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Questions and Answers
What is a primary alkyl halide commonly used in the synthesis of alkynes?
In the synthesis of alkynes, which reagent is used to convert a vic-dibromide to an alkyne?
Which of the following processes is involved in the dehydration of alcohols to alkenes?
When an alkyne reacts with Br2, what type of compound is predominantly formed?
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What is the primary role of NaNH2 in the synthesis reactions described?
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What type of compound is sodium acetylide classified as?
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Which method can be used for the preparation of alkynes from vicinal dihaloalkanes?
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What is the product of the elimination reaction of a geminal dihalide using Na and liquid NH3?
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Which reagent is needed for the syn addition hydrogenation of alkynes to form (Z)-alkenes?
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Which of the following describes the reaction of alkynes with Br2 in CCl4?
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What is the purpose of using Zn in the dehalogenation of tetrahalogenated compounds?
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What type of addition occurs when using Na and NH3 with an alkyne?
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During the dehydration of alcohols, which type of alkene is generally produced?
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What is produced when butene reacts with KMnO4, followed by acidic workup?
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Which mechanism is utilized when alkenes react with Br2 at high temperatures?
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What is the general formula for alkynes?
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What occurs during the reaction of alkenes with Br2 in non-polar solvents at room temperature?
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Which of the following is a product formed from the oxidation of terminal alkenes using KMnO4?
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What type of reaction occurs when an alkene undergoes dehydration?
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Which compound is correctly named as 4,4-Dimethyl-1-pentyne?
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What is a characteristic of substitution reactions of alkenes with Br2?
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Study Notes
Alkynes
- Hydrocarbons containing a triple bond
- General formula is CnH2n-2
- Named using IUPAC nomenclature, ending in '-yne'
- Examples:
- Ethyne or acetylene (HCCH)
- 1-Penten-4-yne (CHCCH2CH2CH3)
- 3-Penten-1-yne (CHCCHCH2CH3)
- 4,4-Dimethyl-1-pentyne (H3CCCH2CCHC(CH3)3)
Synthesis of Alkynes
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From other alkynes with fewer carbons: Can be achieved by adding a carbon to an existing alkyne.
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By elimination reactions:
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Double dehydrohalogenation: This involves removing two hydrogen halides (HX) from a dihaloalkane.
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Vicinal dihalides: These are dihalides where the halogens are on adjacent carbons.
- Example: 1,2-dibromoethane (CH2BrCH2Br) can be converted to ethyne (HCCH) using alcoholic KOH or Na/liq NH3.
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Geminal dihalides: These are dihalides where the halogens are on the same carbon.
- Example: ethylidene chloride (CH3CHCl2) can be converted to ethyne (HCCH) using 2 Na/liq NH3.
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Vicinal dihalides: These are dihalides where the halogens are on adjacent carbons.
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Double dehalogenation of tetrahalogenated compounds:
- Using Zn/acetic acid or NaI/acetone to remove four halogens.
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Double dehydrohalogenation: This involves removing two hydrogen halides (HX) from a dihaloalkane.
Reactions of Alkynes
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Hydrogenation:
- Addition of hydrogen (H2) to the triple bond.
- Can be controlled to produce different products:
- Lindlar's catalyst (Pd/CaCO3): This catalyst produces a syn addition, resulting in a cis (Z)-alkene.
- Pd catalyst: This catalyst produces an anti addition, resulting in a trans (E)-alkene.
- Na/NH3: This catalyst produces an anti addition, resulting in a trans (E)-alkene.
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Halogenation:
- Addition of halogens (like Br2) to the triple bond.
- Occurs in two steps:
- First step: addition of one halogen molecule to form a trans dihaloalkene.
- Second step: addition of another halogen molecule to form a tetrahaloalkane.
- This type of reaction is useful for adding bromine to molecules, but the reaction can also be used to create other dihaloalkanes.
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Reaction with Sodium Amide (NaNH2):
- Forms sodium acetylide (HC≡CNa)
- Sodium acetylide is a strong nucleophile and can be used to alkylate alkynes
- Sodium acetylide can react with primary alkyl halides to form new alkynes with longer carbon chains.
- This is a useful reaction for creating alkynes with specific structures.
Substitution Reactions of Alkenes
- When alkenes react with Br2 at high temperatures (around 500°C), a substitution reaction occurs at the α carbon via a free radical mechanism.
- This reaction results in the replacement of a hydrogen atom on the α carbon with a bromine atom.
- However, if the reaction is carried out at room temperature in non-polar solvents like CCl4, the bromine adds to the double bond via an ionic mechanism.
- This reaction results in the formation of a dibromoalkane.
- The reaction of alkenes with Br2 is temperature dependent, leading to different products depending on the conditions.
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Description
Explore the fascinating world of alkynes, hydrocarbons characterized by triple bonds. This quiz covers their general formula, IUPAC nomenclature, and methods for synthesizing alkynes from simpler molecules. Test your knowledge with examples and elimination reactions.
