9.5 Hydrohalogenation of Alkynes | Organic Chemistry
TLDRThe video script discusses the hydrohalogenation of alkynes, a process similar to that of alkenes but with the unique characteristic that alkynes have two pi bonds, allowing for either one or two additions depending on the amount of reagent used. The lesson begins with a review of hydrohalogenation of alkenes, highlighting the formation of a carbocation and Markovnikov's rule. It then delves into the nuances of hydrohalogenation with alkynes, explaining that terminal alkynes have a more substituted side, which is important for understanding the addition reactions. The script also clarifies that internal alkynes do not follow the same rules due to equal substitution on carbons. The mechanism for these reactions is not fully understood, but it is suggested that it may not involve a carbocation intermediate. The lesson further explores the use of HBr with peroxides, which leads to anti-Markovnikov addition. The summary concludes by noting that one equivalent of reagent can lead to an alkene, while two equivalents result in an alkane, emphasizing the importance of understanding these reactions for organic chemistry.
Takeaways
- ๐ The hydrohalogenation of alkynes is similar to that of alkenes, but alkynes have two pi bonds, allowing for one or two additions depending on the amount of reagent used.
- โ๏ธ In the first step of hydrohalogenation with HBr (or other hydrohalogens), a carbocation is formed by adding hydrogen to the less substituted side, leading to Markovnikov addition.
- ๐ฌ The mechanism for alkynes is less clear than for alkenes, and may not involve a carbocation intermediate due to the instability of such a carbocation.
- ๐๏ธ Terminal alkynes have a more substituted end, which contrasts with internal alkynes where neither carbon is more substituted than the other.
- ๐ค For internal alkynes, the concept of Markovnikov and anti-Markovnikov addition does not apply, as neither carbon is more substituted, allowing for either addition pattern.
- โ When using one equivalent of HBr (or similar), the reaction follows Markovnikov's rule, but the mechanism might be second-order rather than first-order.
- ๐ซ The use of HBr with peroxides results in anti-Markovnikov addition, where hydrogen adds to the more substituted side and bromine to the less substituted side.
- ๐ With peroxides, even with excess or two equivalents of HBr, the reaction still follows anti-Markovnikov's rule, leading to two bromines on the less substituted carbon and two hydrogens on the more substituted carbon.
- ๐ There is no stereoselectivity in the addition to internal alkynes, and both E and Z isomers can be formed.
- โ The general rule for terminal alkynes is that one equivalent of reagent leads to an alkene, while two equivalents lead to an alkane.
- ๐ Students are often not required to know the detailed mechanism of alkyne hydrohalogenation, and simplified or plausible mechanisms may be presented in educational settings.
Q & A
What is the main topic of the lesson?
-The main topic of the lesson is the hydrohalogenation of alkynes.
How does the hydrohalogenation of alkynes differ from that of alkenes?
-The hydrohalogenation of alkynes differs from that of alkenes in that alkynes have two pi bonds, which means they have the potential to undergo the reaction either once or twice depending on the amount of reagent used.
What is the Markovnikov's rule and how does it apply to the hydrohalogenation of alkynes?
-Markovnikov's rule states that in the addition of a polar reagent to an alkene, the hydrogen atom is added to the carbon with the greater number of hydrogen atoms (the less substituted carbon). In the case of terminal alkynes, this rule still applies, but with internal alkynes, neither carbon is more substituted than the other, so the rule doesn't apply in the same way.
What is the difference between a terminal alkyne and an internal alkyne?
-A terminal alkyne has its triple bond at the end of the carbon chain, with one of the carbons being more substituted than the other. An internal alkyne, on the other hand, has its triple bond within the carbon chain, with neither carbon being more substituted than the other.
What happens when one equivalent of a hydrohalic acid is added to a terminal alkyne?
-When one equivalent of a hydrohalic acid such as HBr, HCl, or HI is added to a terminal alkyne, it undergoes Markovnikov addition, resulting in the formation of a vinyl halide (an alkene derivative with a halogen atom).
What is the likely mechanism for the addition of one equivalent of a hydrohalic acid to a terminal alkyne?
-The exact mechanism for the addition is not entirely clear, but it is unlikely to involve a carbocation intermediate due to the instability of carbocations on carbons that are part of an alkene. It may involve a second-order process instead of a first-order process.
What happens when two equivalents of a hydrohalic acid are added to an alkyne?
-When two equivalents of a hydrohalic acid are added to an alkyne, it undergoes a second Markovnikov addition, resulting in the formation of a dihaloalkane (an alkane with two halogen atoms).
How does the use of HBr with peroxide affect the addition reaction to an alkyne?
-When HBr is used with peroxide, the addition reaction to an alkyne follows anti-Markovnikov rules, meaning the hydrogen is added to the more substituted carbon and the bromine to the less substituted carbon.
What is the stereoselectivity of the addition of HBr with peroxide to an alkyne?
-There is no stereoselectivity in the addition of HBr with peroxide to an alkyne. The reaction can lead to both E and Z isomers, although E is more stable than Z.
What are the two typical approaches when adding equivalents of hydrohalic acid to an alkyne?
-The two typical approaches are adding one equivalent to stop at the alkene stage or adding two equivalents (or using an excess from the start) to go all the way to the alkane.
What is the importance of understanding the difference between Markovnikov and anti-Markovnikov addition in alkynes?
-Understanding these differences is crucial for predicting the products of alkyne hydrohalogenation reactions, which can be important in synthetic chemistry and for understanding reaction mechanisms.
Outlines
๐ Hydrohalogenation of Alkynes: Markovnikov Addition and Carbocation Formation
This paragraph delves into the hydrohalogenation of alkynes, highlighting its similarity to the process with alkenes, but with an important distinction: alkynes have two pi bonds, allowing for either one or two additions depending on the amount of reagent used. The discussion starts with a review of hydrohalogenation of alkenes using HBr, where a carbocation is formed by adding hydrogen to the less substituted side, leading to Markovnikov addition. The mechanism for alkynes is less clear and potentially involves a second-order process rather than a first-order one, which is less likely to go through a carbocation intermediate due to its instability. The example of a terminal alkyne is used to illustrate the addition process, emphasizing the difference between terminal and internal alkynes in terms of substitution and the implications for Markovnikov versus anti-Markovnikov addition. The paragraph concludes by noting that while the exact mechanism is not fully understood, students are often not required to know it in detail.
๐ Anti-Markovnikov Addition with Peroxides: Selectivity in Alkyne Hydrohalogenation
The second paragraph focuses on the anti-Markovnikov addition to alkynes in the presence of peroxides. It explains that when using HBr with peroxides, the addition occurs on the more substituted side, contrasting with the Markovnikov rule observed without peroxides. The paragraph uses a terminal alkyne as an example to demonstrate the difference in addition patterns. It is noted that there is no stereoselectivity in this reaction, and both E and Z isomers are possible. The summary also mentions the option to add a second equivalent of HBr to achieve full saturation to an alkane, with two bromines added to the less substituted carbon and two hydrogens to the more substituted one. The paragraph concludes by mentioning the flexibility of stopping at the alkene stage with one equivalent or going to the alkane with two equivalents, and encourages viewers to support the educational content by liking, sharing, and checking out study guides and practice problems on the provided website.
Mindmap
Keywords
๐กHydrohalogenation
๐กAlkynes
๐กMarkovnikov's Rule
๐กCarbocation
๐กTerminal Alkyne
๐กInternal Alkyne
๐กStereoisomers
๐กAnti-Markovnikov Addition
๐กOrganic Peroxides
๐กAlkene
๐กRegioselectivity
Highlights
The hydrohalogenation of alkynes is similar to that of alkenes, but alkynes have two pi bonds, allowing for one or two additions.
The addition can occur either once or twice depending on the amount of reagent used.
Terminal alkynes have a more substituted side compared to internal alkynes, which have equally substituted carbons.
Markovnikov's rule applies to terminal alkynes but not to internal alkynes.
The mechanism for hydrohalogenation of alkynes is less clear and may involve a second-order process.
Carbocations on alkynes are less stable than primary carbocations, making a carbocation intermediate unlikely.
Professors may provide simplified mechanisms for the hydrohalogenation of alkynes, but the exact mechanism is often not required knowledge.
Adding one equivalent of HBr leads to Markovnikov addition, while two equivalents lead to full saturation to an alkane.
With internal alkynes, the addition can occur in either direction, leading to E and Z isomers.
The use of HBr with peroxides results in anti-Markovnikov addition, with hydrogen adding to the more substituted side.
Stereoselectivity is not a factor in the addition of HBr with peroxides to alkynes.
Adding one equivalent of HBr with peroxides yields an alkene, while two equivalents yield an alkane.
The lesson differentiates between terminal and internal alkynes in the context of hydrohalogenation.
The importance of understanding the stability of carbocations in the mechanism of alkyne hydrohalogenation.
The potential for competing mechanisms in the hydrohalogenation of alkynes.
The impact of the length of the carbon chain on the substitution of internal alkynes.
The practical approach to teaching hydrohalogenation of alkynes, focusing on terminal alkynes for clarity.
The option to stop the reaction after one equivalent to form an alkene or continue to form an alkane.
The role of organic peroxides in facilitating anti-Markovnikov addition to alkynes.
Transcripts
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