Question 1

The step reaction polymerizations are fast polymerization reactions

Accepted Answer

Step reaction polymerizations, also known as condensation polymerizations, typically proceed at a slower rate compared to chain growth polymerizations because they involve the gradual reaction of functional groups, often with the elimination of small molecules like water, and require the diffusion of larger and larger molecules as the polymer grows.

Question 2

SN₂ reactions are nucleophilic substitution reactions for which the rate determining step is bimolecular.

Accepted Answer

The given reactions can be classified as nucleophilic substitution reactions (S N ) and the rate determining step is bimolecular, involving the collision of both the nucleophile and substrate.

Question 3

The alkanes are known for their reactivity

Accepted Answer

Alkanes are known for their low reactivity as they have a stable and nonpolar C-C and C-H bond. They undergo reaction mainly through radical reactions under extreme conditions of high heat or UV radiation.

Question 4

The following can be said about the reaction profile of an SN1 reaction: 
I. The rate determining step is formation of carbocation.
II. The reaction profile has one transition state.
III. The slow step is the first transition state.
IV. The second step is attack of electrophile on carbocation.
V.The reaction proceeds in a polar aprotic solvent.

Accepted Answer

I. This is true because the formation of carbocation is the slowest step and therefore is the rate-determining step.
II. This is false. SN1 reactions have two transition states: one between the starting material and the carbocation intermediate, and one between the carbocation intermediate and the final product.
III. This is true because the slow step is the rate-determining step, which is the formation of the carbocation intermediate.
IV. This is true because the second step of the reaction is the attack of the nucleophile on the carbocation intermediate.
V. This is not necessarily true. SN1 reactions can occur in both polar protic and polar aprotic solvents.

Question 5

Which of the following statements correctly describe SN₂ and SN₁reactions? I. SN₂ reactions proceed with retention of configuration. II. SN₁ reactions prefer polar protic solvents III. SN₁ reactions produce racemic products IV. SN₂ reactions are promoted in the presence of a substrate that produces a very stable carbocation V.SN₁ reactions have a unimolecular rate-determining step.

Accepted Answer

SN1 reactions prefer polar protic solvents and produce racemic mixtures due to the planar intermediate. SN1 reactions have a unimolecular rate-determining step.

Question 6

In a substitution reaction an atom, an ion or a group in one molecule is replaced by another atom, ion or group

Accepted Answer

This is the definition of a substitution reaction.

Question 7

Which of the following trends in nucleophilicity are correct? 
I. Everything else being equal, a negatively charged nucleophile will react faster than a neutral one.
II. Generally, species containing elements of high electronegativity are good nucleophiles.
III. Anionic nucleophiles are more reactive in polar and aprotic solvents than in polar and protic solvents
IV. Generally, nucleophilicity is increasing with the increase in size of an atom.
V. Bulky groups adjacent to the nucleophilic atom enhance the nucleophilicity and reactivity of the species.

Accepted Answer

I. Everything else being equal, a negatively charged nucleophile will react faster than a neutral one. This is because a negatively charged nucleophile has more electron density and is thus more likely to react with an electrophile.
II. Generally, species containing elements of high electronegativity are good nucleophiles. This statement is false since electronegativity reflects the ability of an atom to attract electron density towards itself and not give electron density to other atoms.
III. Anionic nucleophiles are more reactive in polar and aprotic solvents than in polar and protic solvents. This is because protic solvents can hydrogen bond with anionic species, reducing their nucleophilicity, while aprotic solvents cannot do so.
IV. Generally, nucleophilicity is increasing with the increase in size of an atom. This is because larger atoms are less electronegative, and their valence electrons are farther away from the nucleus, making them more easily accessible to participate in chemical reactions.
V. Bulky groups adjacent to the nucleophilic atom enhance the nucleophilicity and reactivity of the species. This is because bulky groups can shield the electronegative atoms from the solvent, which reduces their ability to accept a proton and thus enhances their nucleophilicity.

Question 8

Synthesis of alkanes from alkyl halides is a typical example of E2 reaction.

Accepted Answer

Synthesis of alkanes from alkyl halides is a typical example of SN2 reaction, not E2 reaction. In SN2 reaction, the alkyl halide is attacked by a nucleophile, leading to the formation of an alkane.

Question 9

Which of the following steps can be found in E1 reaction mechanism? 
I. Formation of a carbocation
II. Attack of the base and removal of a proton from carbocation
III. Attack of a nucleophile
IV. Formation of a carboanion

Accepted Answer

In an E1 reaction, the first step is the formation of a carbocation (I) through the loss of a leaving group. Then, in the second step, a base attacks the carbocation and removes a proton, leading to the formation of a double bond (II). Attack of a nucleophile (III) and formation of a carboanion (IV) are not involved in E1 mechanism.

Question 10

We can promote SN₂ over SN₁reactions if we: I. use a substrate that gives a primary carbocation and not tertiary carbocation II. use a sterically hindered substrate III. use a polar protic solvent

Accepted Answer

Using a substrate that gives a primary carbocation favors SN2 reactions because primary carbocations are less stable and less likely to form, thus avoiding SN1 pathways.

Question 11

Arrange the following carbocations in order of decreasing stability:

Accepted Answer

The stability of a carbocation depends on the number of alkyl groups attached to the positively charged carbon. The more alkyl groups there are, the more stable the carbocation. In option A, carbocation A has only one methyl group attached to the positively charged carbon, which makes it the least stable. Carbocation C has three methyl groups, making it the most stable. Carbocation B has two methyl groups and is intermediate in stability between A and C. In option B, the order is reversed, which is incorrect. Option C has the same order as option A, which is also incorrect. Option D has the correct order, with carbocation C being the most stable, followed by A and then B. Option E is incorrect as it puts B as the most stable carbocation which is not true.

Question 12

Organic reactions in which atoms or groups that are bonded to adjacent carbon atoms are eliminated are called elimination reactions

Accepted Answer

This is true. Elimination reactions involve the removal of atoms or groups from adjacent carbons in a molecule, resulting in the formation of a double bond. Examples of elimination reactions include dehydration, dehydrohalogenation, and beta-elimination.

Question 13

Molecules add across multiple bonds in another molecule in addition reactions

Accepted Answer

In addition reactions, molecules add across multiple bonds in another molecule.

Question 14

Arrange the following in the order of decreasing nucleophilicity: H₂N^-, HO^-, HCOO^-, CH₃O^-

Accepted Answer

Nucleophilicity generally increases with a decrease in electronegativity and steric hindrance. H₂N⁻ is the strongest nucleophile due to low electronegativity and lack of steric hindrance. CH₃O⁻ is more nucleophilic than HO⁻ due to the electron-donating effect of the methyl group. HCOO⁻ is the least nucleophilic due to resonance stabilization.

Question 15

Finish the sentence: "Elimination reactions....

Accepted Answer

Elimination reactions can follow two different mechanisms: E1 and E2.

Question 16

Nucleophiles are electron poor atoms, ions or groups

Accepted Answer

Nucleophiles are electron rich atoms, ions or groups that are attracted to positively charged atoms or molecules.

Question 17

Find the combination of conditions that are the best for SN₂ reactions.

Accepted Answer

SN2 reactions are favored by primary substrates, polar aprotic solvents, and strong nucleophiles.

Question 18

For which type of reaction is the following an example?

Accepted Answer

The answer of For which type of reaction is the...

Question 19

Reactions between alkanes and HX compounds are typical examples of addition reactions.

Accepted Answer

Reactions between alkanes and HX compounds are examples of substitution reactions, not addition reactions, because in these reactions, a hydrogen atom in the alkane is replaced (substituted) by a halogen atom from the HX compound.

Question 20

Electrophilic substitution reactions are typical reactions of aromatic compounds.

Accepted Answer

Electrophilic substitution reactions involve the addition of electrophiles to the aromatic ring of a compound, resulting in the substitution of one of the hydrogen atoms on the ring. This type of reaction is typical of aromatic compounds.

Quiz 27: Reactions of Organic Compounds