Question

In each reaction box, place the best reagent and conditions from the list below. If you reduce the alkyne in step 3, you would need six steps overall to produce the final carboxylic acid.

Answer 1

General guidance

Concepts and reason

• One or more substances that react together to form a product is called a chemical reaction. Most chemical reactions take place with the help of reagents.

• Reagents are the substances used to transform one form of a substance to another.

• The reaction undergoes a series of steps like deprotonation of an alkyne, substitution, and hydrogenation reactions to attain a final product.

Fundamentals

The reagents play a vital role in all the reactions. Each reagent plays a different role in the chemical reaction.

Deprotonation of alkynes:

An alkyne undergoes deprotonation with the help of the reagent sodium amide$\\left( {{\\rm{NaN}}{{\\rm{H}}_{\\rm{2}}}} \\right)$. Acetylide ion forms when sodium amide reacts with an alkyne. Acetylide ion acts as a good nucleophile which undergoes a substitution reaction with alkyl halides and forms a new carbon chain.

Substitution:

The nucleophilic substitution reaction $\\left( {{{\\rm{S}}_{\\rm{N}}}{\\rm{2}}} \\right)$ takes place when the nucleophile undergoes reaction with alkyl halides.

Hydroboration of alkenes:

Borane in THF is a reagent which is used for the hydroboration of alkenes to give alkyl borane (organoboron) which is treated with \\[{\\user2{H}_\\user2{2}}{\\user2{O}_\\user2{2}}\\user2{, O}{\\user2{H}^\\user2{ - }}\\] forms alcohol.

Oxidation of aldehydes:

Aldehydes are oxidized to carboxylic acids using a suitable reagent such as, \\[\\user2{Cr}{\\user2{O}_\\user2{3}}\\user2{, }{\\user2{H}_\\user2{2}}\\user2{S}{\\user2{O}_\\user2{4}}\\user2{ \\& }{\\user2{H}_\\user2{2}}\\user2{O}\\].

Step-by-step

Step 1 of 4

NaNH2 acetylene synthetic intermediate product 1

Explanation | Common mistakes | Hint for next step

In the given reaction, acetylene is a starting material which reacts with sodium amide$\\left( {{\\rm{NaN}}{{\\rm{H}}_{\\rm{2}}}} \\right)$, undergoes deprotonation and becomes a very good nucleophile which in turn makes the substitution reaction faster.

Step 2 of 4

Explanation | Common mistakes | Hint for next step

Acetylide ion acts as a nucleophile, undergoes a backside $\\left( {{{\\rm{S}}_{\\rm{N}}}{\\rm{2}}} \\right)$attack with alkyl halide (1 \u2013 bromopropane), and forms a synthetic intermediate 2 by leaving a bromide ion.

Step 3 of 4

The reaction is given below:

1. BH3, THF 2. H202, NaOH, H20 synthetic intermediate product 2 synthetic intermediate product 3

Explanation | Common mistakes | Hint for next step

The first step in the reaction is a concerted same side addition of over the carbon-carbon triple bond in the synthetic intermediate product. \\[\\user2{B}{\\user2{H}_\\user2{2}}\\] adds to the carbon with more number of hydrogens, while H adds to the carbon with less number of hydrogen. \\[\\user2{B}{\\user2{H}_\\user2{3}}\\] acts as an electrophile because the boron having an empty p orbital makes it an electron deficient species. After the first step, the oxidation of concerted addition product gives aldehyde. It follows the anti-Markovnikov\u2019s rule.

BH,-H

Step 4 of 4

CrO3, H2SO4, H20 \u043e\u043d synthetic intermediate product 3 pentanoic acid target product

The best reagent and conditions for the given reaction:

1) NaNH2 2) CH CH2CH2Br 3) BH3, THF \u043e\u043d 4) H202, H2SO4, H2o 5) CrO3, H2SO4, H20

The target product is obtained by using \\[\\user2{Cr}{\\user2{O}_\\user2{3}}\\user2{, }{\\user2{H}_\\user2{2}}\\user2{S}{\\user2{O}_\\user2{4}}\\user2{ \\& }{\\user2{H}_\\user2{2}}\\user2{O}\\] reagents on the synthetic intermediate product 3. This reaction is known as oxidation reaction.

Answer

The best reagent and conditions for the given reaction:

1) NaNH2 2) CH CH2CH2Br 3) BH3, THF \u043e\u043d 4) H202, H2SO4, H2o 5) CrO3, H2SO4, H20

Answer only

The best reagent and conditions for the given reaction:

1) NaNH2 2) CH CH2CH2Br 3) BH3, THF \u043e\u043d 4) H202, H2SO4, H2o 5) CrO3, H2SO4, H20