Question

Identify the unknowns in the following reaction sequence. $$ \underset{\text { (Hydrocarbon) }}{\mathrm{P}} \stackrel{\mathrm{Na} / \mathrm{liq} \mathrm{NH}_{3}}{\longrightarrow} \mathrm{Q} \stackrel{\mathrm{O}_{3}}{\mathrm{H}_{2} \mathrm{O}}{\longrightarrow} 2 \mathrm{R} \stackrel{\Delta}{\longrightarrow} 2 \mathrm{CH}_{3} \mathrm{COOH}+2 \mathrm{CO}_{2} $$

Short answer

The unknown compounds P, Q, and R in the given reaction sequence are as follows: 1. Compound P is an aromatic hydrocarbon, which is benzoic acid (HOOC-C6H4-COOH). 2. Compound Q is an alkene formed after the Birch reduction of P, which is butenedioic acid (HOOC-CH=CH-COOH). 3. Compound R is a carbonyl compound, which is oxalic acid (HOOC-CH2-COOH).

Step-by-step solution

Reaction of hydrocarbon P and sodium in liquid ammonia

The reaction of hydrocarbon P with sodium in liquid ammonia (NH3) is a Birch reduction. In this case, P must be an aromatic compound, and the Birch reduction produces an intermediate alkene as compound Q.

Ozonolysis of compound Q and reaction with water

Compound Q undergoes ozonolysis with O3, followed by reaction with water (H2O). This reaction cleaves the double bonds of an alkene (Q) to create corresponding carbonyl compounds, in this case two moles of compound R.

Decomposition of compound R

Compound R undergoes decomposition (Δ) to produce 2 moles of acetic acid (CH3COOH) and 2 moles of carbon dioxide (CO2). Since 2 moles of R produce 2 moles of acetic acid, we can conclude that there is one acetic acid present in the structure of R. Therefore, R can be represented as HOOC-CH2-COOH (oxalic acid).

Determine the structure of compound Q

Using the information about compound R (oxalic acid) and considering the cleavage of a double bond, we can determine the structure of compound Q. After ozonolysis, the following reaction sequence takes place: \( Q \xrightarrow [H_2O]{O_3} 2R \) \( R \equiv \mathrm{HOOC-CH_2-COOH} \) So, the structure of Q would be an alkene with two oxidizable double bonds: Q = HOOC-CH=CH-COOH (butenedioic acid)

Determine the structure of compound P

Now that we have determined compound Q (butenedioic acid), we can find the original aromatic hydrocarbon P. Since the Birch reduction was performed on P, we can infer that the double bonds in Q were derived from a single aromatic ring in P. The structure of compound P must be a benzene ring with two carboxyl groups: P = HOOC-C6H4-COOH (benzoic acid)

Key concepts

Aromatic Compounds

Aromatic compounds are a fascinating class of organic molecules characterized by their unique ring structure and special stability. The most well-known aromatic compound is benzene, which features a six-carbon ring with alternating single and double bonds. This arrangement is known as conjugation and contributes to the overall stability of the aromatic system.

Aromatic compounds adhere to Hückel's rule, which states that a compound is aromatic if it contains \(4n+2\) π electrons, where \(n\) is a non-negative integer. This rule explains why benzene, with six π electrons, displays aromaticity.

Aromatic compounds, due to their stability, often participate in electrophilic substitution reactions rather than addition reactions. In the context of the given exercise, the compound 'P' that undergoes Birch reduction is an aromatic compound. The Birch reduction partially hydrogenates the aromatic ring, converting it into a less stable, non-aromatic structure known as an alkene. This process of reducing aromatic rings selectively reduces certain double bonds, leaving others untouched.

The initial hydrocarbon in the exercise, identified as benzoic acid, is an aromatic compound where two carboxyl groups are directly attached to a benzene ring. During the Birch reduction, the aromatic ring's structure is temporarily altered, allowing for subsequent reactions like ozonolysis.

Ozonolysis

Ozonolysis is a useful chemical reaction for breaking down alkenes into smaller, more manageable pieces, allowing chemists to identify structures more easily. This reaction involves the addition of ozone (O3) to the double bonds of an alkene, resulting in the formation of ozonides. These unstable intermediates are then further reacted with water or other reducing agents, breaking down into carbonyl compounds like aldehydes or ketones.

In the exercise, compound 'Q' undergoes ozonolysis, leading to the cleavage of its double bonds. This processes transform 'Q' (later identified as butenedioic acid) into a compound known as 'R’, oxalic acid in this case. Ozonolysis is particularly useful when wanting to understand unknown molecular structures and can be employed to identify the positions of double bonds in unsaturated carbon chains.

The reaction is highly valuable for synthetic organic chemists when they need to map out or manipulate complex molecular architectures. It highlights the significance of double bonds in chemical transformations and their role in synthesis applications.

Carboxylic Acids

Carboxylic acids are a pivotal functional group in organic chemistry, known for their distinctive sour taste and pungent smell. Their structure is characterized by the presence of at least one carboxyl group \( -COOH \), making them crucial in various biological and chemical processes. Carboxylic acids can undergo various reactions including decarboxylation, reduction, and substitution, which are essential in both laboratory and industrial applications.

In the problem, the end product consists of acetic acid, a carboxylic acid, formed after the decomposition of compound 'R'. Carboxylic acids can act as intermediates, facilitating the transformation from larger molecules, such as aromatic compounds, into simpler compounds. This aspect is emphasized when compounds like butenedioic acid and oxalic acid are eventually broken down into acetic acid post-ozonolysis and thermal decomposition.

The presence of the carboxyl group in these compounds significantly impacts their chemical behavior, including their acidity. Their acidic nature is due to the ability of the carboxyl group to donate a proton (H⁺), influencing reactions and interactions with bases and metals.

• Carboxylic acids tend to have higher boiling points than similar-sized alcohols due to hydrogen bonding.
• They are commonly found in many foods and natural products, playing a role in cellular respiration and metabolism.
• Their salts and esters contribute to flavors and fragrances used in various consumer products.

Understanding these behaviors and properties provides insight into not only the exercise at hand but also the broader role carboxylic acids play in chemistry.