Textbook Question
Without using pKa values, pick out the more acidic compound in each pair. Explain your answer.
(a)
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3. Acids and Bases
Ranking Acidity
6:55 minutesProblem 53a
Textbook Question
Compare the properties of propan-2-ol (I) and the hexafluoro analog (II).
(a) Compound II has almost triple the molecular weight of I, but II has a lower boiling point. Explain.
Verified step by step guidance1
Step 1: Analyze the molecular structures of propan-2-ol (I) and its hexafluoro analog (II). Compound I has two methyl groups (-CH3) attached to the central carbon, while compound II has two trifluoromethyl groups (-CF3) attached to the central carbon. The presence of fluorine atoms in II significantly increases its molecular weight compared to I.
Step 2: Compare the boiling points of the two compounds. Despite the higher molecular weight of II, its boiling point (58 °C) is lower than that of I (82 °C). This suggests that intermolecular forces in II are weaker than in I.
Step 3: Examine the dipole moments of the two compounds. Compound I has a dipole moment of 1.66 D, while compound II has a much lower dipole moment of 0.32 D. The lower dipole moment in II indicates reduced polarity, which diminishes the strength of dipole-dipole interactions between molecules.
Step 4: Consider the role of hydrogen bonding. The hydroxyl group (-OH) in both compounds can form hydrogen bonds. However, in compound II, the electron-withdrawing trifluoromethyl groups (-CF3) reduce the electron density on the oxygen atom of the hydroxyl group, weakening its ability to form hydrogen bonds. This contributes to the lower boiling point of II.
Step 5: Summarize the key factors. The lower boiling point of compound II, despite its higher molecular weight, is primarily due to its reduced polarity (lower dipole moment) and weakened hydrogen bonding capability caused by the electron-withdrawing effect of the trifluoromethyl groups.
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Video duration:
6mKey Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Molecular Weight and Boiling Point
Molecular weight is the sum of the atomic weights of all atoms in a molecule, influencing physical properties like boiling point. Generally, higher molecular weight compounds have higher boiling points due to increased van der Waals forces. However, in this case, despite compound II having a higher molecular weight, its lower boiling point can be attributed to weaker intermolecular forces due to the presence of fluorine atoms, which disrupt hydrogen bonding.
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Intermolecular Forces
Intermolecular forces are the forces of attraction or repulsion between molecules, significantly affecting boiling points. Propan-2-ol (I) can form strong hydrogen bonds due to its hydroxyl group, leading to a higher boiling point. In contrast, the hexafluoro analog (II) has weaker dipole-dipole interactions and lower hydrogen bonding capability, resulting in a lower boiling point despite its higher molecular weight.
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Dipole Moment and Polarity
The dipole moment is a measure of the separation of positive and negative charges in a molecule, indicating its polarity. A higher dipole moment suggests stronger interactions with polar solvents and greater hydrogen bonding potential. Propan-2-ol (I) has a higher dipole moment (1.66 D) compared to the hexafluoro analog (II) (0.32 D), which explains its stronger hydrogen bonding and higher boiling point despite the latter's greater molecular weight.
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