Vivian Marlowe
Alcohols are organic compounds with a hydroxyl (OH) functional group on an aliphatic carbon atom. The general formula for an alcohol is ROH, where R is an alkyl group. In this context, R represents substituents, alkyl, or other attached groups. To add R groups to an alcohol, one method involves using Grignard reagents to react with carbon dioxide, resulting in an alkyl halide (RBr) with an additional carbon in the chain. This alkyl halide can then undergo further reactions to form aldehydes, ketones, or other alcohols. Another approach is to use hydrogen bromide (HBr) or hydrogen chloride (HCl) to convert tertiary or secondary alcohols into alkyl halides. These reactions showcase the synthetic versatility of alcohols and their ability to serve as precursors for various compounds.
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What You'll Learn
Using chromic acid to make aldehyde or ketone from alcohol
Chromic acid, H2CrO4, is a strong acid and a reagent for oxidizing alcohols to ketones and carboxylic acids. It is formed by combining chromium trioxide, hydrochloric acid, and pyridine. The reaction is usually run in acetone or another organic solvent.
When chromic acid is supported on an anion exchange resin, it becomes a selective oxidant for the preparation of aldehydes and ketones. In an aqueous solution, chromic acid converts aldehydes to carboxylic acids. The aldehyde first reacts with water to form a gem-diol, which is then oxidized by chromic acid to a carboxylic acid.
Primary alcohols are oxidized by chromic acid first to aldehydes and then to carboxylic acids. However, under certain conditions, such as cold temperatures, limited chromic acid, and short reaction times, it is possible to obtain an aldehyde as the final product. The oxidation of primary alcohols often forms esters. The primary alcohol is oxidized to an aldehyde, which then forms a hemiacetal with unoxidized alcohol. This hemiacetal is then oxidized to produce the ester.
Pyridinium chlorochromate (PCC) is another chromium(VI) compound that can be used to oxidize primary alcohols to aldehydes. When a primary alcohol is oxidized by PCC, water is absent, preventing the formation of a gem-diol, and the aldehyde is not further oxidized.
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Using pyridinium chlorochromate to make aldehyde from alcohol
Pyridinium chlorochromate (PCC) is a mild oxidizing reagent used in organic synthesis to selectively oxidize primary and secondary alcohols to aldehydes and ketones, respectively. It offers the advantage of being more selective than other reagents, such as Jones' Reagent, which can lead to over-oxidation and the formation of carboxylic acids.
The process of using PCC to make aldehydes from alcohols involves an oxidation reaction, specifically an elimination reaction. The general reaction equation is as follows:
2 [C5H5NH][CrO3Cl] + 3 R2CHOH → 2 [C5H5NH]Cl + Cr2O3 + 3 R2C=O + 3 H2O
In this reaction, one equivalent of PCC is added to the alcohol, resulting in the oxidized version of the alcohol. The byproduct of this reaction is Cr(IV) and pyridinium hydrochloride. It is important to carefully control the amount of water present in the reaction, as water can add to the aldehyde to form a hydrate, which could then be further oxidized by a second equivalent of PCC. This is not a concern with ketones since there is no hydrogen directly bonded to carbon.
The mechanism of the oxidation reaction involves the following steps:
- Attack of oxygen on chromium to form the Cr-O bond.
- Transfer of a proton from the (now positive) OH group to one of the oxygens on chromium, possibly through the pyridinium salt.
- Displacement of a chloride ion, forming a chromate ester.
- Removal of the proton on the carbon adjacent to oxygen by a base, resulting in the formation of the C-O double bond and the breaking of the O-Cr bond, with Cr(VI) becoming Cr(IV).
PCC is commercially available and was discovered by accident. It is prepared by adding pyridine to a cold solution of chromium trioxide in concentrated hydrochloric acid. An alternative method involves changing the order of addition to minimize the formation of toxic chromyl chloride (CrO2Cl2) fumes.
In summary, pyridinium chlorochromate is a valuable reagent for selectively oxidizing primary and secondary alcohols to aldehydes and ketones, respectively. It offers advantages over other reagents due to its mild oxidizing nature and selectivity, making it a useful tool in organic synthesis.
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Using LiAlH4 to make primary alcohol from ester or carboxylic acid
Lithium aluminum hydride (LiAlH4) is a strong reducing agent that can be used to convert carboxylic acids and esters into primary alcohols. This process involves breaking the C-O bonds and forming C-H bonds.
When using LiAlH4 to reduce carboxylic acids to primary alcohols, the following reaction occurs:
- The hydride (H-) from LiAlH4 attacks the proton on the carboxylic acid group, forming a carboxylate ion (COO-).
- Another equivalent of LiAlH4 has its hydride attack the carbon, breaking the double bond between the carbon and oxygen, resulting in an aldehyde.
- The lone pair electrons on the oxygen return to form a double bond, "kicking" off the other oxygen atom.
- A third equivalent of LiAlH4 sends its hydride to attack the same carbon, shifting the electrons back to the oxygen atom, forming the conjugate base of a primary alcohol (an alkoxide).
- Finally, the reaction is quenched with water to protonate the alkoxide and obtain the desired primary alcohol.
Similarly, LiAlH4 can be used to reduce esters to primary alcohols through a reduction reaction. Here's how the reaction proceeds:
- The hydride from the LiAlH4 molecule attacks the ester-carbon, breaking the double bond and sending the electrons to the oxygen.
- The free electrons from the negatively charged oxygen return to reform the carbon-oxygen double bond, breaking the ether carbon-oxygen bond and allowing the ether-oxygen group to leave the molecule.
- A second equivalent of LiAlH4 then sends its hydride to attack the aldehyde-carbon, resulting in the formation of a primary alcohol.
It is important to note that while LiAlH4 is a strong reducing agent, alternative reagents like NaBH4 are not suitable for converting carboxylic acids or esters into alcohols. LiAlH4's ability to reduce carboxylic acids and esters makes it a valuable reagent for synthesizing primary alcohols in organic chemistry.
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Using hydrogen bromide to make alkyl halide from tertiary alcohol
Tertiary alcohols can be converted to alkyl halides by reacting with HX (X = Cl, Br, I) acids. The reaction works for 1°, 2°, and 3° alcohols. The function of the acid is to produce a protonated alcohol. The halide ion then displaces a molecule of water (a good leaving group) from carbon, producing an alkyl halide.
Tertiary alcohols react reasonably rapidly with HCl, HBr, or HI, but for primary or secondary alcohols, the reaction rates are too slow for the reaction to be of much importance. The order of reactivity of the hydrogen halides is HI > HBr > HCl (HF is generally unreactive).
The most common methods for converting 1º- and 2º-alcohols to the corresponding bromo alkanes (i.e., replacement of the hydroxyl group) are treatments with thionyl chloride (SOCl2) and phosphorus tribromide (PBr3), respectively. These reagents are generally preferred over the use of concentrated HX due to the harsh acidity of these hydrohalic acids and the carbocation rearrangements associated with their use.
An alternative method is to use hydrogen bromide. In a triphasic phase-vanishing system comprised of an alkane, perfluorohexanes, and bromine, photoirradiation efficiently generates hydrogen bromide, which undergoes radical addition with 1-alkenes in the hydrocarbon layer to afford terminal bromides in high yields.
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Using alkyl halide to make primary alcohol
Alcohols are organic compounds with a hydroxyl (OH) functional group on an aliphatic carbon atom. The general formula for an alcohol is ROH, where R is an alkyl group.
Alkyl halides can be used to make primary alcohols through a process called nucleophilic substitution. This involves treating the alcohol with an HX reagent, such as HCl, HBr, or HI, which results in the formation of an alkyl halide through an SN2 mechanism. In this reaction, the halide ion displaces a molecule of water from the carbon, producing an alkyl halide.
The order of reactivity of the hydrogen halides is HI > HBr > HCl. Tertiary alcohols react rapidly with these reagents, while primary and secondary alcohols have slower reaction rates.
Another method for converting primary alcohols into alkyl halides involves using thionyl chloride. This reaction has the added benefit of producing gaseous by-products, which simplifies the isolation and purification of the reaction product.
It is important to note that direct displacement of the hydroxyl group does not occur in these reactions because the leaving group would have to be a strongly basic hydroxide ion. Instead, the hydroxyl group is first converted into a better leaving group through the formation of an intermediate compound.
Overall, the use of alkyl halides provides a versatile approach to synthesizing primary alcohols, allowing for the introduction of specific R groups through appropriate reagent selection.
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Frequently asked questions
R groups are substituents, alkyl or other attached, generally organic groups.
Alcohols can be made into alkyl halides and then converted into aldehydes and ketones, which are important starting points for carrying out a Grignard synthesis.
Grignard synthesis is a reaction that affects the C-O bond of an alcohol.
The general formula for an alcohol is ROH, where R is an alkyl group.
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