9.2 Acidity of Alkynes | Organic Chemistry
TLDRThe video script discusses the unique physical property of alkynes, specifically their acidity, which is relevant to subsequent chemical reactions covered in the course. Terminal alkynes, those with a carbon-carbon triple bond at the end of the molecule, are highlighted for their increased acidity compared to alkanes and alkenes, with a pKa in the range of 25 to 26. Despite this, they are still much less acidic than water. The video explains that while a strong base like sodium hydroxide is insufficient to deprotonate a terminal alkyne, sodium amide (NaNH2), a stronger base, can effectively deprotonate it, leading to the formation of an acetylide ion. This ion is noted for its nucleophilic properties, making it useful for SN2 reactions and organic synthesis. The lesson concludes with an invitation for viewers to engage with the content through likes, shares, and comments, and to explore further study materials.
Takeaways
- π§ͺ The acidity of terminal alkynes is a unique property relevant to their chemical reactions.
- π¬ Terminal alkynes have a pKa in the range of 25 to 26, making them more acidic than alkanes or alkenes but still less acidic than water.
- βοΈ The reactivity of terminal alkynes with bases is dependent on the strength of the base; sodium hydroxide is not strong enough to deprotonate them.
- π Sodium amide (NaNH2) is a strong base capable of deprotonating terminal alkynes due to its amide anion, which is a stronger base than hydroxide.
- π The reaction of sodium amide with a terminal alkyne effectively goes to completion, forming an acetylide ion.
- π Acetylide ions are good nucleophiles and are useful in SN2 reactions, which are important for synthetic chemistry.
- π The pKa difference between ammonia (pKa ~37-38) and the terminal alkyne (pKa 26) is what drives the reaction with sodium amide to completion.
- β It's important to recognize that the sodium counter ion is often included in the products when depicting the reaction with sodium amide.
- π For educational purposes, the script is part of a series of organic chemistry lessons released weekly during the 2020-21 school year.
- π Subscribing to the channel and enabling notifications will ensure viewers are updated when new lessons are released.
- π’ The presenter encourages viewers to like, share, and ask questions in the comments section for further engagement and clarification.
Q & A
What is the unique physical property of alkynes that is relevant to their chemical reactions?
-The unique physical property of alkynes that is relevant to their chemical reactions is their acidity, which is significant enough to allow for deprotonation under the right conditions.
What is the approximate pKa range for a terminal alkyne?
-The approximate pKa range for a terminal alkyne is between 25 to 26, making it more acidic than alkanes or alkenes but still less acidic than common acids like water.
Why is sodium hydroxide not strong enough to deprotonate a terminal alkyne?
-Sodium hydroxide is not strong enough to deprotonate a terminal alkyne because the equilibrium lies towards the weaker acid, and water (formed in the reaction) has a pKa of about 15.4, which is stronger than the pKa of the terminal alkyne.
What is the stronger base that can effectively deprotonate a terminal alkyne?
-Sodium amide (NaNH2) is the stronger base that can effectively deprotonate a terminal alkyne, forming an acetylide ion.
What is the name given to the conjugate base formed when a terminal alkyne is deprotonated?
-The conjugate base formed when a terminal alkyne is deprotonated is called an acetylide ion.
Why are acetylide ions considered good nucleophiles?
-Acetylide ions are considered good nucleophiles because they are capable of participating in SN2 reactions, making them useful for synthesis purposes.
What is the pKa value of ammonia that is relevant in the context of the reaction with sodium amide?
-The pKa value of ammonia is around 37 to 38, which is higher than that of the terminal alkyne, making it the weaker acid in the reaction with sodium amide.
Why does the reaction with sodium amide effectively go to completion when deprotonating a terminal alkyne?
-The reaction with sodium amide effectively goes to completion because the pKa of ammonia (the product) is significantly higher than that of the terminal alkyne, shifting the equilibrium far to the right.
How does the presence of a sodium counter ion affect the representation of the acetylide ion in reactions?
-The presence of a sodium counter ion does not change the nature of the acetylide ion; it is simply included in some reaction diagrams for completeness. Whether or not the sodium counter ion is shown, the acetylide ion is the key product of interest.
What is the significance of the pKa values in determining the direction of acid-base reactions?
-The pKa values indicate the relative strength of acids. In an acid-base reaction, the equilibrium will favor the formation of the weaker acid and stronger base. Therefore, a lower pKa value corresponds to a stronger acid and will drive the reaction towards producing the weaker acid.
How can one enhance their understanding of organic chemistry concepts such as the acidity of alkynes?
-One can enhance their understanding of organic chemistry concepts by engaging with educational content like video lessons, subscribing to relevant channels for updates, practicing with problems, and seeking clarification on any doubts through comments or forums.
Outlines
π§ͺ Acidity of Terminal Alkynes and Deprotonation with Sodium Amide
This paragraph discusses the unique physical property of alkynes, specifically their acidity, which is relevant to subsequent chemical reactions covered in the chapter. Terminal alkynes, characterized by a carbon-carbon triple bond at the end of the molecule, have a hydrogen atom attached to an sp hybridized carbon, which is significant in this context. The pKa value for terminal alkynes is between 25 to 26, making them more acidic than alkanes or alkenes but still less acidic than water. The paragraph explains that while sodium hydroxide is not a strong enough base to deprotonate an alkyne, sodium amide (NaNH2) is, resulting in the formation of an acetylide ion, which is a strong nucleophile useful in SN2 reactions and organic synthesis. The difference in reactivity is attributed to the pKa values of the products formed in the reaction, with ammonia (pKa ~37-38) being a weaker acid than the terminal alkyne (pKa 26), thus shifting the equilibrium towards the products.
π’ Engaging with the Channel and Additional Resources
The second paragraph serves as a call to action for viewers, encouraging them to like, share, and engage with the content if they find the lesson helpful. It also invites viewers to ask questions and leave comments for further discussion. Additionally, the paragraph promotes the creator's premium course on organic chemistry for students preparing for finals, directing interested individuals to ChadsPrep.com for more practice problems and a rapid review of the subject.
Mindmap
Keywords
π‘Acidity
π‘Alkynes
π‘pKa
π‘Deprotonation
π‘Sodium Hydroxide
π‘Sodium Amide
π‘Acetylide Ion
π‘Nucleophiles
π‘SN2 Reactions
π‘Organic Chemistry
π‘sp Hybridization
Highlights
The acidity of alkynes is a unique physical property that will be relevant in later chapters.
Terminal alkynes have a pH value in the 25 to 26 range, making them more acidic than alkanes or alkenes.
Despite being more acidic than alkanes and alkenes, terminal alkynes are still much less acidic than water.
Deprotonation of a terminal alkyne is possible but requires a stronger base than sodium hydroxide.
Sodium amide (NaNH2) is a strong base capable of deprotonating terminal alkynes, resulting in the formation of an acetylide ion.
Acetylide ions are nucleophiles that are useful in SN2 reactions, which are important for synthesis.
The success of using sodium amide over sodium hydroxide is due to the weaker acidity of ammonia compared to the terminal alkyne.
The pKa value of ammonia is around 37-38, which is significantly higher than the pKa of a terminal alkyne.
The reaction with sodium amide effectively goes to completion, forming the acetylide ion.
The acetylide ion is represented with or without the sodium counter ion, but it is the key product of the reaction.
For deprotonating a terminal alkyne, sodium amide is the preferred strong base over sodium hydroxide.
The lesson is part of a new organic chemistry playlist released weekly throughout the 2020-21 school year.
Subscribing to the channel and clicking the bell notification ensures viewers are updated with each new lesson release.
Internal alkynes are not addressed in this lesson; the focus is solely on terminal alkynes.
The lesson discusses the potential of deprotonating an alkyne and the conditions required for such a reaction to occur.
The comparison between the reactivity of alkynes, alkenes, and alkanes in terms of acidity is provided.
The importance of the pKa values in determining the direction of acid-base reactions is explained.
The lesson provides insights into the practical applications of acetylide ions in organic chemistry synthesis.
Viewers are encouraged to like, share, and comment with questions for further engagement and support of the channel.
For additional practice and a comprehensive review, the presenter suggests checking out the premium course on Chad's Prep.
Transcripts
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