11.4: Physical Properties of Alcohols
- Page ID
- 490443
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- Explain why alcohols and ethers of four or fewer carbon atoms are soluble in water while comparable alkanes are not soluble.
Alcohols can be considered derivatives of water (H2O; also written as HOH).
Like the H–O–H bond in water, the R–O–H bond is bent, and alcohol molecules are polar. This relationship is particularly apparent in small molecules and reflected in the physical and chemical properties of alcohols with low molar mass. Replacing a hydrogen atom from an alkane with an OH group allows the molecules to associate through hydrogen bonding (Figure \(\PageIndex{1}\)).
![](https://chem.libretexts.org/@api/deki/files/87066/%253D14.2.jpg?revision=1&size=bestfit&width=438&height=378)
Recall that physical properties are determined to a large extent by the type of intermolecular forces. Table \(\PageIndex{1}\) lists the molar masses and the boiling points of some common compounds. The table shows that substances with similar molar masses can have quite different boiling points.
Formula | Name | Molar Mass | Boiling Point (°C) |
---|---|---|---|
CH4 | methane | 16 | –164 |
HOH | water | 18 | 100 |
C2H6 | ethane | 30 | –89 |
CH3OH | methanol | 32 | 65 |
C3H8 | propane | 44 | –42 |
CH3CH2OH | ethanol | 46 | 78 |
C4H10 | butane | 58 | –1 |
CH3CH2CH2OH | 1-propanol | 60 | 97 |
Alkanes are nonpolar and are thus associated only through relatively weak dispersion forces. Alkanes with one to four carbon atoms are gases at room temperature. In contrast, even methanol (with one carbon atom) is a liquid at room temperature. Hydrogen bonding greatly increases the boiling points of alcohols compared to hydrocarbons of comparable molar mass. The boiling point is a rough measure of the amount of energy necessary to separate a liquid molecule from its nearest neighbors. If the molecules interact through hydrogen bonding, a relatively large quantity of energy must be supplied to break those intermolecular attractions. Only then can the molecule escape from the liquid into the gaseous state.
Alcohols can also engage in hydrogen bonding with water molecules (Figure \(\PageIndex{2}\)). Thus, whereas the hydrocarbons are insoluble in water, alcohols with one to three carbon atoms are completely soluble. As the length of the chain increases, however, the solubility of alcohols in water decreases; the molecules become more like hydrocarbons and less like water. The alcohol 1-decanol (CH3CH2CH2CH2CH2CH2CH2CH2CH2CH2OH) is essentially insoluble in water. We frequently find that the borderline of solubility in a family of organic compounds occurs at four or five carbon atoms.
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Summary
Alcohols have higher boiling points than do ethers and alkanes of similar molar masses because the OH group allows alcohol molecules to engage in hydrogen bonding. Alcohols of four or fewer carbon atoms are soluble in water because the alcohol molecules engage in hydrogen bonding with water molecules; comparable alkane molecules cannot engage in hydrogen bonding.