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

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SUMMARY

The discussion focuses on the correlation between molecular mass and the boiling and melting points of alcohols, specifically methanol, ethanol, 1-propanol, 1-butanol, and 1-pentanol. As molecular mass increases, the boiling points rise due to enhanced hydrogen bonding and temporary dipole interactions. The melting points exhibit variability influenced by molecular structure, particularly the degree of branching in the carbon chain, which affects molecular packing and contact. The ideal size for solid alignment appears to be 1-propanol, while branched alcohols like 1-butanol and 1-pentanol demonstrate lower melting points due to reduced intermolecular contact.

PREREQUISITES
  • Understanding of hydrogen bonding in organic compounds
  • Knowledge of molecular structure and polarity
  • Familiarity with the concept of temporary dipole forces
  • Basic principles of boiling and melting point determination
NEXT STEPS
  • Research the role of hydrogen bonding in alcohols and its effect on physical properties
  • Study the impact of molecular structure on boiling and melting points in organic compounds
  • Explore the differences between linear and branched alcohols regarding their physical properties
  • Investigate the concept of temporary dipole forces and their influence on boiling points
USEFUL FOR

Chemistry students, organic chemists, and anyone interested in the physical properties of alcohols and their molecular interactions.

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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!