Question

The properties of alkenes and alkynes related to boiling, melting, polarity and solubility

Answer 1

boiling point

alkene

The boiling point of each alkene is very similar to that of the alkane with the same number of carbon atoms. Ethene, propene and the various butenes are gases at room temperature.  Boiling points of alkenes depends on more molecular mass (chain length). The more intermolecular mass is added, the higher the boiling point. Intermolecular forces of alkenes gets stronger with increase in the size of the molecules.

alkyne

Compared to alkanes and alkenes, alkynes have a slightly higher boiling point. Ethane has a boiling point of -88.6 ^oC, while Ethene is -103.7 ^oC and Ethyne has a higher boiling point of -84.0 ^oC.

melting point

alkene

Melting points of alkenes depends on the packaging of the molecules. Alkenes have similar melting points to that of alkanes, however, in cis isomers molecules are package in a U-bending shape, therefore, will display a lower melting points to that of the trans isomers.

alkyne also follow the same trends as alkane and alkenes

polarity

alkene

Chemical structure and fuctional groups can affect the polarity of alkenes compounds. The sp2 carbon is much more electron-withdrawing than the sp3 hybridize orbitals, therefore, creates a weak dipole along the substituent weak alkenly carbon bond. The two individual dipoles together form a net molecular dipole. In trans-subsituted alkenes, the dipole cancel each other out. In cis-subsituted alkenes there is a net dipole, therefore contributing to higher boiling in cis-isomers than trans-isomers.

alkyne

Alkynes are compounds which have low polarity

solubility

Alkenes are virtually insoluble in water, but dissolve in organic solvents

alkynes are insoluble in water.,They are quite soluble in the usual organic solvents of low polarity (e.g. ligroin, ether, benzene, carbon tetrachloride, etc.)

Most important differences between alkene and alkyne is terminal alkyne is highly acidic than alkene.