Question

When heated, carboxylic salts in which there is a good leaving group on the carbon beta to the carboxylate group undergo decarboxylation/elimination to give an alkene. (a) CC(C)=CCBr (b) Propose a mechanism for this type of decarboxylation/elimination. Compare the mechanism of these decarboxylations with the mechanism for decarboxylation of \(\beta\)-ketoacids. In what way(s) are the mechanisms similar?

Short answer

Answer: Both mechanisms involve the formation of a carbanion on the α-carbon and the creation of a double bond with the β-carbon, resulting in the release of CO₂. Moreover, in both reactions, there is a rearrangement of electrons to form the final products, such as the alkene formation in the elimination reaction and the formation of a ketone in decarboxylation of β-ketoacids.

Step-by-step solution

Identify the beta carbon and leaving group in the given molecule

In the given molecule, we have: CC(C)=CCBr Here, the β-carbon is the one adjacent to the carboxylate group (C in "=C"), and the leaving group is Br- (attached to the β-carbon).

Propose a mechanism for decarboxylation and elimination

The mechanism for this type of decarboxylation/elimination can be proposed as follows: 1. The carboxylate group loses a carbon dioxide molecule (CO_2), leading to the formation of a carbanion on the α-carbon which then forms a double bond with the β-carbon. 2. The leaving group (Br-) on the β-carbon departs, resulting in the formation of the alkene.

Compare the mechanism of these decarboxylations with β-Ketoacid decarboxylation mechanism

The mechanism of decarboxylation of β-ketoacids is as follows: 1. The β-ketoacid loses a proton from its α-carbon, creating a carbanion. 2. The carbanion forms a double bond with the β-carbon, leading to the formation of CO_2 and a ketone.

Identify similarities between both mechanisms

Both mechanisms have some similarities: 1. Both start with the formation of a carbanion on the α-carbon. 2. The carbanion forms a double bond with the β-carbon, resulting in the release of CO_2. 3. In both reactions, there is a rearrangement of electrons to form the final products. In the elimination reaction, the electrons from the departing leaving group on the β-carbon contributes to the alkene formation, while in decarboxylation of β-ketoacids, the carbanion electrons contribute to the formation of a ketone.

Key concepts

Carboxylic Salt Decarboxylation

Imagine a molecule as a tiny factory where atoms are workers doing specific jobs. In carboxylic salt decarboxylation, we are interested in the workers who handle 'carboxylate groups' – parts of the molecule with a carbon attached to oxygen atoms – and in particular, what happens when they start to leave, or decarboxylate.

When heat is applied to carboxylic salts with a good leaving group, such as a halogen, on the carbon that's positioned next to the carboxylate group (known as the beta carbon), these molecules tend to kick off a carbon dioxide molecule. This action transforms the molecule radically. More specifically, the carbon-oxygen bonds reorganize, leading to the formation of a new bond between the alpha and beta carbons. It's like one of our workers deciding to partner up with a neighbor to start a new project, which in our scenario results in the creation of an entirely different molecular structure.

This mechanism showcases the delicate balance of breaking and making bonds, which ultimately results in the removal of a carbon dioxide molecule – the decarboxylation. This process is essential because it simplifies molecules and facilitates the production of different compounds in both laboratory reactions and vital biological processes.

Alkene Formation from Decarboxylation

Now, let's shift our focus to the world of alkenes. Alkenes are molecules characterized by double bonds between carbon atoms, which makes them quite special and useful in various chemical reactions. How do we get there from carboxylic salts? Through a process similar to a factory retooling for a new product line: alkene formation from decarboxylation.

When the carbon dioxide leaves during decarboxylation, it creates an opportunity for the remaining carbon workers to regroup. The alpha carbon, now with an extra electron (forming a carbanion), reaches out and forms a new double bond with the beta carbon. This bond is what defines alkenes. This transformation is critical not just because it forms the alkene, but because it opens up a new world of reactions where the alkene can participate, thereby becoming a starting point for synthesizing a wide variety of chemicals, such as polymers or pharmaceuticals.

Beta-Ketoacid Decarboxylation

Beta-ketoacid decarboxylation takes place in a type of molecule called a beta-ketoacid, where the setup is somewhat different compared to the carboxylic salts. In the factory analogy, think of beta-ketoacids as complexes with several different departments. Here, one section of the molecule actively lets go of a proton (which is a positively charged hydrogen atom), while another department expels carbon dioxide.

The loss of the proton creates a carbanion that bonds with the beta carbon, just like in the decarboxylation of carboxylic salts. Then, as if announcing the completion of a major project, carbon dioxide is released. The big difference in beta-ketoacid decarboxylation is the end product: instead of an alkene, a ketone is formed. Ketones are key players in chemistry, essential in various fields, from fuel production to the fragrance industry.

Leaving Group in Elimination Reactions

Elimination reactions in chemistry are akin to a company downsizing, where a part of the workforce – the leaving group – is let go to streamline operations. In the molecular sense, a leaving group is an atom or group of atoms that detaches from the main structure, carrying with it a pair of electrons.

The stability and willingness of the leaving group to depart is crucial. A good leaving group makes for a smoother transition, similar to an employee who voluntarily resigns. Halogens such as bromine (Br) are excellent leaving groups because they're electronegative and can stabilize the extra electrons they take with them. In the context of forming alkenes, when the leaving group exits the beta carbon, it paves the way for a new double bond to form. Understanding the role of the leaving group is key in predicting and controlling the outcome of many organic reactions, making it a cornerstone concept for any chemist.