Question

Consider the following reactions. For parts \(\mathrm{b}-\mathrm{d}\), see Exercise \(62 .\) a. When \(\mathrm{C}_{5} \mathrm{H}_{12}\) is reacted with \(\mathrm{Cl}_{2}(g)\) in the presence of ultraviolet light, four different monochlorination products form. What is the structure of \(\mathrm{C}_{5} \mathrm{H}_{12}\) in this reaction? b. When \(\mathrm{C}_{4} \mathrm{H}_{8}\) is reacted with \(\mathrm{H}_{2} \mathrm{O}\), a tertiary alcohol is produced as the major product. What is the structure of \(\mathrm{C}_{4} \mathrm{H}_{8}\) in this reaction? c. When \(\mathrm{C}_{7} \mathrm{H}_{12}\) is reacted with \(\mathrm{HCl}\), 1 -chloro-1-methylcyclohexane is produced as the major product. What are the two possible structures for \(\mathrm{C}_{7} \mathrm{H}_{12}\) in this reaction? d. When a hydrocarbon is reacted with \(\mathrm{H}_{2} \mathrm{O}\) and the major product of this reaction is then oxidized, acetone (2-propanone) is produced. What is the structure of the hydrocarbon in this reaction? e. When \(\mathrm{C}_{5} \mathrm{H}_{12} \mathrm{O}\) is oxidized, a carboxylic acid is produced. What are the possible structures for \(\mathrm{C}_{5} \mathrm{H}_{12} \mathrm{O}\) in this reaction?

Short answer

a. The structure of C5H12 in the reaction with Cl2 in the presence of UV light is pentane, which is as follows: H3C-CH2-CH2-CH2-CH3. b. The structure of C4H8 when reacting with water whereby a tertiary alcohol is produced is 2-methylpropene (isobutylene) and represented as: H2C=C(CH3)2. c. The reaction of C7H12 with HCl can yield 1-chloro-1-methylcyclohexane through two possible structures of C7H12 which are 1-methylcyclohexene and 3-methylcyclohexene. d. The hydrocarbon reacting with water followed by oxidation resulting in acetone is also 2-methylpropene (isobutylene): H2C=C(CH3)2. e. The possible structures of C5H12O that get oxidized to carboxylic acids are 1-pentanol: H3C-CH2-CH2-CH2-CH2-OH and pentan-2-ol: H3C-CH2-CH(OH)-CH2-CH3.

Step-by-step solution

Reaction of C5H12 with Cl2 in the presence of Ultraviolet light

C5H12 is a saturated alkane with a molecular formula of CnH2n+2. With five carbon atoms, this compound is pentane. There are no other structural isomers for C5H12 besides pentane itself. The structure of C5H12 in this reaction is: H3C-CH2-CH2-CH2-CH3. b. Determine the structure of C4H8 when it reacts with H2O to produce a tertiary alcohol

Reaction of C4H8 with H2O

C4H8 is an alkene with a molecular formula of CnH2n. In this reaction, a tertiary alcohol is formed. This indicates that an elimination process must occur at a tertiary carbon. The structure of C4H8 in this reaction is 2-methylpropene (isobutylene). The structure of C4H8 in this reaction is: H2C=C(CH3)2. c. Identify two possible structures for C7H12 when it reacts with HCl and produces 1-chloro-1-methylcyclohexane

Reaction of C7H12 with HCl

C7H12 is a seven-carbon alkene (CnH2n) that reacts with HCl to form 1-chloro-1-methylcyclohexane as the major product. The two possible structures for C7H12 that could lead to this product are 1-methylcyclohexene and 3-methylcyclohexene. The two possible structures for the reaction are: 1. 1-methylcyclohexene: Ch3C1 of HCl adds to the double bond on position 1 of the cyclohexene producing 1-chloro-1-methylcyclohexane. 2. 3-methylcyclohexene: H atom of HCl adds to the double bond on position 3 of the cyclohexene leading to a carbocation on position 1 and followed by addition of chlorine producing 1-chloro-1-methylcyclohexane. d. Determine the structure of the hydrocarbon when it reacts with H2O and the major product is oxidized to acetone

Reaction of hydrocarbon with H2O followed by oxidation to acetone

Acetone has a molecular formula of C3H6O. If the major product of the reaction between the hydrocarbon and H2O is then oxidized to acetone, we deduce that the initial hydrocarbon contains four carbon atoms (C4H8). Since a double bond is present, the possible hydrocarbon structure is that of an alkene. The structure of the hydrocarbon in this reaction is 2-methylpropene (isobutylene) as H2O adds across the double bond and oxidized later to 2-propanone (acetone). The structure of the hydrocarbon in this reaction is: H2C=C(CH3)2. e. Identify possible structures for C5H12O, which upon oxidation produces a carboxylic acid

Oxidation of C5H12O to carboxylic acid

C5H12O is a five-carbon alcohol. Alcohols can be oxidized to aldehydes, ketones, or carboxylic acids. In this case, oxidation generates a carboxylic acid. The possible structures of C5H12O that would be oxidized to carboxylic acids are: 1. 1-pentanol: Upon oxidation, this primary alcohol is converted into pentanoic acid. The structure of 1-pentanol is: H3C-CH2-CH2-CH2-CH2-OH. 2. Pentan-2-ol: Upon oxidation, this secondary alcohol is converted to pentanoic acid. The structure of pentan-2-ol is: H3C-CH2-CH(OH)-CH2-CH3.

Key concepts

Alkanes and Alkenes

Understanding alkanes and alkenes is essential for exploring organic chemistry reactions. Alkanes, like the compound \(C_5H_{12}\) in our problem, are saturated hydrocarbons. This means they contain only single bonds. Alkanes have the general formula of \(CnH_{2n+2}\), signifying that for every carbon atom, there are typically two hydrogen atoms plus two more. This saturation leads to generally low reactivity, unless they undergo reactions like halogenation, where a chlorine atom can replace a hydrogen atom in ultraviolet light.
Conversely, alkenes, such as \(C_4H_8\) in our example, are unsaturated hydrocarbons. They have at least one carbon-carbon double bond. This double bond gives alkenes higher reactivity. Alkenes follow the formula \(CnH_{2n}\), indicating a lesser number of hydrogen atoms compared to alkanes with the same number of carbon atoms. In reactions, this double bond can be broken to allow new atoms or groups to attach, making alkenes reactive intermediates.
Understanding these basic structures helps in predicting how a molecule might react under various conditions. Alkanes are usually less reactive, while alkenes with their double bonds are highly reactive and form the basis for many other organic reactions.Takeaways:

• Alkanes are saturated hydrocarbons with single bonds.
• Alkenes contain double bonds and are more reactive.
• Formulas help determine the structure and behavior of these molecules in reactions.

Oxidation Reactions

Oxidation reactions are a key part of transforming organic compounds. In organic chemistry, oxidation often involves the loss of hydrogen atoms or the gain of oxygen atoms in a compound. For alcohols like \(C_5H_{12}O\), oxidation can transform them into different products such as aldehydes, ketones, or carboxylic acids, depending on the structure.
For instance, primary alcohols (like 1-pentanol) oxidize to form carboxylic acids. In our problem, the primary alcohol transforms into pentanoic acid. Secondary alcohols, such as pentan-2-ol, can also form carboxylic acids, although they typically first create ketones. The oxidation of alkenes can be directed to form various products, such as in the formation of acetone from structures in our problem.
These reactions are crucial for creating various compounds from simple hydrocarbons. Whether creating new building blocks for further reactions or understanding the pathways to break down complex molecules, oxidation is a fundamental reaction.
Key Points:

• Oxidation is the gain of oxygen or loss of hydrogen atoms.
• It can transform simple alcohols into acids like pentanoic acid.
• Understanding the initial structure helps predict oxidation products.

Molecular Structure Determination

Determining the molecular structure of organic compounds involves understanding how atoms are arranged. For hydrocarbons like alkanes and alkenes, this means identifying the carbon backbone and any functional groups attached. Identifying these features allows us to predict the types of reactions each molecule can undergo and what products might form.
With practice, one can determine structures based on the reactions the molecules participate in. For example, if a particular hydrocarbon reacts with \(Cl_2\) to produce four chlorination products as mentioned, it suggests the presence of four unique hydrogen positions that can be replaced. Similarly, reacting a compound with water to produce a tertiary alcohol gives insight into the initial presence of an unsymmetrical alkene.
Techniques like these, alongside spectroscopic methods, allow chemists to confirm the structure of unknown molecules. Recognizing patterns in reactivity, such as those provided in textbook examples, is an essential skill for mastering organic chemistry.
Highlights:

• The structure determines reactivity and potential reaction products.
• Knowing molecular structure guides the understanding of reaction pathways.
• Practical exercises enhance ability to deduce structure based on reactions.