Question

\(\beta\) -lactams are: cyclic forms of the least reactive type of carboxylic acid derivative. more reactive than their straight-chain counterparts. molecules with high levels of ring strain. (A) I only (B) Il only (C) Il and III only (D) I, II, and III

Short answer

C

Step-by-step solution

Understand what β-lactams are

β-lactams are a class of antibiotics that have a four-membered lactam (cyclic amide) ring. They are characterized by their high reactivity due to ring strain.

Analyze the statements

To determine the correct answer, evaluate each statement regarding β-lactams: 1. Are they cyclic forms of the least reactive type of carboxylic acid derivative?2. Are they more reactive than their straight-chain counterparts?3. Do they have high levels of ring strain?

Evaluate Statement I

β-lactams are cyclic but are not the least reactive type of carboxylic acid derivatives. They are actually very reactive due to ring strain. Therefore, Statement I is false.

Evaluate Statement II

β-lactams are more reactive than their straight-chain counterparts due to the ring strain imposed by the four-membered ring. Thus, Statement II is true.

Evaluate Statement III

β-lactams have high levels of ring strain because they contain a four-membered ring, which creates angle strain and makes them more reactive. Thus, Statement III is true.

Identify the correct option

Based on the evaluation, Statements II and III are true. Therefore, the correct answer is (C) II and III only.

Key concepts

reactivity of β-lactams

β-lactams are recognized for their high reactivity, which is primarily linked to their chemical structure. At the core of this structure is a cyclized four-membered ring, known as a lactam. This ring is not only small but also opposes the natural bond angles preferred by the atoms within it.
The extreme ring strain present in β-lactams contributes significantly to their reactivity. Ring strain is essentially the tension that arises from the deviation of bond angles from the ideal values. In a four-membered ring, the bond angles are forced to be around 90 degrees.
This angle is far removed from the typical 120 degrees found in sp² hybridized carbon atoms, and 109.5 degrees for sp³ hybridized carbon atoms. The energy stored in this strained system makes β-lactams much more likely to participate in chemical reactions, seeking to release the built-up strain energy by breaking the ring open.
This is why β-lactams are often more reactive than their acyclic (non-cyclic) counterparts, which do not possess the same level of ring strain.

ring strain in β-lactams

Ring strain is a paramount factor in the chemistry of β-lactams. Due to the small size of the ring, β-lactams experience significant angular strain.
Angular strain arises because the bond angles in a four-membered ring do not match the preferred tetrahedral angle of 109.5 degrees in sp³ hybridization, nor the 120 degrees in sp² hybridization.
Additionally, torsional strain cannot be overlooked. This strain stems from the eclipsing interactions between adjacent hydrogen atoms or substituents on the carbons in the ring.
The combination of angular and torsional strain in β-lactams elevates the overall ring strain, making these molecules energetically unfavorable in their current form. This high level of strain thus pushes β-lactams towards chemical reactions that can alleviate this tension.
Such reactions typically involve opening the ring, which allows the ring strain to be released and the molecule to achieve a more thermodynamically stable state.

β-lactams compared to straight-chain counterparts

Comparing β-lactams to their straight-chain counterparts offers a clear perspective on their unique reactivity. While both structures contain the same functional groups, the spatial arrangement of these groups fundamentally alters their properties.
Straight-chain carboxylic acid derivatives lack the significant ring strain present in β-lactams. Without this strain, straight-chain molecules are more stable and hence less reactive under similar conditions.
The absence of angular strain and minimized torsional strain in acyclic compounds means these molecules do not have the compulsion to react as swiftly as β-lactams.
In essence, the ring strain pent up within the β-lactam ring makes it far more reactive. This heightened reactivity is particularly useful in pharmaceutical applications, where the ability to react swiftly with biological targets can enhance the effectiveness of antibiotic β-lactams.
Hence, understanding this comparison underscores why β-lactams are chosen for certain chemical and medicinal applications over their straight-chain analogs.