Alkanes

What are the reasons for an alkane's lack of reactivity?

- C-C and C-H bonds are strong. - C-C bonds are non-polar. -The electronegativity of carbon and hydrogen is so similar that the C-H bond can be considered non-polar.

What is a sigma bond?

-A sigma bond is the result of the overlap of two orbitals, one from each bonding atom. -Each overlapping orbital contains one electron, so the sigma bond has two electrons that are shared between the bonding atoms. -A sigma bond is positioned on a line directly between bonding atoms. -Each carbon atom in an alkane has four sigma bonds.

What is the covalent bond in an alkane called?

-A sigma bond.

What is further substitution?

-After CH3Br is formed, another bromine radical can collide with a bromomethane molecule, substituting a further hydrogen atom to form dibromomethane, CH2Br2. -Further substitution can continue until all hydrogen atoms have been substituted. -The result is a mixture of these compounds.

What are alkanes?

-Alkanes are the main components of natural gas and crude oil. -They are amongst the most stable organic compounds, and their lack of reactivity has allowed crude oil deposits to remain in the Earth for many millions of years. -Saturated hydrocarbons.

What is combustion?

-Alkanes can react with a plentiful supply of oxygen to produce carbon dioxide and water - this is combustion. -All combustion processes give out heat, and alkanes are used as fuels because they are readily available, easy to transport, and burn in a plentiful supply of oxygen without releasing toxic products.

What is the general formula for an alkane?

-CnH2n+2

What is carbon monoxide?

-Colourless, odourless, and highly toxic gas. -Combines irreversibly with haemoglobin in red blood cells to form carboxyhaemoglobin, which prevents the haemoglobin from transporting oxygen around the body. -Severe CO poisoning will often turn a victim's lips a cherry-pink. -The danger lies in the lack of any odour, so a person can be poisoned without noticing any danger.

What is the shape of an alkane?

-Each carbon atom is surrounded by four electron pairs in four sigma bonds. -Repulsion between these electron pairs results in a 3D tetrahedral arrangement. -Each bond angle is approximately 109.5 degrees. -The sigma bonds act as axes around which the atoms can rotate freely, so these shapes are not rigid.

What are alkanes mainly used as?

-Fuels, exploiting their reaction with oxygen to generate heat.

What are the limitations of radical substitution in organic synthesis?

-Further substitution. -Substitution at different positions in a carbon chain.

What is substitution at different positions in a carbon chain?

-If the carbon chain is longer than ethane, we will get a mixture of monosubstituted isomers by substitution at different positions in the carbon chain. -With further substitution there are even more possibilities.

When will alkanes react with halogens?

-In the presence of sunlight. -The high-energy UV radiation present in sunlight provides the initial energy for a reaction to take place. -These are substitution reactions, as a hydrogen atom in the alkane is substituted by a halogen atom.

What are the three stages of radical substitution?

-Initiation. -Propagation. -Termination.

What is the effect of branching on boiling point?

-Isomers of alkanes have the same molecular mass. -If you compare the boiling points of branched isomers with straight-chain isomers, you find that the branched isomers have lower boiling points.

Why does the boiling point of alkanes increase with chain length?

-London forces act between molecules that are in close surface contact. -As the chain length increases, the molecules have a larger surface area, so more surface contact is possible between molecules. -The London forces between the molecules will be greater and so more energy is required to overcome the forces.

What is the mechanism for forming halogenoalkanes?

-Radical substitution.

What happens when there is a limited supply of oxygen?

-The hydrogen atoms are always oxidised into water. -Combustion of the carbon may be incomplete, forming the toxic gas carbon monoxide or even carbon itself as soot.

What is initiation?

-The reaction is started when the covalent bond in a bromine molecule is broken by homolytic fission. -Each bromine atom takes one electron from the pair, forming two highly reactive bromine radicals - a radical is a species with an unpaired electron. -The energy for this bond fission is provided by UV radiation.

What is propagation?

-The reaction propagates through two propagation steps, a chain reaction. -In the first propagation step, a bromine radical reacts with a C-H bond in the methane, forming a methyl radical and a molecule of hydrogen bromide. -In the second propagation step, each methyl radical reacts with another bromine molecule, forming the organic product bromomethane together with another bromine radical. -The new bromine radical then reacts with another methane molecule as in the first propagation step, and the two can continue to cycle through in a chain reaction. -In theory the propagation steps could continue until all the reactants have been used up.

Why do branched isomers of alkanes have lower boiling points?

-There are fewer surface points of contact between molecules of the branched alkanes, giving fewer London forces. -Another factor is the shape of the molecules. The branches get in the way and prevent the branched molecules getting as close together as straight-chain molecules., decreasing the intermolecular forces further.

What is termination?

-Two radicals collide, forming a molecule with all electrons paired. -There are a number of possible termination steps with different radicals in the reaction mixture. -When two radicals collide and react, both radicals are removed from the reaction mixture, stopping the reaction.

In practice, when is propagation terminated?

-Whenever two radicals collide. -It has been estimated that up to a million propagation cycles take place before a termination stops the reaction.