Physical state Ethene, Propene, and Butene exists as colorless gases. Members of the 5 or more carbons such as Pentene, Hexene, and Heptene are liquid, and members of the 15 carbons or more are solids. Density Alkenes are lighter than water, therefore, are insoluble in water. Alkenes are only soluble in nonpolar solvent. Solubility Alkenes are virtually insoluble in water, but dissolve in organic solvents. The reasons for this are exactly the same as for the alkanes. Boiling Points 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. All the rest that you are likely to come across are liquids. 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. 0 1 Compound Boiling points (oC) Ethene -104 Propene -47 Trans-2-Butene 0.9 Cis-2-butene 3.7 Trans 1,2-dichlorobutene 155 Cis 1,2-dichlorobutene 152 1-Pentene 30 Trans-2-Pentene 36 Cis-2-Pentene 37 1-Heptene 115 3-Octene 122 3-Nonene 147 5-Decene 170 In each case, the alkene has a boiling point which is a small number of degrees lower than the corresponding alkane. The only attractions involved are Van der Waals dispersion forces, and these depend on the shape of the molecule and the number of electrons it contains. Each alkene has 2 fewer electrons than the alkane with the same number of carbons. Melting Points 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. 0 1 Compound Melting Points (0C) Ethene -169 Propene -185 Butene -138 1-Pentene -165 Trans-2-Pentene -135 Cis-2-Pentene -180 1-Heptene -119 3-Octene -101.9 3-Nonene -81.4 5-Decene -66.3 Polarity Chemical structure and fuctional groups can affect the polarity of alkenes compounds. The sp 2 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. Contributors Trung Nguyen Jim Clark ( Chemguide.co.uk )