Explain in terms of molecular structure why sugars have a relatively high melting point compared to hydrocarbons of a similar size.
Sugars have so many OH bonds that hydrogen bonding can take place easily which makes it harder to break the crystal lattice.
To understand why sugars have a relatively high melting point compared to hydrocarbons of a similar size, we need to examine their molecular structures. Sugars are classified as carbohydrates and consist of carbon, hydrogen, and oxygen atoms arranged in a specific way. On the other hand, hydrocarbons contain only carbon and hydrogen atoms.
Sugars have a general formula of (CH2O)n, where n represents the number of carbon atoms in the molecule. For example, glucose, one of the simplest sugars, has the formula C6H12O6. In glucose, there are six carbon atoms, twelve hydrogen atoms, and six oxygen atoms.
The presence of oxygen atoms in the sugar molecules creates polar bonds between carbon and oxygen. Unlike hydrocarbons, sugars can form intermolecular hydrogen bonds due to the presence of hydroxyl groups (-OH) on their molecules. Hydrogen bonding is a strong type of intermolecular force that occurs between a hydrogen atom and a highly electronegative atom, such as oxygen or nitrogen.
These hydrogen bonds play a significant role in raising the melting point of sugars. When sugars are heated, the energy supplied breaks the weaker intermolecular forces, such as van der Waals forces, between the sugar molecules. However, the stronger hydrogen bonds require more energy to break, hence leading to a higher melting point.
In contrast, hydrocarbons lack polar bonds and do not have the ability to form hydrogen bonds. Instead, they have weaker intermolecular forces, such as van der Waals forces, which are relatively easy to overcome. Therefore, hydrocarbons have lower melting points compared to sugars of similar molecular size.
In summary, the presence of polar bonds and the ability to form stronger hydrogen bonds in sugar molecules contribute to their relatively high melting point compared to hydrocarbons of a similar size.