Addition Reactions Questions

Addition reaction is a chemical reaction in which two or more reactants combine to produce a bigger product.

Our daily lives involve chemical reactions. When left exposed to the air for several days, iron items typically become flaky and brown in color. This is because iron combines with the air’s current moisture and oxygen to generate iron oxide. This is known as iron rusting.

Addition Reactions Chemistry Questions with Solutions

Q1: What is the function of peroxides in the addition of HBr to the alkenes?

Answer: The peroxides serve as the radical chain initiator.

Q2. Which of the following reactions of addition to the alkenes occurs in an anti-manner?

1. Hydroboration oxidation
2. Addition of Br₂
3. Addition of H₂
4. None of the above.

Answer: (b.)

Explanation: The addition of Br₂ takes place by the attack of the alkene on Br₂ forming a bromonium ion. The bromonium ion thus formed is attacked from the backside by the remaining bromide (Br^–) ion. Hence, resulting in an anti-product.

Q3. What is the test for the unsaturation of alkenes?

Answer: Bromine water is used for the detection of alkenes in the given solution. Bromine water indicates the presence of a double bond by forming dibromoethane due to which the original brown-red color of the solution fades away and the solution becomes clear.

Q4. Which reaction takes place when HCN reacts with a ketone?

Answer: The reaction is the nucleophilic addition reaction. The HCN reacts with the C=O double bond in the ketone. The CN^– ion attacks the electrophilic carbon of the C=O bond.

Q5. What is the correct IUPAC name of the following compound?

Answer: The given compound has two chiral carbons having S configurations. Both of these carbons have S- a configuration. Hence, the IUPAC name of the given compound is (1S, 3S)-1-chloro-3-methylcyclohexane.

Q6. Which intermediate has the highest energy?

Answer: Intermediate C has the highest energy. As the E_a required for the formation of C is much much higher than that in the case of E and the energy released as a result of the formation of C is also very less.

Q7. What is the significance of adding H₂SO₄ during the addition of H₂O to an alkene?

Answer: Since pure water is not acidic. Water is not capable of itself producing a strong enough electrophile (H⁺ ions) to initiate the reaction with the alkenes. Hence, sulphuric acid is mixed with water to make it acidic and to begin the reaction.

The addition of acidic water to an alkene leads to the formation of alcohols.

Q8. What happens when propene reacts with methanol in the presence of sulphuric acid?

Answer: Propene reacts with methanol in the presence of sulphuric acid to form 2-methoxypropane.

Q9. Consider the given S_N1 reaction:

(CH₃)₃C-Cl + I^– → (CH₃)₃C-I + Cl^–

What happens when the amount of I^– is doubled?

Answer: In the S_N1 reactions, the nucleophile is not included in the rate-determining step. Thus the rate does not change with the change in the nucleophile. Hence, no change occurs when the amount of I^– is doubled.

Q10. What is the relation between the given two structures?

Answer: The two given compounds are Diastereomers. Diastereomers are non-superimposable, non-identical stereoisomers. They have the same molecular formula and the same sequence of the bonding of atoms/ groups within the molecule but different spatial arrangements.

Q11. The specific rotation due to (R)-2-butanol is -13.5^o. If the specific rotation of the mixture is -5.4^o, what percentage of the mixture of the given two enantiomers will be the (S)-2-butanol?

Answer: In this case, the observed specific rotation is arising from the enantiomeric excess(e.e.). The percentage of e.e. can be calculated as:

% e.e. = [(-5.4^o) / (-13.5^o)] x 100 % = 40 %

Hence, (100 – 40) % = 60% of the remaining solution remains as the racemic mixture.

Thus, the total % of (R)-2-butanol = (40 + 60/2) % = 70%.

% of (S)-2-butanol = 100 % – 70 % = 30%.

Q12. Predict which of the following RX will undergo S_N2 reaction most quickly?

1. CH₃CH₂F
2. CH₃CH₂I
3. CH₃CH₂Br
4. CH₃CH₂Cl

Answer: (b.)

Explanation: During the S_n2 reactions, Nu^– attacks from the opposite of the leaving group. The easier and the faster the leaving group leaves, the quicker will be the addition of Nu^–. Among halogens, I^– ion is the best leaving group. Hence, CH₃CH₂I will react most quickly in S_N2 reactions.

Q13. What is the relation between the given structures?

1. Identical
2. Diastereomers
3. Structural isomers
4. Enantiomers

Answer: (b.)

Explanation: Both the structures have the same molecular formula and the same sequence of bonded atoms/ groups. but they are non-identical and non-mirror images of each other. Hence, the two compounds are Diastereomers.

Q14. Which among the following shall undergo S_N1 reaction most quickly?

1. CH₃CH₂CH₂CH₂Br
2. CH₃CH₂CH(CH₃)Br
3. C(CH₃)₃Br

Answer: (c.)

Explanation: The ease of the reaction depends on the stability of the carbocation formed as the reaction intermediate.

Q15. What is the driving force for the electrophilic addition reaction?

Answer: The driving force for the electrophilic addition reaction is the generation of an electrophile (E⁺) which attacks on the C=C. The E⁺ forms a covalent bond with the C=C leading to the formation of a carbocation.

The carbocation due to the positive charge density is then attacked by a nucleophile (Nu^–). The Nu^– forms another covalent bond with the carbocation.

Thus, in this process, 1 double bond breaks off and two new covalent bonds are formed.