequence of steric effects. Steric hindrance is the slowing of chemical reactions due to steric bulk. It is usually manifested in intermolecular reactions, whereas discussion of steric effects often focuses on intramolecular interactions. Steric hindrance is often exploited to control selectivity, such as slowing unwanted side reactions.

Given reaction-

In the given reactant, the two groups which are present on the benzene rings (OCH₃​ and CH₃​) are electron-donating and ortho- para-directing groups.

 Thus they both will direct the Br group on ortho- and para-positions. But due to the steric hindrance, the compound given in option 2 will be a major product.