Question

What is responsible for the differences in chemical shift observed in carbon 4 in 4-fluoroheptane and carbon 4 in heptane? View the provided images of heptane and 4-fluoroheptane with the fourth carbon atom indicated by an arrow. Check all that apply.

a. The electronegative fluorine atom generates a partial positive charge on carbon 4, resulting in a downfield shift of the carbon signal.
b. The hybridization of carbon 4 has changed, and this change in hybridization is responsible for the downfield shift of the carbon signal.
c. There is no difference in the chemical environment between carbon 4 in heptane and 4-fluoroheptane, so there is no difference in chemical shift.
d. The electronegative fluorine atom generates a partial negative charge on carbon 4, resulting in an upfield shift of the carbon signal.

Answer 1

The chemical shift difference between carbon 4 in 4-fluoroheptane and heptane is due to the electronegative fluorine atom in 4-fluoroheptane, which induces a partial positive charge on carbon 4, leading to a downfield shift in its NMR signal. The correct option is (a).

Step-by-step explanation:

The difference in chemical shift observed in carbon 4 of 4-fluoroheptane compared to carbon 4 in heptane is due to several factors:

• Electronegativity: The fluorine atom in 4-fluoroheptane is highly electronegative, creating a significant difference in electronegativity between it and the carbon atom. This pulls electron density away from carbon 4, leading to a partial positive charge on this carbon, and resulting in a downfield shift in the carbon's NMR signal.
• Hybridization: The hybridization of carbon 4 does not change with the addition of a fluorine atom. Both in heptane and 4-fluoroheptane, the carbon atoms are sp³ hybridized.
• Chemical Environment: The presence of a fluorine atom changes the chemical environment of carbon 4 in 4-fluoroheptane compared to heptane. In heptane, there is no highly electronegative substituent affecting the chemical shift of carbon 4.

Option a is correct because the partial positive charge induced by the electronegative fluorine causes a downfield shift. Option b is incorrect because the hybridization remains the same (sp³ hybridization). Option c is incorrect as the chemical environments are different due to the presence of the fluorine atom. Option d is incorrect because the electronegative fluorine creates a partial positive charge on carbon 4, not a partial negative charge, and this results in a downfield shift not an upfield shift.