Alkanes

Naming Alkanes Step 1 : find the longest continuous chain of carbons and name it according to the number of carbon atoms it contains. Step 2: number the carbon atoms in the main chain, beginning at the end nearer to the first branch. Step 3: Identify the branching substituents and number each according to the carbon to which it is attached. Step 4.: Write the name as a single word, using hyphens to separate numbers from prefixes and commas to separate numbers. If two or more different substituentsp are present, list them in alphabetical order. If two or more identical substituents are present, use of the prefixes, di, tri, tetra, etc.

Drawing the Structure from the Name 1. Draw the structure of the parent chain according to the name. 2. Place the substituents on the carbons as indicated in the name.

Properties of Alkanes Alkanes contain non-polar C-C and C-H bonds, so the only intermolecular forces are London dispersion forces. First four alkanes are gases at room temperature and pressure. Alkanes with 5-15 carbons are liquids and those with 16 or more carbon atoms are waxy solids. Because they are non-polar, they are insoluble in water, but soluble in non- polar organic solvents.

Properties of Alkanes Because they are generally less dense than water, they float on its surface. Alkanes with low molecular weights are volatile and their vapors are flammable. Mixtures of alkane vapors and air can explode when ignited by a single spark. They are not very reactive.

Reactions of Alkanes Alkanes don’t react with acids, bases, or most other common lab reagents (they are not very reactive). Their only major reactions are combustion and halogenation.

Combustion of hydrocarbons Any hydrocarbon burned in the presence of oxygen will yield carbon dioxide and water. CxHy + O2 → CO2 + H2O When hydrocarbon combustion is incomplete, carbon monoxide and soot are also produced.

Combustion of hydrocarbons

Halogenation of Alkanes Halogenation is a substitution reaction in which a hydrogen atom is replaced by one of the two atoms of a halogen molecule. This reaction takes place in the presence of ultraviolet light or heat. If neither U.V. light nor heat is present, there is no reaction.

Halogenation of Alkanes For example, methane will react with chlorine to form chloromethane and hydrogen chloride.

Halogenation of Alkanes The UV light will split the chlorine molecule into 2 chlorine atoms, each with an unpaired electron (these are called free radicals) The process of splitting the chlorine molecule is called homolytic fission.

Halogenation of Alkanes

Halogenation of Alkanes

Halogenation of Alkanes So, once the chlorine radicals are formed, they will start a chain reaction in which a mixture of products including the halogenoalkane (CH3Cl) is formed.

Halogenation of Alkanes Similar reactions occur with other alkanes and bromine. The change in color of the bromine water from brown to colorless when it reacts with an alkane occurs only in the presence of UV light. This reaction is sometimes used to distinguish between alkanes and alkenes.