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Magazine Chapter 16: Problem 17
Write chemical equations, showing all necessary reagents, for the preparation of 2 -butanol by each of the following methods: (a) Hydroboration-oxidation of an alkene (b) Use of a Grignard reagent (c) Use of a Grignard reagent different from that used in part (b) (d-f) Three different methods for reducing a ketone
Short Answer
Expert verified
Use hydroboration for 1-butene, Grignard reaction with acetaldehyde, and reductions using NaBH鈧�, LiAlH鈧�, or hydrogenation to prepare 2-butanol.
Step by step solution
01
Hydroboration-Oxidation of an Alkene
The hydroboration-oxidation process involves two main steps. First, an alkene undergoes hydroboration with borane, BH鈧�, resulting in the formation of an organoborane intermediate. The reagent typically used is BH鈧兟稵HF. Then, an oxidation step is performed using hydrogen peroxide (H鈧侽鈧�) and sodium hydroxide (NaOH). For the preparation of 2-butanol from an alkene, use 1-butene. The overall reaction is:\[\text{1-butene} + \text{BH}_3 \cdot \text{THF} \xrightarrow{\text{THF}} \text{Tri-n-butylborane} \xrightarrow{\text{H}_2\text{O}_2, \text{NaOH}} \text{2-butanol}\]
02
Use of a Grignard Reagent (Method 1)
A Grignard reagent is formed by reacting an alkyl or aryl halide with magnesium in dry ether. To prepare 2-butanol, you can start with ethyl magnesium bromide (CH鈧僀H鈧侻gBr) and react it with acetaldehyde (CH鈧僀HO). The reaction proceeds as follows:\[\text{CH}_3\text{CH}_2\text{MgBr} + \text{CH}_3\text{CHO} \xrightarrow{\text{Ether}} \text{(Addition of Grignard)} \xrightarrow{\text{H}_2\text{O}} \text{2-butanol}\]
03
Use of a Grignard Reagent (Method 2)
In this alternative method, use a different Grignard reagent such as methyl magnesium iodide (CH鈧僊gI) and react it with propanal (C鈧侶鈧匔HO). The reaction is shown below:\[\text{CH}_3\text{MgI} + \text{C}_2\text{H}_5\text{CHO} \xrightarrow{\text{Ether}} \text{(Addition of Grignard)} \xrightarrow{\text{H}_2\text{O}} \text{2-butanol}\]
04
Reduction of a Ketone (Method 1: Catalytic Hydrogenation)
Use a ketone such as butan-2-one and perform catalytic hydrogenation over a catalyst like Pd/C in the presence of hydrogen gas. This will reduce the ketone to 2-butanol. The reaction can be represented as:\[\text{CH}_3\text{COCH}_2\text{CH}_3 + \text{H}_2 \xrightarrow{\text{Pd/C}} \text{2-butanol}\]
05
Reduction of a Ketone (Method 2: NaBH鈧� reduction)
Sodium borohydride (NaBH鈧�) is a mild reducing agent used for the reduction of ketones to alcohols. Butan-2-one can be reduced by NaBH鈧� to form 2-butanol. The chemical equation is:\[\text{CH}_3\text{COCH}_2\text{CH}_3 + \text{NaBH}_4 + \text{H}_2\text{O} \rightarrow \text{2-butanol}\]
06
Reduction of a Ketone (Method 3: LiAlH鈧� reduction)
Lithium aluminium hydride (LiAlH鈧�) is a stronger reducing agent than NaBH鈧� and will also convert butan-2-one to 2-butanol. The procedure is as follows:\[\text{CH}_3\text{COCH}_2\text{CH}_3 + \text{LiAlH}_4 \xrightarrow{\text{Ether}} \text{(Reduction)} \rightarrow \text{2-butanol}\]
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Hydroboration-Oxidation Process
Hydroboration-oxidation is an organic reaction that transforms an alkene into an alcohol. It proceeds in two key steps:
- Hydroboration: This involves the addition of borane (BH鈧�), usually as a complex with tetrahydrofuran (THF), to the alkene. The result is the formation of a trialkylborane intermediate, like tri-n-butylborane.
- Oxidation: Afterwards, the organoborane is oxidized by hydrogen peroxide (H鈧侽鈧�) in the presence of sodium hydroxide (NaOH) to yield the desired alcohol, such as 2-butanol from 1-butene.
Grignard Reagent Synthesis (Method 1)
Grignard reagents are organomagnesium compounds critical in organic synthesis, allowing for the formation of carbon-carbon bonds. To synthesize a Grignard reagent, react an organohalide, such as ethyl bromide (CH鈧僀H鈧侭r), with magnesium in a dry ether solvent. One common use of a Grignard reagent is to react with aldehydes to form secondary alcohols.
- The reaction of ethyl magnesium bromide (Grignard reagent) with acetaldehyde (CH鈧僀HO) is a classic pathway to synthesize 2-butanol. The Grignard reagent first adds to the carbonyl carbon, forming an alkoxide intermediate.
- Subsequent protonation with water yields the alcohol, in this case, 2-butanol.
Grignard Reagent Synthesis (Method 2)
By changing the components in the Grignard reaction, different alcohols can be synthesized. In this second method, a different Grignard reagent, methyl magnesium iodide (CH鈧僊gI), is used.
- In this route, methyl magnesium iodide reacts with propanal (C鈧侶鈧匔HO) to produce 2-butanol.
- Once again, the Grignard reagent adds to the carbonyl carbon, forming an intermediate alkoxide.
- Afterward, protonation with water completes the synthesis, giving 2-butanol.
Ketone Reduction: Catalytic Hydrogenation
Catalytic hydrogenation is a powerful method for converting ketones into alcohols. This reduction uses molecular hydrogen (
H鈧�) in the presence of a metal catalyst such as palladium on carbon (Pd/C).
- The ketone, such as butan-2-one, undergoes direct hydrogen addition at the carbonyl group, leading to the formation of an alcohol 鈥� here, 2-butanol.
- This method is highly efficient but typically requires high pressures of hydrogen gas.
NaBH鈧� Reduction
Sodium borohydride (NaBH鈧�) is a widely used reagent for the mild reduction of carbonyl compounds like ketones into alcohols. The key feature of NaBH鈧� is its selective reactivity.
- NaBH鈧� reduces ketones by hydride transfer to the carbonyl carbon. Butan-2-one is thus converted into 2-butanol with the aid of water.
- The reaction with NaBH鈧� is gentle enough to not affect other functional groups like esters or carboxylic acids in the same molecule.
LiAlH鈧� Reduction
Lithium aluminum hydride (LiAlH鈧�) is a very strong reducing agent, capable of reducing a wide range of functional groups, including ketones, to give alcohols. Unlike NaBH鈧�, LiAlH鈧� reduction is more comprehensive and powerful.
- It reduces ketones like butan-2-one to alcohols, such as 2-butanol, with high efficiency.
- Since LiAlH鈧� reacts vigorously with water and provides a highly reactive hydride ion, anhydrous conditions using dry ether are necessary to avoid side reactions.
