Theo Nash
Alcohols can undergo a variety of reactions, including oxidation, substitution, and dehydration. The specific reaction pathway depends on the structure of the alcohol, particularly the nature of the carbon atom attached to the hydroxyl (-OH) group. This topic will explore the substitution mechanisms favoured by different alcohols, specifically focusing on primary, secondary, and tertiary alcohols. By understanding the factors that influence reaction selectivity, we can predict the predominant substitution mechanisms for various alcohol substrates.
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What You'll Learn
Nucleophilic substitution reactions
Alcohols can undergo nucleophilic substitution reactions, where they act as nucleophiles. In these reactions, the -OH group is protonated by a strong acid catalyst, converting it into a good leaving group. This protonation step is crucial as it facilitates the subsequent substitution reaction.
There are two main types of nucleophilic substitution reactions: SN1 and SN2. In the SN1 mechanism, the first step is the protonation of the alcohol, forming an oxonium ion. This step improves the leaving ability of the -OH group, making it more favourable for dissociation. Subsequently, a carbocation is formed, which then reacts with a nucleophile (a halide ion) to complete the substitution. SN1 reactions are favoured by tertiary alcohols.
On the other hand, in SN2 reactions, primary alcohols are preferred. The SN2 mechanism involves a backside attack by the nucleophile, which replaces the -OH group. The rate of the SN2 reaction decreases as the number of groups around the leaving group increases, creating steric hindrance.
Additionally, alcohols can undergo a halogen substitution reaction, where they act as nucleophiles. For example, cyclohexanol can react with thinoyl chloride to yield ethoxycyclohexane.
It is important to note that the protection of alcohols is also a crucial aspect of nucleophilic substitution reactions. One common method of protecting alcohols is by converting the -OH group to a -O-TMS (trimethylsilyl) group using a silylating reagent such as TMS chloride. This prevents the alcohol from undergoing undesirable reactions and allows for selective functionalization.
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SN1 reactions
The abbreviation SN1 stands for "unimolecular nucleophilic substitution". SN1 reactions are a type of nucleophilic substitution reaction, which involves the replacement of one nucleophile with another. In the context of alcohols, this typically involves the replacement of a hydroxyl group (-OH) with another functional group.
In SN1 reactions, the rate of reaction depends only on the concentration of the substrate (the alkyl halide) and not on the concentration of the attacking nucleophile. This is because the reaction begins with the loss of a leaving group, which results in the formation of a carbocation. The carbocation is then attacked by the nucleophile to form a new bond. Since the nucleophile attacks the carbocation only after the leaving group has departed, there is no need for a backside attack, and the nucleophile can attack from either side. This results in the formation of both enantiomers in the reaction, leading to a racemic mixture.
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SN2 reactions
The reactivity of alcohols in SN2 reactions follows the order: primary (fastest) > secondary > tertiary (slowest). This trend is due to the increasing steric hindrance associated with the additional alkyl groups in secondary and tertiary alcohols, which hinders the backside attack of the nucleophile.
While primary alcohols tend to favour the SN2 mechanism, secondary alcohols can exhibit a mixture of SN1 and SN2 reaction characteristics. This is because secondary alcohols can form a secondary carbocation, which is more stable than a primary carbocation. However, the SN2 reaction is still possible if the secondary alcohol is first protonated to form the conjugate acid, thereby providing a good leaving group.
Tertiary alcohols, on the other hand, are resistant to SN2 reactions due to the high steric hindrance of the three alkyl groups attached to the carbon bearing the -OH group. Instead, tertiary alcohols typically undergo SN1 reactions, which involve the formation of a carbocation through the departure of a leaving group, followed by the attack of a nucleophile.
In summary, the SN2 reaction is a nucleophilic substitution reaction that is favoured by primary alcohols due to the lack of steric hindrance, allowing for an unhindered backside attack by the nucleophile. Secondary alcohols can exhibit a mix of SN1 and SN2 characteristics, while tertiary alcohols predominantly undergo SN1 reactions due to the high steric hindrance associated with multiple alkyl groups.
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E1 reactions
In the second step, a base removes a proton, forming the alkene. This follows Zaitsev's rule, which states that the more substituted alkene is the major product. The final product is an alkene, with a byproduct of HB. The E1 reaction is regiospecific but not stereospecific, meaning that a hydrogen is not required to be anti-coplanar to the leaving group.
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E2 reactions
The E2 reaction follows Zaitsev's rule, which states that the most stable alkene will be the major product. The stereochemistry of the C-H bond and the leaving group is always "anti", and the leaving groups must be coplanar to allow for the formation of a pi bond. The carbons go from sp3 to sp2 hybridization states. The product can be both eclipsed and staggered, depending on the transition states.
Primary alcohols can undergo E2 reactions, but only in the presence of a hindered (bulky) base. This is because primary alcohols have less steric hindrance, making them more accessible to backside attacks by nucleophiles, which is characteristic of SN2 reactions.
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Frequently asked questions
Primary alcohols are favored in SN2 reactions.
Secondary alcohols can potentially undergo all four SN1/SN2/E1/E2 reactions.
Tertiary alcohols are favored in SN1 reactions.
Benzyl alcohol can be protonated to give the conjugate acid, and the leaving group would be water.
