ORGANIC CHEMISTRY I CHEM 2323

Chapter 2 - METHANE

HYDROCARBONS

Hydrocarbons contain hydrogen (H) and carbon (C)

Hydrocarbons can be divided into the following groups

STRUCTURE OF METHANE

Methane is CH₄

Structure = sp³

PHYSICAL PROPERTIES

non-polar (Van der Waals forces)

mp -183^oc

bp -161.5^oc

Attractive forces (intermolecular) are easily overcome by thermal energy.

Colorless, less dense than water, only slightly soluble in H₂O.

SOURCE

Methane is the end product of the decay of plants (swamp gas and natural gas)

Natural gas (fossil fuel) is approximately 97% methane.

Methane is separated from natural gas through fractional distillation.

REACTIONS

• Oxidation

• Halogenation

reactivity of X₂ : F₂ > Cl₂ > Br ( > I₂ unreactive)

OXIDATION: Heat of Combustion

Heat of Combustion - the amount of heat evolved when one mole of a Hydrocarbons are burned to carbon dioxide and water.

HALOGENATION: Chlorination

Halogenation is a substitution reaction. (RH + X₂ ---> RX + HX)

CONTROL OF HALOGENATION: Chlorination

Use a large excess of CH₄.

Halogenation is more likely to occur on the methane than a methane which has already been halogenated.

REACTION WITH VARIOUS HALOGENS

• Bromine (Br₂) is somewhat slower than chlorine (Cl₂).
• Iodine (I₂) does not react.
• Fluorine (F₂) reacts too rapidly, therefore, explosive.
• Reactivity of halogens F₂>Cl₂>Br₂>(I₂)

RELATIVE REACTIVITY

Relative reactivity is the rate of reaction (kinetics).

As reactivity increases, the faster the reaction occurs.

REACTION MECHANISM

A reaction mechanism is a step by step description of how a chemical reaction takes place.

MECHANISM OF HALOGENATION: Free radical

An atom or group of atoms processing an odd (unpaired) electron is called a free radical. The dot represents the free radical.

XŽ (energy rich)

MECHANISM OF HALOGENATION: chain reactions

A chain reaction is a series of steps which occur in a sequence causing each step to occur following the previous step.

• Chain - initiating steps:

steps in which the chain reaction starts

• Chain propagating steps:

steps which keep to the reaction going

• Chain - terminating:

steps which might occur which causes the chain reaction to stop.

INHIBITORS

Inhibitor - a substance which slows down or stops a reaction even though the inhibitor is present in small amounts.

Inhibition period - time during which the inhibitor lasts.

Example: If oxygen is present during halogenation, the oxygen slows down the reaction.

This breaks the cycle (propagating steps) and slows down the reaction.

When the oxygen molecules are all reacted (inhibition period), the reaction then speeds up.

HEAT OF REACTION

Exothermal Reaction - energy is given off

Endothermal Reaction - energy is consumed

ENERGY OF ACTIVATION

The minimum amount of energy that must be provided by a collision for a reaction to occur (E_act)

This minimum energy is greater than the heat of reaction.  Sometimes only one collision in forty has sufficient energy to provide the E_act.

Also the collisions must be orientated correctly (aligned).