Why do the boiling and melting points of alcohols increase with molecular mass?

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Discussion Overview

The discussion centers on the trends in boiling and melting points of alcohols with increasing molecular mass, specifically examining the molecular structure and intermolecular forces involved. Participants explore the relationship between molecular size, hydrogen bonding, and the physical properties of various alcohols.

Discussion Character

  • Homework-related
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant notes the boiling and melting points of several alcohols and suggests that the hydrogen-carbon and carbon-hydroxyl bonds contribute to understanding these trends.
  • Another participant challenges the initial explanation, emphasizing the importance of hydrogen bonding and questioning the role of C-H bonds in comparison to O-H bonds.
  • A different participant proposes that hydrogen bonding is key, suggesting that larger molecules may fit together less tightly in solids, with 1-propanol being an ideal size for solid alignment of hydroxyl groups.
  • Another participant observes that as molecular mass increases with additional -CH2 groups, boiling points rise, and introduces the concept of temporary dipole forces while noting that the shape of alcohols may affect melting points, particularly for branched versus linear structures.
  • This participant also expresses uncertainty about why boiling points do not follow the same pattern as melting points for branched alcohols.

Areas of Agreement / Disagreement

Participants express differing views on the factors influencing boiling and melting points, particularly regarding the significance of hydrogen bonding, molecular shape, and the role of temporary dipole forces. The discussion remains unresolved with multiple competing perspectives.

Contextual Notes

Participants do not fully agree on the mechanisms behind the observed trends, and there are unresolved questions regarding the influence of molecular structure on boiling and melting points, particularly for branched alcohols.

doggbAT
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Homework Statement


The boiling points (ºC) of methanol, ethanol, 1-propanol, 1-butanol, and 1-pentanol are 64.7, 78.5, 97.2, 117 to 118, 137.5, respectively. The melting points are -97.8, -114.1, -1277.0, -90.0, and -79.0, respectively. Explain these trends with reference to molecular structure.


Homework Equations


N/A


The Attempt at a Solution


The bonds in alcohols are hydrogen-carbon bonds and the carbon-hydroxyl bonds. By looking at the H-C bond, one can understand that the H end is partially positive. In the hydroxyl group, the O is partially negative, and has two pairs of electrons, making it quite negative.
 
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What you wrote doesn't explain observable trends. Besides, C-H bond is almost not polarised (especially when compared with O-H bond).

Think - what forces keep these molecules together in solids and liquids?
 
Okay, so it's the hydrogen bonding, and as the molecules grow larger, they can fit together less tightly. Now I'm going to take a guess and say that for solids, the ideal size carbon chain comes with 1-propanol; that is, the molecules can fit together in a way that aligns hydroxyl groups best. For liquids this doesn't matter because they have some movement, and the smaller the molecule, the more chance there is of hydoxyl contact.


Is this correct?
 
what can be observed is that when the molecular mass increases, ie you are adding more -CH2 groups, the boiling point is increasing. you can use a knowledge of temporary dipole forces of attractions.

probably for the solids, the shape the alcohols take matters more. methanol, ethanol and propan-1-ol are linear. but butan-1-ol and pentan-1-ol may be branched. so the values of melting points for the two last alcohols may be averaged values.

the branched alcohols would normally have lower melting points because of less contact compared. this gives a lower average value.

but i wouldn't be able to explain why this is not the case for the boiling points!