Arielle Cruz
Alcohols are organic compounds with one or more hydroxyl groups (-OH) bound to a single-bonded alkane. They are easily synthesised and transformed into other compounds, making them important intermediates in organic synthesis. The most common reactions of alcohols are oxidation, dehydration, substitution, esterification, and reactions of alkoxides. Alcohols can be converted into esters through reaction with carboxylic acids, and they can also be converted into aldehydes, ketones, and carboxylic acids. The oxidation of primary alcohols produces aldehydes, while the oxidation of secondary alcohols produces ketones. Tertiary alcohols, on the other hand, are resistant to oxidation. Alcohols can also undergo dehydration reactions to form alkenes or ethers.
| Characteristics | Values |
|---|---|
| Alcohol oxidation | Aldehydes, ketones, and carboxylic acids |
| Alcohol dehydration | Alkenes or ethers |
| Alcohol substitution | Alkyl halides |
| Alcohol esterification | Esters |
| Alcohol reactions of alkoxides | Salts |
| Alcohol reaction with sodium metal | Sodium ethoxide and hydrogen gas |
| Alcohol reaction with tosylate | Stereochemical configuration differs from alcohol starting material |
| Alcohol reaction with carboxylic acid | Ester |
| Alcohol reaction with thionyl chloride | Inorganic ester |
| Alcohol reaction with potassium dichromate | Carboxylic acid |
| Alcohol reaction with manganese dioxide | Aldehyde |
| Alcohol reaction with Sarett-Collins reagent | Alderhyde |
| Alcohol reaction with pyridinium chlorochromate | Alderhyde |
| Alcohol reaction with potassium permanganate | Carboxylic acid |
| Alcohol reaction with chromium trioxide in acetic acid | Ketone |
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What You'll Learn
Oxidation of alcohols
Alcohol oxidation is a collection of oxidation reactions in organic chemistry that convert alcohols to aldehydes, ketones, carboxylic acids, and esters. The reaction applies mainly to primary and secondary alcohols. The oxidation of primary alcohols produces aldehydes or carboxylic acids, while secondary alcohols are oxidized to ketones. Tertiary alcohols are generally resistant to oxidation.
The oxidation of primary alcohols to aldehydes can be achieved using mild oxidizing agents such as pyridinium chlorochromate (PCC) or Collins reagent. PCC is a milder version of chromic acid that is suitable for converting primary alcohols into aldehydes without oxidizing them further to carboxylic acids. The reaction mechanism involves the attack of alcohol oxygen on the chromium atom to form a Cr-O bond, followed by the transfer of a proton from the OH group to one of the oxygens of chromium. This reaction can also be carried out using Dess-Martin periodinane (DMP), which offers advantages such as higher yields and less stringent reaction conditions.
Stronger oxidizing agents such as potassium permanganate (KMnO4), sodium dichromate (Na2Cr2O7), or ruthenium tetroxide are required to oxidize primary alcohols to carboxylic acids. The reaction with KMnO4 is typically carried out by adding KMnO4 to a solution or suspension of the alcohol in an alkaline aqueous solution. The so-called Jones reagent, prepared from chromium trioxide (CrO3) and aqueous sulfuric acid, also oxidizes alcohols to carboxylic acids, often with substantial amounts of esters as byproducts.
Secondary alcohols, on the other hand, are oxidized to ketones. This oxidation can be achieved using chromic acid (H2CrO4) or its milder version, pyridinium chlorochromate (PCC). Heating a secondary alcohol, such as propan-2-ol, with a sodium or potassium dichromate(VI) solution acidified with dilute sulfuric acid, results in the formation of a ketone called propanone. This reaction is not affected by changes in conditions, and further oxidation is not possible as it would require breaking a C-C bond, which demands a significant amount of energy.
The identification of primary, secondary, and tertiary alcohols can be achieved through an oxidation test using sodium dichromate (Na2Cr2O7). Primary alcohols are easily oxidized to aldehydes and can be further oxidized to carboxylic acids. Secondary alcohols are oxidized to ketones, but they do not undergo further oxidation. Tertiary alcohols do not react with sodium dichromate and are resistant to oxidation.
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Dehydration of alcohols
Primary alcohols undergo bimolecular elimination (E2 mechanism), while secondary and tertiary alcohols undergo unimolecular elimination (E1 mechanism). The hydroxyl oxygen donates two electrons to a proton from sulfuric acid (H2SO4), forming an alkyloxonium ion. The conjugate base, HSO4–, then reacts with one of the adjacent (beta) hydrogen atoms, while the alkyloxonium ion leaves, forming a double bond.
Secondary and tertiary alcohols dehydrate through the E1 mechanism. The secondary and tertiary –OH protonate to form alkyloxonium ions, which then leave and form a carbocation as the reaction intermediate. The water molecule then abstracts a proton from an adjacent carbon, forming a double bond.
The dehydration reaction of alcohols to generate alkenes involves heating the alcohols in the presence of a strong acid, such as sulfuric or phosphoric acid, at high temperatures. If the reaction is not sufficiently heated, the alcohols do not dehydrate to form alkenes but instead react with one another to form ethers.
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Formation of esters
Esters are organic compounds found in oils and fats. They are derived from carboxylic acids, which contain the --COOH group. In the formation of an ester, the hydrogen in this group is replaced by a hydrocarbon group, often an alkyl group. This reaction is called Fischer esterification and it involves combining an alcohol with an acid (with acid catalysis) to yield an ester and water.
The general formula for the esterification reaction is:
> RCOOH + R'OH → RCOOR' + H2O
Where R and R' can be the same or different. For example, to make ethyl ethanoate, you can react ethanoic acid (RCOOH) with ethanol (R'OH) in the presence of an acid catalyst, such as sulphuric acid (H2SO4), to form ethyl ethanoate (RCOOR') and water (H2O). This reaction can be written as:
> CH3COOH + CH3CH2OH → CH3COOCH2CH3 + H2O
The esterification reaction is slow and reversible, and it requires heat and a catalyst to generate the required energy. The catalyst lowers the activation energy, making it easier for the reaction to occur. The most common catalyst used is sulphuric acid, but other acids such as tosic acid (TsOH) can also be used. The reaction can be carried out in a test tube placed in a hot water bath for a few minutes.
The ester formed has a sweet and pleasant smell, which is often used in perfumes, food flavourings, and cosmetics. The smell can be detected by pouring the mixture into water, as the ester is the only component that does not form hydrogen bonds, and thus has the weakest intermolecular forces.
Esters can also be formed through the reaction of alcohols with acyl chlorides (acid chlorides) or acid anhydrides. For example, adding liquid ethanoyl chloride to ethanol results in a vigorous reaction, producing the ester ethyl ethanoate and hydrogen chloride. This reaction can be violent at room temperature, producing clouds of steamy acidic fumes.
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Conversion to alkyl halides
Alcohols can be converted to alkyl halides through various methods. The most common methods for converting primary and secondary alcohols to the corresponding chloro and 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 hydrohalic acids and the carbocation rearrangements associated with their use. Both of these reagents form an alkyl halide through an SN2 mechanism. The mechanism for both reactions starts by converting the hydroxide of the alcohol into a better leaving group through the formation of an intermediate. Thionyl chloride creates an intermediate chlorosulfite (-OSOCl2) compound, and phosphorus tribromide makes an intermediate dibromophosphite (-OPBr2) compound. These intermediate compounds can subsequently be eliminated as a leaving group during an SN2 reaction with the corresponding nucleophilic halide ion.
Alternatively, we can transform an alcohol group into a sulfonic ester using para-toluene sulfonyl chloride (Ts-Cl) or methanesulfonyl chloride (Ms-Cl), creating what is termed an organic tosylate or mesylate. The laboratory synthesis of isopentenyl diphosphate—the 'building block' molecule used by nature for the construction of isoprenoid molecules such as cholesterol and b-carotene—was accomplished by first converting the alcohol into an organic tosylate (step 1), then displacing the tosylate group with an inorganic pyrophosphate nucleophile (step 2). Tosylate and mesylate group's retention of conversion during formation makes them an important source of stereochemical control in organic synthesis.
In the context of SN1 reactions, the conversion of secondary alcohols to secondary alkyl halides by HX is an excellent opportunity to bring up the subject of carbocation rearrangements. This falls under the purview of the SN1 pathway. A good rule of thumb is to assume that secondary alcohols treated with HX will proceed through an SN1 mechanism. The SN1 mechanism is illustrated by the reaction of tert-butyl alcohol and aqueous hydrochloric acid (H3O+, Cl-). The first two steps in this SN1 substitution mechanism are the protonation of the alcohol to form an oxonium ion. Protonation of the alcohol converts a poor leaving group (OH-)- to a good leaving group (H2O), making the dissociation step of the SN1 mechanism more favourable.
On an industrial scale, the conversion of primary and secondary alkyl halides by SN1 substitution can be considered. However, it is important to pay attention to the bonds that form and break. Memorization is key, and with enough practice, the patterns will become more apparent, and memorization will become less crucial.
In terms of stereochemistry, if you need a specific stereochemistry for your final product, it is important to know whether the reaction will retain, invert, or scramble the configuration, creating a racemic mixture. For example, if you took (S)-butane-2-ol and treated it with HBr in SN1 conditions, you would end up with a racemic mixture instead of an enantiomerically pure final product. If stereochemistry is not a concern, and carbocation rearrangements are not an issue, the SN1 methods can be utilised. If stereochemistry is important, the SN2 methods should be employed.
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Alcohol as a nucleophile
Alcohols can act as nucleophiles and electrophiles. The hydroxyl groups of R-OH are not particularly reactive nucleophiles or electrophiles. However, the conjugate acid is a better leaving group, and the conjugate base is a better nucleophile.
When alcohols act as nucleophiles, they can undergo nucleophilic substitution reactions to form alkyl halides. This can occur through an SN1 or SN2 mechanism, depending on the nature of the substrate (alkyl group), leaving group, nucleophile, and solvent. The SN1 mechanism involves the formation of a carbocation intermediate, while the SN2 mechanism involves the formation of a transition pentavalent state.
In the SN1 mechanism, the alcohol is first protonated to form an oxonium ion, which can be viewed as a Lewis acid-base complex. This protonation converts the poor leaving group OH- to a good leaving group H2O, making the dissociation step more favorable. The carbocation then reacts with a nucleophile (a halide ion) to complete the substitution.
In the SN2 mechanism, the function of the acid is to produce a protonated alcohol. The halide ion then displaces a molecule of water from carbon, producing an alkyl halide. This mechanism is favored by primary alcohols and methanol.
Additionally, alcohols can undergo halogen substitution reactions, such as the reaction of cyclohexanol with 1-bromoethane to yield ethoxycyclohexane.
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Frequently asked questions
The primary alcohol is oxidised to form an aldehyde.
Carboxylic acids are formed.
Ketones are formed.
They can dehydrate each other to form an ether.
Esters are formed, often using a strong acid catalyst.
