Functional Groups and Their Properties
What is a functional group? A structural arrangement of atoms which determines the physical and chemical properties in a given molecule There are 3 main types of functional groups Organic molecules are grouped into organic families based on the functional groups present All the molecules in an organic family will have similar structural characteristics and properties
1. Multiple Bonds Between C Atoms -C=C- and -CC- Unlike single C-C bonds, double and triple bonds allow atoms to be added to the chain. Double bonds are more reactive than single bonds. The increased reactivity of multiple bonds is due to the fact that they are weaker than single bonds. The electrons in a single bond are localized between 2 atoms resulting in a strong attraction between the two carbon nuclei (a sigma bond). Multiple bonds are weaker as they form when electrons are delocalized all around the carbon nuclei. These bonds are known as pi () bonds.
2. Carbon Bonded to a More Electronegative Atom (O, N, halogen) C-O, C-N, C-Cl, C-Br, C-F These atoms cause an unequal sharing of electrons resulting in polar bonds. The polar bonds increase the intermolecular forces raising melting and boiling points. C-OH, C-NH- These groups enable hydrogen bonding, increasing solubility in polar substances (i.e. water).
3. Carbon double-bonded to an O atom C=O This bond is extremely polar which increases the melting and boiling points due to higher intermolecular forces.
Why does increasing the number of polar groups in a substance increase the melting and boiling point? Polar groups increase the dipole-dipole interactions within a substance. This causes the molecules in a substance to have a greater attraction to each other (i.e. greater intermolecular forces). The increased intermolecular forces need more energy to break them therefore increasing the melting and boiling points.