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

In summary, the boiling points and melting points of alcohols follow a trend where as the molecular mass increases, so does the boiling point. This is due to the increase in temporary dipole forces of attraction. For solids, the shape of the alcohol molecule may also play a role, with branched alcohols having lower melting points due to less contact. However, this trend is not observed in boiling points.
  • #1

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


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.
Physics news on Phys.org
  • #2
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?
  • #3
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?
  • #4
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!

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

What is the definition of "Alcohol Property Trends"?

"Alcohol Property Trends" refers to the study and analysis of changes and patterns in the market for alcoholic beverages, including sales, consumption, production, and pricing.

Why are alcohol property trends important to study?

Studying alcohol property trends can provide valuable insights for businesses in the alcohol industry, such as identifying emerging markets, understanding consumer preferences, and predicting future demand. It can also help policymakers make informed decisions about alcohol regulation and taxation.

What factors influence alcohol property trends?

Several factors can impact alcohol property trends, including economic conditions, consumer behavior, marketing strategies, government policies, and cultural attitudes towards alcohol consumption.

How can alcohol property trends be analyzed?

Alcohol property trends can be analyzed through various methods, such as conducting market research, tracking sales and production data, and using statistical models to identify patterns and correlations. Social media and online reviews can also provide insights into consumer perceptions and preferences.

What are some current alcohol property trends?

Some current alcohol property trends include the rise of craft and artisanal spirits, the increasing popularity of low- and no-alcohol options, and the growing demand for sustainable and ethically-produced alcoholic beverages. There is also a trend towards premiumization, with consumers willing to pay more for high-quality and unique alcoholic products.