Question

Draw structural formulas for the alkenes formed by acid-catalyzed dehydration of each alcohol. Where isomeric alkenes are possible, predict which is the major product. (a) 2-Methyl-2-butanol (b) 1 -Methylcyclopentanol

Short answer

Question: Draw the structural formulas for alkenes produced from the acid-catalyzed dehydration of 2-methyl-2-butanol and 1-methylcyclopentanol, and predict the major product when isomeric alkenes are formed. Answer: The structural formulas for the alkenes produced are CH3-C(CH3)=CH-CH3 for 2-methyl-2-butanol and a six-membered ring for 1-methylcyclopentanol. The major product for 2-methyl-2-butanol is CH3-C(CH3)=CH-CH3 due to Zaitsev's Rule, while the major product for 1-methylcyclopentanol is the six-membered ring as it is the only possible product.

Step-by-step solution

Identify the structure of the given alcohols

We will first need to identify the structure of both alcohols based on their given names. a) 2-Methyl-2-butanol: We have a butanol molecule, which has four carbon atoms and an alcohol group (-OH). A methyl group is attached to the second carbon atom from either end. So, the structure is CH3-C(CH3)-CH2-CH3 with the -OH group attached to the carbon with the methyl group. b) 1-Methylcyclopentanol: The base molecule is cyclopentanol, which consists of 5 carbon atoms in a ring formation with an -OH group attached to one of the carbon atoms. There is a methyl group attached to the same carbon with the alcohol group. So, the structure is a 5-membered ring with an -OH and CH3 group attached to one of the carbons.

Determine the possible alkene products

Now, we will identify the possible alkene products formed via the elimination reaction. a) For 2-methyl-2-butanol, an elimination reaction will occur when the hydrogen atom adjacent to the carbon with the -OH group is removed along with the –OH group, resulting in a π bond: (i) Removing a hydrogen from the CH2 group next to the carbon with the -OH group, we get: CH3-C(CH3)=CH-CH3 (ii) Alternatively, removing a hydrogen from the CH3 group on the carbon with the -OH group, we get: CH3-C(=CH2)-CH2-CH3 b) For 1-methylcyclopentanol, there is only one possible alkene product by removing a hydrogen atom adjacent to the carbon with the -OH group: (i) A six-membered ring is formed after removing a hydrogen from the CH2 group next to the carbon with the -OH group.

Predict the major product using Zaitsev's Rule

We will now predict the major products in cases where isomeric alkenes are possible using Zaitsev's Rule, which states that the major products will be the ones with the most highly substituted double bond. a) 2-Methyl-2-butanol: (i) CH3-C(CH3)=CH-CH3: The double bond has one methyl group on one end and two methyl groups on the other end. It is a tri-substituted alkene. (ii) CH3-C(=CH2)-CH2-CH3: The double bond has one methyl group on one end and no substituents on the other. It is a mono-substituted alkene. According to Zaitsev's Rule, the major product will be CH3-C(CH3)=CH-CH3 as it is more highly substituted. b) 1-Methylcyclopentanol only has one possible alkene product, therefore, it is the major product.

Key concepts

Structural Formulas Explained

Structural formulas are visual representations of molecules that show not only the kinds of atoms present but also their relative placement and the bonds between them. To comprehend acid-catalyzed dehydration of alcohols and the resulting formation of alkenes, it is essential to be able to interpret these formulas.

Consider the alcohol 2-Methyl-2-butanol. Its structural formula is based on the linear chain of butanol, with a crucial defining detail: the position and attachment of a methyl group (–CH₃) on the second carbon. This is crucial for understanding how molecules will rearrange during a chemical reaction. For 1-Methylcyclopentanol, the structural formula depicts a five-membered ring indicating the cyclic nature of the compound, with the –OH and –CH₃ groups attached to the same carbon atom. This structural visualization helps predict the behavior of the molecule during reactions, such as the creation of alkenes through dehydration.

Zaitsev's Rule

Zaitsev's Rule is a fundamental principle in organic chemistry, particularly when predicting the outcomes of elimination reactions, which often lead to the formation of alkenes. It states that in an elimination reaction, the most stable alkene product (usually the most substituted one) is the most likely 'major' product.

During the dehydration of alcohols, an –OH group and a hydrogen atom are removed from adjacent carbon atoms, causing a double bond to form between those carbons, resulting in an alkene. For instance, when analyzing 2-Methyl-2-butanol, you encounter two possibilities for which hydrogens could be removed. Zaitsev's Rule helps us predict that the major product will be the tri-substituted alkene—where the double bond is surrounded by more carbon groups—as it's more stable than the mono-substituted alternative.

Alkenes

Alkenes are hydrocarbons that contain a carbon-carbon double bond. Their general formula is C_nH_2n and they are known for being highly reactive due to their double bonds, which can participate in various chemical reactions, including polymerization and addition reactions.

Using the dehydration of alcohols as an example, alkenes are formed as a result of the elimination of a water molecule (-H and -OH), leaving behind a double bond. For 2-Methyl-2-butanol, the dehydration leads to either a tri-substituted or a mono-substituted alkene depending on where the water molecule is removed. The more substituted, and usually the more stable alkene, often dominates as the major product, adhering to Zaitsev's Rule. Alkenes are central in organic synthesis and material science due to their versatility and reactivity.

Substitution

Substitution reactions are a type of chemical reaction where one functional group in a molecule is replaced by another. In the context of alcohols, an acid-catalyzed dehydration is a form of substitution where the hydroxy group (-OH) is replaced by a double bond to form an alkene.

Acid-catalyzed dehydration typically involves the substitution of a water molecule, consisting of the -OH from the alcohol and a hydrogen atom typically from an adjacent carbon. The 'leaving group' in this case is not just the -OH, but the entire water molecule. Understanding substitution reactions and the properties of leaving groups is crucial for predicting and explaining the outcomes of reactions in organic chemistry, such as the formation of different isomers in dehydration reactions.