Alkynes and Their Synthesis

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Questions and Answers

What is a primary alkyl halide commonly used in the synthesis of alkynes?

Tetrafluoroethane

Ethylidene chloride (correct)

Vinyl bromide

Dichloromethane

In the synthesis of alkynes, which reagent is used to convert a vic-dibromide to an alkyne?

Zinc and acetic acid (correct)

Sodium acetylide

Lithium aluminum hydride

Sodium borohydride

Which of the following processes is involved in the dehydration of alcohols to alkenes?

Electrophilic addition

Elimination reactions (correct)

Nucleophilic substitution

Hydrogenation

When an alkyne reacts with Br2, what type of compound is predominantly formed?

Dihalide Signup and view all the answers

What is the primary role of NaNH2 in the synthesis reactions described?

Base for deprotonation Signup and view all the answers

What type of compound is sodium acetylide classified as?

Alkyne Signup and view all the answers

Which method can be used for the preparation of alkynes from vicinal dihaloalkanes?

Dehydrohalogenation using alcoholic KOH Signup and view all the answers

What is the product of the elimination reaction of a geminal dihalide using Na and liquid NH3?

Alkyne Signup and view all the answers

Which reagent is needed for the syn addition hydrogenation of alkynes to form (Z)-alkenes?

H2 / Pd / CaCO3 Signup and view all the answers

Which of the following describes the reaction of alkynes with Br2 in CCl4?

Electrophilic addition to form tetrahaloalkane Signup and view all the answers

What is the purpose of using Zn in the dehalogenation of tetrahalogenated compounds?

To facilitate double elimination Signup and view all the answers

What type of addition occurs when using Na and NH3 with an alkyne?

Anti addition Signup and view all the answers

During the dehydration of alcohols, which type of alkene is generally produced?

E-alkene Signup and view all the answers

What is produced when butene reacts with KMnO4, followed by acidic workup?

Propionic acid and CO2 Signup and view all the answers

Which mechanism is utilized when alkenes react with Br2 at high temperatures?

Free radical mechanism Signup and view all the answers

What is the general formula for alkynes?

CnH2n-2 Signup and view all the answers

What occurs during the reaction of alkenes with Br2 in non-polar solvents at room temperature?

Electrophilic addition to the double bond Signup and view all the answers

Which of the following is a product formed from the oxidation of terminal alkenes using KMnO4?

CO2 Signup and view all the answers

What type of reaction occurs when an alkene undergoes dehydration?

Elimination reaction Signup and view all the answers

Which compound is correctly named as 4,4-Dimethyl-1-pentyne?

A five-carbon alkyne with two methyl groups at the fourth carbon Signup and view all the answers

What is a characteristic of substitution reactions of alkenes with Br2?

They occur readily at low temperatures with a non-polar solvent Signup and view all the answers

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

From other alkynes with fewer carbons: Can be achieved by adding a carbon to an existing alkyne.
By elimination reactions:
- Double dehydrohalogenation: This involves removing two hydrogen halides (HX) from a dihaloalkane.
  - 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.
  - 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.
- Double dehalogenation of tetrahalogenated compounds:
  - Using Zn/acetic acid or NaI/acetone to remove four halogens.

Reactions of Alkynes

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.
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.
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.