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9.4.3. Preparation
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Good morning, class! Today, we’re starting our journey into alkynes. Can anyone tell me what alkynes are?
Alkynes are hydrocarbons that contain a triple bond between carbon atoms, right?
Exactly! And what’s the general formula for alkynes?
I think it's CnH2n−2?
Great job! This formula helps us understand the relationship between alkynes and alkanes. Now, what’s an example of a simple alkyne?
That would be ethyne, commonly known as acetylene!
Correct! It’s widely used in welding. Now, let’s talk about how we can prepare these compounds.
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First, let’s discuss preparation from calcium carbide. How do we form ethyne from calcium carbide?
We combine it with water, right? It produces acetylene and calcium hydroxide.
That’s right! The reaction is CaC2 + 2H2O → Ca(OH)2 + C2H2. Now, what can you tell me about using vicinal dihalides in alkyne synthesis?
They can undergo dehydrohalogenation to form alkenes first, and then we can convert those alkenes into alkynes!
Excellent! This two-step process shows the versatility in synthesizing alkynes. Can anyone summarize these steps?
Sure! First, we eliminate hydrogen halide from vicinal dihalides using KOH, then treat the resulting alkene with sodium amide to yield an alkyne.
Well explained! Just remember this multi-step approach when studying synthesis.
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Create a free accountFinally, let’s touch on how alkynes are used in various industries. Why do you think they are important?
They’re used in welding, making plastics, and even in pharmaceuticals!
Exactly! Their versatility makes them essential in many applications. Can someone elaborate on their usage in welding?
Sure! Acetylene produces a very hot flame when burned in oxygen, making it perfect for melting metal.
Perfect understanding! As a recap, we explored the methods of preparation for alkynes and their significant roles across different fields. Remember these key points for your upcoming assessments.
Overview
Short Summary
This section discusses the preparation methods for alkynes, including reactions from calcium carbide, vicinal dihalides, and other relevant chemical processes.
Medium Summary
The preparation of alkynes is explored through various chemical methods such as the reaction of calcium carbide with water to form acetylene and the dehydrohalogenation of vicinal dihalides. These methods are essential for synthesizing alkynes in both laboratory and industrial settings.
Detailed Summary
Preparation of Alkynes
Alkynes are unsaturated hydrocarbons characteristically containing at least one triple bond between carbon atoms. One practical source of alkynes, particularly ethyne (or acetylene), involves treating calcium carbide with water:
1. From Calcium Carbide
Calcium carbide (CaC2), produced by heating quick lime with coke, reacts with water to yield ethyne:
- Reaction:
This reaction provides a significant method for synthesizing ethyne on an industrial scale.
2. From Vicinal Dihalides
Vicinal dihalides, which have halogen atoms attached to adjacent carbon atoms, can also be converted into alkynes through dehydrohalogenation. This involves removing a molecule of hydrogen halide by using alcoholic potassium hydroxide (KOH), leading to the formation of an alkene first, which can then be further treated with sodium amide (NaNH2) to yield the desired alkyne.
- General Steps:
- Dehydrohalogenation:
- Formation of Alkyne:
This method allows for a versatile route to obtain various alkynes based on the starting materials used.
Reference YouTube Videos
Audio Book
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Create a free account- From calcium carbide: On industrial scale, ethyne is prepared by treating calcium carbide with water. Calcium carbide is prepared by heating quick lime with coke. Quick lime can be obtained by heating limestone as shown in the following reactions:
CaCO3 ∆ CaO + CO2 (9.55)
CaO + 3C CaC2 + CO (9.56)
CaC2 + 2H2O Ca(OH)2 + C2H2 (9.57)
Detailed Explanation
Ethyne, commonly known as acetylene, can be produced from calcium carbide. The process initiates by heating limestone (CaCO3) to produce quick lime (CaO) and carbon dioxide. Then, quick lime is combined with coke (carbon) to produce calcium carbide (CaC2). When calcium carbide is treated with water, ethyne is generated alongside calcium hydroxide.
Examples & Analogies
Think of making a cake. First, you need to bake the flour (limestone) to create a base (quick lime). Then, adding sugar (coke) helps create the cake (calcium carbide). Finally, adding icing (water) brings out the delicious outcome (ethyne). Just like with baking, different steps must occur in the right order to yield the final product.
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Create a free account- From vicinal dihalides: Vicinal dihalides on treatment with alcoholic potassium hydroxide undergo dehydrohalogenation. One molecule of hydrogen halide is eliminated to form an alkenyl halide which on treatment with sodamide gives alkyne.
Detailed Explanation
Vicinal dihalides are compounds where two halogen atoms are attached to adjacent carbon atoms. When these compounds react with alcoholic potassium hydroxide, they lose a molecule of hydrogen halide, resulting in the formation of an alkenyl halide. The next step involves treating this alkenyl halide with a strong base like sodamide which leads to the formation of an alkyne through elimination of another hydrogen halide.
Examples & Analogies
Imagine a pair of twins (the vicinal dihalides) who are neighbors (attached to adjacent carbons). If they decide to move out (lose hydrogen halide), they can transform into a new beginning (alkenyl halide) and by further encouraging them to leave together with a strong push (treatment with sodamide), they create a single entity (alkyne).
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Key Concepts
Core takeaways and short definitions to help you quickly recall the key ideas from this section.
Preparation Methods: Alkynes can be sourced from calcium carbide through a reaction with water, and vicinal dihalides via dehydrohalogenation.
Ethyne: The simplest member of alkynes, commonly used for various industrial applications like welding.
Vicinal Dihalides: Compounds that provide an intermediate step in forming alkynes when treated with strong bases.
Examples
Memory Aids
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Stories
Flash Cards
Glossary
Alkyne
A type of unsaturated hydrocarbon that contains at least one triple bond between carbon atoms.
Ethyne
The simplest alkyne, also known as acetylene, commonly used in welding and chemical synthesis.
Calcium Carbide
A chemical compound (CaC2) used as a key precursor in producing ethyne when it reacts with water.
Vicinal Dihalides
Dihalides that have halogen atoms attached to adjacent carbon atoms, which can be converted to alkynes through dehydrohalogenation.
Dehydrohalogenation
A chemical reaction where a hydrogen halide is eliminated from a vicinal dihalide compound.
Sodium Amide
A strong base (NaNH2) used in organic synthesis to generate alkynes from alkenes.