Make Alkyl Halides Bromoethane and Bromohexane

Summary

In this video, the creator demonstrates the process of making alkyl halides, specifically bromoethane and bromohexane, from alcohols using hydrobromic and sulfuric acids. The reactions follow classic nucleophilic substitution mechanisms, with a focus on distillation techniques and addressing challenges with volatile and high-boiling compounds. The creator also explores the purification of the products, handling unwanted side products like diethyl ether and bromine. The video highlights the importance of careful temperature control and explains the chemistry behind these reactions, which are used to create grignard reagents for further applications.

Takeaways

• 😀 Alkyl halides are compounds containing an alkane carbon bonded to a halogen like fluorine, chlorine, bromine, or iodine.
• 😀 Alkyl halides are used in various applications such as pesticides, flame retardants, drugs, lubricants, solvents, and refrigerants.
• 😀 The video demonstrates the synthesis of bromoethane by reacting ethanol with hydrobromic acid and sulfuric acid in an Sn2 nucleophilic substitution reaction.
• 😀 A distillation setup is used to separate the volatile bromoethane from the reaction mixture.
• 😀 The reaction involves protonation of the alcohol group, making it a good leaving group for nucleophilic attack by bromide ions.
• 😀 Sulfuric acid is used to absorb water and drive the reaction forward, while also creating small amounts of bromine, contributing to the orange color of the distillate.
• 😀 Crude bromoethane is purified using a wash with water and sodium bicarbonate to remove bromine and other impurities.
• 😀 The yield of bromoethane can vary, with some amateur chemists achieving up to 90% yields using the same method.
• 😀 The video also covers the process of synthesizing bromohexane from 1-hexanol, utilizing sodium bromide and sulfuric acid in a reflux setup.
• 😀 Unlike bromoethane, bromohexane has a higher boiling point, requiring reflux instead of distillation to drive the reaction to completion.
• 😀 Bromohexane is purified by separating it from water, sodium bicarbonate, and other aqueous impurities, and fractional distillation can be used to remove excess water.

Q & A

What are alkyl halides and why are they important in chemistry?

Alkyl halides are organic compounds containing an alkane carbon bonded to a halogen like fluorine, chlorine, bromine, or iodine. They have vast applications in chemistry, including in pesticides, flame retardants, drugs, lubricants, solvents, and refrigerants.

What is the purpose of using hydrobromic acid in the synthesis of bromoethane?

Hydrobromic acid is used to provide the bromide ion necessary for the nucleophilic substitution reaction with ethanol, leading to the formation of bromoethane. It is also essential for protonating the alcohol group, making it a better leaving group.

What role does sulfuric acid play in the preparation of bromoethane?

Sulfuric acid serves two main purposes: it protonates the alcohol group in ethanol, turning it into a better leaving group, and it helps drive the reaction forward by absorbing water, preventing the formation of unwanted products.

Why is it important to control the temperature during the distillation process of bromoethane?

Controlling the temperature during distillation ensures that the bromoethane is condensed properly and prevents it from evaporating or being lost. It also helps in maintaining a gradual rate of distillation, which is crucial for obtaining a good yield.

How does the presence of water in the distillation process affect the bromoethane yield?

Water in the distillation process helps to purify the bromoethane by removing dissolved hydrogen bromide and ethanol. The distillation of water also helps drive the reaction forward by removing water, which is a product of the nucleophilic substitution reaction.

Why was an ice water bath used during the setup for making bromoethane?

An ice water bath was used to cool the boiling flask initially to prevent the volatile bromoethane from evaporating before the distillation process. It ensures that bromoethane is only vaporized when the heating process begins.

What is the purpose of adding sodium bicarbonate solution during the purification of bromoethane?

The sodium bicarbonate solution reacts with the bromine present in the crude bromoethane to form sodium hypobromite, which dissolves in the aqueous layer, helping to purify the bromoethane by removing the bromine contamination.

What challenges are presented when making alkyl halides that have a higher boiling point than water, such as bromohexane?

When making alkyl halides like bromohexane with a higher boiling point than water, simple distillation is not practical because the alkyl halide will not easily separate. Instead, a reflux setup is used to complete the reaction without losing the alcohol or product due to evaporation.

How does the reaction to create bromohexane differ from the process used to create bromoethane?

The bromohexane synthesis uses sodium bromide as the bromide source, combined with sulfuric acid, whereas bromoethane is synthesized with hydrobromic acid. Additionally, bromohexane requires refluxing due to its higher boiling point, while bromoethane can be distilled directly.

Why is the bromohexane reaction refluxed for six hours instead of being distilled immediately?

The bromohexane reaction is refluxed because bromohexane has a higher boiling point than water and hexanol. Refluxing allows the reaction to proceed to completion without losing hexanol or bromohexane, which would occur if the mixture were distilled prematurely.