Question

(a) While many \(\alpha, \beta\) -unsaturated carboxylic acids undergo decarboxylation upon heating, the \(\alpha, \beta\) -unsaturated acid shown below is immune to such a reaction. What does this suggest about the mechanism of decarboxylation of \(\alpha, \beta-\) unsaturated acids?

Short answer

The given α, β-unsaturated carboxylic acid cannot undergo decarboxylation due to steric hindrance caused by the two methyl groups present at the β-carbon, which prevents the necessary 6-membered cyclic transition state and [3,3]-sigmatropic rearrangement for the reaction to proceed. This suggests that the mechanism of decarboxylation for α, β-unsaturated carboxylic acids depends on a specific structural arrangement which enables a [3,3]-sigmatropic rearrangement via a 6-membered cyclic transition state.

Step-by-step solution

Recognize the structure of the given compound

The given compound is an α, β-unsaturated carboxylic acid, which means it has a carbon-carbon double bond between alpha (α) and beta (β) carbons and has a carboxylic acid functional group. The given molecule has the structure: ``` H3C-C(CH3)=C-COOH ```

Understand the mechanism of decarboxylation in α, β-unsaturated carboxylic acids

For an α, β-unsaturated carboxylic acid to undergo decarboxylation, it needs to proceed through a 6-membered cyclic transition state via a [3,3]-sigmatropic rearrangement. This transition state involves the breaking of the C-C bond between the carbonyl carbon and the α-carbon and the formation of a new C-C bond between the α-carbon and another carbon atom in the molecule. The rearrangement results in the loss of a carboxyl group (COOH) as carbon dioxide (CO2) and the formation of a new π-bond between the α- and the γ-carbon, leading to the formation of an alkene.

Identify the factors that prevent the given molecule from undergoing decarboxylation

The reaction cannot proceed via the 6-membered cyclic transition state, as there is no possible interaction between α-carbon and another carbon atom in the molecule for the formation of a new C-C bond. This is due to the steric hindrance caused by the two methyl groups present at the β-carbon. These two methyl groups effectively block the approach of the α-carbon to any other carbon atom in the molecule, and prevent the necessary rearrangement to take place. Considering these structural features, it becomes clear that the mechanism of decarboxylation in this particular α, β-unsaturated carboxylic acid is not possible, which indicates that the molecule cannot undergo decarboxylation upon heating.

Key concepts

Decarboxylation Mechanism

Decarboxylation is a chemical reaction that involves the removal of a carboxyl group from a molecule, particularly from carboxylic acids. For α, β-unsaturated carboxylic acids, this process generally requires forming a specific type of transition state. This state is called a 6-membered cyclic transition state and is integral to the mechanism.
Typically, the mechanism follows a [3,3]-sigmatropic rearrangement. This term describes the shifting of a bond from one position in the molecule to another, resulting in the loss of carbon dioxide (CO₂). It's as if the molecule rearranges its internal structure to remove a part of itself in a stable manner. The end product after this change is often an alkene, a simple molecule with a carbon-carbon double bond.
The requirement of a 6-membered ring configuration makes this process quite specific and limits the types of molecules that can successfully undergo decarboxylation. Factors like steric hindrance and molecular symmetry can disrupt this mechanism, preventing decarboxylation from occurring.

Stability of Unsaturated Acids

Unsaturated acids, such as α, β-unsaturated carboxylic acids, have some distinctive stability features. These acids contain a double bond between the alpha and beta carbons, which contributes to their structural uniqueness. Stability is often enhanced due to resonance. Resonance allows for the dispersion of electron density across the molecule, providing some resistance against reactions that might destabilize the acid.
For many α, β-unsaturated carboxylic acids, the double bond's position relative to the carboxylic group enhances stability by allowing potential interaction between these groups. Such interactions can impact the reactivity, including the likelihood of decarboxylation occurring. However, structural disruptions, like excessive steric hindrance, can compromise these stability benefits, as found in our example where the β-carbon is crowded by additional groups.
This structural crowding prevents the necessary interactions for the transition state, consequently blocking decarboxylation. Thus, the stability of unsaturated acids is a balancing act between electronic effects and physical structural constraints.

Steric Hindrance

Steric hindrance refers to the physical obstruction that large groups bonded to a central molecule create, which can deter certain chemical reactions. In our scenario, the presence of two methyl groups at the β-carbon creates significant steric hindrance.
These methyl groups physically block the approach of the α-carbon to form the required new bond in the 6-membered transition state needed for decarboxylation. This obstruction prevents the molecule from undergoing the necessary [3,3]-sigmatropic rearrangement.
Steric hindrance is significant because it highlights how the three-dimensional shape of molecules can impede certain reactions. Even if a reaction is chemically favorable due to resonance effects, too much bulkiness (like in our example) will make the process physically impossible. Consequently, considering steric hindrance is crucial in predicting the reactivity of organic molecules.

Organic Chemistry Transition States

In organic chemistry, a transition state represents a high-energy, unstable configuration that molecules form temporarily during a reaction. For α, β-unsaturated carboxylic acids undergoing decarboxylation, this state involves a temporary ring structure.
This 6-membered cyclic state serves as an intermediate that facilitates the breaking and making of bonds required for decarboxylation. The challenge with forming a transition state lies in achieving the right geometric configuration, which, if hindered by steric factors or incorrect molecular orientation, will prevent the reaction from progressing.
Understanding and predicting these transition states can reveal much about the reactivity and behavior of organic molecules. Molecules that fail to achieve the necessary transition state will not complete the reaction, as seen with unsaturated carboxylic acids hindered by steric factors. This aspect is pivotal in designing and predicting the outcomes of chemical reactions in organic chemistry.