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Chapter 9: Problem 31

Show by writing appropriate chemical equations how each of the following compounds could be converted to 3 -hexyne: (a) 1-Butene (b) 1,1 -Dichlorobutane (c) Acetylene

Short Answer

Expert verified
Convert initial compounds to 1-butyne, and then to 3-hexyne via nucleophilic substitution with ethyl bromide.

Step by step solution

01

Convert 1-Butene to 1-Butyne

1-Butene, a linear alkene, can be converted to 1-butyne, an alkyne, by first converting it to a dihalide and then performing a double elimination. First, add Br2 to form 1,2-dibromobutane: \[ \text{CH}_3\text{CH}_2\text{CH} = \text{CH}_2 + \text{Br}_2 \rightarrow \text{CH}_3\text{CH}_2\text{CHBr}-\text{CH}_2\text{Br} \]. Then, treat with a strong base like sodium amide (NaNH2) to perform a double dehydrohalogenation, forming 1-butyne: \[ \text{CH}_3\text{CH}_2\text{CHBr}-\text{CH}_2\text{Br} + 2\text{NaNH}_2 \rightarrow \text{CH}_3\text{C} \equiv \text{CH} + 2\text{NaBr} + 2\text{NH}_3 \].
02

Convert 1-Butyne to 3-Hexyne

React 1-butyne with sodium amide (NaNH2) to generate the acetylide ion: \[ \text{CH}_3\text{C} \equiv \text{CH} + \text{NaNH}_2 \rightarrow \text{CH}_3\text{C} \equiv \text{C}^-\text{Na}^+ + \text{NH}_3 \]. Then, perform a nucleophilic substitution with ethyl bromide: \[ \text{CH}_3\text{C} \equiv \text{C}^-\text{Na}^+ + \text{CH}_3\text{CH}_2\text{Br} \rightarrow \text{CH}_3\text{C} \equiv \text{CCH}_2\text{CH}_3 + \text{NaBr} \]. This yields 3-hexyne.
03

Convert 1,1-Dichlorobutane to But-1-yne

React 1,1-dichlorobutane with excess sodium amide (NaNH2) to perform a double dehydrohalogenation, converting it to but-1-yne: \[ \text{CH}_3\text{CH}_2\text{CHCl}_2 + 2\text{NaNH}_2 \rightarrow \text{CH}_3\text{C} \equiv \text{CH} + 2\text{NaCl} + 2\text{NH}_3 \].
04

Convert But-1-yne to 3-Hexyne as in Step 2

Following the same procedure as in Step 2, react but-1-yne with sodium amide (NaNH2) to form the acetylide ion, then perform a nucleophilic substitution with ethyl bromide: \[ \text{CH}_3\text{C} \equiv \text{C}^-\text{Na}^+ + \text{CH}_3\text{CH}_2\text{Br} \rightarrow \text{CH}_3\text{C} \equiv \text{CCH}_2\text{CH}_3 + \text{NaBr} \]. Thus, obtaining 3-hexyne.
05

Convert Acetylene to 1-Butyne

React acetylene with sodium amide (NaNH2) to produce acetylide ion: \[ \text{HC} \equiv \text{CH} + \text{NaNH}_2 \rightarrow \text{HC} \equiv \text{C}^-\text{Na}^+ + \text{NH}_3 \]. Then, add an ethyl group using ethyl bromide: \[ \text{HC} \equiv \text{C}^-\text{Na}^+ + \text{CH}_3\text{CH}_2\text{Br} \rightarrow \text{CH}_3\text{C} \equiv \text{CH} + \text{NaBr} \].
06

Convert 1-Butyne to 3-Hexyne as in Step 2

Using the method from Step 2, convert 1-butyne to 3-hexyne by generating the acetylide ion with sodium amide (NaNH2), then reacting it with ethyl bromide: \[ \text{CH}_3\text{C} \equiv \text{C}^-\text{Na}^+ + \text{CH}_3\text{CH}_2\text{Br} \rightarrow \text{CH}_3\text{C} \equiv \text{CCH}_2\text{CH}_3 + \text{NaBr} \].

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Dehydrohalogenation
Dehydrohalogenation is a key reaction in organic chemistry used to form alkynes. It involves the elimination of hydrogen halides (HX) from a molecule. This process converts dihalides (compounds with two halogen atoms) into alkynes by removing the halogen and adjacent hydrogen atoms.
This reaction is typically performed in the presence of a strong base such as sodium amide (\( \text{NaNH}_2 \)).
  • In the exercise, we see how 1-butene is first converted into a dihalide through the addition of bromine (\( \text{Br}_2 \)) to form 1,2-dibromobutane.
  • Subsequently, treating it with (\( \text{NaNH}_2 \)) induces a double elimination, creating 1-butyne, an alkyne.
The reaction is split into two key eliminations: one for each halogen-hydrogen pair. The mechanism strengthens the triple bond of the resulting alkyne. Understanding the conditions and steps for dehydrohalogenation helps in effectively synthesizing different alkynes from dihalides.
Nucleophilic Substitution
Nucleophilic substitution reactions are fundamental to transforming molecules. In this reaction type, a nucleophile replaces a leaving group on a molecule.
  • In the context of synthesizing 3-hexyne, nucleophilic substitution is used after dehydrohalogenation.
  • For example, after converting 1-butyne into its sodium acetylide, this ion reacts with ethyl bromide.
The acetylide ion acts as the nucleophile, attacking the electrophilic carbon bonded to bromine (a good leaving group). In the process, the acetylide ion displaces the bromine atom, forming 3-hexyne.
This step embodies the significance of nucleophilic substitution in constructing carbon-carbon bonds, particularly when aiming to elongate carbon chain structures.
Acetylide Ion
The acetylide ion is an essential intermediate created during the synthesis of alkynes. It is formed by deprotonating a terminal alkyne using a strong base like sodium amide (\( \text{NaNH}_2 \)). This ion is a powerful nucleophile due to the negative charge on the carbon.
  • In our exercise, the transformation of 1-butyne or acetylene to its sodium acetylide ion is a critical step.
  • This ion enables the subsequent nucleophilic attack on an alkyl halide, like ethyl bromide, to build a longer alkyne chain.
Understanding the properties of the acetylide ion is pivotal for managing reactions involving alkynes. Its ability to react with different electrophiles underpins many synthesis strategies in organic chemistry.
Chemical Equations
Chemical equations offer a concise way to describe chemical reactions. They use chemical formulas and symbols to convey information about reactants, products, and reaction conditions.
  • In alkyne synthesis, equations illustrate each step, from dehydrohalogenation to nucleophilic substitution.
  • For example, \( \text{CH}_3\text{CH}_2\text{CH} = \text{CH}_2 + \text{Br}_2 \rightarrow \text{CH}_3\text{CH}_2\text{CHBr}-\text{CH}_2\text{Br} \) shows bromine addition to 1-butene, and \( \text{CH}_3\text{C} \equiv \text{CH} + 2\text{NaNH}_2 \rightarrow \text{CH}_3\text{C} \equiv \text{CH} + \text{2NaBr} + 2\text{NH}_3 \) represents the double dehydrohalogenation.
Learning to interpret and write chemical equations helps visualize the flow and conversion of molecules throughout a reaction. This skill is crucial for predicting reaction outcomes and constructing complex organic molecules efficiently.

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Most popular questions from this chapter

Show, by writing an appropriate series of equations, how you could prepare propyne from each of the following compounds as starting materials. You may use any necessary organic or inorganic reagents. (a) 2 -Propanol (d) 1,1-Dichloroethane (b) 1 -Propanol (e) Ethyl alcohol (c) Isopropyl bromide

Write structural formulas and give the IUPAC names for all the alkynes of molecular formula \(\mathrm{C}_{5} \mathrm{H}_{8}\)

Show by writing appropriate chemical equations how each of the following compounds could be converted to 1 -hexyne: (a) 1,1 -Dichlorohexane (c) Acetylene (b) 1 -Hexene (d) 1-Iodohexane

Write structural formulas for all the alkynes of molecular formula \(\mathrm{C}_{8} \mathrm{H}_{14}\) that yield 3 -ethylhexane on catalytic hydrogenation.

Assume that you need to prepare 4-methyl-2-pentyne and discover that the only alkynes on hand are acetylene and propyne. You also have available methyl iodide, isopropyl bromide, and 1,1 -dichloro-3-methylbutane. Which of these compounds would you choose in order to perform your synthesis, and how would you carry it out?
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