Discuss the effect of the 'extra' hydroxyl groups on the physical properties of diols and triols, compared to alkanols that have only one hydroxyl group.
HINT: Consider the polar nature of the hydroxyl group
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To understand the effect of the 'extra' hydroxyl groups on the physical properties of diols and triols compared to alkanols with one hydroxyl group, we need to consider the polar nature of the hydroxyl group.
First, let's briefly discuss the properties of alkanols, which have just one hydroxyl group. Alkanols, such as methanol (CH3OH) and ethanol (CH3CH2OH), exhibit higher boiling points, melting points, and solubilities in water compared to hydrocarbons of similar molecular weight. This is because the hydroxyl group in alkanols is polar, containing an electronegative oxygen atom bonded to a hydrogen atom. As a result, the oxygen atom is partial negative, and the hydrogen atom is partial positive.
Now, let's consider diols and triols. Diols, also known as glycols, have two hydroxyl groups, while triols have three hydroxyl groups. These additional hydroxyl groups provide more polar sites in the molecule, leading to stronger intermolecular forces like hydrogen bonding.
Due to the presence of multiple hydroxyl groups, diols and triols have even higher boiling points, melting points, and solubilities in water compared to alkanols. The more hydroxyl groups there are, the stronger the intermolecular forces become. Hydrogen bonding occurs between the OH groups of neighboring molecules, creating a network of intermolecular attractions.
The stronger intermolecular forces in diols and triols result in higher boiling points because more energy is required to break these bonds and convert the substance from a liquid to a gas phase. Similarly, the stronger forces also result in higher melting points because more energy is needed to break the intermolecular bonds and convert the substance from a solid to a liquid phase.
Furthermore, the increased polarity due to the multiple hydroxyl groups also enhances the solubility of diols and triols in polar solvents like water. The polar OH groups of diols and triols can form hydrogen bonds with water molecules, allowing for better mixing and dissolution.
In summary, the presence of 'extra' hydroxyl groups in diols and triols increases the polarity and strengthens the intermolecular forces, resulting in higher boiling points, melting points, and solubilities compared to alkanols with only one hydroxyl group.