Arielle Cruz
Alcohols are a versatile family of chemical compounds with a wide range of applications. They are defined by their hydroxyl (-OH) group, which is bound to an alkane. Alcohols can be classified as primary, secondary, or tertiary, and this distinction is important when considering their reactivity. Alcohols can be converted into a variety of other compounds, including esters, ethers, and alkenes, through reactions such as oxidation, dehydration, substitution, and esterification. The oxidation of primary alcohols yields aldehydes, which can be further oxidised to form carboxylic acids, while the oxidation of secondary alcohols produces ketones. Tertiary alcohols, on the other hand, are resistant to oxidation without breaking carbon-carbon bonds. Alcohols can also be converted into alkyl halides through reactions with halogen acids, and they play a crucial role in organic synthesis as important intermediates.
| Characteristics | Values |
|---|---|
| General formula | -OH |
| Acidity | Weaker than water |
| K a value | 1 x 10 −16 |
| Oxidation | Aldehydes, ketones, and carboxylic acids |
| Dehydration | Alkenes or ethers |
| Esterification | Carboxylic acid and alcohol heated with a mineral acid catalyst |
| Primary alcohols | RCOH |
| Secondary alcohols | Oxidised to ketones |
| Tertiary alcohols | R3COH, cannot be oxidised |
| Alkyl halides | S N1 and S N2 reactions with halogen acids |
| Grignard reagent | Reaction with ethylene oxide to produce primary alcohol |
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What You'll Learn
Oxidation of primary alcohols
The oxidation of primary alcohols is a fundamental organic reaction that transforms a primary alcohol (\CH_{3}CH_{2}OH) into an aldehyde (\CH_{3}CHO) or a carboxylic acid (\CH_{3}COOH). This reaction is of immense significance in synthetic organic chemistry, as it allows for the manipulation and derivation of various compounds.
Primary alcohols, due to the presence of the hydroxyl group (\OH) attached to a primary carbon atom, exhibit a high degree of reactivity. The oxidation process involves the removal of hydrogen from the hydroxyl group, resulting in the formation of either an aldehyde or a carboxylic acid moiety. The nature of the oxidizing agent and reaction conditions play a pivotal role in determining the outcome of the reaction.
Mild oxidizing agents, such as copper(II) chloride (\CuCl_{2}) or chromium (III) oxide (\Cr_{2}O_{3}), are commonly employed for the oxidation of primary alcohols to aldehydes. These reactions are typically carried out in a solvent system, with the alcohol substrate undergoing a change in electron distribution, leading to the cleavage of the O-H bond and the formation of the carbonyl group characteristic of aldehydes.
For instance, the reaction between ethanol (\CH_{3}CH_{2}OH) and copper(II) chloride proceeds as follows:
\CH_{3}CH_{2}OH + \CuCl_{2} → \CH_{3}CHO + \CuCl_{2}H + \HCl
In this reaction, ethanol is oxidized to yield ethanal (\CH_{3}CHO), along with the formation of copper(I) chloride (\CuCl) and the release of hydrochloric acid (\HCl).
To obtain carboxylic acids from primary alcohols, stronger oxidizing agents are employed, such as potassium permanganate (\KMnO_{4}) or chromic acid (\H_{2}CrO_{4}). These reactions often require more vigorous conditions, including higher temperatures and prolonged reaction times. The oxidation of primary alcohols to carboxylic acids is a critical step in many synthetic routes, providing essential intermediates for further derivatization.
As an illustration, the oxidation of ethanol to acetic acid (\CH_{3}COOH) using chromic acid occurs as follows:
3\CH_{3}CH_{2}OH + 4\H_{2}CrO_{4} → 3\CH_{3}COOH + 4Cr^{3+} + 7\H_{2}O
In this reaction, three moles of ethanol react with four moles of chromic acid to yield three moles of acetic acid, along with the reduction of chromium (VI) to chromium (III) ions and the release of water.
It is imperative to recognize that the oxidation of primary alcohols is a complex process influenced by numerous factors, including the structure of the alcohol, the choice of oxidizing agent, reaction conditions, and the presence of other functional groups in the molecule. Consequently, a comprehensive understanding of these factors is essential to predict the outcome of a specific reaction and to devise synthetic routes that yield the desired products.
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Oxidation of secondary alcohols
The oxidation of secondary alcohols is a fundamental synthetic transformation in organic chemistry. This process involves the conversion of secondary alcohols to ketones. The oxidation of secondary alcohols is more favourable than that of primary alcohols due to the less crowded environment around the former.
Several reagents and methods have been developed for the oxidation of secondary alcohols, with varying levels of selectivity, reaction times, and yields. One method involves the use of chromium(VI) reagents, such as the Jones reagent, which is a solution of chromic acid (H2CrO4) formed from chromium trioxide (CrO3) or sodium dichromate (Na2Cr2O7) in the presence of sulfuric acid. The Jones reagent is a strong oxidizing agent that converts primary alcohols to carboxylic acids and secondary alcohols to ketones. However, caution must be exercised to prevent overoxidation, which can cleave carbon-carbon bonds if the temperature and concentrations are not carefully controlled.
Another method employs nitric acid and iron(III) chloride as catalysts in fluorinated alcohol solvents, such as 2,2,2-trifluoroethanol (TFE) and 1,1,1,3,3,3-hexafluoropropan-2-ol (HFIP). These solvents have unique properties, including high ionizing power and high hydrogen bond donor ability, which facilitate the reaction. This method selectively oxidizes secondary alcohols to ketones while leaving primary alcohol groups untouched.
In addition, the Stevens oxidation method utilizes sodium hypochlorite (household bleach) in acetone to efficiently convert secondary alcohols in the presence of primary alcohols. This method offers a practical alternative to other strong oxidizing agents. Furthermore, a rapid and selective oxidation of secondary alcohols to ketones can be achieved using trichloroisocyanuric acid and pyridine at room temperature in ethyl acetate.
It is worth noting that the presence or absence of water can impact the outcome of the oxidation process. Mild oxidizing agents, such as pyridinium chlorochromate (PCC) and pyridinium dichromate (PDC), are commonly used to stop the oxidation once the carbonyl group is formed, resulting in the production of aldehydes from primary alcohols and ketones from secondary alcohols.
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Oxidation of tertiary alcohols
Tertiary alcohols are organic compounds where the hydroxyl (-OH) group is attached to a carbon atom that is connected to three other carbon atoms. This structure makes them relatively stable and less reactive towards oxidation under normal conditions. However, they can be oxidised under certain conditions.
Under normal circumstances, tertiary alcohols are inert towards oxidation. The carbon atom in a tertiary alcohol is already attached to four other groups (including O), so it cannot be further oxidised without breaking the C-C bonds of the molecule, which requires a lot of energy. Acidified sodium or potassium dichromate(VI) solution does not oxidise tertiary alcohols because there is no hydrogen atom bound to the carbon in tertiary alcohols, and the formation of a carbon-oxygen double bond requires the elimination of two unique hydrogen atoms.
However, there are certain conditions under which tertiary alcohols can be oxidised. For example, they can be burned, which is a form of oxidation. Tertiary alcohols can also undergo unique reactions when they are allylic, such as allylic shifts, which allow for oxidation. Bobbitt's reagent is a specialised oxidising agent that is particularly effective for oxidising allylic alcohols. It facilitates the conversion of alcohols to carbonyl compounds through a mechanism that often involves the formation of a cyclic intermediate.
Photocatalysis of tertiary alcohols under highly defined vacuum conditions on a titania single crystal can also result in unexpected and new reactions. This process involves the disproportionation of the tertiary alcohol into an alkane and the respective ketone. The presence of platinum loadings not only increases the reaction rate but also opens up a new reaction channel: the formation of molecular hydrogen and a long-chain alkane resulting from the recombination of two alkyl moieties.
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Dehydration of alcohols
Alcohols are a family of chemical compounds with one or more hydroxyl (-OH) groups bound to a single-bonded alkane. They are amphoteric, meaning they can act as both acids and bases. This is due to the lone pair of electrons on the oxygen atom, which makes the -OH group weakly basic.
The dehydration of alcohols is a reaction that forms alkenes. Alcohols undergo E1 or E2 mechanisms to lose water and form a double bond. The reaction involves heating the alcohol in the presence of a strong acid, such as sulfuric or phosphoric acid, at high temperatures. The required temperature range decreases with increasing substitution of the hydroxy-containing carbon. If the reaction is not sufficiently heated, the alcohols may react with each other to form ethers instead.
The dehydration process involves the -OH group in the alcohol donating two electrons to the H+ from the acid reagent, forming an alkyloxonium ion. This ion then leaves to form a carbocation. The deprotonated acid (the nucleophile) then reacts with the hydrogen adjacent to the carbocation, forming a double bond.
Different types of alcohols may undergo dehydration through slightly different mechanisms. Primary alcohols undergo bimolecular elimination (E2 mechanism), while secondary and tertiary alcohols undergo unimolecular elimination (E1 mechanism). Tertiary alcohols are more resistant to oxidation due to their chemical structure.
The dehydrated products are a mixture of alkenes, with and without carbocation rearrangement. The most stable alkene is usually the major product formed, following Zaitsev's Rule. More-substituted and trans-substituted alkenes are generally favoured due to their higher stability.
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Esterification
The Fischer esterification mechanism involves six steps, each of which is reversible, with the starting materials and final products in equilibrium. In the first step, the carbonyl oxygen of the carboxylic acid is protonated with acid to form an oxonium ion. This protonated carbonyl is a stronger electrophile than a neutral carbonyl carbon. The second step involves the addition of a neutral nucleophile (ROH) to the protonated carboxylic acid, resulting in the formation of a tetrahedral intermediate. This step involves the formation of a new C-O bond and the breaking of a C-O bond.
The next two steps are collectively known as proton transfer, as they result in the net movement of a proton (H+) from one oxygen atom to another. This involves the deprotonation of the O-H group from the alcohol, followed by the protonation of the same O-H oxygen atom. The fifth step involves the deprotonation of the ester H atom by a base, leading to the formation of an OH group. Finally, another base deprotonates the last H atom on the newly formed O-H group, resulting in the formation of the carbonyl group of the ester.
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Frequently asked questions
Sodium ethoxide and hydrogen gas are produced.
Aldehydes and carboxylic acids are formed.
Ketones are formed.
A primary alcohol containing two more carbon atoms than the original Grignard reagent is formed.
Esters are formed.
