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Chapter 2: Problem 105

The number of structural and stereoisomers of a bromo compound \(\mathrm{C}_{5} \mathrm{H}_{9} \mathrm{Br}\) formed by the addition of \(\mathrm{HBr}\) to pent- 2 yne are respectively (a) 4 and 2 (b) 1 and 2 (c) 2 and 4 (d) 2 and 1

Short Answer

Expert verified
Answer: 2 structural isomers and 4 stereoisomers.

Step by step solution

01

Identify the starting compound

Draw the structure of starting alkene - pent-2-yne. It has the molecular formula C5H8 and the structure is CH3-C≡C-CH2-CH3. A triple bond is present between the second and third carbon atoms.
02

Add HBr to form a bromo compound

The addition of HBr to an alkyne results in an alkene, this is called a hydrohalogenation reaction. In this case, the alkyne is pent-2-yne and HBr is added, which will result in the formation of a bromoalkene.
03

Determine structural isomers

When HBr is added to pent-2-yne, the carbocation intermediate can be formed at two different positions, and then the Br- ion can attach to either of the carbocation sites. This will give two different structural isomers, which are: 1. 2-Bromo-2-pentene (Br is added to 2nd carbon and a double bond exists between carbon 2 and 3) 2. 3-Bromo-2-pentene (Br is added to 3rd carbon and a double bond exists between carbon 2 and 3) There are 2 structural isomers.
04

Determine stereoisomers

Each of the above structural isomers in step 3 can have two different spatial arrangements (stereoisomers) due to the possibility of cis (Z) and trans (E) configuration around the double bond. For trans: Terminal groups (Br and H) are on opposite sides and for cis: Terminal groups (Br and H) are on the same side 1. 2-Bromo-2-pentene : - E-2-Bromo-2-pentene - Z-2-Bromo-2-pentene 2. 3-Bromo-2-pentene : - E-3-Bromo-2-pentene - Z-3-Bromo-2-pentene There are 4 stereoisomers. Hence, the correct answer is (c) 2 structural isomers and 4 stereoisomers.

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Most popular questions from this chapter

Which of the following is the correct statement? (a) Acetylene is less reactive than ethylene in electrophilic addition reaction. (b) The hydrolysis of \(\mathrm{Mg}_{2} \mathrm{C}_{3}\) (Magnesium carbide) produces acetylene. (c) Ozonolysis of acetylene produces \(\mathrm{CO}_{2}\) and \(\mathrm{H}_{2} \mathrm{O}\). (d) Reduction of dialkyl acetylene using Lindlar's catalyst gives a trans olefin.

(A) and (B) are respectively (a) But-2-yne and but-2-ene (b) But-2-ene and but-2-yne (c) Butane and but-2-ene (d) But-2-ene and butane

\(\mathrm{CH}_{3}-\mathrm{CH}=\mathrm{CH}_{2}+\mathrm{Br}_{2} \rightarrow(\mathrm{A}) \stackrel{\mathrm{NaNH}_{2}}{\longrightarrow}(\mathrm{B}) \stackrel{\mathrm{H}^{+}}{\longrightarrow}(\mathrm{C}) .\) The structure of (C) is (a) \(\mathrm{CH}_{3} \mathrm{C} \equiv \mathrm{CH}\) (b) \(\mathrm{H}_{3} \mathrm{C}-\mathrm{C} \equiv \mathrm{C}-\mathrm{CH}_{3}\) (c) \(\mathrm{BrCH}_{2}-\mathrm{CH}_{2}-\mathrm{CH}_{2} \mathrm{Br}\) (d) \(\mathrm{H}_{2} \mathrm{C}=\mathrm{C}=\mathrm{CH}_{2}\)

(a) Write the structure of organic product formed by heating \(\mathrm{C}\left(\mathrm{CH}_{2} \mathrm{I}\right)_{4}\) with zinc dust. (b) How is buta- 1,3 -diene prepared from but- 1 -ene (c) What are the products expected when ethylene is passed into an aqueous solution of bromine containing dissolved sodium chloride?

Which of the following will have zero dipole moment? (a) Cis 1, 2 dichloroethene (b) 1,1 dichloroethene (c) Cis-2-butene (d) trans-1, 2 dichloroethene
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