7.4 - Alcohols and Phenols
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Acid-Catalyzed Hydration of Alkenes
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Today, we'll examine how we can form alcohols from alkenes, starting with acid-catalyzed hydration. Can anyone tell me what alkenes are?
I think alkenes are hydrocarbons with at least one double bond.
Exactly right! Alkenes react with water in the presence of an acid catalyst. This follows Markovnikov's rule. What do you recall about Markovnikov's rule?
It states that when adding to an alkene, the more substituted carbon gets the positive charge.
Great! This means that the water will add across the double bond such that the alcohol ends up on the more substituted carbon. Can anyone recall the steps in the mechanism?
I remember that it starts with the protonation of the alkene to form a carbocation.
Correct! Then, water attacks this carbocation, and what happens next?
It gets deprotonated to form the alcohol!
You all have done well! So we noticed that through this reaction, we can systematically create alcohols from alkenes.
Hydroboration-Oxidation of Alkenes
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Now, let’s talk about hydroboration-oxidation. Can anyone explain how this process differs from acid-catalyzed hydration?
I think it involves diborane instead of acid, and it adds differently to the double bond.
Yes! In hydroboration, boron attaches to the carbon with more hydrogen atoms. This is opposite to Markovnikov's rule. What's the next step after the diborane formation?
It’s oxidized using hydrogen peroxide in aqueous sodium hydroxide.
Correct! This method is known for providing alcohol in excellent yields. Can you visualize how this mechanism works?
Yes, I can see how the addition happens across the double bond.
Perfect! So, we have two powerful techniques for preparing alcohols. Let’s keep building on that!
Reduction of Carbonyl Compounds
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Next up, we will look at how alcohols can be formed by reducing carbonyl compounds. Does anyone remember what types of carbonyl compounds we're discussing?
Aldehydes and ketones!
Great! How do we reduce them to alcohols?
We can use hydrogen with a catalyst, or reagents like sodium borohydride!
Exactly! And do you know what types of alcohols these reactions yield?
Aldehydes produce primary alcohols, and ketones give secondary alcohols.
Well done! Now, what can you tell me about reducing carboxylic acids?
They can be reduced to primary alcohols using lithium aluminium hydride.
Exactly! Even though it is an expensive reagent, it achieves excellent results.
Grignard Reagents in Alcohol Synthesis
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Now let's discuss Grignard reagents. Who can describe what happens when they react with carbonyl compounds?
They perform nucleophilic addition to carbonyl groups!
Correct! After this addition, what happens to the adduct?
It undergoes hydrolysis to form an alcohol.
Fantastic! Can you think of the types of alcohol produced when different aldehydes or ketones are used?
A primary alcohol from methanal, secondary from other aldehydes, and tertiary from ketones.
Exactly! Grignard reagents are versatile and provide a rich area of study in alcohol synthesis.
Introduction & Overview
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Quick Overview
Standard
Alcohols can be prepared through several chemical reactions: hydration of alkenes via acid and hydroboration-oxidation, reduction of carbonyl compounds such as aldehydes and ketones, and by using Grignard reagents. Each method varies based on the type of starting materials and conditions required.
Detailed
Preparation of Alcohols
Alcohols are important organic compounds with various applications. This section outlines several methods used to prepare alcohols, which include:
1. From Alkenes
(i) Acid-Catalyzed Hydration
- Alkenes are hydrated in the presence of an acid catalyst, following Markovnikov’s rule.
- Process Mechanism:
- Protonation: The alkene is protonated to form a carbocation with the electrophilic attack of 3O 3.
- Nucleophilic Attack: Water acts as a nucleophile, attacking the carbocation.
- Deprotonation: The resulting species is deprotonated to yield an alcohol.
(ii) Hydroboration–Oxidation
- In this method, diborane reacts with alkenes to form trialkyl boranes, which are oxidized to yield alcohols.
- Here, addition occurs such that boron attaches to the carbon atom with more hydrogen atoms, leading to alcohol formation opposite to Markovnikov’s rule.
2. From Carbonyl Compounds
(i) Reduction of Aldehydes and Ketones
- Aldehydes and ketones are reduced using hydrogen in the presence of a catalyst or through reagents like sodium borohydride or lithium aluminium hydride.
- Aldehydes yield primary alcohols, while ketones yield secondary alcohols.
(ii) Reduction of Carboxylic Acids and Esters
- Carboxylic acids can be converted to primary alcohols through reduction using lithium aluminium hydride. For commercial applications, they are often reduced via ester intermediates.
3. From Grignard Reagents
- These reagents react with aldehydes and ketones. The Grignard reagent nucleophilically adds to the carbonyl group, forming an adduct that upon hydrolysis yields an alcohol, producing different types of alcohols depending on the carbonyl structure.
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Preparation of Alcohols
Chapter 1 of 3
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Chapter Content
Alcohols are prepared by the following methods:
1. From alkenes
(i) By acid catalysed hydration: Alkenes react with water in the presence of acid as catalyst to form alcohols. In case of unsymmetrical alkenes, the addition reaction takes place in accordance with Markovnikov’s rule.
Mechanism
The mechanism of the reaction involves the following three steps:
Step 1: Protonation of alkene to form carbocation by electrophilic attack of H3O+.
Step 2: Nucleophilic attack of water on carbocation.
Step 3: Deprotonation to form an alcohol.
(ii) By hydroboration–oxidation: Diborane (BH3)2 reacts with alkenes to give trialkyl boranes as addition products. This is oxidised to alcohol by hydrogen peroxide in the presence of aqueous sodium hydroxide.
The addition of borane to the double bond takes place in such a manner that the boron atom gets attached to the sp carbon carrying a greater number of hydrogen atoms. The alcohol so formed looks as if it has been formed by the addition of water to the alkene in a way opposite to the Markovnikov’s rule. In this reaction, alcohol is obtained in excellent yield.
Detailed Explanation
Alcohols can be produced through two main methods: acid catalyzed hydration of alkenes and hydroboration-oxidation. In the first method, alkenes react with water under acidic conditions to form alcohols, following specific rules like Markovnikov’s rule for unsymmetrical alkenes. This consists of three steps: first, the alkene is protonated, forming a carbocation; second, water attacks this carbocation to form an alcohol; finally, a proton is removed, resulting in the alcohol. The second method, hydroboration-oxidation, involves adding borane (BH3) to the alkene, creating trialkyl boranes, which are then oxidized with hydrogen peroxide and sodium hydroxide to yield alcohols. Interestingly, this method favors the formation of alcohols in a manner contrary to Markovnikov's rule, providing high yields of the desired alcohols.
Examples & Analogies
Imagine you're trying to bake a cake (the alcohol) by mixing different ingredients (the alkenes and water). If you follow the recipe but accidentally add the eggs (acid) first, you might end up with a fluffier mixture than intended. This mirrors how wearing certain conditions can influence the product we ultimately make—just like in chemistry, where the reaction conditions can shift yields and products!
Preparation of Phenols
Chapter 2 of 3
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Chapter Content
Phenol, also known as carbolic acid, was first isolated in the early nineteenth century from coal tar. Nowadays, phenol is commercially produced synthetically. In the laboratory, phenols are prepared from benzene derivatives by any of the following methods:
- From haloarenes
Chlorobenzene is fused with NaOH at 623K and 320 atmospheric pressure. Phenol is obtained by acidification of sodium phenoxide so produced (Unit 6, Class XII). - From benzenesulphonic acid
Benzene is sulphonated with oleum, and benzene sulphonic acid so formed is converted to sodium phenoxide on heating with molten sodium hydroxide. Acidification of the sodium salt gives phenol. - From diazonium salts
A diazonium salt is formed by treating an aromatic primary amine with nitrous acid (NaNO2 + HCl) at 273-278 K. Diazonium salts are hydrolysed to phenols by warming with water or by treating with dilute acids (Unit 9, Class XII). - From cumene
Phenol is manufactured from the hydrocarbon, cumene. Cumene (isopropylbenzene) is oxidised in the presence of air to cumene hydroperoxide. It is converted to phenol and acetone by treating it with dilute acid. Acetone, a by-product of this reaction, is also obtained in large quantities by this method.
Detailed Explanation
Phenols are usually derived from benzene through various synthetic pathways, including the following steps: Haloarenes that react under intense conditions with sodium hydroxide create phenols through a transformation called hydrolysis. Another common method involves converting benzenesulphonic acid to sodium phenoxide through heating, followed by an acidification which yields phenol. Additionally, diazonium salts can be produced from primary amines, which can be hydrolyzed to yield phenols as well. Finally, cumene is a significant commercial route for phenol production where oxidation and acid treatment are employed to extract phenol and produce acetone.
Examples & Analogies
Think of producing chocolate from cocoa beans. Just as there are different methods (like roasting or grinding) to create the desired chocolate bars (the phenols), chemists use various routes to transform simple benzene derivatives into functional compounds like phenol. Each method has its own process and conditions, which dictate the quality and efficiency of the end chocolate (phenol) produced.
Physical Properties
Chapter 3 of 3
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Chapter Content
Alcohols and phenols consist of two parts, an alkyl/aryl group and a hydroxyl group. The properties of alcohols and phenols are chiefly due to the hydroxyl group. The boiling points of alcohols and phenols increase with an increase in the number of carbon atoms (increase in van der Waals forces). In alcohols, the boiling points decrease with an increase of branching in the carbon chain (because of decrease in van der Waals forces with decrease in surface area). The –OH group in alcohols and phenols is involved in intermolecular hydrogen bonding. The high boiling points of alcohols are mainly due to the presence of intermolecular hydrogen bonding in them which is lacking in ethers and hydrocarbons.
Detailed Explanation
The properties of alcohol and phenol are influenced by their structure, specifically the presence of the hydroxyl (-OH) group. This group allows for hydrogen bonding, which significantly affects boiling points. Generally, as the carbon chain length increases, boiling points increase due to greater van der Waals forces. However, if the carbon chain branches, the boiling points tend to decrease due to a reduction in surface area, which leads to fewer van der Waals interactions. Compared to ethers and hydrocarbons, alcohols and phenols have notably higher boiling points due to this capability to form strong hydrogen bonds.
Examples & Analogies
Consider how water (which can form hydrogen bonds) has a higher boiling point than similar-sized molecules that cannot, like oxygen or nitrogen. When cooking (similar to heating substances), the capacity for compounds to attract each other (via hydrogen bonds) will keep them together longer before they vaporize! This bond strength is what we see reflected in the boiling points of alcohols and phenols.
Key Concepts
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Acid-Catalyzed Hydration: A method where alkenes react with water in the presence of an acid to form alcohols following Markovnikov's rule.
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Hydroboration-Oxidation: An alternative method where diborane adds to alkenes to yield alcohols opposite to Markovnikov's rule.
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Reduction Reactions: Aldehydes and ketones can be reduced to alcohols using hydrogen and catalysts.
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Grignard Reagents: Specialized reagents that react with carbonyl compounds to yield alcohols.
Examples & Applications
In acid-catalyzed hydration, propene (an alkene) can combine with water to yield isopropanol.
Using sodium borohydride, the aldehyde formaldehyde can be reduced to methanol.
Memory Aids
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Rhymes
If you want alcohol from alkenes clear, just add acid, never fear!
Stories
Imagine alkenes at a party, where water and acid mingle, creating a cozy bond that turns hydrocarbons into alcohols—Markovnikov is the name of the host!
Memory Tools
Remember A-HA! for acid hydration: A for Alkene, H for Hydrogen, A for Alcohol.
Acronyms
H.A.-R.A.-G. for Hydroboration (-Aldehydes, -Reduction, -Grignard) can lead to forming alcohols.
Flash Cards
Glossary
- Alcohol
An organic compound containing one or more hydroxyl (-OH) groups.
- Alkene
A hydrocarbon with at least one carbon-carbon double bond.
- Markovnikov’s Rule
A principle stating that in the addition of HX to an alkene, the hydrogen atom will attach to the carbon with the most hydrogen atoms already.
- Aldehyde
An organic compound containing a carbonyl group (C=O) bonded to at least one hydrogen atom.
- Ketone
An organic compound containing a carbonyl group (C=O) bonded to two carbon atoms.
- Grignard Reagent
An organomagnesium compound used to create carbon-carbon bonds in organic synthesis.
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