Question

Teflon-coated frying pans routinely endure temperatures that would cause polyethylene or polypropylene to oxidize and decompose. Decomposition of polyethylene is initiated by free-radical abstraction of a hydrogen atom by O₂. Bond-dissociation energies of C—H bonds are about 400 kJ/mol, and C—F bonds are about 460 kJ/mol. The BDE of the H—OO bond is about 192 kJ/mol, and the F—OO bond is about 63 kJ/mol. Show why Teflon (Figure 7-5) is much more resistant to oxidation than polyethylene is.

Accepted Answer

Step 1: Analyze the bond-dissociation energies (BDEs) provided in the problem. The C—H bond in polyethylene has a BDE of approximately 400 kJ/mol, while the C—F bond in Teflon has a higher BDE of about 460 kJ/mol. This indicates that the C—F bond is stronger and less likely to undergo free-radical abstraction compared to the C—H bond. Step 2: Compare the stability of the radicals formed during oxidation. When a hydrogen atom is abstracted from polyethylene, a carbon-centered radical is formed, which is relatively reactive. In contrast, the fluorine atoms in Teflon do not easily form radicals due to the high electronegativity and strong C—F bonds. Step 3: Examine the bond-dissociation energies of the H—OO and F—OO bonds. The H—OO bond has a BDE of 192 kJ/mol, making it relatively weak and prone to decomposition. The F—OO bond, however, has a much lower BDE of 63 kJ/mol, but the formation of such bonds is unlikely due to the inert nature of fluorine in Teflon. Step 4: Consider the chemical structure of Teflon (poly(tetrafluoroethylene)). The presence of fluorine atoms in Teflon creates a highly stable polymer due to the strong C—F bonds and the shielding effect of fluorine atoms, which prevent oxidative attack by O2 or other radicals. Step 5: Conclude that Teflon is much more resistant to oxidation than polyethylene because its chemical structure and bond strengths make it less susceptible to free-radical reactions and decomposition under high temperatures.

비슷한 문제에 대한 검증된 영상 답변:

주요 개념

Bond Dissociation Energy (BDE)

Free Radical Mechanism

Polymer Structure and Stability