Structure and Properties Hydrocarbons Structure and Properties

Hydrocarbons Alkanes Alkenes- have one or more C=C double bonds single C-C bonds if all C’s have H’s attached molecules are called saturated hydrocarbons Alkenes- have one or more C=C double bonds Alkynes- have one or more CΞC triple bonds alkenes & alkynes are called unsaturated hydrocarbons because they have fewer than the maximum possible number of H atoms

Hydrocarbons can form either a straight chain or a cyclic (ring) structure hydrocarbons which are attached to the main structure are called alkyl groups and are named according to the number of carbons

Hydrocarbons a 4th group are the aromatic hydrocarbons which have a unique ring structure simplest is benzene and all others are derivatives of benzene

Physical Properties of Alkanes

Alkanes are insoluble in water Solubility Alkanes are insoluble in water This is because water molecules are polar, whereas alkanes are non-polar (all the C - C and C - H bonds are nearly purely covalent) The O - H bond in a water molecule is highly polar due to the high electronegativity of oxygen δ- H O H δ+ δ+

Solubility The hydrogen atoms in one water molecule are strongly attracted to the oxygen atoms in other water molecules, and the small size of the H atoms allows the molecules to approach each other very closely. This special attraction is called hydrogen bonding Due to alkanes non- polar nature they do not mix with the polar water molecule.

Similar hydrocarbons are found in beeswax Solubility The mutual insolubility of alkanes and water is used to advantage by many plants. Alkanes often make up part of the protective coating on leaves and fruits (C27H56 and C29H60). Similar hydrocarbons are found in beeswax The major function of plant waxes is to prevent water loss from the leaves and fruit.

This is due again to alkanes being non-polar. Boiling Points Alkanes have lower boiling points than most other organic molecules with a similar weight. This is due again to alkanes being non-polar. Alkane molecules are only held together by weak intermolecular forces called van der Waals attractions (London forces). Electrons are always moving and sometimes for a instant they can be unevenly distributed in a molecule and cause temporary dipole (positive and negative end).

Boiling Points These attractions are weak, so the process of separating the molecules (boiling) from one another requires little energy.

Boiling Points Since these attractive forces can only operate over short distances between the surface of molecules: The boiling points of alkanes rise as the chain length increases and fall as the chains become branched.

Boiling Points Note: In a homologous series there is a gradual increase in boiling point as the number of carbon atoms increases, for the very same reason. Organic compounds in the a homologous series vary by -CH2