Chapter 12 Alkanes Flashcards
What are alkanes?
Saturated hydrocarbons which contain carbon and hydrogen atoms only, joined together through C-C single bonds
How are the atoms in an alkane bonded together?
Each carbon atom is joined to four other atoms by single covalent bonds known as sigma bonds σ.
Each carbon atom has 4 sigma bonds.
What is a sigma covalent bond (σ)?
A sigma bond is the result of the overlap of two orbitals, one from each bonded atom. Each overlapping orbital contains one electron, so the sigma bond has 2 electrons which are shared between the bonding atoms. The sigma bonds are positioned on a line directly between two bonding atoms.
What happens to the boiling point of straight chain alkanes?
The boiling points will increase as the Carbon chain gets longer due to the London forces between the molecules. This is because the number of atoms in the chain increases so there are more electrons in the molecule which increases the strength of the London Forces providing a higher boiling point as a larger amount of energy is required to overcome the bonds.
Additionally, as the chain length increases the alkanes will have a larger surface area so there is more surface contact between the molecules increasing the strength of the London Forces.
What does the overlap of Sigma covalent bonds allow in alkanes?
Allows free rotation
How do Branched isomers affect the boiling point of alkanes?
Branched chain isomers have the same number of electrons, however they have a lower boiling point than straight chain alkanes, this is because branching reduces the surface contact between the molecules. Therefore, the London forces are weaker and don’t require a lot of energy to be overcome.
Why are alkanes unreactive?
The C-C and C-H σ covalent bonds are strong, they have high bond enthalpies
C-C bonds are non-polar
The electronegativity of Carbon and Hydrogen is similar so it is also considered non-polar.
Therefore, the σ bonds are low in polarity.
What does the complete combustion of alkanes produce?
When alkanes burn in a plentiful supply of oxygen, carbon dioxide and water is produced.
Why are alkanes useful as fuels?
Readily available
Easy to transport
Burn in a plentiful supply of oxygen without releasing toxic products
What does the incomplete combustion of alkanes produce?
When alkanes burn in a limited supply of oxygen carbon monoxide or carbon and water is produced.
What do alkanes react with halogens in the presence of?
UV light provides the initial energy for a reaction to take place.
What mechanism do alkanes proceed through?
Free radical substitution
What are the stages of Free radical substitution?
Initiation
Propagation
Termination
What happens during the initiation stage of Free radical substitution?
Initiation- UV light provides the energy required to break the covalent bond of the halogen via Homolytic fission. Both of the bonded atom will take one of the shared pair of electrons each, so two free radical will be formed.
What happens during the two propagation stages of Free radical substitution?
The free radicals are used up and regenerated
First propagation step: the halogen radical reacts with the C—H bond in the alkane, forming a new radical and another molecule.
Second propagation step: the new radical that was formed reacts with another halogen molecule forming an organic product and a halogen radical.
The propagation step is terminated when two free radical collide.
